ScottCommutes

Sheldon Brown also provides a mathematical formula that gives the mechanical advantage at the straddle cable. The formula is 1/sin(yoke angle). Yoke angle is the angle between one side of the straddle cable and the horizontal.

A common position might be a yoke angle of 45 degrees, where the two sides of the straddle meet at a 90 degree angle. This provides a mechanical advantage of about 1.4x.

A long straddle cable could limit this mechanical advantage to about 1.1x. Going lower is essentially impossible without an infinitely long straddle cable. Even then, you are still at 1.0x.

The interesting thing is that shortening the straddle cable has a greater effect. If you can shorten your straddle cable down to a yoke angle of 30 degrees (120 degree angle where the two sides of it meet), you have increased your mechanical advantage to 2.0x (double).

Also, the cable angle you see in pictures doesn't mean diddly. What matters is the cable angle when the lever is pulled and the brakes start to work. This changes as the pads wear. Compensating with the barrel adjuster keeps the pads close, but it doesn't preserve your mechanical advantage - your straddle cable effectively becomes longer. Everyone loses the mechanical advantage of a short cable every time they wear down the pads, and there is no easy adjustment for this.

Foolish me. I figured this was an easy question. Turns out there is a lot to it.

Edit: Also worth noting that the straddle length is personal - no one else could possibly set it for you. You need your body weight, your fingers, and your tires on the bike to actually test how the bike stops at a particular setting.