Subject: Re: [CR] Short Reach Front, Long Reach Rear?
I've seen this done on 70's Brit and French (but really Swiss) centerpulls, and Italians from several decades using sidepull and centerpull. I'm going to pass along how it was explained to me by an insightful LBS mechanic when I asked the same question three decades ago.
This is a purposeful framebuilder's choice/opinion rooted in the physics of braking. You need more stopping power transmitted to the front wheel. Every cyclist knows this. Every motorcyclist knows this. And you certainly are replacing more front brake pads than rear on your off-topic four-wheel-disc-brake auto.
We've all experienced the hard dual-wheel panic braking situation that locks up the rear wheel quicker than you expected at the worst time. With all other caliper variables held constant, the longer reach caliper arm has less mechanical advantage, and will transmit less braking force to the rim than the short reach will (for equally applied pressure to matched brake levers at the handlebar). Assumptions are made here about the pivot point geometry .. but they apply for all applications I've seen.
When you see a long rear / short front caliper combo, it was a designed-in attempt to compensate for the extra force required at the front, and offer the rider more modulation control on both wheels without having to overthink the applied pressure at the brake lever. Even if the builder did not push the choice to different spec calipers, it's not uncommon to see machines from the 70's that pushed the range to honor this physics. I have two Italians in the basement that have the same caliper arm spec, but they were brazed up to set the rear at 57ish and the front at 53ish.
The observation about longer rear drop-outs being involved has a lot of merit, but this comes up far more quickly for Mixte location rear calipers than traditional brake bridge locations. If Mixte location ... then yes ... the long-armed caliper choice is almost essential to accommodate the variable wheelbase adjustment of longer slots while looking for that freewheel-derailleur sweet spot.
Just sharing, I think the math will support the observation ... but I would certainly value a frame-builders opinion on this subject if my observations don't pass muster for this well-informed group.
Tory Werne
Woodstock, Georgia
USA