Originally Posted by
Bob Pringle
Purevl,
It appears to me that if, in an extreme case, the cable run is lowered until it's in line with the brake arm pivot, it would have zero mechanical advantage and be unable to produce any torque at all on the brake arm. Conversely, the further above the pivot the cable run is, the more leverage it would have in producing torque on the brake arm, and therefore more force at the brake pad. The best case would be if each brake arm had a cable pulling straight up, in which case the mechanical advantage would be appx. 3:1. A relatively long straddle cable would approach this scenario.
Regards,
Bob P.
Moment or torque is a product of the length of the moment arm and the amount of force applied
perpendicularly to the arm. The arms in this case are the portions of the caliper above and below the pivots. The force is obviously applied through the cable. The only way to get zero net torque would be to pull the cable directly along a line extending from the anchoring points to the pivot, which is not what you are saying. The best case would definitely not be running the cable straight up, but having the cable pull be exactly perpendicular to the upper arm when the pad is contacting the rim. It's unfortunate that so many words in the common vernacular such as "leverage" either have different meanings in physics or mean nothing at all. Sorry, but I'm just not sure where you are getting your numbers. I have to leave town to help my sister move, but I will draw a sketch of what's happening whenever I get home if that will help you understand.
Cheers
