EVERYTHING YOU NEED TO KNOW ABOUT DISC BRAKES

OLD TECHNOLOGY THAT’S STILL NEW TO MANY

In the pantheon of stupid things said about the bike industry, some of the more stupid have been the response to disc brakes becoming standard equipment on road bikes. The silliest has been the conspiracy theory that disc brakes have only became a thing because the bike industry wanted to sell more bikes. Selling more bikes—um, is there a problem with that?!

Even though RBA has been accused of being in on the nefarious plot (because we can sell more magazines), we’re still not sure what exactly is wrong with the bike industry trying to sell more bikes. Curious, too, is how the comeuppance of disc brakes is any more an insidious sales ploy than the arrival of carbon fiber or electronic drivetrains?! 

No doubt some of the road world’s reluctance to embrace disc brakes evolved from the UCI’s early ban on their use in competition, as well as many Euro road teams’ continued reliance on caliper brakes. What many roadies among us forget is that whether or not a Grand Tour is won with disc brakes, it has no practical bearing on what brakes are best for the rest of us to use. Remember, it’s not that the rim brakes don’t work; it’s just under all conditions and for all riders, disc brakes simply work better.

FROM WHENCE WE ARRIVED

When it comes to dealing with disc brakes, the road world seems to have ongoing difficulties and misperceptions understanding how they work and how they are maintained. But remember, disc technology is over 100 years old and, in bicycle-specific applications, has been continually refined ever since Robert Reisinger unveiled his cable-pull Pro Stop disc brake (as a necessary complement to his upside-down suspension fork) in 1990. 

Thanks to the mountain bikers’ embrace (and need) for improved braking, in the years that followed the Pro Stop, bicycle disc brakes would continue to evolve with Wayne Lumpkin’s Avid BB5 mechanical version, assuming the greatest level of pre-hydraulic success. 

Thankfully, after SRAM purchased Avid and rebranded the BB5, they knew that not only were mechanical (cable-pull) brakes not the ultimate stopping solution, but that proficient braking for road bikes could, and would, be just as important as a step forward. 

And despite suffering a serious technical glitch when they launched their first hydraulic road disc brake in 2013, SRAM can nonetheless be credited with finally moving the road world forward with the same level of confident and efficient braking that mountain bikers had been enjoying for years. 

WHAT WE HAVE HERE

Since the major manufacturers have made the choice of running disc brakes easy by simply discontinuing frames designed for caliper brakes, we thought it was the perfect time to break it all down to help ensure everyone that disc brakes really aren’t that complicated.

Unlike mountain bikes that have separate braking and shifting assemblies, road bikes use integrated brake/shift units, which for roadies there are three major brands of disc-brake systems to consider: Shimano, SRAM and Campagnolo. This can make it harder for brake-specific brands like TRP, Magura, Hope and Formula to compete like they do in the mountain bike market. 

Important to note is that in terms of the actual mechanical function, the Shimano, Campy and SRAM systems are more alike than they are different. They all rely on a hydraulic system to actuate the pistons in the caliper. This in turn moves the pads a few millimeters against the rotor. In short, it’s not rocket science. 

DISC-BRAKE TECH TIPS

Here’s a handy how-to guide to best ensure that your disc brakes brake well

For sure, disc brakes are more complicated to work on than caliper brakes, but that doesn’t mean you need to be a master mechanic to ensure that they are set up and functioning properly. And helping you understand the basics is what we’ve dedicated the following pages to. For the sake of simplicity, we’re going to cover hydraulic brake systems here, but many of the techniques and tips would apply to mechanical brake versions, too. 

When looking to resolve disc-brake issues, slow down and start simple. Before you mess with replacing brake pads, truing rotors, stacking shims or bleeding the system, make sure your wheels are properly installed in the fork and frame. A paint chip, dirt or loose thru-axle will wreak havoc on braking performance. 

Pull the wheel out, inspect the dropout, replace the wheel and snug things up. This will resolve a lot more issues than people realize, as there is very little room for misalignment. Also, remember that all of these are open-system hydraulic brakes and will self-adjust for wear. 

