6 4 Associated CointreeAlex on 01/02/2022 - 14:07 cointree.com (142...Read More
Love it or hate it, disc brakes on drop-bar bikes are here to stay. With all major drivetrain manufacturers now fully committed to the cause, and bike sales figures showing little demand for rim brake bikes, there’s a high chance your next bike will feature disc brakes. However, the number of advantages they offer do bring about a number of quirks and maintenance demands.
In this article, we seek to arm you with all the knowledge (and then some) you need for owning a disc-equipped bike, regardless of the cycling discipline. This article may not be the most exciting to read, but we hope it’s at least useful.
Note: This is an updated version of an article first published in May 2018, and a lot has changed since then!
A disc brake is a braking system where the brake pads are contained in a caliper and create friction against a disc (rotor) that is attached directly to the wheel. This friction between the two surfaces slows the wheel.
Disc brakes are the most common braking system on modern cars and motorbikes. Disc brakes have been around on bicycles for a decent amount of time – as early as the 1950s, in fact. Mountain biking really brought discs to popularity in the early 2000s, and they’ve been the standard braking system there for over twenty years.
In concept, disc-brake calipers aren’t all that different from rim-brake calipers, either, except that the caliper bodies are much smaller, and the pads are made of much harder materials than the various rubbery compounds used in rim brakes.
Similar to the brakes found in modern cars or motorbikes, disc brakes are widely thought to provide more consistent braking control versus a rim-based brake. Moving the braking surface away from the rim means that the rim no longer needs to serve the double duty of handling braking heat, retaining the tyre, and resisting friction-based wear. And doing so allows the braking surface to be made from consistently-surfaced steel or a similar effective heat-managing material, which in turn means the brake pads can be made from a harder and more durable material, too.
The consistent surface of the disc brake goes a long way to not having pulsing or similar tight spots in the braking. Similarly, the rotor is further away from the muck and spray of the tires in wet weather, and being far smaller, it’s quicker for the braking surface to clear itself of unwanted elements. By contrast, your stopping distance with a rim brake is greatly increased because the larger circumference of a rim needs to be cleaned off by the pads before maximum friction is achieved.
Hydraulic brakes introduce other leverage and friction-based advantages that allow for less effort from the rider. They are also a sealed system that won’t degrade as a result of dust and grit, and similarly, they offer automatic adjustment as the brake pads wear.
Lastly, there are the system benefits. Disc brakes help to open up clearance for wider tyres and unique rim shapes. The rims, no longer needing to handle braking duties, can be made lighter and/or more aerodynamic. Frame designs no longer need to be symmetrical to accommodate the mounting of rim brakes. And engineers can now play with the flex in the areas of the frame that were previously required to be stiff for secure rim brake performance.
There are a few notable disadvantages to disc brakes.
Firstly, hydraulic disc brake technology is obviously more complicated and intricate than a mechanical rim brake system. This, of course, introduces its own setup and servicing idiosyncrasies (covered later on), and when done wrongly can lead to excessive braking noise, brake rub, or poor performance.Disc brakes carry increased costs, something that’s especially applicable to road and gravel bikes where the hydraulic master cylinder must be integrated with the shifter.
Similarly, disc brake technology has more components to it and therefore, is more expensive. And at least on drop-bar bicycles, disc brakes typically introduce a small weight penalty, too.
There’s also the aerodynamic argument – I’ll return to this point.
A mechanical disc brake is much like a traditional rim brake, using a mechanical brake lever and cable that is connected to the brake caliper. When the brake lever is pulled, it pulls a cable that then actuates a lever that pushes a single brake pad, flexing the rotor enough for it to get squeezed between the moving brake pad and a fixed one. Some mechanical disc brakes, such as the TRP Spyre, work by pushing both brake pads simultaneously against the rotor.
A hydraulic brake (pictured at the top of the article) is more like what a modern car would use, where fluid is used to transfer force. Here, fluid is stored in a reservoir called the master cylinder, which resides inside the brake lever body. When the lever is pulled, it pushes a piston that forces fluid out of the master cylinder and through the brake hose to the caliper. That fluid then pushes the caliper pistons (to which the pads are attached) against either side of the rotor.
Mechanical brakes are simple, easy to repair, and highly reliable, and as such are often the choice for touring and bike-packing use. However, the cables and housing are less efficient at transferring power from the lever to the caliper, they’re more susceptible to contamination, and they have to be manually adjusted as the pads wear.
By contrast, hydraulic disc brakes automatically compensate for pad wear, they’re generally lighter as a system, and they’re fully sealed from the elements. Additionally, the hydraulic design produces less mechanical friction in the system, and impressive force amplification can be engineered in, too, meaning a light effort at the lever equates to a whole lot of braking force generated at the wheel. Meanwhile, recent trends in high-end dropbar and mountain bike designs require the flexible nature of hydraulic hoses for more intricate concealed cable routing.
More importantly, the biggest players of the industry are deeply invested in the development and sale of hydraulic disc brakes, while these big brands typically only offer mechanical disc bakes as a budget entry-level option.
No, but you can connect a mechanical brake lever to a semi-hydraulic brake. Options are limited for this, but both TRP and Yokozuna offer such a thing. Both of these feature a hydraulic caliper where the fluid master cylinder is in the caliper body itself. Your mechanical brake lever pulls a mechanical cable, which pulls on the piston rod at the caliper. It’s still a compromise, with cable friction losses and increased weight as issues. But if you’re looking for improved braking performance on your mechanical disc-equipped bike, or looking to re-use some existing mechanical brake lever/shifters on a new bike, it’s a viable option.
Meanwhile, a number of more entry-level Giant road and gravel bikes offer another example, where a mechanical road shifter/brake lever is connected to a stem-mounted hydraulic master cylinder. The mechanical cable from the brake lever connects directly to this master cylinder, which then has a hydraulic hose that connects to a hydraulic disc brake caliper. This system aims to offer performance near to that of a full hydraulic brake system but at a lower price point. In reality, these semi-hydraulic systems do perform better than a mechanical brake, but carry an additional weight penalty and don’t ease service requirements.
This is a tough question, and it depends.
