Road Gear

Technical FAQ: Disc brakes in road racing

This week, Lennard Zinn addresses the UCI's announcement that it will allow disc brakes to be used in the pro peloton this summer

Converting existing wheels for discs?

Dear Lennard,
With the UCI announcing its strategy for road discs, I guess it’s now no longer a matter of “if” but “when.” For those of us with some nice carbon wheels we’d like to keep riding on that new super-disc frame, what’s your opinion on rebuilding (with the appropriate cross-lacing of course) the rims with disc hubs? OK, some math required on whether selling the wheelset and replacing is more economical, but the range of disc wheels isn’t that great right now. For example, would there be issues with different loads on the spoke holes? I’d accept that any warranty would be long gone.
— Phillip

Dear Phillip,
I think that is a poor idea, because the spoke drilling in the rims was not done with disc brakes in mind. That the warranty would be void would be the least of your problems.

First off, the spoke count would probably be too low for all but super-light riders to safely use disc brakes with those rims. And you aren’t apt to find disc-brake hubs with fewer than 32 holes to attach them to.

Secondly, many radially-spoked front carbon wheels have a spoke count that is not a multiple of four. This is also a non-starter; if your wheel has 18 or 22 spokes, you can’t lace it with a crossing spoke pattern.

Thirdly, unless you have crossed spokes on both sides of both the front and rear wheels, the angles at which some or all of the spoke holes in the rims were drilled will be wrong. On radially-spoked wheels, the holes will have been drilled radially — oriented directly from the rim toward the center of the hoop. The carbon at the V of a deep-section carbon rim is quite thick, so if you come out of such a hole with a spoke nipple directed radially inward and then try and lace it to a spoke coming at a crossing angle, it will not line up. It will bend the spoke and tear the carbon at the spoke hole when you tension the spoke. And if you have a radial wheel with just a carbon “skin” forming the deep-section part of it, the hole in the rim bed for the nipple lines up with the hole in the skin and the center of the hub flange, so trying to make the spoke curve at the edge of the skin to a hole in the hub flange for a crossing pattern will just distort and tear the skin as soon as tension is applied to the spoke.

Sorry to be the bearer of bad tidings, but you have to start with rims drilled for a crossing pattern, and they will probably have to be 32-hole.
― Lennard

Reader comments about disc brakes in road racing

Dear Lennard,
The letter from Ante, which appeared in the April 15 Tech FAQ, illustrates two common misconceptions about disc brakes on road bikes. Firstly, Ante’s letter contends that smaller rotors can be on road bikes because road tires have less traction than MTB tires. This is generally not the case — road tires on smooth pavement can usually provide more traction than knobby tires on dirt. Road tire traction is high enough that in practice, the braking limit on a road bike is often not the traction limit, but the tip-over point — the point when the forward weight shift from braking causes the rear wheel to lift off the ground and the bike begins to do an endo. (You can test this yourself by riding at a slow speed on smooth pavement and clamping the front brake trying to induce a front wheel skid — if you brake hard enough, you’ll find that the rear wheel lifts off the ground before the front wheel can begin to skid). In other words, road tires often have more traction than can actually be used for braking.

The second misconception is that the size of the rotor is selected primarily based on required braking force. In fact, the largest factor for selecting rotor size is more likely to be the ability to dissipate energy. Brakes work by converting a bike’s energy of motion (kinetic energy) into heat. Since kinetic energy increases with the square of speed, every doubling of speed will increase kinetic energy four-fold. Since road bikes typically go faster than mountain bikes, they will have far more kinetic energy that needs to be dissipated when stopping. Combine that with the very long road descents often found in the mountains, and there can be a very large amount of energy transferred to the brakes. Larger rotors may be required to absorb and dissipate this larger heat energy.

If, as contended in the letter, disc brakes are to be weakened as a safety factor to prevent riders from going over the bars, this might be accomplished by reducing the overall system leverage rather than reducing rotor size. This would have the additional advantage of increasing pad clearance, improving wheel interchangeability, and speeding up wheel exchanges.
— Mark

Dear Lennard,
I come again to the same conclusion as I did back in last November and even last summer when road disc brakes have been discussed. “Road Disc Brakes??? We don’t need no stinkin’ road disc brakes!”