WHEN TO CHANGE THE PADS

The biggest worry for many that are new to disc are brake pads. The material on the pads is so much thinner than those from rim brakes that it can be hard to tell how much life is left. On average, each disc brake pad will have about 2mm of material when new. 

 

The pads will also have a return spring that helps assist in centering and the return movement of the pad. This is about 0.5mm thick and a good indicator of pad wear. This means you don’t want to wear the pads past about 0.6mm or about 70 percent. You can normally shine a light into the brake caliper and visually see how much pad is left, but removing the wheel can make it easier. 

Another thing to check is that pad wear is even on each side and front to back. Many times, a caliper or its mounting points might not be perfectly aligned, and this can cause uneven and accelerated wear.

In most cases, the hydraulic system is only made to operate correctly with as little as 0.4mm of pad material left per side. Then the lever and caliper pistons will no longer be able to actuate the pad the appropriate distance. This will result in a lever that will hit the bar before the pads are able to completely contact the rotor. 

Another common issue with pads is that they get contaminated and lose their ability to create friction. This will feel like the brakes are moving correctly, but no matter how much pressure you apply to the brake lever, it is not enough to stop. Sometimes this can be from road grime (and can be sanded off with sandpaper), and the most likely culprit can be from using incorrect cleaners when washing a bike. It is very difficult to recover a contaminated pad, and the usual result is them needing to be replaced. 

PICKING THE CORRECT REPLACEMENT PAD

Choosing the correct pad for your system could be the hardest part of the whole job. Each brand’s calipers have a specific shape, which means there are a lot of pads on the market. Additionally, there are usually a few options in the type of brake-pad material as well as backplate. 

Shimano offers two brake-material compounds—resin and metallic. Resin is the most common for road applications and is quieter, while also resulting in less brake dust. Metallic is more common in off-road applications and is more abrasive, which can lead to more noise. Metallic pads are also not compatible with all rotors, so check the specifications of the rotor you are using.

Shimano offers backing plates in alloy and stainless steel, as well as with or without cooling fins for dissipating heat. Like Shimano, SRAM also offers two braking materials—organic and sintered. They also come in alloy or steel backing. Organic pads are quieter with better initial bite and modulation, while sintered are more consistent under heavy braking or wet conditions.

Campagnolo currently only offers one style of pad that is an organic compound with steel backing. They have a built-in wear indicator, and instead of a return spring, the brake-pad return is accomplished by a magnetic system in the caliper piston. Also unique to Campagnolo is a special metal plate positioned between the pad and the caliper piston that is said to cushion the vibrations during braking.

There are lots of aftermarket brands—Galfer, Swissstop, Kool-Stop, Jagwire, Alligator and more—offering pads that fit all the systems. Some claim better stopping in specific conditions, or lighter weight, while others just offer an economic price point.

HOW TO CHANGE THE PADS

1. Changing pads is fairly easy and starts with removing the wheel from the bicycle. Be sure that when the wheels are removed that you do not actuate the brakes. If it does happen, this could result in the extension of a caliper piston past its designed functioning limit and can cause damage to piston seals.

2. Next, you need to manually retract the caliper pistons, as they have most likely auto-adjusted over time to compensate for the wear on the pad. You can do this with the old pads in or out and would be determined by the tool you are using. 

If all I have is a flathead screwdriver I leave the pads in and leverage the screwdriver against the worn pad material and backing plate for each side. This can damage the remaining pad material, so make sure you have
a replacement.

The preferred method is to use a plastic tire lever against the piston after you remove the pads. When doing either method, ensure the caliper is at the lowest point in the braking system.

3. Remove the brake pads from the caliper. This will start with the security pin that will vary in design on each system. These normally go through the backing plate of each pad and either thread to the caliper or have some sort of locking clip that secures the pads from falling out of the caliper body. Then simply pull the pads out of the caliper.

Remember, if you didn’t already push the caliper pistons back into the caliper manually, this is the time to do it with something less abrasive like a plastic tire lever. You don’t want to damage the caliper piston or seal.

4. Clean the caliper and surrounding area so it is free of dirt, debris and brake dust. Isopropyl alcohol works here, but soapy water works too. 

