Evan Mcneely’s complete guide to disc brakes on road bikes
January 7, 2016
Norco Factory Team rider Evan Mcneely is more than just a professional mountain bike and cyclocross racer – he’s also a fourth-year mechanical engineering student at Carleton University. Who better to explain the hot topic of disc brakes on road bikes!
Writing for Canadian Cycling Magazine, Evan recently put together “The Complete Guide to Disc Brakes on Road Bikes” looking at the physics behind braking and what discs mean for your next road ride. The article coincides with the UCI’s recent announcement to continue testing discs on road bikes for 2016.
Read an excerpt from Evan’s article below.
By Evan Mcneely
On April 14, 2015, the UCI issued a statement announcing that they would allow professional road cycling teams to begin testing disc brake-equipped bikes, at two events of their choosing, during August and September. This news came after nearly every major bike manufacturer had introduced a disc brake-equipped bike into its line. The machines were essentially marketed with “have what the pros can’t have.” Since then, a few professional teams have experimented with disc brakes in competition.
Recently, retired professional road racer David Millar took to Twitter on the subject, claiming that the difference in power of hydraulic disc brakes and traditional rim brakes means nothing if the tire is already maxed out, sliding in a skid on the tarmac. Millar raises a very interesting point, but it is not the whole story.
Brakes slow you down by applying a “moment” onto your wheel in the opposite direction that the wheel is spinning. A moment is a fancy engineering term for a force acting around the centre of rotation of a wheel. The road is also applying a moment on the wheel at the point of contact the tire makes with the road’s surface. The wheel also gains a moment of inertia once it starts spinning. The moment of inertia is the wheel’s desire to keep spinning once it has started spinning, and is also known as Newton’s First Law. I will call these moments the braking moment, road moment and inertial moment, respectively. If the braking moment applied to the wheel is greater than the road moment and inertial moment combined, the wheel will lock up and you will enter a skid. In this instance, David Millar is correct in saying that it doesn’t matter whether you’re on disc brakes or rim brakes, the distance that it takes you to stop is only dependent on the friction between the road and tire. However, when bombing down a hill at 80 km/h, the wheel gains a large inertial and road moment. This situation means you can apply a huge braking moment without entering a skid. In this scenario, disc brakes can greatly decrease the distance it takes to come to a complete stop. Traditional rim brakes can’t apply anywhere near the braking moment that a hydraulic disc brake can.
Read the rest of Evan’s article here.