Designing a frame for either 650b OR 700c
 

I am generally a sports-tourer type of rider and recently finished building my first lugged steel frame. I am thinking about my second frame and I think I would like to build a frame that could take either 650b or 700c wheels; a good number of people who share my riding style rave about 650b, so I figured that it would be good to give it a try.

Anyway, what types of things should I consider when building a frame that will work with up to 700x32's and 650x38's? Or should I just design a 700c frame with short-ish brake reach (49mm, most likely) and an average amount of BB drop (75mm...on the low side, I know), and know that 650's will work with longer reach brakes? Would any geometry changes from 73 degrees parallel and 50mm or so of fork rake be advised?

Thanks,

Pete

700Cx32 has a 622+64=686 mm dia, and 650Bx38 has a 584+76=660 mm dia. So you'll have 13mm BB drop diff between the two. For an average of 75 one will be 68.5 and the other 81.5.

You'll also need 19mm more brake reach to go from 700c to 650b. If you set it at 49mm you'll need a super long reach for the 650b, and I don't think you'll find good brakes at that reach? Furthermore, as a fail safe it's better to shoot for the middle of the brake reach (say 44mm).

Bottom line, I think the conversion is better when you go from 700cx23 to 650b. Plus you'll have a hard time putting fenders on 32s with short reach brakes. (not to mention the fender clearance would change a lot from one wheel to the other).

So I think it's better to choose a given wheel size when designing a frame. I'll try 650b one of these days :P

If you don't mind the look and want the function, how about designing it for long reach calipers 700C and put canti or center pull on it studs for 650B? 650B will automatically have less trail with the smaller wheel size which could work well work the wider 650B tires. Or of course could go with disk brakes and maybe for long reach on 700C.

From fat 650s to skinny 700s, the difference in tire diameter and BB height would be negligible. I`m pretty sure that`s the point of 650s- feel free to shoot me down if I`m way off base here.

I really dig Wayne`s idea of cantis + sidepulls for two different wheel sizes. I dunno what kind of complications might arise, but it certainly sounds feasible to me (and cool). I was thinking along the same lines as a way to change between fat 406s and skinny 451s on the same frame. It just sounds like fun.