For road bikes, there is an undeniable weight penalty to disc brakes. Simply comparing something like the Shimano Dura-Ace R9100 brake levers and rim-brake calipers (including an approximate weight for cables) with the equivalent disc-brake system, you’ll see the disc brakes are approximately 200g heavier, accounting for cables, housing, and brake hoses.Much of the weight gain related to disc brakes is due to the rotors.
Additionally, disc-brake hubs are heavier since they require interfaces for mounting the rotors, and disc-compatible wheels are typically built with a higher spoke count or heavier gauge spokes to handle the twisting forces introduced by discs. However, we’re beginning to see some of this increased weight offset with a lighter rim, since it no longer has to include a braking surface and/or a flat exterior shape.
Conversely, some – but not all – modern disc-compatible road frames are actually lighter than their rim-brake counterparts, but the stiffer thru-axles usually paired with them add some of that weight back. Overall, many top-tier brands claim the weight difference between rim-brake and disc-brake frames is between 30-90g.
Taking everything into account, you’re typically looking at a 300g disadvantage with disc brakes, and that’s really a best-case scenario. More often, and where less generous budgets are involved, the weight gain is typically closer to 500g (1.1lb) or more.
However such weight gain isn’t the case for all cycling disciplines, and there was a turning point for disc brakes in cross-country mountain bike racing about 15 years ago, where disc brakes as a complete system (including wheel weights) became lighter than equivalent rim-brake options. While we may never see that happen in road disciplines, for sure the gap will continue to narrow.
Being able to lock a wheel is not a sign of great braking performance. Rather, great braking performance should be measured by the ability to have fine control of your braking power at the traction limit of your tires. This is often referred to as ‘modulation’, and it’s the ability to more precisely apply the power that reduces your stopping distance. In a race, this allows you to brake later into corners and have the confidence to carry more speed out of them.
See above for other advantages of disc brakes.
All things being equal, disc brakes alone are marginally less aero than rim brakes. However, an increasing number of bike brands, such as Giant with their Propel Disc or Factor and their Ostro, claim that by removing the clutter of rim brakes and being able to open up room around the fork, the aero effects of a disc rotor can be more than balanced out. Meanwhile, hydraulic disc brake hoses have made it simpler for integrated cockpits with concealed cabling to be used – something that has fast become a normal sight at the top end of road cycling.
More importantly, some of the fastest aero bikes have traditionally used rim brakes that perform with dubious quality, not to mention the fact that braking performance on carbon rims (the go-to for performance racing wheels) isn’t as good as on aluminium ones.
Yes, as of the 1st of July 2018, disc brakes are authorised for use in all UCI-governed events. And given just about every amateur event, including Gran Fondos, looks to the UCI rules for guidance, disc brakes are now wholly accepted.
As of 2022, there are only a small handful of professional teams in the WorldTour remaining on rim-brake-equipped bikes. The rest have made the move to disc brakes.
Sort of, yes.
Disc brakes are now the norm in all off-road cycling disciplines and you’ll struggle to find riders that are keen to go back to rim brakes. Meanwhile, despite greater resistance from many riders, the trend to disc brakes on the road is equally clear, and most of the biggest bicycle manufacturers have now drastically reduced or discontinued offering rim brake bikes.
However, while most of the big brands have shifted away from rim brakes, you’re still able to work with any number of custom builders, or smaller brands, to get a bike with rim brakes. And there’s always the used market.
Meanwhile, sourcing rim brake components has become slightly more challenging in the past couple of years. All three major drivetrain manufacturers still offer groupsets with rim brake options, but they’re no longer investing in progressing the technology. Similarly, all new wheel releases are of wheels for disc-brake use only, meaning the technology progression for rim brake wheels arguably stopped prior to 2020.
In theory, yes, and also, no. Frame builder Rob English has jokingly argued this point nicely before.
Just to ruin a good joke, the advantages of disc brakes lie not in the size of the rotor, but in how efficiently force is transferred from the lever to the braking surface. The friction coefficients of carbon fibre and aluminium rims aren’t nearly as good as steel disc-brake rotors. Meanwhile, rim-brake calipers and housings are much more prone to power-robbing flex than compact hydraulic disc-brake calipers. Exceptions do exist, such as some specially treated rim-brake sidewalls and hydraulic rim-brake calipers, but for the most part, it’s really not the same thing.
Disc brake calipers require specific mounting points on the fork and frame, while the wheels need specific hubs onto which the rotor can be installed. While there were a handful of older (mostly cyclocross) bikes manufactured during the nascent days of disc brakes that could accept either format, the chance that you can install disc brakes to your existing rim-brake bike is extremely small.
In order to understand the automatic pad wear adjustment in a hydraulic disc brake caliper, you need to understand how the caliper pistons operate.
The pistons in a hydraulic caliper feature rubber seals that not only help to keep dirt on the outside and fluid on the inside, but they also flex back and forth with the piston. When you pull the brake lever, a piston at the lever creates fluid pressure down the brake line and into the caliper, which in turn sees the caliper pistons move. The caliper piston seals flex to allow the pistons to move out, and then once the fluid pressure is relieved (letting go of the lever), these seals then flex back into shape, pulling the pistons back with them.
These calliper piston seals have a specific amount of flex engineered into them. So for example, some seals may allow 1.5 mm of back-and-forth piston travel, meaning the piston (and attached brake pad) will extend out by 1.5 mm for the pad to contact the rotor, and then retract back by the same amount to stop the pad from rubbing the rotor when not in use.
As the brake pads wear and the brake is operated, the rubber seals hit their flex limit and can no longer can hold onto the piston before it hits the contact point with the rotor. At this point, the piston slides forward within the seal until contact with the rotor is made. Once this contact is made, the piston seal will then retract the piston back by 1.5 mm to its new starting point. And that is automatic pad wear compensation in a hydraulic disc brake.
Disc brakes pads are commonly available in a few different materials, namely metallic (aka sintered), organic (aka resin), or a combination of the two. Both materials offer different characteristics, and your choice should depend on your riding style, discipline, location, and propensity for riding in foul weather.
Resin – otherwise known as organic – disc pads generally offer improved modulation, quieter braking, and greater initial bite when cold. Some brands, such as SRAM and Campagnolo, also claim they dissipate heat better than metallic pads.
