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by Dave Rome
May 23, 2018
Photography by Matt Wikstrom, James Huang & David Rome
Love it or hate it, disc brakes on drop-bar bikes are here to stay. With all major drivetrain manufacturers now committed to the cause, and an increasing number of brands pushing the advanced braking system, there’s a high chance your next bike (or bike after that) will feature disc brakes. However, the number of advantages they offer do bring about a number of quirks and unique maintenance demands.
The same questions regularly appear with nearly every mention of disc-equipped bikes, so we figured it’s time to answer them. This article may not be the most exciting to read, but we hope it’s at least useful. Let us know if we missed anything; This is the endless FAQ to road disc brakes, after all, and despite the length, there’s always room to add more.
Last updated: August 16, 2019.
What is a disc brake?
Disc brakes have been around on bicycles for a decent amount of time – as early as the 50s, in fact. Mountain biking really brought discs to popularity, and they’ve been the standard braking system there for nearly twenty years. But in terms of what makes them different, it’s really quite simple. Similar to the brakes found in modern cars or motorbikes, disc brakes move the braking surface from the rim to a dedicated metal (usually steel) rotor attached to the hub – that’s all.
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.
Mechanical versus hydraulic disc brakes: what’s the difference?
A mechanical disc brake uses a traditional brake lever, brake housing, and inner cable.
A mechanical disc brake is much like a traditional rim brake, using a standard road bike brake lever and cable that are connected to the brake caliper. When the lever is pulled, the cable actuates a piston 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 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.
Why is hydraulic braking superior?
Mechanical brakes are highly reliable, however the cables and housing are less efficient at transferring power from the lever to the caliper, they’re more susceptible to contamination, and have to be manually adjusted as the pads wear.
By contrast, hydraulic disc brakes automatically compensate for pad wear, they’re generally lighter, 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.
Can I turn a mechanical brake lever into a hydraulic one?
They TRP Hy/Rd is one example of a hydraulic brake caliper that can be used with a mechanical lever.
Simply put, no. However, you can connect a mechanical brake lever to some hydraulic calipers. Options are limited for this, but both TRP and Yokozuna offer such a thing. Both of these feature a hydraulic caliper where the 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 shifters on a new bike, it’s a viable option.
How much heavier is a disc-brake system?
This is a tough question.
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.
However, disc-brake hubs are also heavier since they require interfaces for mounting the rotors, and disc-compatible wheels are also heavier since they have to handle more twisting loads. As a result, disc wheels are typically built with a higher spoke count, crossed spoke lacing and often thicker gauge spokes, too. Some of that can be offset with a lighter rim, since it no longer has to include a braking surface, but it’s still a net gain in mass.
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 stricter budgets are involved, the weight gain is typically closer to 500g (1.1lb).
That’s likely to change over the coming years, though. For example, 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 on the road side, for sure the gap will continue to narrow.
I can already lock up a wheel with rim brakes, so why should I even bother with discs?
Being able to lock a wheel is not a sign of great braking performance, and yes, ultimate braking power is dictated by the amount of traction your tires have on the road surface. However, disc brakes provide finer control of your braking power just before lock-up, which is where you get the most effective braking. Better known as modulation, this ability to more precisely apply the power reduces your braking distance. In a race, this allows you to brake later into corners and have the confidence to carry more speed out of them.
Those who ride in wet conditions will also benefit from improved braking confidence regardless of the weather, as well as reduced maintenance. Nearly all hydraulic disc brakes will automatically adjust to pad wear, and you won’t suffer from worn rims due to gritty braking.
Hydraulic disc brakes require less hand effort, too, meaning a single finger is often all that’s needed to keep your speed in check.
Enve’s SES 4.5 AR Disc rim is not only wider than its rim-brake counterpart, but it’s lighter, too.
Lastly, there are the engineering freedoms disc brakes bring. By removing the brake caliper from the fork crown or seatstay bridge, manufacturers have been able to greatly increase tyre clearance and improve frame compliance. Likewise, hydraulic disc brake lines can be routed with tighter bends than mechanical cables and housings, which greatly eases routing complications. Wheel manufacturers also have the potential to build lighter rims (although in practice, those differences are very minor at present).
