Not long ago Alto Cycling posted their test of a series of carbon clinchers undergoing their braking test. See below:
This is a great test for a bunch of reasons, however it presents a lot of data without much context (which as an Engineer I can confidently say is the primary challenge of Engineers everywhere). So, all of these wheel fail, except the Alto’s of course, but what does that mean for you and me? All these rims failing doesn’t really jive with experience, lots of people ride all these wheels all over the world with an acceptably low level of catastrophic failure, so what gives?
So first off, I’m going to assume a constant 1200W braking force. The video isn’t 100% clear about this, physics wise a constant 20mph speed and 1200W driving force would require the same braking force regardless of the 5 lbf or 9 lbf braking force they mention (I’m guessing the 1200 was not the same between the two tests).
1200 Watts seems like a fair amount of braking watts and it generally is. Assuming your rear brake cable is broken (oh no!) and you only have one wheel to slow down with you can get a required braking power to maintain 20 mph at various gradients and weights:This means a 200 lb rider (with bike) would need to descend at a constant gradient of 15 in order to achieve the 1200 watts from the test. In the worst case test (Knight 60) that means 120 seconds of braking or 2/3 of a mile.
This is a fairly scary number, however remember some important assumptions about this: only one wheel, no air flow over brake surface, no drag from the rider, and constant braking (which is not typical in real world riding). Air resistance is 150 watts – 250 watts at 20 mph, braking is typically split 80% – 20%, taking just these two considerations into account the picture becomes a little different, the constant 2/3 mile descent the 200 lb rider would need to find is now 22.5!!!!
It’s worth pausing to give some perspective. The Hincapie Fondo specifically bans the use of carbon clinchers for use in their Fondo. The reason is this descent: Strava Link
Let me say, this descent is gnarly with a capital G, there’s a bunch of steep hair pins and stretches of 40+ mph downhill. The first year of the Fondo there was at least one carbon clincher blow out (I don’t know any details beyond that) which warranted the ban. However in the Alto test of respected wheels the rider would have to be over 300 lbs to fail on Skyuka under the one wheeled braking criteria.
In order to determine what sort of conditions would be required to bust a wheel on Skyuka we need to re-tool the analysis a little and get a little more physics intensive. I’m speaking of heat transfer, aka q = h*A*dT. It’s interesting to note that the test reports cooling time, from this observation we can approximate a heat transfer coefficient given the heat added equals time till failure multiplied by power driving the wheel. Assuming a constant heat loss from max temp to room temp we can get a linear approximation of heat transfer away from the wheel dependent on rim temperature. This gives us one side of the equation, that a rim can dissipate, that these rims can dissipate 60-120 watts of heat under stationary air conditions. Forced convection (air flow over the rim) would increase this heat dissipation but we’ll leave that consideration aside. Another approximation is the mass of rim being heated, this was somewhat guesstimated given the above procedure to be 100 grams, this intuitively seems reasonable since CF has pretty poor conductivity so only the area local to the braking surface would be affected, a more detailed modeling would be useful here. So Given a 250 lb rider on Skyuka the heat addition to the two wheeled air drag scenario would look something like this:
This calculation indicates that at constant braking the minimum failure temperature seen by the ENVE (130 C) would never be reached on Skyuka.
COUPLE OF NOTES HERE:
- Don’t take this to mean go bombing down a mountain on your clinchers, peak braking puts a lot more heat into a rim (without associated cooling time)
- Rims will develop ‘hot spots’ that are typically points of failure due to simple manufacturing variances
- Carbon Clincher rims typically give some indication before failure, mainly the strong pulsation in the braking power you’ll feel before they start de-laminating
- The Alto test could have picked particularly good or bad wheels from each of the manufacturer, so it’s hard to extrapolate from a single test for each.
The real solution if you’re worried is to stick to alloy’s or get disc brakes, also a more reputable brand wheel set will have better quality control. Also ride smart, don’t drag your brakes all the way down a descent, sit up and use smooth gradual braking.
The test is maybe a bit overly grueling, most carbon clinchers have a weight limit of 185 for good reason and unless you live in the mountains east of the Mississippi you’ll likely never see such intense braking. The B-17 Flying Fortress from WWII was famous for it’s ability to take large amounts of damage and return home, however this durability came at a cost of high structural weight. This begs the question if the B-17 would have been better suited with a durable structure, or would have fared better with lighter structural weight traded for more gunner positions. The point here being that seldom is a mechanical system designed do just one thing, and over-design isn’t always necessarily beneficial.