www. Aidan Reilly .net

Bicycle brakes

Because brakes can be quite important, and many people don't know enough about them. And we’re all prone to acting like sheeple.

The hot brakes myth

This actually applies just as much to cars as it does to bikes, and is possibly more relevant there, since the loads are so much greater that heat dissipation is more of a problem. Although wheels and brake blocks on a bike can get pretty hot — don’t touch your wheel rims after a long alpine descent! — it’s very rare indeed that they ever get hot enough to cause a problem. The only cases where I’ve heard of it happening, and then only third hand, are on tandems, and on bikes with hydraulic disc brakes. That said, it’s unlikely to be problem with a modern car, either.

The myth that I’m concerned with, is the common belief that ‘you shouldn’t drag your brakes’; however this is expressed, it generally refers to the idea that braking hard and intermittently on a long descent is better than leaving brakes on gradually for the whole descent, in order to avoid overheating. I think this is wrong — as a little bit of common sense and basic thermodynamics can show.

When you’re descending, you build up a lot of kinetic energy. In order to stop, the brakes take that energy and, using friction, convert it into heat. The amount of energy that they convert to heat is equal to the amount of kinetic energy that they dissipate. It doesn’t matter whether you stop from 50 km/hr to zero in 1 second or 10, the amount of energy is the same. Where does that heat go once you’ve stopped? It dissipates into the air around, rather slowly compared to the braking time. The difference between stopping in 1 s and in 10 is the power: stop from 50 km/hr to 0 in 1 s, and your brakes will be working at around 9 kW. Stop in 10, and they will working at less than 900 W. 9 kW over 1 s will heat up your brakes to higher temperature than 900 W over 10 s, as the longer time gives more time for heat to transfer to the air around. So stopping quickly is actually worse!

There is one minor effect of air resistance that works in favour of braking sharply at the end of a descent, but it’s not usually significant. If you’re freewheeling so fast that you’re reaching close to terminal velocity, then by leaving the braking until the end, you are allowing air resistance to do more of the work of slowing you down, leaving less total work for your brakes. But if your descent is straight enough that you approach terminal velocity, braking probably isn’t an issue anyway.

In short, feel free to ‘drag your brakes’ — it’s much safer than sudden, hard braking at the end of a descent! (Of course, if your brakes are dragging, constantly, when you’re on the flat, that’s an entirely different problem, and one that definitely needs sorting out.)

The disc brake myth

This is related to the previous section about bicycle braking techniques. I was inspired to write this when I bought my new bike (affectionatly nicknamed Reuben); the shop where I found Reuben seemed to be a decent bike shop, with knowledgeable and helpful staff. However, the guy I was speaking to was very keen to sell me the later model. That’s understandable — it was £400 more expensive, after all — but one of the key selling points he was making was that the later model came with disc brakes.

Now, there are good reasons to choose disc brakes over rim brakes: the primary one, is that they are less susceptible to clogging up with mud, sand and gravel, which is a definite hazard if you like to take your mountain bike down muddy, sandy, gravelly paths. It’s less clear that that applies to read bikes. (If you like to take your road bike down gravelly paths on a regular basis, you should probably buy a mountain bike.)

Rim brakes, on the other hand, are more aerodynamic and lighter (substantially lighter, since the front fork can also be lighter with rim brakes). Hydraulic disc brakes also suffer from the problem that the hydraulic fluid can overheat and boil, causing a sudden and complete loss of brake power. Of course, that will happen when you’re on a steep descent and really need them. Although this may not be a problem with modern, larger rotors it’s still worth thinking about: see here for an example of what can happen.

The real error the man in the shop made, in my opinion, was to assume that the brakes on a mountain bike need to be stronger than those on a road bike. They don’t. And that’s a very common misconception, it seems. On a road bike, it’s possible to reach much higher speeds than on a mountain bike, and the road surface offers much better grip. On a gravel or mud path, even comparatively gentle braking will lock the wheels, and from that point on it doesn’t matter how much more the brakes grip the wheels, your deceleration will be governed by the contact between the tyre and the surface. On tarmac, the limit is usually you going head-over-heels over the handlebars rather than friction. In either case, with rim brakes or disc brakes, your stopping distance is determined by other factors. Provided they are properly set up, the stopping distance will be exactly the same regardless of the type of brakes you have fitted. The current fashion for disc brakes on road bikes is exactly that: a fashion, with no logical underpinning whatsoever.

I should add one small caveat: there may be other advantages for disc brakes when used with carbon wheel rims, which offer a very poor braking surface. That’s a different matter, though the addition of an aluminium overlay to the rim adds very little weight. But that might be the subject for another day.

I’m not the only one who thinks this: have a look here and here for other opinions about disc brakes (including a summary of the disc brakes that were tried in the 1970s — they’re certainly not a new invention!)