Installing "U-brake" bosses:

!Jones said:

I'm building a fixture to braze on U-brake bosses to an existing
frame. A mandrel through the drop out is zero and the wheel radius is
R. In terms of R, how far do I offset for modern brakes?

Jones

I haven't done this and don't have any frames or brakes in front of me
to measure, but I will give it a shot since you haven't gotten any
replies. First of all, understand that there aren't a lot of folks in
BMX, yes even brake designers, that are completely on the ball with
numbers and details, so you really should just measure the brakes
you're planning on using. From looking here: http://www.tektro.com/02products/12fx30.php
and also having my buddy measure an Odyssey Evo2 over the phone, it
seems like most brakes max out the reach at around 44-48mm. The 907A
on that page is probably the single most commonly specced 990-style
brake and their measurement is 30-47mm. The Evo2 measured around
35-48mm. A quick search of the framebuilder's list (
http://search.bikelist.org/getmsg.asp?Filename=framebuilders.10602.0049.eml
) mentions using an axle-to-boss dimension of 9 1/8" (231.775mm) with
a 406 wheel all the way forward in the dropouts. That number would
make sense alongside the brake reach numbers I mentioned.

It might also be worth noting that any well-thought-out standard axle-
to-boss dimension used by folks who make BMX frames is going to take
into account both the large range of rim wall heights that a bike
might run, the amount of axle adjustability offered by the frame, and
whether the brake will be SS or CS mounted. So if this was, say, a
bike with vertical drops and no possibility of chunky rims, then just
using someone else's numbers but adapted to whatever wheel size you're
using may be less than optimal although it will probably work.

Nate Knutson said:

!Jones said:

I'm building a fixture to braze on U-brake bosses to an existing
frame. A mandrel through the drop out is zero and the wheel radius is
R. In terms of R, how far do I offset for modern brakes?

Quoted message said:

Jones

I haven't done this and don't have any frames or brakes in front of me
to measure, but I will give it a shot since you haven't gotten any
replies.

[...]

Very useful advice! Thanks!

Jones