Originally Posted by
brooklyn_bike
before you get too deep in wheel replacement : make sure to verify the front wheel is not 650A (vs 650B) most of the triathlon frames with a smaller wheel i've seen are 650A
There's been a number of 650C front wheeled tri/TT bikes made too.
Here's a few axle to center of brake track dimensions of the rim sizes we have mentioned (thanks to Southerlands). 700C- 311mm, 650A- 295, 650B- 292, 650C- 286 and MtB 559- 279mm.
Assuming (yes, I know...) the front brake would have the same reach as the rear you could do the math and discover which rim gives you the reach of the rear caliper. Remember that on the fork there's a couple of mms difference between the center of the steerer/crown at the brake hole WRT the axle as compared to the same axle to but instead taken to the front face of the crown at the hole. Andy