blingshock

Does anyone know why some cantilever brakes come with different length link wires for front vs rear? For example, the Shimano Altus cantilever BR-CT91 front brakes come with length B 82mm and the rear brakes come with length A 73mm.

hokiefyd 

I think the idea is front and rear brake posts (the posts to which the brake arms are mounted) often have wider spacing on the fork than they do on the seat stays. Using Shimano's idea of an ideal link wire configuration, this requires a longer link wire on the front than on the rear. Shimano's link wires will work, but are usually far from optimal in terms of mechanical advantage. Especially with low profile cantilevers like the CT91s, the lower you can get that straddle or yoke cable, the better (in terms of mechanical advantage).

FBinNY 

The yoke cable's length is part of the leverage geometry and along with arm length determines the final leverage. The goal is usually to balance maximum leverage against lever and brake shoe travel considerations.

A manufacturer will tune yoke cable length to accommodate different caliper arms, or by design to reduce leverage in either brake to better balance braking forces when levers are squeezed equally. Lastly, on some frames, the yoke length has to be set to accommodate physical factors, ie. going around a tube when necessary, or because the hanger is too close to the brake.

My advice is to assume the maker knows what they're doing and use what's spec'd if it fits, until/unless you have the expertise to change to better suit your personal preferences.

oldbobcat

Generally, a 90-degree angle at the yoke gives the best combination of responsiveness and braking power. Longer yoke means more power but requires more lever movement. Shorter yoke means less power but quicker action.

Andrew R Stewart 

Not sure this is correct. A longer straddle/link cable will result in less leverage on the pad but the lever will feel more solid. A shorter cross cable will have more leverage but the lever will feel mushier. Pick your poison.

Francis's comments about dimensional constraints is a good one. I have made quite a few smaller sized frames and working the cable routing, rack struts, cable hangers is more challenging on smaller bikes. At the front end this dimensioning stuff is rarely an issue. Andy

Bill Kapaun

Maybe the best "average", on paper.
Everybody's different and may find a slightly different combination is best for them. I know what you're trying to say, but you're trying to be too absolute.

oldbobcat

No. Work is the pressure from the pad on the rim. Force to perform the work comes from the hand squeezing the lever. Think of your gearing. You shift to a low gear when you want to climb a hill. You have to apply more revolutions (distance) to the cranks to perform a given measure of work, but the force you apply to the cranks is lower. You are trading force for distance.

A short straddle cable will be quicker, (like a high gear) but for a given amount of force from the hand that's squeezing the lever, a long cable allows more work (like a low gear). Unless, of course, the cable is so long that the lever bottoms out on the handlebar before the pads touch the rim. The pad moves farther with the shorter cable, but the force that can be applied on the rim is less because the power from the hand is not infinite.

Variations in straddle cable length will also relate to the intended application of the caliper. An MTB will have greater distance between the pivots than a cyclocross or road touring bike.

FBinNY 

Sorry (I truly am) but No.

Work is defined as the product of force and distance. Other than the movement against the spring (not much force, not much work) and later the movement from the distortion of the downstream hardware, ie. cable, brake shoes, brake shoe compound, (plenty of force, virtually no distance) NO (meaningful) WORK IS DONE here.

The Work happens at the rim where movement of the rim against the friction generated by the brake shoes, produces significant heat energy. If squeezing brake levers were "work" (in the technical sense) you'd have burned palms. While riders produce work turning pedals to propel bicycles, it's the bicycle itself that's producing work during braking, not the rider and not the brakes. That's why your legs get tired and your useless lazy hands don't.

As for Andrew's comments about firmness vs. mushiness, it's about leverage. Greater leverage makes you "stronger" and more able to distort the hardware, and that's why things feel mushy. FWIW ---Mushiness, however undesirable doesn't matter other than robbing usable lever travel. Force into the cable always equals force out the other end. So, as I posted early on, it's always about balancing maximum braking force against practical travel considerations.