By contrast, metallic – also known as sintered – pads are made of harder materials and have a higher metal content than organic pads. As a result, metallic pads offer greater raw stopping power when hot and last longer than organic pads, albeit at the expense of noisier running and increased rotor wear.
Mountain bikers have long had this choice, and the improved durability and better power when hot means they often use sintered metallic pads. With less grit, less need for raw stopping power, and demand for silent braking, the majority of disc users on the road prefer resin pads.
Note: Some cheaper Shimano-equipped bikes come with “Resin only” disc rotors, which are made with fewer stages of heat treatment and therefore cannot be used with metallic pads. You’ll need better rotors if you decide to use metallic pads with these “resin-only” rotors.
The use of these fancier materials is in reference to the backing plate of the pad, and while most disc brakes pads use steel backing plates, some brands offer aluminium or even titanium-backed pads for weight savings. Meanwhile, the critical friction material of the brake pad is commonly unchanged. Looking to Shimano’s range of brake pads, high-end resin pads will often use aluminium backing plates, while the metallic versions will use titanium backing plate. Between the two, the aluminium ones are lighter.
This is not to be confused with Shimano’s Ice Tech finned pads, which are detailed below.
Brake pads are commonly brake manufacturer and model specific and so it’s important to match the brake pad to your brake caliper. Most brake manufacturers offer handy compatibility charts for what brake pad is required for your calipers.
If you’re unsure of your brake caliper model, then you’ll need to remove the pads to cross-reference the pad shape.
All major brake manufacturers offer high-quality brake pads that have been designed to work ideally with their own brake system and rotor. However, there are countless aftermarket and generic brake pad options intended to be an alternative to your brake manufacturer’s provided pad.
Some more premium aftermarket pad companies, such as SwissStop and KoolStop, claim to provide performance benefits such as reduced noise, improved longevity or greater bite. Meanwhile, most aftermarket options simply aim to be a cheaper alternative (and can introduce performance trade-offs).
The first letter in Shimano’s brake pad naming designates the pad shape, fitment and/or pad compound. Certain brake calipers call for certain pads, and so if you’re unsure, match the model name found on your caliper to the suggested list from Shimano. Shimano Dura-Ace R9100, Ultegra R8000 and 105 R7000 disc calipers use an L-series pad. Previous generation R785 calipers use a J-series pad, which is the same as many previous generation mountain bike calipers.
The letter at the end of the brake pad model name designates the backing plate material. For example, “A” is for alloy (aluminium), “S” is for steel, “Ti” is for titanium and “C” is for cooling (typically steel backing plate with alloy cooling fins).
If you’re shopping for new resin disc brake pads to suit your newer Shimano road calipers, you may notice a choice between L02A, L03A, and L05A. The “02” are the early generation resin pad that were superseded by the 03 which claimed to offer 50% better durability without giving up anything else. Shimano Dura-Ace and Ultegra 12-speed saw the introduction of the latest L05A pads which are backward compatible. If given the choice, pick the newer “03” or “05” option.
This is the brake pad spring.
Most riders assume this is what helps to push the pads back into place after using the brake, but that task is almost entirely handled by the caliper piston seals. Rather these springs keep the brake pads pushed up against the respective pistons and free from rattling.
Shimano and SRAM use these springs for their pads, while Magura and Campagnolo often rely on magnets to keep the pads from rattling.
We’ve seen disc-brake pads disappear within a single wet gritty ride, and then other pads last many seasons of casual riding.
Generally speaking, if ridden in conditions where grit isn’t an issue, disc-brake pads will go further than rim-brake ones, especially if you use metallic pads (see the section about brake pad types above). However, any amount of wet grit will see the pads abrade away decently quick.
The brake pad consists of the braking material and the backing plate. You can see how much pad material is left by peering through the caliper (a flashlight can be helpful). Experienced mechanics will typically judge pad wear based on sight, and a helpful indicator of this is how closely the pad thickness is to the pad spring – the pad thickness must sit proud of the pad spring. However, it may be necessary to take the wheel out for a closer visual inspection.
With Shimano pads, it’s suggested that you replace your pads when there is less than 0.9mm of braking material left. For SRAM, the minimum measurement is 2.5mm including the backing pads. Campagnolo makes it easier with a wear groove through the center of its pads; replacement is required as soon as the groove is no longer visible.This pad is almost worn out and was at a good point for replacement. You should replace your pads while the pad thickness protrudes out further than the steel pad spring.
In addition, look for uneven wear on the pad surface or rotor, as this is typically a sign of a poorly adjusted brake or a caliper mounting surface that needs to be faced.
And some brake pads are designed to make noise once they’re worn. For example, Shimano brake pads will typically begin to squeal when they have reached the recommended replacement point.
It’s a common occurrence for bicycle mechanics to find disc brake pads that have been used beyond their intended life.
Most brake systems will typically provide some form of warning that you’re running out of brake pad. As already mentioned, some will begin to make more noise under braking and most will present a softer feel at the lever with less power on tap. In the case of extreme wear, you’ll likely hear a ticking sound as the rotor catches the pad spring.
Riding past this point will see the pad material run out and you’ll begin to use the backing plate as a friction material – something that will require a new disc rotor. And in the most extreme cases, I’ve even seen the backing plate worn through to the point the piston is then used as a friction material, something that then requires new pads, a new rotor, and a new caliper.
Leaving your brake pads to wear beyond their useable thickness will likely see your fluid reservoir (master cylinder) compensate for a lack of capacity by drawing in air. A brake bleed is often required if pads are severely worn.
Disc brakes are hypersensitive to any form of contamination. Even the oil on your fingertips can be enough to upset the braking performance. Car brakes are arguably no different, but they offer far more surface area, and often produce more heat that burns away such issues.Take care when degreasing your drivetrain. It’s best to remove the wheel and cover the brake caliper if you’re going to be spraying degreaser.
Keeping your disc perfectly clear of any form of oil contamination is the key to disc-ownership happiness. This means not using spray lubes on your chain, and also being careful when washing your bike in terms of the methods and detergents used. When cleaning your drivetrain it’s a good idea to replace the rear wheel with a dummy hub and cover the brake caliper with a plastic bag or similar.
