Disc Brake Primer - Part 1

There’s no other way to start this, so let’s get straight to the point: Disc brakes are here. What I mean by that is that they’re officially A Thing for road and triathlon bikes. Looking back only twenty years ago, it seemed highly improbable, if not impossible, that this would ever happen. To put it mildly, the debate surrounding this development has come on strong with considerable fire and protest. Yet another disc-related discussion popped up in our reader forum not long ago.

The two polar opposite positions seem to be:

1) Disc brakes are completely unnecessary for road and tri, have zero value, and should die immediately. Thus far, there is insufficient data to show that they’re on an equal playing field of aerodynamics. They add weight, and they definitely add cost, service time, and complication to the bike. They slow wheel changes. They make noise. They’re usually coupled with several different thru axle standards, which require special adapters to use a repair stand, roof rack, or indoor trainer.

2) Disc brakes are a clear technological advancement that will eventually take over the market entirely, and that’s a good thing for everyone. Look what they’ve done on mountain bikes. The extra investment of time and money for parts and service are clearly worthwhile when you look at the improved braking performance. If you don’t know how to deal with disc brakes, you probably shouldn’t be working on your bike anyways – so take it to a pro mechanic.

Those are the ends of the spectrum, but as with most things, there’s much more nuance to the discussion than you see on the surface (and the truth for most people will lie somewhere in-between the extremes).

This will be a two-part article that will investigate and explain disc brakes. In this first article, we’ll distill the lore of disc brakes into some take-home messages. What are the different types? What type of performance can I expect? What generalizations can we reasonably make to help aid beginners in making purchasing decisions? In the second segment, we’ll cover some of the take-homes for what you actually have to do to live with disc brakes. What type of tools do I need? How much more time can I expect to spend working on my bike, or how much will my repair bills increase if I switch to discs? What about storing my bike or traveling with my bike?

Note that these articles are aimed at folks who are relatively inexperienced with discs – just to give a heads-up on some of what you can expect. I’m not here to convince you that you should or shouldn’t use disc brakes (I don’t have a dog in the fight, and am old enough to know that my preference might not be the same as yours). I’m just providing basic information so you can make your own decision.

Setting the Stage

The first thing I’d like to do is set the stage, provide some definitions, and clear up some of the sweeping generalizations that are running amok.

Unless you started cycling very recently, you’re probably used to rim brakes. These simply pinch the rim with two pads to help slow your bike down (pictured below). The rim IS the braking surface. Braking effectiveness is highly influenced by the rim and pad material, and also by the design of the brake caliper itself.

In contrast, disc brakes have a rotor that’s attached to the wheel’s hub. The caliper is located down near the hub, and houses the pads that squeeze down on the rotor to slow you down (the rotor turns along with your wheel).

Regardless of whether you’re talking about disc brakes or rim brakes, there are options for either cable-actuated (also called ‘mechanical’) and fluid-actuated (also called ‘hydraulic’).

The thing we all need to get out of our heads right now is that ALL disc brakes perform better than ALL rim brakes (or vice versa). It’s just not the case. If someone is emphatically making that point to you, I suggest running away or otherwise not wasting your time. There are a TON of factors that influence brake performance, such as rim material, pad material, installation/setup, method of actuation (cable or hydraulic), rotor size, and even frame/fork stiffness. If you want to read more, this thread in our reader forum covers the topic – including a test done by Tour Magazine which concluded that hydraulic disc brakes resulted in 2-3% shorter stopping distances than standard cable rim brakes, but was far from testing all possible variables.

After all my years of wrenching and riding, these are the generalizations I’m comfortable making regarding the debate of rim vs disc brakes as of 2018:

1. Disc brakes, over their lifetime, will make more noise than rim brakes used on an aluminum rim. When you introduce carbon braking surfaces, this one gets a little muddier (some carbon rims are loud with rim brakes). Resin (a.k.a. organic) disc pads tend to be quieter but wear out more quickly than metallic (a.k.a. sintered) disc pads. Regardless, I’ve yet to find a disc system with any pad that makes less noise than quality rim brakes on an alloy rim. I’m open to the possibility that this could change some day, but we’re not there yet.

2. Disc brakes, over their lifetime, will require a minimum of 3-4 times more service than standard rim brakes. Note that I’m looking at all aspects of ownership – installation, adjustment, cleaning, pad changes, rotor truing, bedding in disc brake pads, etc.

3. Most disc brakes perform better than poorly-executed rim brakes. What I'm referring to by “poorly executed” are many of the tiny/hidden/scissor-style brakes that found their way on to triathlon and time trial bikes, peaking in about 2010 (example below). Some of these were awful in terms of performance, feel, and mechanical difficulty, and are a key reason that discs have migrated to triathlon bikes.

4. Disc brakes perform better than most rim brakes that are used with a carbon fiber braking surface (but note that there are plenty of great wheels with an aluminum braking surface). The performance gap closes with a high quality rim brake caliper, pad, and rim material.

5. Cable-actuated disc brakes perform roughly on-par with a good quality cable-actuated rim brake (such as Shimano Ultegra or Dura Ace) in normal weather conditions.

6. Hydraulic disc brakes perform roughly on-par with hydraulic rim brakes (such as SRAM’s Hydro-R, pictured below) in normal weather conditions.

