POSTFACH, Germany (BRAIN) — Disc brakes on road bikes? Not so fast, says Germany’s brake specialist, Magura.
Magura has launched a study logging how a number of test riders use disc brakes over miles of European roads before deciding whether the technology is an option for the company.
“The biggest concern is overheating since average and top speeds are much higher for road bikes than mountain bikes,” said Stefan Pahl, Magura’s bicycle product manager and engineer.
“As you know from physics, energy is equal to mass multiplied by the square of the speed. So if you double the speed, you generate four times the energy which you have to destroy by braking—converting it into heat,” he said.
As Pahl walked about Eurobike and Interbike looking at disc brake-equipped road bikes, he winced whenever he saw bikes spec’d with 140- or 160-millimeter rotors.
Rotor size and type are vital when considering heat dissipation. And Pahl said he’s doubtful small rotors—currently fashionable on road bikes—can dump enough heat on long descents to keep braking systems from boiling.
That’s one reason Magura only offers hydraulic rim brakes for road use—rims are the largest rotor possible.
“All you have to do is look at motorcycle brakes. Dirt bikes have single small rotors up front while road motorcycles usually have huge twin rotors. That tells you a little about braking needs on the road,” he said.
There are requirements for disc brakes in the current European bicycle standards, EN 14781, that were adopted in 2005. But there are omissions as well.
“While there are testing procedures for disc caliper mounting tabs for road forks, there are no similar tests for rear frame mounts,” said Michael Baker, ACT Labs’ sales and marketing director. ACT Labs provides third-party testing to manufacturers seeking to comply with European standards.
“Companies developing disc-brake road frames ask us to modify the fork-mount test to test their rear frame mounts because there is nothing in the road standard covering it,” he added.
The static disc brake torque test for road forks required by the EN 14781 standard is more rigorous than the EN 14766 standard for mountain bikes.
Road bike forks must carry an 8 percent higher load than a mountain bike fork and a road fork’s braking cycle fatigue test is 66 percent longer than for a mountain bike fork.
“I don’t know why the road disc fork testing is to higher loads and cycles than a mountain bike fork, but it does reflect higher road speeds and less tire slippage of a road tire on pavement,” Baker said.
While road standard tests are for higher peak loads and greater cycle frequency compared with mountain bike discs, the heat dissipation requirement is the same. Road and mountain brakes must absorb 75 watt-hours of energy over a 15-minute period without failure to be legal for sale in Europe.
To put this in perspective, a 200-pound bike and rider traveling at 25 mph dissipates 3 watt-hours of braking energy to stop, and no matter how many times they stop they are unlikely to overheat brakes.
However, the same rider descending a mountain pass hitting 50 mph in stretches between switchbacks and who brakes to 15 mph for corners uses 11 watt-hours to slow down. If the descent averages a 7 percent grade, add 2 watt- hours to each brake application.
After braking for seven or more corners within a 15-minute period, the rider’s front brake could have absorbed much more than 75-watt hours of energy required by the standard.
“Road disc brakes are still in their infancy with regard to spec and with regards to UCI- or USCF-sanctioned racing. There is still a great deal of learning to be done,” said Stefan Berggren, ASTM’s representative to the ISO TAG meetings and Trek’s global standards and compliance engineer.
“As for testing and issues with heat, oil in the caliper, and or energy application for the system, these are currently not addressed in the standards,” Berggren added.
Brake and bicycle manufacturers have greatly increased the amount of in-house testing on road disc systems, Berggren added.