5. Now it is time to install new pads. It is important that you do not touch the pad compound, as this could contaminate them. For Shimano and SRAM pads, place the retraction spring between the pads, then with two fingers pinch the outside of each backing plate and slide them into the caliper as one complete unit. Do not try to insert them one at a time. For Campy, there is no spring, so place the pads together, compound against compound, and slide it in. The magnetic system will latch to the pad when it is in place. 

For all the systems, check the backing plate for markings to ensure you have the correct pad on the correct side of the caliper. Most pads will not fit correctly if installed backwards. Reinstall the security pin, and if your new pads came with new hardware, use it as long as it matches what you removed. 

Some Shimano pads come with a split pin instead of the circlip. I prefer the circlip, but if you must use the split pin, make sure to bend the slightly longer tail so it doesn’t slide out.

6. Now you need to reinstall the wheel and ensure the caliper with the new pads is centered with an even gap on each side. Now pump the lever a few times. This will reset the pads to the correct distance from the rotor.

7. The last step is the most important and most overlooked. Before you go out and test your new pads, you need to perform a bed-in procedure to gain optimal performance. Accelerate the bike to a moderate speed, and then firmly apply the brakes until you are at walking speed. Try and remain seated while applying the brakes. Repeat this process about 20 times. 

Then accelerate the bike to a faster speed and apply the brakes until you are at walking speed. Repeat this process about 10 times. It’s important that during this process you never come to a complete stop or lock up the wheels at any point. Coming to a complete stop causes a build-up of pad material in one spot that can lead to pulsing and noisy brakes.

TRUE YOUR ROTORS

This might sound funny, but the rotor of a disc brake needs to be perfectly true. There is very little space on either side of the rotor before it contacts the brake pad. If the rotor is even a tiny bit off, it can be a real ride killer, as the rotor scuffs the pad repeatedly. Most wheel-truing stands have an attachment that can be used to true a rotor. The process can be done on a bike, but using the correct tools will lead to better results. When bending the rotor using a rotor truing tool, don’t overdo it. Make small adjustments as you move around the rotor.

BLEEDING THE SYSTEM

Since all of these brakes utilize an open-system hydraulic setup, bleeding them and ensuring proper fluid level are important. In an open system, piston slippage/pad advancement manages the gap between the pads and the rotor as mentioned earlier. Surplus fluid in the reservoir moves into the system, ensuring that there is always an adequate volume of fluid between the lever pistons (master) and caliper pistons (slave pistons). This allows the system to operate consistently as parts wear. 

If the system is not bled correctly or air is introduced into the system, then problems will definitely arise. A few key signs you might need to bleed your brakes include the following: 

1. The pads still have plenty of braking material, but the lever hits the bar. This can be caused by a lack of fluid in the reservoir, and as the pad gap is automatically adjusted, there is not enough fluid for the system to operate.

2. The pads start to rub after a long braking period that builds heat. This is likely a sign that there is air in the system, and as the heat rises, the gas expands and doesn’t let the caliper piston retract all the way. Another sign that there is air in the system is a mushy feel at the lever. That is because the air is compressing and not letting pressure build in the system. 

3. Another sign that it’s time to bleed the brakes is, when you pull the lever, it is slow to return or you have to pull it back to its stationary position. This is likely caused from dirty fluid that has been heat cycled too many times. The brakes usually still work, but the fluid is not moving as it normally should and needs to be flushed. This is normal, and I recommend a full flush at least once a year.

WHAT ABOUT BRAKE FLUID?

It should also be noted that the type of fluid in a brake system is defined by the brake manufacturer. Both Shimano and Campagnolo use mineral oil in their systems, while SRAM uses DOT fluid. The biggest difference in mineral oil is that it’s hydrophobic and does not absorb moisture, while DOT fluid does. This means that mineral oil is slightly easier to bleed, but if there is air or moisture introduced to the system, it will likely show signs of malfunction immediately. 

DOT fluid will absorb moisture and air, making it a longer process to separate and bleed the fluid. It is controlled by standards set out by the Department of Transportation (DOT) and is widely used in the automotive industry. There are two main glycol-based brake fluids—DOT 4 and DOT 5.1—that are used. The main difference between these two brake fluids is in their boiling points. 

