Technical FAQ: Can we make disc brakes safer?

After the Paris-Roubaix disc brake injury, Lennard Zinn answers a litany of questions about whether discs are safer or better than rim brakes.

Dear Lennard,
With all the controversy about Fran Ventoso’s injury at Paris-Roubaix supposedly caused by a brake disc, I’m wondering if rim brakes are any different in terms of injury potential. Specifically, if I remember correctly, Joseba Beloki’s crash in the 2003 Tour de France was or might have been caused by a tire rolling off the rim, possibly because of high temperatures on the road, or possibly from overheated rims caused by heavy braking on the descent. I have had the experience on Mount Ascutney in Vermont of tubulars rotating on the rim because of the braking necessary — luckily never to the point of ripping the valve stem off, but making it necessary to stop and swap the rotation direction of the front wheel a few times on the way down. What do you think of this? Do the pros have that problem in races on hot days with long, steep descents?
— Bob

Dear Bob,
Yours was one of umpteen letters I got this week regarding Fran Ventoso’s Paris-Roubaix injury and road disc brakes.

You have a valid point; under some conditions, rim brakes have also been the cause of injuries. Yes, Joseba Beloki’s crash on that Tour de France stage into Gap was a function of his rim brakes. Locking the wheel up was the big problem; I don’t think he could have avoided the crash even if his tire had stayed on the rim. The rolled tire was a result of the combination of locking up his brakes, the wheel jumping up and landing hard at an angle, and the carbon rims being very hot, making the tubular rim cement more liquid.

I do think the brake locking up was a complete surprise to Beloki; he had not expected his brake to behave that way. The road surface had a lower coefficient of friction than he expected due to the high temperatures softening the asphalt, and when he touched the brake, the wheel started sliding. It was not entirely due to a grabby brake; we all know how easily this will happen on ice, even with the perfect brake. The road surface was not as slick as ice, nor was it as grippy as asphalt at lower temperatures. The brake pads were most certainly grabbier than normal, as the rim was so hot. The carbon brake pads of 13 years ago tended to not be as good at managing heat as they are now; 2003 was early on in the development of carbon rims and brake pads for them.

My guess is that, indeed, a disc brake of today would have greatly reduced the likelihood of this happening to Beloki. While the road would have been just as slick, his rim would not have been as hot due to braking. Consequently, his tire would not have been as hot and hence would not have been at such a high pressure, which would have given him marginally more grip. And with the disc brake, the cooler rim would have greatly reduced the likelihood of rolling the tire. However, had he locked up the wheel like that with a disc brake, I think he still would have fallen; once you stop a wheel and cross it up relative to the direction of travel at high speed like that, it is very hard to ride it out unscathed.

Carrying your argument further, the tires blowing in the heat on the stage 5 neutralized descent in the 2015 Tour of Oman were obviously a function of rim brakes. I went into great detail on that at the time.

As for the tire sliding around the rim from rim braking, I suspect that most riders with a lot of mountain miles on tubulars will have had this experience. I have had a valve stem tear on a tubular due to this, and the tire deflates very fast. I have also torn off valve stems on a number of occasions on clincher tires from the tire sliding around the rim on super-steep dirt while riding rim brakes, both on road bikes and on mountain bikes. I’m certain that none of these occurrences would have happened at all, had I had disc brakes. I remember descending the steep Silver Canyon Road from Mt. Whitney down to Bishop, California on mountain bikes on a hot summer day long ago. I was an early adopter with disc brakes and had no problems, and the guy I was with had rim brakes. He tore the valve stem out of every tube we had by the bottom, even though by the end we were stopping every few minutes to turn his front wheel around and slide his rear tire forward on the rim again to straighten out the valve stem.

These are cases where rim brakes certainly made the bike ride more dangerous than a disc brake would have. That is not to say a disc brake is immune to heat problems, but a hot disc does not also heat the tire.
― Lennard

Dear Lennard,
I don’t understand way disc brakes have to be sharp and resemble a table saw blade. The working part is the flat surface, not the edge. The edge could be rubber or some other material. I am just puzzled by designers that ignore safety considerations.
— Bob

Dear Lennard,
Regarding the riders’ safety concerns with disc brakes in the peloton, I have always wondered why manufacturers don’t machine a full radius edge on discs after they are laser cut. It’s one extra machining operation, so it’ll add a little cost, but in an insanely litigious world I’m shocked the manufacturers haven’t been doing it since day one for the consumer market. I also believe it would’ve prevented or greatly reduced the injury suffered by Fran Ventoso in Paris Roubaix.
— Ante

Dear Ante, Bob, and all of the rest of you who asked me this same question,
Here are a few responses from disc brake makers. This is a question that clearly comes to mind immediately for many cyclists worrying about discs in road crashes, judging by the flood of mail I got asking about this.
― Lennard

From TRP:
For obvious reasons, I’ve been having this conversation today with TRP engineers.