You shouldn’t need to, although maybe the occasional wiping away of brake dust with a clean and lint-free rag will be needed. A trickle of water can be good, too.Isopropyl (or rubbing) alcohol is the commonly recommended cleaner for disc brakes. I keep it in a refillable spray bottle.
If you do need to clean them, use isopropyl alcohol, and preferably the 90-100% varieties, not the diluted stuff you usually find in pharmacies. Yep, it’s not that cheap, nor easy to find, but it’s guaranteed to clean the braking surfaces without leaving any oily residue or causing seal damage.
Automotive disc brake cleaners can often be problematic. Some leave an oily residue, while others strip life out of rubber seals. And just about all degreasers will cause more harm than good.
Bicycle-specific disc-brake cleaners are more likely to be fine to use, but unfortunately, it’s not always the case. Some are just rebadged automotive cleaners, while others are just isopropyl alcohol at a higher price. They’re probably fine, but then again, isopropyl alcohol is still the recommended cleaner by all major disc brake manufacturers.
Brake bed-in, or burn-in, is the process of transferring pad material to the brake surface for proper function. It’s a crucial step any time new brake pads or brake rotors are involved, and skipping it will often result in poor brake power and constantly squealing. Read our brake bed-in article for more on this topic.
A disc brake rotor is the friction plate directly attached to the hub of the bicycle wheel. Slowing the disc rotor slows the wheel at the same rate.
These are the two common ways to mount disc rotors to hubs. Center Lock uses a splined interface to hold the rotor, and a lockring (which either uses a cassette lockring tool or an external bottom bracket tool) secures it in place. It’s an elegant system introduced by Shimano and now offered by most manufacturers of hubs and brake rotors. Both SRAM and Campagnolo (called AFS) have adopted it as the standard for performance road bikes.
Typically found on mountain bikes or cheaper road bikes, six-bolt rotors are simpler and see the rotors attach to the hub with six bolts. A Torx T25 is the common tool for these bolts.
You can identify either rotor type by simply looking at either the hub or rotor. A splined interface is Center Lock, and six-bolt is, well, six-bolt.A Center Lock adaptor allows a six-bolt rotor to fit onto a Centerlock (splined) hub.
It’s worth mentioning that six-bolt rotors can be installed on to splined hubs with the proper adapter, but the reverse isn’t possible.
The earliest Center Lock lockrings were designed at a time when most bikes had skinny quick-release axles and so the internal spline lockring (that used a regular Shimano HG cassette tool) was the standard. However, soon bikes were coming with 15 or even 20 mm diameter thru-axles that didn’t leave space for the cassette tool or the matching splined lockring. Shimano’s first answer was a non-standard lockring and special tool, but that never quite caught on. The next solution was to merely move the tool engagement point to the outside of the lockring.An internal lockring (top centre) versus an external lockring (bottom centre).
Today most 12 mm hubs used on disc-equipped road bikes work just fine with the original “internal” lockring (HG cassette tool required). Meanwhile, an increasing number of hubs with oversized axle end caps or those with 15/20 mm axles often demand the use of an “external” lockring. These external-type aluminium lockings will work with the vast majority of Center Lock hubs (except Fulcrum/Campagnolo AFS) and rotors on the market, including those using quick release, 12 mm, 15 mm and 20 mm thru-axles. The external rotor lockrings are driven by a regular 16 notch 44 mm bottom bracket tool (such as the Park Tool BBT-9, Wolf Tooth Pack Wrench, or Abbey Bike Tools “Common” socket).
Shimano now often gives you the choice between internal or external lockring types when buying rotors. Do note that not all road forks have the clearance to be used with all external type lockrings, and some wheel brands even include special low profile external lockrings to solve this issue.
Campagnolo brands its version of Center Lock as AFS. It uses the identical spline interface as a regular Center Lock rotor, but the lockring works in a different way. Where normal Center Lock lockrings thread into an internal thread on the hubshell, AFS uses an external thread that a specific lockring threads over the top of.An example of AFS, as seen on a Fulcrum wheel road disc wheel. Shimano, SRAM and other brands of centerlock rotors fit this hub without issue.
The AFS system is only found on Campagnolo and Fulcrum wheels. All Campagnolo “AFS” rotors will work on regular Centerlock hubs with a regular lockring, while all regular Centerlock disc rotors will work with Campagnolo and Fulcrum AFS hubs when used with the special lockring that is provided with the wheels.
Ice Tech is the heat management technology built into specific models of Shimano brake pads and rotors.
In the rotors, Ice Tech refers to a three-layer rotor construction, with an inner aluminium layer sandwiched between stainless steel braking surfaces. Since aluminium is a better heat conductor than steel, the claim is that this design dissipates braking heat better than an all-steel rotor. On higher-end models, such as the RT-99, this is combined with Freeza technology, where the aluminium core is also formed into a finned heat sink. Shimano claims these Freeza rotors to be approximately 50% more efficient at cooling than rotors with just Ice Tech.
In the brake pads, Ice Tech refers to backing plates with built-in heat sinks. As with the rotors, these help pull heat away from the brake pad and dissipate into the surrounding air. The disadvantage of the finned brake pads is increased weight and cost.
While it may not be fitted on the bike you’re buying, most of Shimano’s road-specific brakes are Ice Tech-compatible. Sometimes an upgrade in brake pads is all that’s required.
Rotor size selection is commonly based on the discipline of riding, the terrain, the rider’s ability, and personal preference. Most importantly, a larger rotor offers more material and therefore provides better heat management. The larger-diameter rotor also provides more leverage, and therefore increased braking power, than a smaller one. Meanwhile smaller rotors are lighter, less likely to get damaged in a wreck, offer a more gentle braking action, and are more aerodynamic.
There isn’t a standard size for mountain biking and riders typically choose rotor size based on how much heat management and stopping power they require. These days, 160-180 mm rotors are commonly used for cross country and trail riding, 180-200 mm rotors are common for trail and endurance riding, while larger 220-230 mm rotors are now commonly seen on e-MTBs and downhill bikes.
