Originally Posted by twilkins9076
Can you be more specific as to how the spring improves performance? From what I see, it should give system a little bit of "snap" as the brakes are released, and help those brakes that tend to drag a little bit, but what else it might do, I'm not sure.
What else would you want it to do?
Seriously though, the Avid ball bearing mechanical disc brakes were not designed for use on tandems and the internal return spring is not strong enough to deal with the added static and dynamic drag of the significantly longer brake cable run leading from the brake lever to the caliper's torque arm. Using "slick wire" and Avid Flak Jacket or Aztec cable housing can eliminate some of the dynamic drag, but they don't resolve the static drag that comes from the extra 2.5' - 3' of brake cable that runs the full length of a tandem.
The use of a compression spring sized to fit between the Avid caliper's integrated cable guide and the torque arm provides just enough additional pre-loading on the caliper's torque arm to remove excess slack from the brake cable while, at the same time, helping the torque arm to return to it's fully open position.
What this means to you when operating the brake is that you're no longer losing the first few mm's of brake lever movement to take up the slack in the brake cable which provides a more positive lever feel and linear brake action from the caliper. As you modulate your rear brake the action is also more positive and it allows the torque arm to uniformly return to its fully open position, hence the elimination of the slight dragging noise that is often found on tandems with rear discs.
There are some dealers and owners who have opted to install in-line QBP / World Class Travel Agents or SideKick BPBs (brake pull boosters) on tandems equipped with the Avid BB discs to increase the mechanical advantage of the road brake levers, similar to how linear-pull or V-brakes are integrated with STI and Ergo road levers on tandems. Personally, I think that it's overkill and an ineligant solution that yields a somewhat odd lever feel; however, many teams with the 'brake boosters' report good results. The brake boosters simply pick-up the excess slack that still remains in the brake cable run (that is, unless you also install the compression spring) with the roller-cam's higher mechanical advantage... so much so that the Avid's brake pads don't need to be set as close to the rotor as they do on a "stock" installation to preclude brake drag. The increased mechanical advantage also reduces the intervals between brake pad adjustment which may a be good or a bad thing. If you don't keep after your brake pad adjustment with the built-in red knobs you may start hearing a metallic sound as you operate your rear disc. It's not that the pads have worn down so far that they need to be replaced; rather, they've just worn down enough on the inboard /fixed side to where the outboard / actuating side is pushing the rotor up against the caliper body instead of the brake pad. A turn on the large red, inboard adjusting knob will quickly resolve that problem.
So, at least for me, when I'm asked how to "tweak" an Avid I'll always suggest the compression spring as the first "mod" beyond making the most out of the internal adjustment features. If that still doesn't provide an owner with the right feel or performance -- noting that it really does take about 20 good, hard braking events to start bedding in the pads and rotor -- then I'll offer up the brake booster mods, but only as a last resort. IMHO, simple solutions are always the best.
Last edited by TandemGeek; 03-25-07 at 10:31 PM.