Converting Street to Off-road (700C -> 26")

[[email protected]]

When I couldn't find replacement forks for my 1" steerer MTB frame, I
decided to move all the (quality) MTB bits onto my former commute bike
(a 700c MBK dual purpose - essentialy a MTB frame) with a 1 1/8"
steerer head, cuz I could get reasonably cheap and useful sus forks for
it (I got Suntour XCP 75, good value for money...).

I solved all the problems like chain-line, etc., but I'm stuck with the
rear brakes: the braze-ons for the v-brakes are too high for the
smaller 26" wheels - they are almost exactly inline with the rim
itself. I don't want to fit discs or starting welding the frame, so...

Does anyone know if there are any conversion plates or anything that
will fix this? Something like a brake booster that would bolt onto the
existing braze-ons but offer canti sockets further down might do the
trick.

Otherwise, I could use a caliper brake, if there is one that'd fit
around a 2" tire (BMX?).

Any ideas greatly appreciated...

 

[JeffWills]

[email protected] wrote:
>
> Does anyone know if there are any conversion plates or anything that
> will fix this? Something like a brake booster that would bolt onto the
> existing braze-ons but offer canti sockets further down might do the
> trick.
>
> Otherwise, I could use a caliper brake, if there is one that'd fit
> around a 2" tire (BMX?).
>
> Any ideas greatly appreciated...

Something like this?
http://harriscyclery.net/site/page.cfm?PageID=49&SKU=BR9121

Depending on the size of tire, you'll probably lower your bottom
bracket by 1/2" to 2". This may be a problem if you're in the habit of
pedaling through corners.

Jeff

 

[dvt]

[email protected] wrote:
> I solved all the problems like chain-line, etc., but I'm stuck with the
> rear brakes: the braze-ons for the v-brakes are too high for the
> smaller 26" wheels - they are almost exactly inline with the rim
> itself. I don't want to fit discs or starting welding the frame, so...

The only similar solution I've heard about is a homemade job, mentioned
in this very newsgroup by Chalo:

> ...the frame... was
> intended to be a K2 comfort bike with 26" wheels....
> An evening on the CNC mill yielded a brake mount that used the canti
> boss pads to hold U-brake studs instead, positioned to work with a 20"
> wheel.

He made an adapter from 26" -> 20" brake mounts, so you might be able to
make the same thing for 700 -> 26". Or maybe the old brake bosses would
interfere with the new brakes -- I haven't measured that.

If you don't have easy access to CNC machines, you might be able to move
the bosses and repaint the frame at less expense than this.

--
Dave
dvt at psu dot edu

 

[Chalo]

[email protected] wrote:
> When I couldn't find replacement forks for my 1" steerer MTB frame, I
> decided to move all the (quality) MTB bits onto my former commute bike
> (a 700c MBK dual purpose - essentialy a MTB frame)
<snip>
> Does anyone know if there are any conversion plates or anything that
> will fix this? Something like a brake booster that would bolt onto the
> existing braze-ons but offer canti sockets further down might do the
> trick.

Nope. That won't work; you'd have to chop off the old canti bosses or
else contrive a mount that sticks way out from the frame. Even then,
you would have to figure out a way to tie it down in a third location
or risk bending/breaking the frame bosses from the additional leverage.


> Otherwise, I could use a caliper brake, if there is one that'd fit
> around a 2" tire (BMX?).

That would work, if the canti bosses were not in the way. In my
experience, 700c frame bosses are located pretty much directly
alongside a 26" rim's edge. That puts them in the approximate location
you want the brake pads or arms to be.

If it doesn't hurt your feelings to do so, you can cut and file away
the old bosses, then use a BMX caliper, canti or U-brake frame adapter
plate, or something a little more elegant like this:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=7182463950

I use one of these on one of my bikes, and it's surprisingly
non-terrible for a long reach caliper.

Another way to cope would be to build up a new rear wheel with a drum
brake of some kind, like a Shimano Nexave roller brake freehub or an
old Sachs VT5020 threaded hub. That would allow you to leave your
brake bosses intact for later variations.

there's also nothing wrong in principle with using a 700c wheel on the
back and a 26" wheel on the front.

Chalo Colina

 

[[email protected]]

Thanks for the suggestions, guys. I have ordered up a "Big Cheese
Linear pull brake mount. Vertical distance from mounting bolt to brake
stud adjusts from 108 - 117mm (BR9121)" like Jeff suggested.
Perfect, as it picks up the fender bolt hole above as well as the old
canti braze-ons.
Unfortunatley, Sheldon's shop was out of stock, but I found it at:
http://aebike.com/site/page.cfm?PageID=30&SKU=BR9121