Seriously, there is no problem that they adequately address. Road discs are poor options in many ways. And they cause a lot of problems that didn’t exist before. I really wish the product managers would stop trying to force them on the market. If someone must run a lighter weight carbon rim, they better find a rim that has an aluminum braking surface (a la Mavic), or bone up for tubulars. Otherwise, just get a nice set of aluminum clinchers. In many ways, they are tough to beat. It’s hard to find new and additional problems with either of those two approaches. Let’s get behind this and bury those road discs in a giant hole in the desert! (This time with no ranger in a power wagon to get their way out.)
— Kurt

Dear Lennard,
One of the touted cons about disc brakes in road racing is that wheel changes would be complicated and slow. The racing motorcycle crews have solved this by having the complete chain and sprocket assembly retained on the frame/fork by a dedicated bearing carrier (see video). On some motorcycle versions, the brake caliper and rotor also stay secured in the frame/fork. This way a wheel is just a hub and rim with an engineered interface to slot into the brake/drive systems, which are retained in the frame. The motorcycle example is obviously heavy but some elegant design could make a bicycle version almost the same weight as the current standard disc wheel designs and invisible when assembled.

If properly designed and implemented across the peloton, this could enable faster wheel changes than currently experienced with rim brakes (certainly for the rear). Also, brake and drive system alignments would be irrelevant.
— Mike

Dear Lennard,
I’ve been following this discussion about overheated disc brakes, and I just have to say that the day these things show up on the start line of amateur crits is the day I quit racing. We already take stupid risks when we do our little local races; why in the world should we add one so potentially devastating? So that disc brake makers can make their millions (or whatever they make, I have nothing against them, per se)?

Please, USA Cycling and UCI, keep these things out of road racing! They’re not needed!
— Thom

Dear Lennard,
I believe the debate over disc brakes on road bikes and the comments regarding their safety is ridiculous and overblown.

Disc rotors are not really that much of a safety concern in regards to getting sliced or burned. For one, a spoked wheel is way more dangerous than a disc rotor. For two, the buildup of heat, especially in the pro peloton, is another non-issue. If a rider is dragging his brakes, enough to heat them to a scorching degree, he is doing something wrong and should re-evaluate his skills.

I could see some heat problems on a casual rider, who may drag their brakes, though, but better braking is a good thing.

The only issue I see is the thru-axle or standard QR dropouts being used; I know the thru-axle is a better way to go, but it would slow wheel changes.
— Jeff

Dear Lennard,
If riders are so scared of those sharp brake discs, maybe they should stop using chainrings. Lets all switch to belt drive and hub gears. Maybe we should stop racing on the road and its nasty harsh surface and start racing inside velodromes lined with cotton wool.

Are these people serious? It’s the fear of the unknown.
— Ray

Dear Lennard,
Anyone who’s gone down a long descent with discs knows just how hot a disc can get. But truly, how common are mass pileups in pro races at the bottom of long descents? It seems like most crashes on long descents take out a couple of riders as they are single file and more often than not occur in the rain, when the disc would be cooler anyway.

Most mass pileups seemingly occur when the group is bunched up riding almost tempo or when they are going flat out. I would assume the discs leading up to the initiation of the pileup are at a fairly cool/ambient temperature. So it seems to me that at worst, the rider is going from say 35 mph to 10 mph (not 0 mph because then they would’ve stopped and not crashed!) in a matter of less than a couple seconds. Assume too that the rider is not skidding the tire. How fast can a disc heat up in less than 2 seconds? If the rider is skidding the tire, then even less heat is getting into the disc.

The edges of discs are indeed sharp. But the femoral artery isn’t exactly near the surface. There is not a lot of space between the disc and the wheel/frame/fork to actually get the disc into the artery. I’m not saying it is impossible, it just isn’t likely. I don’t see discs being any more dangerous than a cassette or chainrings in that regard. Those types of injuries do occur — I crashed in a mountain bike race earlier this year and sliced a vein in my ankle with the chainring and I’ve seen a couple of people get stitches in their faces from chainrings from mass pileups on the road over the last 25 years or so. Do you think a disc is intrinsically more dangerous than a cassette or chainring in that regard?
— Craig