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August 02, 2013
Next time someone yells "give me a break," take the opportunity to show a complete system. What is a brake system, anyway? Well, first you need a caliper and a control lever to actuate it.
Next are cables and housing to connect the two. Is that everything? While this is what you get when you buy a bicycle brake system in a box, it is not the complete brake. What is missing? Most lightweight bicycle brakes are technically speaking, disk brakes. Not drum brakes with internally expanding shoes, or coaster brakes with internal clutch mechanisms, or band brakes with straps that tighten on hub mounted drums.
No, bicycle brakes are disk brakes and, for most, the disk is your wheel. So add the disk to complete the brake system:
The rotor is mighty large, but the function is disk pure and simple. The trend in brake design is towards disks that are small in diameter. We all expect that to extend to road bikes which will free the rim from huge variations in temperature and the melting and burning dangers that pneumatic tires cannot well manage.
So, what are the important issues with bicycle brakes? There are only three whose misunderstanding really handicaps our scene.
Center of gravity
Compared to every terrestrial vehicle with practical function, the bicycle has a very impractical distribution of weight. Notice where the center of gravity is compared to the wheelbase.
Before any serious deceleration can occur, this geometry wants to tip forward. The tip over danger in cycling did not end with high wheelers. Owing to its motor, no motorcycle has a high center of gravity. This instability makes you wonder if public cycling would be permitted were it invented in the 21st century. This also means that bicycle brakes have little to do.
A super effective bicycle brake (imagine a dual disk brake from a racing motorcycle) would not stop us faster than our relatively weak systems. That's because a front tip over occurs at such low brake forces. There's nothing we can do about it except ride recumbents!
As this rolly poly shape starts to topple forward, the rear wheel becomes light. All the vehicle weight moves to the front wheel which overloads its traction and it begins to skid. A skidding front wheel has no directional stability and a single track vehicle without steering falls down. So even if you escape a tip over, you'll fall down trying to stop too fast.
Best, of course, to take advantage of your tall posture and be aware your surroundings, anticipate danger, and move with finesse. That's what we do and why cycling can be so safe. We're safe NOT because we have such powerful brakes (like cars) but because we ride smart.
Weight transfer and traction
What does all this mean for your wheels? They are spared serious brake forces. With a rim brake, here is the effect on spoke tension.
Due to the bike's inability to get much force to the ground (weight distribution and traction) the wheel thinks it is enduring a light radial load. Like you just put on a backpack. Easy to resist.
A hub brake, like a disk, is also spared serious loads and shares the advantage that all spokes are engaged in supporting torque forces in a wheel. Braking forces never exceed those of pedaling so any wheel built to survive pedaling torque is well equipped to manage a disk brake.
Why do wheels fail in braking?
Of all the six functions your wheels perform, their role as disk in a disk brake system is the least demanding. Important, sure. Demanding, no. Then why do some wheels fail during peak brake events?
If your wheel begins to skid, loses directional stability, it can become turned from the direction of travel. In the rear we call this fish tailing. In the front it's an instant loss of control and usually a fall. These out of control slides can overload the wheel with side force. It's not the braking itself that hurts the wheel. Any loss of control can cause damage.
There's lots more to the story but if you keep reminding yourself of the underlying principles you'll be better at picking, using, tuning, and designing brakes.
Next comes Wheel Trick Four - the key role it plays as a transmission element. Coming soon!
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