But yes, there are notable disadvantages to disc brakes. They’re heavier, the initial setup is more complicated, and they have their own idiosyncrasies. And yes, the technology is still more expensive.
Are disc brakes more or less aero than rim 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, claim that by removing the clutter of rim brakes, the aero effects of a disc rotor can be balanced out.
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 isn’t as good as on aluminium ones. Regardless, disc brakes are opening up possibilities for aero benefits elsewhere on the bike, and we’re only starting to learn how.
Can I race with disc brakes?
Come 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 widely accepted.
Is the industry going to force us onto disc brakes?
Not as long as rim-brake bikes are still being sold. There’s no denying the trend toward disc brakes on road, and a few brands, such as Giant, have been extremely vocal in stating that rim-brake bikes have a limited lifespan.
Even if the big brands shift away from rim brakes, which will take a number of years, you’ll still be able to work with any number of custom builders, or smaller brands, to get a bike with rim brakes.
Yes, more and more disc-equipped options will become available over the next couple of years, and it’ll come at the cost of fewer and fewer rim-brake bikes. But if you’ve got a rim-brake bike now, or you’re looking to buy one, it won’t become obsolete any time soon.
Can I fit discs to an existing bike?
Neither traditional road hubs, nor frames, have provisions for adding discs.
Simply, no. 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 (most 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 are extremely small.
If I imagine my wheel as a disc, surely I have disc brakes already, don’t I?
In theory, yes. However, the advantages of disc brakes lie not in the size of the rotor, but 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, and 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.
What is the difference between a Center Lock and six-bolt disc rotor?
Six-bolt on the left, Center Lock on the right.
These are the two common ways to mount rotors to hubs. Center Lock uses a splined interface to hold the rotor, and a lockring (which typically uses a cassette lockring tool, but sometimes an external bottom bracket tool) secures it in place. It’s an elegant system introduced by Shimano and now offered by the likes of SRAM and Campagnolo, too.
Typically found on cheaper 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 on to a 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.
What is Ice Tech?
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 them 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. Like 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 marginally 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.
What is the difference between flat-mount and post-mount calipers? Are they compatible?
An example of a post-mount brake.
Post-mount calipers were adopted from mountain bikes in the early days of road disc brakes. Here, the brake caliper mount onto threaded posts on the frame, with the bolts running through the caliper. The system allowed for easy repositioning and adjustment of the caliper, as well as shared caliper bodies in the early days of road disc brakes.
By contrast, the road-specific flat-mount standard is smaller and requires less frame space.
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 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.
Post-mount calipers can be installed for use on some flat-mount frames; however, adapters for going the other way aren’t generally available.
What is the standard rotor size?
This one is still up to debate, and varies based on which manufacturer you ask. A larger-diameter rotor provide more leverage, and therefore increased braking power, than a smaller one. Additionally, a larger surface area means better heat management. On the flipside, smaller rotors are lighter, less likely to get damaged in a wreck, offer a more gentle braking action, and are more aerodynamic.
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.
There is a trend, borrowed from mountain bikes and other disc-equipped vehicles, to mismatch rotor sizes on a bike, since as much as 70% of your braking power comes from the front wheel. An example of this is with the new Specialized Tarmac Sl6 Disc, where a larger 160mm rotor is featured up front for increased brake power and heat control, with a lighter 140mm rotor at the back.
Regardless of the argument, frames are typically optimised for a particular size. A bike like the Giant Propel Disc uses 140mm rotors, while others provide adaptors to size up.
What are two-piece rotors?
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 braking surface.
What’s the difference between metallic and organic pads?
Most brake manufacturers offer a choice of pad material. These are commonly resin (left) and metallic (right).
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 and quieter braking. 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 and last longer than organic pads, albeit at the expense of noisier running and faster rotor wear.
Mountain bikers have long had this choice, and the improved durability means they often use sintered metallic pads. With less grit, less need for raw stopping power, and a 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 of less-durable steel alloys. You’ll need better rotors if you decide to use metallic pads.
I see premium pads offered with aluminium or titanium; what’s the story?
This is all about weight savings. Disc-brake pad materials are usually bonded to steel backing plates, but ones made of aluminium or titanium will obviously save a few grams. Generally speaking, 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 Ice Tech finned pads, which are detailed above.
