Putting on the Brakes -- Part Two

In my last post, I looked at the latest and greatest thing in brakes for road bikes – hydraulic rim brakes – which stop the bike roughly the same way as the brakes we already have, except that they are operated with hydraulic fluid instead of cables. Readers will be glad to know that this new “breakthrough” lifts bicycles out of the “dark ages” where they have “languished with the likes of horse shoes and buggy whips” (actual SRAM ad copy). Indeed. According to the folks at SRAM, we should now be able to engage in space exploration with our bicycles, or something like that. It all just kind of makes my head hurt. And it’s almost enough to even make BikeSnobNYC into a Retrogrouch.

Today, I’d like to look at the other trend in braking that is making its way onto more road bikes today: disc brakes.

Coming soon to a road bike near you!
(but probably not one of mine)

People who are familiar with cars are well aware how much of an improvement disc brakes are over the drum brakes they replace. Discs are used on all high performance cars today, and on most normal passenger cars and trucks – at least on the front (some less expensive models still use drums on the back). Discs offer better stopping power, better modulation, and are less prone to overheating, and therefore less prone to fade under hard braking. Most motorcycles use disc brakes now too, for the same reasons.

Knowing what an improvement they make on cars, they should obviously be much better for bicycles, right? Well. . . that’s not so clear and easy. Keep in mind that the force necessary to stop a 3-4000 lb. car is many times higher than what’s needed to stop a bicycle. Given the options available, disc brakes are the best one found that can handle that task on cars. Current technology rim brakes manage the task on bicycles so well that it’s hard to really improve them very much.

What exactly are the problems with rim brakes that disc brakes are supposed to solve? Here are two: First, when riding in wet and muddy conditions where the wheel rim – which is also the braking surface – is constantly sloshing through the mud and water, rim brakes can lose some of their effectiveness. One applies the brakes, and the brake pads first have to essentially “squeegee” the rims clean before they can really begin to grab. This is why disc brakes have been sweeping through the mountain bike world. OK, score one for discs.

Second, on long fast descents, if one isn't careful in their brake use, the rims can become hot enough to cause a tire blowout. A legitimate concern, although that’s one of those things one hears more warnings about than actual incidents (I’m not saying it doesn't happen, but I've never met or even heard of anyone it’s actually happened to). I do believe tire blowouts from overheated rims would be more of a danger with a heavily loaded touring bike or on a tandem – either of which can generate some pretty serious momentum on a long mountain descent. That is why many tandems are equipped with some type of “drag brake” – either a drum or disc brake – to scrub off some speed without overheating the rims – but even then, rim brakes are often used for most of the stopping duties and are more than effective enough in most conditions even with the extra weight of a tandem.

That’s two problems with rim brakes, both “solved” by disc brakes – but not completely, and not without other drawbacks.

Look at the wet weather problem. Getting the brake surface up out of the water and mud means stopping power should be degraded less. That's good, but brake performance is still diminished in wet conditions, just not as much as with rim brakes. Then again, I've seen tests that show rim brakes still stop faster and more controllably in most other conditions. Bicycle Quarterly, which I believe is more independent than most bicycling publications, does tests that are more objective, with more measurable and repeatable results -- and they found that under most conditions good rim brakes stop faster and more predictably than discs, despite the claims by the manufacturers. (Vol. 11, No. 4, Summer 2013)

What about the heat problem? Well, as I've already pointed out, that’s more likely to be a problem on heavily loaded bikes and tandems – and even then, only if they’re being ridden over exceptionally hilly terrain. Most road bikes aren't subjected to those conditions, and most people don’t ride the kinds of long mountain passes that would be able to generate that kind of heat. And heat buildup is still a problem with disc brakes – it just presents itself in a different way – in the form of warped rotors and dangerous fade. That's right -- the very same thing that makes disc brakes superior to drums on cars makes them actually inferior to rim brakes on bicycles!

When it comes to disc brakes, one important factor in their performance is the size of the brake disc, or rotor. All other things being equal, the larger the rotor, the more surface area it has, and the better it will handle heat -- all of which mean better stopping power with less fade. One thing people overlook about rim brakes on bicycles is that functionally they ARE disc brakes. The wheel rim is the rotor -- a huge rotor that is 622 mm in diameter (559 for MTBs), as opposed to 140 - 200 mm diameter as found on most disc brakes. And those disc brake rotors are only a couple of millimeters thick. One doesn't need to be descending mountain passes to overheat those rotors to the point of warping. A few hard stops, or dragging the brakes on even a moderate hill can heat the rotors up to the point that they will blister skin (so don't touch 'em!). That heat means warping and fade -- both of which are every bit as bad for braking and safety as a blown tire, and perhaps more likely to happen because the heat builds up so much faster. The manufacturers could make them to better withstand the heat, but that would mean making the rotors much thicker and larger and therefore much heavier and nobody would want them.

Then there are other problems with disc brakes, such as poor modulation. Good quality rim brakes have terrific modulation -- smooth, linear, and predictable. Many independent tests (like the BQ one I mentioned) find such modulation to be lacking in disc brakes. I've ridden bikes equipped with mechanical disc brakes and found them to be incredibly "grabby." It was as though they had only two "speeds": Go and Stop RIGHT NOW! Even a light touch was easily enough to lock up the wheels -- too easy. It's possible that hydraulic versions might offer better feel or linear control, but the jury's still out on that one.

It seems to me that disc brakes are also more finicky about adjustment. Of the bikes I tried with them, I found one of them had brake pads that must have been out of adjustment to the point of constantly and audibly scuffing on the disc with every rotation, and no amount of fiddling could get it to go away. The other bike had, to my estimation, a slightly warped rotor (which seems to be a fact of life with discs), which led to more scuffing and a bit of "shudder" while braking -- much worse than I've experienced with rim brakes on an out-of-true rim. These were not old abused and neglected bikes, either.

So let me summarize the disc brake vs. rim brake pros and cons: Disc brakes offer somewhat improved stopping power in wet conditions, but somewhat less stopping power in most other conditions, along with inferior modulation. Disc brakes make heat-induced tire blowouts (which I believe may be fairly rare) virtually impossible, but make heat-induced rotor warping and fade much more likely. I think we'll continue to see disc brakes on mountain bikes, where some may feel the strengths outweigh the weaknesses, but even on a mountain bike I think I'd prefer a good set of cantilever rim brakes to get the best possible brake feel and feedback. Discs may be a good choice on urban commuter bikes, especially those that are ridden in wet climates and are rarely pushed to the limit. Discs may make a good "drag-brake" for tandems. But on a performance road bike, give me a set of good rim brakes, whether side pulls or cantilevers, any time.