These standards are the minimum temperatures that the brake fluid must perform before the brake fluid starts to boil, which can lead to complete brake failure. Also worth nothing with DOT 5, which is silicone-based, is that it should never be mistaken for 5.1, as they are not interchangeable.

ADJUST THE CALIPER POSITION

To be honest, adjusting the position of the caliper is lower on my list, because it should be something that is done less often. Once set, this should not need to be adjusted or modified unless there is a change in the system, like new rotors or wheels that could change the spacing and alignment. 

Too often it is the first thing that people jump to when their brakes make noise or perform below par. The easiest way to adjust and center a caliper is to set the mounting bolts just loose enough to move the caliper, but tight enough that it will only take minimal rotation to secure the position.

Then pump the brake lever with one hand and add a slight bit of tension to each mounting bolt on the caliper. In some cases, this doesn’t exactly get it spot-on, and after the first addition of tension to the bolts, a slight manual adjustment and alignment by eyeing the gap on both sides are needed.

It is important to not wrench down on the bolts, but instead just switch back and forth, incrementally adding more tension each time. This will minimize the likelihood that the caliper shifts and pulls itself out of alignment. 

START FRESH

Like all hydraulic systems, disc brakes rely on a small amount of fluid in plastic hoses that are repeatedly put under pressure and heat-cycled. Sometimes a hose will get a soft spot or a fitting will not fit perfectly. Over time these can lead to failures or reduced performance. When I am having issues with a brake and I have worked through the steps above, I just start fresh with new hoses, fittings, fluid and pads. That almost always fixes it, and if it doesn’t, then I likely am tired and have missed something simple.

DO I NEED SHIMS?

In the early days of disc brakes, shims were mandatory since production tolerances for brake mounts were all over the place. In the modern age of disc we rarely need shims, and using them can cause more problems. The only thing that you may want to shim on a modern disc brake is the rotor spacing from the hub. 

If you have two sets of wheels that you want to swap between but the rotors do not align the same in the caliper, you can shim the one that is narrower out. It is important to remember that if you are utilizing a six-bolt-mounted rotor that all six bolts must have the same amount of shims so there is no warping.

TOO MANY ROTOR OPTIONS

When it comes to rotors, the industry is stacked full of options. There are different sizes, designs, materials and mounting styles. This can be a place of customization with anodized alloy spiders or slight performance gains with alloy fins and heat management. No matter what route you choose, there are a few key things to consider.

1. Size is by far the most important. Many frames have a minimum and maximum rotor size. You cannot just switch rotor sizes without switching other components, specifically caliper adapters. I recommend to always try and match your front and rear rotor sizes. This is to minimize the likelihood that you install them wrong, and means you only need to have one as a spare since it could work for both front and rear. The 160mm has become the standard for road, but many prefer the look of the 140mm. For me, the bigger it is, the more material there is to shed heat and maximize performance.

2. The materials used in the construction of the rotor. As we mentioned above, some rotors require the use of specific pads. Most rotors are steel on the friction portion to maximize durability. To shed weight, this could be paired with an alloy spider or carrier. 

Shimano used their own Ice-Technology rotors, which have an aluminum and stainless-steel-layered brake track with alloy spiders to shed even more heat at the cost of accelerated wear.

3. Last thing to consider is the mounting style with two options—six-bolt or center lock. This is normally defined by the hubs and wheels you choose, but it should be noted that a six-bolt rotor can be adapted to center lock but not
vice versa. 

While there is a lot of info here, remember that disc brakes have been on the market for a long time. The systems have been refined, and the odds of having issues is very low. If you feel like resolving the issue is beyond your current abilities, then don’t worry, because your local bike shop has years of experience with these systems. In many cases they can have the issues remedied before you can pull your hand from your pocket and feed their tip jar.

Warning: Disc brakes work by converting the speed and energy of the bicycle into heat. When you have been on your brakes hard, they will generate a significant amount of heat—in the rotor, pads, caliper and even brake fluid. So, before touching any part of your brake system, make sure it has cooled down.

THE DISC BRAKE FAMILY