My bike looks like a bird’s nest. Can I trim the hydraulic brake hoses?
Brake hoses use metal barbs and olives that get compressed to make for an air-tight fit. You’ll need to install new ones each time the hose is shortened.
Yep! Being able to trim the brake hoses is a feature of all systems on the market. However, it can be a more involved job.
At a minimum, you’ll need a way to trim the reinforced hose cleanly, and replacement barbs and olives to push into the fresh end of the hose. This is what creates 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.
Can I mix and match components between brands?
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 not all brake levers pull the same amount of cable for a given amount of lever movement, and certain combinations will work better than others. Shimano 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.
Brake rotors are a different story, and while you’ll likely get the best performance by matching the rotor to the brake system (especially with higher-end Shimano models), mixing brands rarely results in any major issues.
Is there an easy way to swap between wheelsets on a disc-brake bike?
If the wheels you’re swapping between have the same hubs, or maybe you’re lucky, there should be no difference in rotor placement, and you’ll be fine to swap between wheels. But 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 rotor shims. A variety of brands (e.g. Syntace) produce shim kits for 6-bolt rotors while November Bicycle recently launched the first shim kit 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.
How long do brake pads last for?
It depends. We’ve seen disc-brake pads disappear within a single wet gritty ride, and then other pads last two full seasons.
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.
How do I check for brake pad wear?
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. 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.
In addition, look for uneven wear on the pad surface or rotor, as this is typically a sign of a poorly adjusted brake.
And some brake pads are designed to make noise once they’re worn. For example, Shimano brake pads will begin to squeal when they have reached the recommended replacement point.
Which brake pads do I need for my brakes?
Even within the same brand, different pad shapes for different models are common. Pictured are two brake pads, each for Ultegra-level brakes but one generation apart.
Disc-brake pads vary greatly based on the brand and model of brake. It’s extremely important to match the pad with the brake caliper model.
Once you’ve done this, you’ll find a number of third-party brands offering aftermarket pads. Some do it to be a cheaper alternative than original manufacturer pads. Others do it claiming superior performance, longevity, and noise reduction (such as SwissStop and KoolStop).
If you want to play it safe, it’s generally recommended to stick with the authentic brand of disc pads (rim brakes are a different story) to match your caliper. It’s what your brakes were engineered to use, and frankly, I’ve had too many mixed experiences to suggest otherwise.
What’s the difference between Shimano brake pad models?
The first letter in Shimano’s brake pad naming designates the pad shape and fitment. 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 J02A, J03A, or L02A and L03A. The “02” are the previous version resin pad, now superseded by the 03 which claim to offer 50% better durability without giving up anything else. If given the choice, pick the newer “03” option.
Is it really that important to keep oils away from my disc-brake pads?
Yep. 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 a far higher 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 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. If you insist on spraying degreaser everywhere, don’t put it on the discs, and better yet, use a clean freezer bag (held with an elastic band) to cover the brake caliper.
How do I clean my disc brakes?
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.
If you do need to clean them, use isopropyl alcohol, and preferably the 100% variety, not the slightly 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, isopropanol is still the recommended cleaner by all major disc brake manufacturers.
What is brake bed-in and why is it needed?
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.
How do I know when my brakes need to be bled?
Spongy-feeling levers or inconsistent brake feel are both sure-tell signs your brakes could use a bleed. Both of these symptoms are the result of air in the system, which can sometimes happen 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 fine.
Another reason for re-bleeding is 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, but we know plenty of people who ignore this without issue.
What fluid can I use?
Only use the fluid that your specific brake recommends. Shimano, Campagnolo, Magura, and a few others use specific types of mineral oil. SRAM brakes use automotive DOT 5.1 fluid.
Mineral oil and DOT fluid are not cross-compatible, and using the wrong fluid will cause seals to swell and the brakes to fail. This is certainly something to be very careful with, and be sure to not cross-contaminate systems by sharing bleed kits.
How do I bleed my brakes?
The process will vary depending on the brake model. The following links should help. Generally speaking, bleeding brakes involves a few brand-specific attachments, some basic tools, the right fluid, and a few syringes. Bleed kits are available from each brand as well as aftermarket suppliers, which offer a one-stop-shop option.
Why are my rotors a weird colour?