7. Disc brakes perform better than rim brakes in rain, snow, and poor weather without changing the brake pads out (there are several rim brake pad compounds made for these weather conditions that can level the playing field, but most people don’t bother to change their pads out). In my opinion, this shouldn’t be your only reason for switching to discs. If you think you’ll save time by avoiding the hassle of swapping pads out on your rim brakes, you will more than make up for the time because disc pads wear out much more quickly, and disc brakes generally require more fiddling and adjustment.

Update / Clarification:

I consider this point to be true if we're talking about organic / resin disc pads, which tend to offer the lowest noise in most conditions, at the expense of faster wear and worse hot-weather performance and modulation. They're what I normally use, and I combat the problems by checking/changing the pads often, and using proper technique to avoid heat buildup. If you really want to optimize your brakes' performance - regardless of rim or disc - you're going need at least two pad compounds at your disposal.

8. Hydraulic brakes (both rim and disc) get the nod in terms of modulation and feel over cable-actuated brakes. High quality cable-actuated brakes, pads, and proper setup can help to close the gap.

9. High quality cable-actuated standard rim brakes (such as Shimano Ultegra or Dura Ace) require the least service time to perform very well over their lifetime.

Basic Implications

Hopefully we’ve cleared the air just a tad on what you can expect to gain (or not) from a switch to discs. In a nutshell, if your internal braking performance baseline was set by a really bad rim brake / pad / rim, you will definitely enjoy a substantial boost in performance by switching to a disc brake bike. If you’re accustomed to really nice rim brakes/pads/rims, there could be little-to-no performance benefit to discs. Regardless, a switch to discs will require more maintenance. How much depends on what system you choose.

Now let’s cover some of the basic-level changes you can expect in your day-to-day, if you buy a disc-equipped bike.

Race Wheels

If you already bought a set of fancy carbon race wheels for your rim brake bike, you cannot use them on a disc brake bike. The hubs must be specifically made for discs, and need a location on which to mount a rotor. In addition, the rear axle spacing is different on most disc bikes, with a 135mm-wide axle (rim brake road wheels use a 130mm-wide axle). Also, many disc brake bikes come with thru axles, which are another article by themselves – requiring adapters to use your bike on most indoor trainers, roof racks, and repair stands.

There are now many models of race wheels that are built for discs. Just keep in mind that you must get the correct axle for your bike (which may not be the same as your spouse’s bike – if you share wheels). Some wheels provide different end caps that may be quickly swapped out to accommodate various thru axle styles, but some do not. When in doubt, ask your mechanic for help.

Service Schedule

Disc brake pads tend to wear out more quickly than rim brake pads. If you’re using organic/resin disc pads (as most people do, because they are quieter), it’s safe to say that the pads should be checked a minimum of every 6 months. If you live in a mountainous area, I’d cut that in half to every 3 months. The last thing you want is to completely wear out the pads while in the middle of a steep mountain descent. Err on the side of caution.

If you’re using mechanically-actuated disc brakes, you also must manually adjust the pads as they wear out. If you don’t, you could get into a situation where your brake lever pulls all the way to the handlebar, and you can’t stop the bike. Hydraulic disc brakes automatically adjust for pad wear.

Depending on the system you’re using, you may also have to change your fluid out. Some manufacturers use automotive DOT fluid for their hydraulic brake systems (SRAM, for example). It performs very well in terms of operating temperatures, but it’s also hygroscopic, meaning that it absorbs moisture from the air over time. As this happens, the fluid degrades. A good rule of thumb with such systems is to replace the fluid once per year (most cars recommend every ~3 years, but the fluid volume in a bicycle is significantly less). You need to do this even if you don’t ride the bike at all – moisture still works its way into your brake system.

Other hydraulic brake systems use mineral oil (e.g. Shimano and Magura), which does not absorb water and generally lasts the lifetime of your brakes. I personally prefer mineral oil systems for this reason, as it eliminates the need for annual fluid changes and extra work.

Mechanical / cable disc brakes do not require any fluid (obviously), but I have found that some require rebuilding the calipers periodically. My normal go-to cable disc brake is the Avid BB7. Overall, they’re the best combination I’ve found of relatively low noise and ease of adjustment. However, I had a set of them that just wouldn’t stay quiet – the squeal, sqwawk, and screech just kept coming back no matter what I did – surgical-level cleaning, different pad compounds, different rotors, fastidious pad bedding procedures, and so on. I eventually decided to spend the few hours required to completely rebuild the calipers, reinstall, and readjust them – and they finally quieted down. Who knows – perhaps there was some vibration happening inside the caliper that resulted from a lack of assembly grease (i.e. it got washed away over time).

A Note on Cleanliness

Perhaps the biggest change that you’ll need to get used to with a disc brake bike is the need to step up your cleanliness game. Disc brakes do NOT like grease or oil of any kind. Don’t touch the brake rotors with your bare hands. Don’t spill chain lube on them. Don’t smudge grease on them accidentally. Anything you do to get the brakes greasy or oily has the potential to contaminate your brake pads, causing noise and poor braking performance. At a minimum, this requires removing the brake pads to clean them, and also cleaning the rotor (auto parts brake cleaner or 90% isopropyl alcohol do the job). However, if things get badly contaminated, sometimes the only solution is to replace the pads entirely and start over.

I highly recommend buying some nitrile or latex gloves, and using them any time you’re near the brakes. Keep your work area clean, and always have isopropyl alcohol and clean rags / paper towels handy.

Stay tuned for Part 2, in which we’ll cover more specifics about your day-to-day dealings with a disc brake bike – tools, storage, travel, and best practices.