Sunday, June 24, 2007

Equipment - Single Pivot Brakes

One of my first epiphanies happened when I adjusted a "sidepull" brake for the first time. The dang thing wouldn't center until I turned the center bolt using a cone wrench. And suddenly the brake was centered and it stayed centered!

That was great.

Nowadays, with the dual pivot brakes, such mechanical knowledge became, well, sort of obsolete.

Recently Campagnolo came out with their combination brakesets - the front was a dual pivot, the rear a single pivot. The rear brake acted funny to contemporary mechanics - when you twisted it to center it, it simply sprung back into its uncentered state.

What the heck was happening?

Centering:
A single pivot brake needs tools to center (rather than a firm nudge with your hand). This is because there is a two sided spring (left-right) held by the center bolt. If you push on the brake arms, they simply spring back in place. The center bolt position determines where the brake arms center. In contrast, the dual pivot brakes have one spring for one side, another for the other, and they are mounted on a piece which you can tilt, tool-free, from one side to the other.

If the center bolt on a single pivot brake is tilted to one side (say the right side of the spring is tilted down towards the rim), the corresponding brake pad will rub. You can use your fingers to twist the brake back in place till you use the brake again, but as long as the center bolt is tilted, the brakes will always recenter around that "tilt".

To center a single pivot brake, you must center the center bolt. Once you have the brake centered properly, and the center bolt is tightened sufficiently, it will stay centered.

Advantages Single Pivot:
1. Lighter (less stuff in brake).
2. Simple (less stuff).
3. Cheaper (less stuff).

Disadvantages Single Pivot:
1. Can't re-center brake during a ride unless you carry around a wrench.
2. Can't re-center brake without tools if you put in a wheel that is slightly "off center" to the brake's current setting - this is significant if you get a wheel change and the new wheel is not centered in your brake.

Braking Power:
There is actually a difference in braking power, simply because the dual-pivot brakes have more leverage. The same difference exists between "normal" cantilever brakes and the "V-brake" setup on mountain bikes. Normal cantilevers have rapid pad movement but less leverage. The V-brakes have tons of leverage. In fact they were so strong compared to normal brakes that many companies had to redesign their frames after the brakes caused the seat stays to buckle outwards. The problem with V-brakes is that the pad takes a while to move - it takes more cable travel to make the brake pad move the same distance. In behavior, normal cantis=single pivot and V-brakes=dual pivot.

The improved braking power is not apparent in current sidepull brake designs since all brakes seem to work pretty well. The extreme "low braking power" example for single pivot brakes is the old Aero-Gran Compe. The brakes had such little leverage that slowing firmly was an adventure in itself - the brake single handedly defined "anti-lock brakes" for bicycles. The rider got instant pad movement but could not exert a lot of force on the pads. It didn't help that the pads were small, of poor material, and of unique shape with no real aftermarket pads available.

For ultimate braking power, both single- and dual-pivot brakes work fine (excepting the aforementioned Aero Gran Compe). With maximum power applied to the brake lever, any current brake will lock up its wheel. This is actually the legal definition of what a "proper" brake will do on a bicycle here in Connecticut - a bicycle brake must be powerful enough to lock up the wheel. It's sort of illogical since locking up a wheel either skids a tire (if you lock the rear) or dumps you on your face (if you lock the front).

Locking the rear is not as dangerous as locking the front - to the point where a lot of uneducated riders think that using the front brake is "dangerous". Safety, in this case, is a matter of control. Nothing changes the fact that virtually all of your braking is done with the front brake.

The real question is which design allows you to use the maximum effective brake pressure? In that respect the easily-modulated dual pivot brake wins for a front brake. Hence Campy puts the heavier but easier to control brake in front. The rear brake, being less important, recieves the lighter but a bit less controllable single pivot brake. A great combination of reducing weight, reducing complexity, and maintaining control.

Until, of course, you need to re-center that rear brake.

2 comments:

suitcaseofcourage said...

Excellent technical post! I'd wondered about that, esp. since I just recently upgraded my brakes from the stock Cannondale Omega to Campy Veloce (last year's version) to match my levers(!) Could be my imagination, but they seem to work much better.

Anonymous said...

Agree with that comment. Just got a new bike with campy and thought there was a problem with the rear brake because the arms were "floppy" unlike my old shimano rear brake. This post totally answers my questions