Meanwhile, rotor size choice on the road and gravel will often vary based on which manufacturer you ask. Most companies, including SRAM and Campagnolo, suggest 160mm-diameter front rotors on the road, as a conservatively safe option for managing heat on extended downhills. However, Shimano says that most riders will be fine on a 140mm-diameter front rotor thanks to the temperature-reducing Ice Tech technology.
Given that as much as 70% of your braking power comes from the front wheel, there is a trend, borrowed from mountain bikes and other disc-equipped vehicles, to mismatch rotor sizes on road and gravel bikes. An example of this is with the Specialized Tarmac SL6 Disc, where a larger 160 mm rotor is featured up front for increased brake power and heat control, with a lighter 140 mm rotor at the back. While an off-road-going gravel bike such as the BMC URS is designed with a 180 mm rotor on the front, and a 160 mm rotor at back.
It’s important to note that frames and forks are typically optimised for a particular size, and larger rotors can create more stress than the product may have been designed to handle. If you’re looking to size up, consult your frame and/or fork manufacturer for the maximum recommended rotor size.
Let’s first cover one-piece rotors. Most of these are stamped from a single piece of steel. Many cheaper six-bolt rotors are one-piece.
Two-piece rotors see the braking surface attached to a separate carrier or spider. This is often done to allow for a splined Center Lock attachment, but on higher-end models, it can also reduce weight, increase rotor stiffness, and help with heat management, helping draw the heat away from the steel braking surface and into the aluminium carrier.An example of a two-piece rotor, note how to the steel braking track (silver) is riveted to an aluminium (black) carrier. A one-piece rotor would be stamped from a single piece of steel (typically silver).
Swapping wheels between a disc brake bike may cause unexpected disc rotor rub. This is because the specific placement of the rotor in relation to the axle isn’t an industry standard.
If the wheels you’re swapping between have the same hubs, or maybe you’re just lucky, there should be no difference in rotor placement, and you’ll be fine to swap between wheels.
However, with so little clearance between the brake pads, and just enough variability between different makes and models of hubs and wheels (and sometimes even within the same make and model), brake rotors can often be placed in a different position relative to the dropout. If you only rarely swap wheels, then readjusting the caliper is the obvious answer. However, if you’re looking to swap regularly, then it’ll be worth spending the time to set the wheels up so that all of the rotors are in exactly the same plane.
For where there is a difference in placement, the answer is to use rotor shims on the wheel that has the rotor sitting more inboard than the other. A variety of brands (e.g. Syntace) produce shim kits for 6-bolt rotors, while November Bicycle and a few other brands offer shim kits for Center Lock rotors. In both instances, all that is needed is to add a shim(s) to the wheel(s) where the rotor sits inboard until it no longer rubs on the inboard brake pad.Specific rotor spacers exist for six-bolt and centerlock.
Most brake manufacturers believe that you’ll likely get the best performance by matching the rotor to the brake system. However mixing rotor brands rarely results in any major issues (as long as the caliper is adapted for the correct rotor diameter), and some riders have found that certain rotors offer better bite (power) or reduced noise than others.
Unfortunately, the topic of which rotors are best is a complicated one as it varies based on the caliper, brake pads being used, and personal preference. Often the easiest solution is to stick with what your brake manufacturer recommends.
Similar to mixing and matching drivetrains, disc brake systems officially have very limited cross-compatibility, but there are some exceptions.
Cross-compatibility is one of the key benefits of mechanical disc-brake systems, and mixing and matching different levers and calipers isn’t just possible, it’s often baked into the design of the caliper. However, be aware that mechanical mountain bike brake levers and mechanical road brake levers are designed to pull a different amount of cable, and there are mechanical disc brake calipers to suit both. Equally, even amongst road bike brake levers there are some differences with the amount of cable pulled for a given amount of lever movement, and certain combinations will work better than others. For example, newer Shimano road levers will generally produce a firmer lever feel with more pad clearance, but less power, while SRAM and Campagnolo levers will typically yield excellent power, but less pad clearance and a softer lever feel.
Hydraulic systems are far more restrictive. Fluid capacity, brake fluid types, piston diameters, and hose fittings are enough reason to match the caliper manufacturer with the lever. There are some cases where you’re able to mix and match between different generations from within the same brand, but again, the manufacturers rarely suggest doing so.
This was also covered in the brake pad section above but it’s worth reiterating.
Brake bed-in, or burn-in, is the process of transferring pad material to the brake surface for proper function. It’s a crucial step any time new brake pads or brake rotors are involved. Read our brake bed-in article for more on this topic.
Assuming they’re not just dirty, a change of colour is often a sign of heat damage. Heat-damaged rotors will range in colour from a pale yellow, darker yellow, to a progressively darker colour based on the damage, moving into brown, purple, or even blue if truly roasted. If your rotors progress past yellow, you should look at ways to better manage the heat, such as using IceTech pads for Shimano systems and/or running larger rotors.
Yes, they certainly do. Look closely at a Shimano rotor and you’ll see “1.5mm” stamped on it. This is the minimum recommended thickness prior to replacement. Most brands hover around this mark. For example, SRAM suggests 1.55mm minimum thickness for most of its rotors.
The good news is that the stainless steel braking surface of disc rotors is inherently durable, and you’ll likely go through a few sets of disc pads prior to needing new rotors. And when you do, just be happy it’s not your rims that are needing replacement.
Correctly measuring the thickness of a rotor can be slightly tricky as you need to measure the centre of the brake track (likely the thinnest part) and be extremely accurate. Recommended tools for measuring brake rotor wear have been covered in a Cool Tool Tuesday feature.
Post-mount calipers are the common standard on modern mountain bikes and were used in the early days of road disc brakes. Here, the brake caliper mounts onto threaded posts on the frame, with the bolts running through the caliper. The Post-mount system allows for easy repositioning and adjustment of the caliper.
Flat-mount is a newer road-specific disc brake mount and is more compact and lighter than post-mount. With flat-mount, the rear caliper is affixed with two bolts running through a flattened frame surface and then threading directly into the caliper (or adapter affixed to the caliper). At the front, the caliper is first bolted to an adapter, which is then bolted to two threaded inserts on the fork. Flat-mount’s more compact sizing has allowed disc brake calipers to be tucked into the corner before chainstays and seatstays, however the system is known to be a little more fussy to align.
