At 6:15 PM -0800 1/20/09, Kurt Sperry wrote:
>Thus I expect any significant advances in braking power since the '50s
>to have been either down to lower friction cables/housings, better
>brake friction pads/rims or simply changes in the lever ratio
>compromises chosen.
A few things have changed since the 1950s...
- braking has become more important. On gravel roads, the limit is the adhesion of the tires on the road, not the power of the brakes. You can't let the bike roll on a gravel descent, then apply the brakes hard just before reaching a hairpin. (Touring bikes and tandems often used drum brakes to allow continuous braking without overheating rims.)
- brakes now have shorter reach. In the 1950s, many racing bikes had huge clearances even with wide tires and fenders.
- Caliper flex is a real concern. Short-reach brakes work much better than long-reach brakes. Disc brakes really are just ultra-short reach brakes. (The disc is a second, tinner rim, without tire, that can be clamped by a very small caliper.) Long-reach brakes work well if you move the pivots to shorten the lower arms.
- Powerful brakes must be strong, as the forces generated are high. In the 1940s, numerous brake calipers broke in the Tour de France. I suspect this was because they used cast aluminum calipers. Mafac's Racer proudly wrote "Dural Forge" ("forged high-strength aluminum") on their calipers to show the difference.
- Brake design wasn't always very brilliant. Many makers used the same upper arms for short- and long-reach brakes. Of course, the long-reach brakes had much less mechanical advantage. Has anybody experienced differences in braking power between short-reach and long-reach Campagnolo Nuovo/Super Record sidepulls?
- As Kurt pointed out, if you increase the mechanical advantage, you have to put the pads closer to the rim. This is OK if your brake arms always return to the same spot. If they move over time, as they do on a sidepull, you'll have pads rubbing the rims after a short period of time. By having the brakes open further, you increase the amount of time between having to re-center the brakes. (Dual pivot brakes, cantilever and centerpulls return to the same spot every time.)
A slight increase of mechanical advantage as you apply the brakes is useful. This is built-in with cantilever and centerpull brakes. (Campagnolo's Delta brake had this, too, but it was negated by the friction in all those pivots, which required great hand power to overcome. Also, I believe the Deltas had too much mechanical advantage - they use only a small portion of the available lever travel.)
- cable routing is important. Modern handlebars often have very sharp bends. With "aero" routing, I suspect you loose significant braking power due to friction. (I once had a Bike Friday with a particularly tortuous and long cable routing to the rear cantilever brake, which had so much friction and compressed so much that the brake had no discernible effect - it did not slow the bike down. The same brake on the front, with a very clean cable run, was very powerful.)
If all brakes were equal, then why did so many racers switch to Mafac centerpulls almost immediately after they were introduced? From what I have heard, Mafac did not sponsor anybody, the racers had to buy the brakes with their own money...
From that, I conclude that the 1940s sidepull brakes were pretty lousy. By the 1960s, when Campagnolo introduced their sidepull, shorter brake reach and closer manufacturing tolerances made sidepulls competitive again (albeit not as good as centerpulls in my experience).
Jan Heine
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