With a rotor thickness of only 1.8mm, the radius on the edge would have to be so tight that it wouldn’t really help; in fact, it could make it worse. Currently in the manufacturing process, rotor edges are smoothed and polished and aren’t really “razor sharp,” so we don’t think there is much room for improvement in the shape of the edge.

If what we’ve seen online today is true, with a crash in a crowded peloton, rotors can cause real damage and this needs to be addressed. I think everyone is surprised that we haven’t seen this before in MTB or CX, but obviously WorldTour racing, and specifically Paris-Roubaix with a 198-rider peloton, is the ultimate test of this technology.

We’re looking into possible safety improvements, motocross-style covers, etc., but so far haven’t hit on a good solution that addresses this issue and is still acceptable in terms of weight, complexity, and aerodynamics.
— Lance Larrabee
Managing director at TRP/Tektro USA

From FSA:
I was actually expecting an email from you based on the news regarding the UCI in regards to road disc brakes.

It is unfortunate that a rider was hurt due to the rotor. That said, I am confident that we (disc brake manufactures) have solutions to protect the rider from injury due to the rotor edge and arms. Given the rotor width (1.8mm for most brands) and mount standards are somewhat fixed, we can look at rotor edge profiles for sure. Additional protection in the form of guards may also be investigated. This solution may seem simple on the surface, but it will increase the weight and aerodynamic challenges that disc brakes are already facing.

The real question is, should professional road races be using disc brakes? Disc brakes certainly add value to the rider’s experience on endurance, CX and gravel/adventure road bikes, but the optimized race bike may not ultimately benefit from the increased braking performance. Until they are proven to offer better aerodynamics, I do not see the racers embracing the product.
— Joel Richardson
FSA disc brake product manager

From SRAM:
There may be some merit to it even though the rotor is only 1.8mm thick. We are always looking at ways to improve.
— Paul Kantor
SRAM brake product manager

Dear Lennard,
A couple of years ago I bought a set of carbon clinchers from a very well-known and reputable brand. One of my first rides was a 100 miler involving some very long and steep descents and while I’m not a novice by any means, I am also not the world’s greatest descender, and in the course of the event, my rear tire blew out when the tube overheated due to sustained braking. Luckily I managed to come to a stop with no further drama. In hindsight I sort of wish I’d gone with aluminum clinchers but the carbons do ride nice, so …

Anyhow, besides realizing that I needed to learn how to descend more like the pros, I also took the step of running thicker and larger Continental Cross 28 tubes in my tires. I’ve not had any further mishaps to-date but admittedly I am anxious about jumping out on another road where I might be putting my descending skills to an extreme test. It occurred to me that perhaps adding a couple of ounces of water to the inside of the tubes might also help to keep things cooler within the carbon rims but I’m not sure whether that would actually provide any benefit or perhaps make things worse.

Do you have any view on this?
 — Bill

Dear Bill,
I tossed your question into this column due to its similarity with the first question. I doubt you’d get the result you’re hoping for by putting water in the tires.

First off, if you put enough in to absorb a significant amount of heat, you’ll be adding enough weight to make the purchase of lightweight carbon rims in the first case relatively superfluous.

Secondly, while it would slow how rapidly the tire pressure went up with braking, once the water got hot, it would tend to retain the heat more than air alone. And steam can also create a lot of pressure on the vessel containing it. If it couldn’t, the Industrial Revolution would have been a lot longer in the making.
― Lennard

Feedback on last week’s column:

Dear Lennard,
Regarding spacers in some cartridge bearings. To get the balls between the races, all the balls are placed adjacent to each other in the outer race filling less than half the circumference of the bearing. This allows the inner race to be slipped in. Once the balls are spaced circumferentially, the races cannot come apart.

In an assembled bearing, you can pop out the spacer, move all the ball to one side, then slip out the inner race.
— Lou

Dear Lou,
Good point. I did know this, but it did not occur to me when I made the comment about the costs of making a cartridge bearing. You are absolutely right that you could not produce a cartridge bearing if you had the balls touching all of the way around. And with fewer balls, to avoid them moving around and throwing the inner race off center, the spacers are a must.
― Lennard

Dear Lennard,
In addition to telling Paul that Shimano levers will not work with his existing Campy setup, it would have been helpful to tell him that SRAM levers will work, as the cable pull is the same as Campy (a trick I learned from your previous columns). SRAM levers should also be equally effective as Shimano in avoiding arthritic thumb pain.

I was surprised to learn just how many riders share the problems of arthritic thumbs, which caused me to switch away from Campy several years ago.
— Doug

Dear Doug,
Good idea. The cable pull is slightly different between SRAM and Campy, but it is close enough to often work quite well with a drivetrain mixed between those brands.
― Lennard