Post-mount calipers can be installed for use on some flat-mount frames with the appropriate adapters; however, adapters for going the other way aren’t widely available due to common frame clearance limitations.
A third brake mount type is the Internal Standard (IS), something that was the successor to Post Mount. This mounting standard has two bolts that sit alongside the caliper, and alignment is done via the use of shims. The IS system is still occasionally found on small brand steel frames or similar niche bikes.
Yep! Being able to trim the brake hoses is a feature of all systems on the market. However, it can be an involved task.
At a minimum, you’ll need a way to trim the reinforced hose cleanly, and you’ll require replacement barbs and olives (matched to the brake system) to push into the freshly cut end of the hose. It’s the barb and olive that create the airtight seal for wherever the hose is attached. While it is possible to cut most hoses without having to bleed the system, it’s more likely a bleed will be needed.
The topic of tools related to hose trimming and barb pressing was covered in a dedicated Cool Tool Tuesday article.
The process for replacing pads in a hydraulic disc brake will depend on the specific brake model in question. Consult the manual for your brake model for specific instructions.
That said, the trickiest part to brake pad replacement is often related to resetting the caliper pistons back into their bores (as a reminder, they progress out further as the brake pads wear). Many riders will simply slide a blunt tool between the old pads to push the pistons apart, however, this can also force dust and dirt into those critical seals. As a result, the best practise is to clean the pistons and caliper prior to pushing resetting them. The topic of piston cleaning and tools for pushing pistons has been covered previously.
There are two types of fluid used in bicycle hydraulic disc brakes: DOT and Mineral Oil. They are strictly not cross-compatible with each other.
The most common fluid type in the bicycle industry is mineral oil, something used by Shimano, Magura, Campagnolo, Tektro, TRP, some Formula brakes, and some of the latest SRAM brakes, too. Each brake manufacturer specifies its own unique mineral oil brake fluid, and it’s common for them to warn against using anything else. Mineral oil is hydrophobic and doesn’t absorb water; if you were to pour mineral oil into a jug of water, you’d see it float to the surface. Because of this, mineral oil offers a superior shelf-life and carefree maintenance versus brakes that use DOT fluid. Mineral oil also offers a high boiling point (important for a brake fluid) and is non-corrosive to paint.
By contrast, the likes of SRAM, Avid (owned by SRAM), Hope, and Hayes use glycol-based DOT-fluids, typically DOT 4 or DOT 5.1. DOT (Department of Transportation) are brake-specific fluids used in the automotive industry that, when fresh, offer an incredibly high boiling point. Some brake manufacturers suggest that the regulated nature and easy accessibility of DOT fluids is enough reason to use it, while others suggest the real advantage is in the use of the required stronger and more flexible EPDM seals that provide a brake with a snappier and higher-performance feel to it. However, DOT fluids are hygroscopic and absorb water, and that eventual water absorption will see the fluid lose its high heat resistance. It’s also worth noting that DOT fluid is corrosive to paint.
It’s the need for those EPDM seals with DOT fluid that is why you must never mix mineral and DOT fluids together. Both fluids will cause irreversible seal damage to the opposite system, something that often presents itself with swollen seals that prevent the brake from functioning.
SRAM’s own marketing materials suggest that brakes running on Mineral oil are perhaps best for those who don’t want to do basic maintenance or only ride their bikes infrequently, while they maintain that brakes using DOT fluid are a higher performance product but need more frequent fluid replacement (bleeding). The likes of Shimano dispute such a claim and state that mineral oil is a superior option for bicycle brakes.
It’s commonly thought that mineral oil is safer to handle while DOT is toxic, however, both fluid types are to some degree toxic and not safe for aquatic life. It’s recommended to wear protective gloves and eyewear when handling either fluid, and to dispose of them responsibly.
It’s most likely that your brake calls for a specific type of glycol-based DOT fluid to be used, typically DOT 3, DOT 4, or DOT 5.1. All of these are cross-compatible with each other. The exception is DOT 5, which is silicon-based and not cross-compatible with other fluids or recommended for use with bicycle brakes.
The various DOT fluids carry different minimum boiling points, with the standards calling out temperatures for both dry (fresh and no water content) and wet (old and water absorbed) fluid. DOT 3, DOT 4, and DOT 5.1 each have a minimum dry boiling point of 205°C, 230°C, and 270°C respectively, meanwhile the wet boiling points are higher for each respective fluid, too. It’s because of this higher boiling point that SRAM uses and recommends DOT 5.1 with its DOT brakes.
Spongy-feeling levers, inconsistent brake feel, or a sudden loss of brake power are sure signs your brakes could use a bleed. These symptoms are likely the result of air in the system, which can happen from a brake that wasn’t bled correctly to begin with or if you let your brake pads wear too far.
One easy way to check for this is to tip the bike upside down while repeatedly squeezing the brake lever. Does it change the feel? Did the lever just pull to the bar? If so, you’ve got air in the system and need a bleed. If it feels exactly the same, you’re likely fine.
Another reason for re-bleeding is the degradation of the fluid, often caused by excessive heat. Downhill mountain bikers know this too well, with brakes changing feel after extended downhills where rotors become red hot (where’d you be risking blowing a tyre with rim brakes). Another reason is that DOT fluid – such as that used in SRAM systems – absorbs water over time, which eventually lowers the boiling point of the fluid.
Whatever the reason, a system bleed will flush out the old fluid and purge air from the caliper, hose, and lever. Most brake manufacturers recommend this to be done annually, a good interval for anyone with DOT-based brakes. Meanwhile, we know plenty of people who ignore this interval with mineral-oil based brake systems. Whatever brake system you have, your brakes are likely to offer an improved feel and performance with fresh fluid.
The goal of bleeding a brake is to replace the old fluid with new and to remove any pockets of air from the system. The specific process and tools required will vary depending on the brake model. The following links should help.
The topic of tools related to bleeding disc brakes has been covered in-depth as part of the Cool Tool Tuesday series.
Often the cause of mechanical disc brake sponginess is related to compression from the brake cable housing. You can buy compressionless brake housing from Yokozuma, Jagwire, and a few other brands that should go a long way toward improving the feel and performance of your mechanical disc brakes.
