The Science of Stopping | Street Savvy

The basics of braking sound simple enough. Preload the front brake lever smoothly to settle the suspension, squeeze hard enough to achieve the desired rate of deceleration and finish with an equally smooth release. The first and last steps are important, but mastering the main part of the process is vital: modulating lever pressure to generate maximum braking without locking the front wheel.

Constant pressure on the lever during that main phase doesn’t slow things down very quickly, but as speed drops, the rate of deceleration quickens. The same gentle squeeze that barely slows the bike at high speed generates an abrupt stop when you’re going slow.

It makes sense to apply more lever pressure at high speed. The big question is, how much more? Is there a reliable way to judge how much lever pressure to apply? How do you avoid locking the front wheel? Not knowing the right answers keeps riders from braking aggressively enough at high speeds. Why? Braking hard at high speeds is frightening. Braking hard at slow speeds feels safer, creating a tendency to over-brake. Under-braking at high speeds increases stopping distances, while over-braking at low speeds increases the chances of locking the front wheel.

Why does the same lever pressure let you decelerate differently at different speeds? In order for the bike to slow down, it needs to shed the kinetic energy of the combined bike/rider moving mass (M). Kinetic energy (E) is proportional to the square of the speed (V): E=MV². We shed this kinetic energy by squeezing the front brake lever, which presses brake pads against rotors, which creates enough frictional force to slow the bike. At any speed (V), the work (W) being done (kinetic energy burned per unit of time) is directly proportional to the force (F) being applied to the bike, and not to the square of such force: W=FV.

Double your speed and you need four times as much lever pressure to generate the same rate of deceleration. Compare two reductions of 20 mph—one from 30 to 10 and another from 100 to 80: In the first, the brakes need to shed the energy of the bike/rider mass expressed as M: M30²-M10²=900M-100M=800M. For the second, it's M100²-M80²=10000M-6400M=3600M. Dropping 20 mph at high speed requires energy of 3600M while doing so at slow speed requires only 900M.

As the bike slows, lever pressure should be gradually reduced. Say a rider applies 1 lb. of force to the lever at 20 mph to get a given deceleration rate. It takes 4 lbs. of pressure to achieve that same rate at 40 mph. And at 80 mph, it takes 16 lbs. To brake effectively from 80 mph to a stop, the rider needs to apply 16 lbs. of lever pressure at 80, easing to 9 lbs. at 60, 4 lbs. at 40, 1 lb. at 20 and almost no pressure at full stop. How do you keep the front wheel from locking up? Why does it lock up at all?

The tire forms a rolling bond with microscopic road imperfections at the contact patch, similar to the one between a chain and sprocket. Traction is the tire’s ability to withstand sliding forces. Braking loads the tire with sliding forces. Weight transfer adds downward force to the front contact patch, increasing traction. That’s one reason the front tire can generate more braking force despite its smaller contact patch. Conversely, hard braking reduces downward force on the rear tire, _decreasing _traction.

The key to effective braking is a constant rate of deceleration. If you can brake hard at slower speeds, you can achieve the same rate of deceleration at higher speeds, but there is a real-life caveat to this rule. Road imperfections shake the tire more at higher speeds, allowing rapid fluctuations in available traction. That means maximum braking forces (not exceeding available traction) are reduced slightly. The real object of effective braking is to achieve somewhat less than maximum deceleration as soon as possible, reducing lever pressure as the bike slows to bring the deceleration rate to maximum.

If this technique seems counterintuitive, give it time. Practice by _gradually _adjusting your current approach. And don’t forget about phase one and three. Your preload and release should be smooth and gradual, even if each one only lasts a second or two.

As our test rider so deftly demonstrates, the forward weight transfer of hard braking enhances the front tire’s ability to put braking force on the pavement, making the rear tire irrelevant.