I have two 1980s Jack Taylor tandems, one 27" (originally with Shimano, now with Mafac cantis), one with 700C (originally with Shimano, now with Campy cantis). To my surprise, the frame dimensions (bridges, brake boss placements) are identical. Both brake fine, the 700C bike somewhat better (due to the newer pads, I believe).
This seems to indicate that in _some_ cases, switching from one size to another is not a problem. It seems that Taylor built them so the bosses were in between the "correct" locations, resulting in a 2 mm "error" for each wheel size. (The pads on the Mafac-equipped 27" tandem point ever-so-slightly upward.)
The problem Peter mentions is a shortening of the lever arm, thus results in too little brake pad travel per given lever travel (great braking force, but your levers may bottom out). Assuming you can adjust the brake so the pad hits the rim correctly, you can increase the pad travel per lever travel in at least two ways: - playing with the straddle cable length - I believe it's a shorter straddle cable that gives more leverage (details on Sheldon Brown's web site) - using canti-specific levers.
Another possibility are the Mafac "tandem" (long-arm) cantilevers. In my experience, they work best with a road bike lever, because they offer too much pad travel (and not enough braking force) with a canti-specific lever (including MAFAC!). Most French tandems used these with Campy NR-type levers. Using these with canti-specific levers would solve the problem. If you need an NOS set of these, contact me ($ 65 without pads).
That said, relatively nice Panaracer Pasela tires in 27" appear to be available, too, so you can always postpone the switch.
Your mileage may vary.
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Jan Heine, Seattle
Editor/Publisher
Vintage Bicycle Quarterly
http://www.mindspring.com/
700c rims are 4mm smaller in radius than 27" rims. So, if the studs are properly located for 27" rims, they will be 4mm too high for 700c rims. This will make the pivot point too close to the rim, making the brakes difficult if not impossible to setup correctly. And it will significantly reduce the pad travel for a given cable travel.
I'd stay with 27" rims. Sun still makes the CR-18 in 27", which will look fine on a period bike.
Peter Jon White Peter White Cycles 24 Hall Rd. Hillsborough, NH 03244 603 478 0900 Phone 603 478 0902 Phax http://www.PeterWhiteCycles.com
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