Another common reason is that the disc rotor is being bent across to the opposite pad upon brake use. The solution to this is to adjust the brake caliper so that the fixed pad (the one that doesn’t move when you pull the lever) sits as close to the rotor as possible without rubbing.
That said, everyone on the CyclingTips tech team agrees that no matter what parts or tuning tricks are used, there’s simply no mechanical disc brake on the market that rivals the braking performance of leading hydraulic disc brakes.
This only applies to hydraulic systems. It’s due to the self-adjusting nature of the hydraulic systems, where the pistons are designed to push out until they contact an immovable object (the disc rotor) and then retract a certain amount.
If you squeeze the brake lever without a rotor in place, the pistons will be pushed out until the two pads contact each other. It’s not the end of the world if that happens, though; in most cases, you’ll just need to reset the pads by slowly pushing them back into the caliper body with a brake pad wedge (or a clean flat blade screwdriver and a lot of care).
However, if your pads are severely worn (or aren’t in place), it’s possible that you could push the pistons beyond their intended travel, in which case you’ll spew fluid everywhere and unseat the pistons. Seriously, this is bad news.
If you do need to take a wheel out for travel, either be careful to not touch the lever, or use a pad spacer/travel wedge. These are plastic wedges supplied with every bike and brakeset (your shop should have some spare, if they didn’t give you some already). They simply clip into the brake caliper for travel and prevent the pads from being pushed together. Always ensure they’re clean prior to using them.A pad spacer (typically brake model specific) prevents you from accidentally pushing the pistons out.
Nope. It’s a non-pressurized sealed system, and the relatively minor changes in atmospheric pressure associated with air travel are no big deal.
It’s mostly a myth, but with reason.
The earliest disc-brake systems on mountain bikes could suck air into the hoses if the bike was turned upside down. However, none of the newer brake systems on the market use this design, and in theory, there’s no air that can migrate into the lines.
However, modern brakes that are improperly bled can leave air trapped in the fluid reservoir at the lever, which will only present itself when the bike is inverted and the air travels to a point in the brake where it can be compressed under pressure (when you squeeze the lever). If you have a bike with modern disc brakes and the levers go soft when the bike is flipped upside down, that’s a good sign you’re due a bleed.
It’s not uncommon to accidentally bend a rotor when travelling with a boxed or bagged bike. Thankfully an increasing number of bike travel cases now provide built-in disc rotor protectors, although some are certainly better than others.
If your bike travel case or bag doesn’t have a means for protecting rotors, then I’d suggest taking steps to keep the rotors safe. The first option is a disc rotor protector, something that many disc-equipped bikes are shipped with from the factory. Ask your local shop for some, assuming they haven’t made it to the bin yet. However, these will only do so much, and so my advice is to remove your rotors for travel.
Removing your brake rotors is easy with Center Lock rotors, and there are even lightweight tools designed specifically for this, such as the Wolf Tooth Components Pack Tool range. Six-bolt rotors are still easy enough to work with, just slower and fiddly.
Once removed, store the rotors in clean resealable plastic bags. This will ensure the rotors don’t get contaminated by touching anything else in your case.
It’s recommended to remove the brake caliper rather than split the brake hose, however, in rare cases where this isn’t possible, then there are a few solutions.
Formula’s Speed Lock is considered the gold standard as a quick-connect solution and the company sells hose kits to suit SRAM (DOT fluid), Shimano (mineral oil), and other brakes.
Another option is No.22’s own Brake Break which is designed to work with the company’s travel coupler frames, however, this hose disconnect component is both quite large and expensive. Meanwhile, SRAM offers its own hose disconnect at OE level (original equipment), a fairly elegant solution that’s unfortunately quite tricky to source.
More often than not, this is due to either glazed pads or contamination. Sure, sometimes it’s poor setup, sometimes it’s a warning of needing new pads, or that you’re just using noisy pads, but nearly always, it’s contamination or glazing.
Glazed pads are often the result of skipping the initial brake bed-in process and/or getting too much heat into the system. Taking the pads out and lightly sanding them until the glass-like finish is gone may solve the issue.
Otherwise, it’s likely contamination, something that’s surprisingly easy to do. Keeping any form of oil, degreasers, and bike cleaners away from your disc brake pads and rotors will go a long way to keeping them squeak-free. If you fear they’re contaminated, then try cleaning the rotor with Isopropyl alcohol and rubbing the brake pads on sandpaper. Unfortunately, badly contaminated brakes will likely require new pads and rotors.
Sounds like you’ve got a warped disc rotor. They’re often less than two millimetres thick, so it’s expected that they can bend. Because of this, it does pay to be careful: don’t lean the bike up against the rotor, and take extra care when transporting the bike in a vehicle or case to keep the rotors from being damaged.
If this has happened directly following a long descent, it may be heat-related. Give the rotor time to cool, and if still warped, continue with the advice below.
If the rotors are not terribly bent, they can be pulled or pushed back into shape. This is typically done with the wheel in the bike, using the brake pads as a gauge. Use a piece of clean paper towel to touch the disc surface with, and gently use your thumb at the point of rubbing to push the rotor in the opposite direction. Go easy; it doesn’t take much force to bend the rotor back.
Alternatively, just about every tool brand offers a specific rotor truing tool for this job. Or you can use an adjustable spanner, but just make sure it’s super clean before you do.
Did you just have the wheel out and now they’re rubbing like crazy? If so, refer to our section above about travelling with discs. The answer here is to push the brake pads (and pistons) back into the caliper to reset the system.
If this wasn’t the case, you’ve likely got a brake caliper that needs aligning. Peer through the caliper and see whether the rotor is rubbing on one specific side. Yes? Ok, it’s time to slightly loosen the two bolts holding the caliper in place. Once loose, many calipers will automatically centre by pulling the brake lever and then tightening the two bolts while holding the lever. Sometimes this doesn’t work, but there are plenty of other tricks to overcome this. Sighting through the caliper and carefully tightening the caliper may work (with a white piece of paper on the ground, to make visibility easier).
Another option is to use a business card between the rotor and pad that is rubbing. With the business card in place, repeat the process above. Loosen the caliper bolts, squeeze the lever, and tighten the bolts while holding the lever. This may take some trial and error, but it’ll typically fix the issue when the brakes are not automatically positioning themselves correctly.
