Quote:
Originally Posted by GregL
As others have noted, older cross bikes with comparatively narrow brake post widths place the brake arms in non-vertical positions when wider rims are used. I partially compensated for this with spacer placement.
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This should be a good thing. The issue with using linear pull brakes with road levers is that they have a higher mechanical advantage (leverage ratio) than the road calipers that road levers were designed for. Anything that reduces effective arm length of linear pull brake should improve pad clearance.
Single pivot road calipers have a leverage ratio of about 1:1. Dual pivot brakes with a single offset pivot have a leverage ratio of about 1.5:1. Dual pivot brakes with dual offset pivots have a leverage ratio of about 2:1. Mini-V linear pull brakes typically have leverage ratios of 3:1, depending on arm length and mounting stud location. So the cable travel of road levers designed for side pull calipers will always be a poor match to mini-Vs. But the closest match will be road levers designed for dual pivot brakes with dual offset pivots (such as Shimano 11spd levers).
A Travel Agent will increase cable travel by a ratio of about 2:1, making the overall leverage of a Min-V + Travel Agent about 1.5:1, or about the same as a dual pivot brake with a single offset pivot.
Coming from a different angle, the leverage ratio of a typical low-profile cantilever brake is about 1:1. This is a better match to most road levers (and partly why cantilever brakes were used with road levers on 'cross bikes for so long). A cantilever brake with longer arms will increase its leverage ratio. The leverage ratio can also be increased by lower the straddle height, but this carries the disadvantage of a falling leverage rate (leverage ratio decreases as the brake is actuated).
The leverage ratio of a Mini-V is only affected by the height of the mounting studs, the height of the brake track, and the vertical height of the cable attachments*. Moving spacers around on the pads will not change the heights of the mounting stud or brake track, and will have only a minor affect on the vertical height of the arms. Moving the pads spacers around will also have no affect on the arc of travel of the pads at the rim - this is only affected by the relative locations of the mounting studs and brake track**. However, a wider rim will decrease the downward arc of the pad (but will still have no effect on leverage ratio).
*This was one of the advantages of V-brakes over cantilevers: V-brakes have fewer variable, so it is easier to set up, and harder to set up wrong. But the caveat here is that levers have to be matched to the brake, because there is no way easy way to match their leverages.
**The exception here is for V-brakes with the pads actuated on parallograms - the parallograms keep the pad travel more horizontal, and reduce pad dive at the rim.