Direct Pull (V-Brake) vs. Center Pull Cantilevers (pros and cons)
I am looking around for an old mountain bike to ride this winter (and this summer when it is wet).
I was just browsing around and was curious what the difference was between
Direct pull brakes:
And Center-pull Cantilevers:
Sheldon Brown has a good description of both kinds of brakes but he does not give pros and cons. Or suggest which are better.
The main reason that the direct pull (V brake) was invented is that it approximates a center-pull cantilever in terms of balanced force and leverage, while not requiring that the cable be anchored at some point above the wheel. This is important for front wheels on suspension forks, but not significant for most other uses.
Another slight advantage of the V brake is that it doesn't generally protrude out of the profile of the bike quite as much and is thus somewhat less likely to be damaged in off-road use.
Neither advantage is of any importance for a road bike without a suspension fork. However, there is the problem that the two require different levers (because the cable travel length is different) and the old cantilevered levers are getting hard to find, should you break one.
You've missed out the main marketing point of direct pull. By having the bowden cable pull in the same direction as the direction of travel on the brake, more of your pull translates of braking power (some physics fluff about F being proportional to the normal (perpendicular) force). Centre pull loses about a third of the pull by pulling upwards as well (which is useless).
@Aron - You must have missed the place in physics class where levers were explained. Arguments about perpendicular force, et al, are pure bull -- work = force x distance. Period.
I must have missed that class between Quantum Mechanics, Advanced Particle Physics and General Relativity. I always thought that E = F . x or E=Fd cos theta. The fact that the Bowden cable pulls at an angle to the two arms of the Y made me think that cos theta was no longer unity. Thank you for schooling me in basic Physics.
Besides, this has nothing to do with the work function on the brake pads, but the normal contact force of the on the brake pads and the coefficient of friction.
Moment is conserved in a lever. That means that you could increase the leverage as much as you like, but the travel of the pad would decrease, the result is much smaller tolerances.
@Aron - Right, the other factor in the equation is pad clearance. But most setups can tolerate a 2x difference in pad clearance. (And the V-brake is actually *worse* in this regard -- less clearance than your typical canti.)
A canti with the same force output would have even worse clearance, as I said moment is conserved, and the cantilever setup has less moment.
Another advantage of a V-brake is that you can adjust them a lot faster than a cantilever brake.
If you want to calculate on forces, don't forget the difference in cable tension and travel. Canti brake levers put more tension on the cable with less travel compared to V brake levers for the same force you apply to them. In the end, given conservation of moment, if you apply the same force with the same amount of travel at the brake lever, and the pad clearance is the same, then the force applied to the pad must be the same (minus friction).