If you’ve just installed new pads, and now the gap is minimal, try pushing the pistons back into the bores of the caliper (use a plastic tyre lever, or the ring-end of a small spanner). Sometimes your brake system will have too much fluid in it, which can occur if your brakes were given a quick bleed or top-up with worn pads in place. A fresh bleed, or carefully opening the system to release fluid (without letting air in!) will be required.
Discs brakes run on very tight tolerances, with typically only a millimeter or less of space between the rotor and the pads. This doesn’t leave much room for error. If your brakes make rubbing noises at random times, it’s either a sign of flex, a brake needing servicing, or outside elements.
Hub and frame/fork/axle flex can be the cause of the rubbing. Do you only ever hear it when leaning the bike over in a specific direction or in a sprint? If so, it’s flex. Checking that your axles are done up tight is the first step. Also check that there’s no play in your hubs while you’re at it. If that doesn’t take care of the issue, check that the caliper is properly positioned with an even gap to the pads on both sides of the rotor. If that doesn’t fix it, it may just be a flexy axle or dropout setup.
Sometimes pads rub because the pistons behind them are getting sticky. This can be due to a build-up of debris, or sometimes just with long-term use. But a brake service (where the pistons are reset and cleaned) and the brake bleed will usually solve the problem.
Lastly, mud, grit, or just plain water can be enough to make sounds on your discs. If you just rode through a puddle and now they’re rubbing, don’t stress about it. Applying your brakes will probably clear the muck and bring back the silence.
Assuming the caliper adjustment is right, and your rotor is straight, then it sounds like your frame/fork mount is to blame. Speak to your frame manufacturer about a solution, but most likely they’ll suggest getting the frame surfaces faced.The Park Tool Disc Tab Facing Tool (DT-5.2) is designed to face all common disc mount standards. All tools of this kind are intended to be shop tools.
Park Tool, Cyclus, VAR and a number of other specialist tool brands make disc tab facing tools to solve this very issue. Sometimes paint, or just crummy manufacturing tolerances can cause alignment issues. The disc tab facing tool will remove material until the brake caliper mounting surface is parallel to the wheel axle. It’s an extremely expensive tool, so look for a skilled mechanic or service shop that has one.
Bugger. Are you sure it wasn’t the race fencing? Actually, it was probably the chainring. Either way, you should probably seek medical attention.
A few things could be causing this, but most likely, it’s one of the following issues. To diagnose the issue, first confirm whether your brakes have a firm and consistent feel at the lever, or if it’s soft and spongy.
If your brakes offer a firm and consistent lever feel, then it’s most likely your braking surface is contaminated. Any amount of oil, even body oil from your fingertips, can upset the performance of disc brakes. A contaminated brake won’t just lack power, it’ll often squeal loudly under hard braking. Cleaning the system may help, but if it’s badly contaminated, new pads and a new rotor are likely the best answer.
Another cause can be an incorrect bed-in process, causing glazing or similar on the pads and rotors. See our article about brake bed-in to learn more.
Speaking of glazing, it’s possible for this to happen if the brakes are dragged on an extremely long descent. This can be identified with a glass-like finish to your brake pad surfaces. Remove the pads and rub them along some clean sandpaper; now repeat the bed-in process.
Heat can be another factor, with some brake pads (particularly metal sintered) requiring some heat in the system before they begin to bite at full capacity. Alternatively, it may be that your brake rotors aren’t big enough for your riding style or body weight, especially if your bike is currently equipped with 140mm-diameter rotors. Speak to your shop about whether your bike can support moving to a larger rotor that will offer greater leverage.
If your brakes have a soft or spongy lever feel, then it’s likely air in your system that’s preventing power from being transferred efficiently from the lever to the caliper. A brake bleed will solve this; see our section about brake bleeding above.
If this issue has only recently surfaced then it’s likely a sign that your brake pads and/or rotor are wearing thin and needing replacement.
If it’s not a new issue, then it sounds like there’s not enough fluid in the system. Check whether a bleed is needed by following the recommendation in the “How do I know when my brakes need bleeding?” section above.
If that’s not the cause, then this is known as lever stroke. You have a few other options.
Firstly, some newer and more premium brakes offer lever stroke adjustment. If this is an available option, there’s likely an adjustment screw that can be used to reduce the lever stroke and provide a firmer lever feel.
The process is a little trickier for brakes without a lever stroke feature. For this the goal is to advance the pads closer to the rotor, reducing the distance the pads needed to travel with the pull of the lever blade. To do this, remove the wheel (and rotor) and carefully pull the brake until you see the pads move. Re-install the wheel and check lever feel. Repeat this process until the lever is as desired. Alternatively, you can speed up the process by using a pad distance setting tool. Do be warned that doing this will make it harder to get a rub-free setup, and as the pads wear it may reset itself.
This is known as brake fade. It’s sometimes a sign that the brake fluid is getting too hot, and is starting to boil. When that happens, the once-incompressible fluid becomes a compressible gas, and you’ll quickly lose power. If this occurs, it’s strongly recommended that you move to a larger rotor size, or if available, heat-dissipating brake pads and rotors, such as Shimano’s Ice Tech range.
Alternatively, it may be a sign that you have air in your system. See the section above on bleeding brakes.
Well, that’s not good!
This is an extremely rare occurrence. The most likely cause of this is a bad bleed that has resulted in air being left in the brake. The first thing to do to rectify this in an emergency is to try to pump the brake, this is simply a matter of squeezing the lever quickly on and off. Assuming it’s air in the brake causing your sudden brake woes, then pumping the lever will hopefully move the bubble out of the caliper or hose and up into the reservoir.
If that doesn’t work, then most likely you’ll be left with one good brake. Use it carefully to come to a stop and then proceed with absolute caution. Speak to your local shop about the issue and have them check the system. Although extremely rare, a wrongly installed or faulty component can be the culprit for a full failure.
What did we miss? What are your top tips when it comes to disc brakes?
Disc brakes are a technical product and require a good amount of knowledge and ability to be serviced. With any of the tips above, be sure to consult with a professional if at any point you’re unsure. Functioning brakes do matter.
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