Assuming they’re not just dirty, a change of colour is 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).
Do brake rotors wear out?
Yep. 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 its rotors.
The good news is that the stainless steel braking surface of disc rotors is inherently durable, and you’ll likely go through many 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.
I heard I need to be careful when removing a disc-brake wheel; why?
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 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, it’s possible that you could push the pistons out back the seals, 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 sure they’re clean prior to using them.
Is the brake fluid an issue when traveling?
Nope. It’s a non-pressurized sealed system, and the relatively minor changes in atmospheric pressure associated with air travel are no big deal.
I was told not to turn a disc brake bike upside down? Is it true?
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. 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 for a bleed.
What can I use to protect the disc rotors in a travel case?
Lightweight cassette tools exist, meaning easy rotor removal and little weight gain to your luggage. Pictured are tools from Abbey Bike Tools (left) and Wolf Tooth (right)
Disc rotor protectors do exist, and many disc-equipped bikes are shipped with such things. Ask your local shop for some, assuming they’ve 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, but far 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.
Why do my brakes squeal?
More often than not, this is due to contamination. Sometimes it’s poor setup, sometimes it’s a warning of needing new pads, sometimes it’s because your pads are glazed or of the wrong material, but nearly always, it’s contamination.
Keeping any form of oil, degreasers, and bike cleaners away from your disc brake pads and rotors is a sure way to keep them squeak-free. And of course, a proper bed-in process is a must, too.
Why do I occasionally hear brake rub at random times?
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 be 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 blded 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.
My brakes rub, but only at one point of the wheel. What do I do?
A warped brake rotor is the cause of a brake rubbing every revolution of the wheel.
Sounds like you’ve got a warped disc rotor. They’re only about two millimeters 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.
For a little more precision, just about every tool brand offers a specific tool for this job. Alternatively, you can use an adjustable spanner, but just make sure it’s super clean before you do.
My hydraulic brakes constantly rub; what can I do?
Did you just have the wheel out and now they’re rubbing like crazy? If so, refer to our section above about traveling 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 making 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 a 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.
I’ve had a few people attempt to adjust my brakes, but no matter what, the pads are not contacting the rotor square. What now?
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 (and hopefully, they’ll foot the bill).
The Park Tool Disc Tab Facing Tool (DT-5.2) is designed to face all common disc mount standards. This is very much a shop tool.
Park Tool makes a tool called the DT-5.2 Disc Tab Facing Tool for 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 axle. It’s an extremely expensive tool, so look for a workshop that has one (and unfortunately, few of them do).
My spinning disc brake cut off my left leg. What do I do?
Bugger. Are you sure it wasn’t the race fencing? Actually, it was probably the chain ring. Either way, you should probably seek medical attention.
My disc brakes lack power. What can I do?
A few things could be causing this, but most likely, it’s one of the following issues.
Firstly, it’s most likely your braking surface is contaminated. Any amount of oil, even that of your body oil from your fingertips, can upset the performance of disc brakes. Cleaning the system may help, but if it’s badly contaminated, new pads and a thoroughly cleaned rotor are likely the best answer.
Another cause can be from 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; that should do it.
It may also be air in your system that’s preventing power from being transferred efficiently fro the lever to the caliper. A brake bleed will solve this; see our section about brake bleeding above.
Lastly, 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.
What can I do if my hydraulic brake lever pulls very close to the bar?
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.
Pumping the brake pads to run closer to the rotor will provide less lever travel. To do this, remove the wheel (and rotor) and carefully pull brake until you see the pads move. Re-install the wheel and check lever feel. Repeat this process until the lever is as desired. Do be warned that doing this will make it harder to get a rub-free setup, and it’ll only last until the pads wear.
Some brakes also offer lever stroke adjustment. If this is an available option, there’s likely a adjustment screw that can be used to reduce the lever stroke.
Why do my brakes lose power when braking for long periods?
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.
What do I do if my brakes suddenly lose power?
Well that’s not good!
The first thing to do is to try and pump the brake. This is simply a matter of squeezing the lever quickly on and off. If it’s air causing your sudden brake woes, then pumping the lever will hopefully move the bubble out of the 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 faulty component is a likely culprit of a full failure.
What did we miss? What are your top tips when it comes to disc brakes?