On Jul 13, 2006, at 11:20 AM, majorhahn wrote:

>> I have heard of people running 26 in. wheels, like big apples, on 29
> in. mountain bikes which have been xtracycled with disc brakes. Has
> anybody actually done this and is it really a superior configuration
> to say running the same wheels on a 26 in. bike? Are there any
> downsides? Especially in the handling and feel, like do the pedals
> hit ground when cornering, does it affect acceleration? Thanks.
> Robert

When you put an Xtracycle on a bike, one consequence is that more of
your weight will be borne on the front wheel than before. If the
steering felt light before, it will feel "more stable" (heavier)
after the conversion, sort of cruiser-ish. If it felt heavy to start
with (such as MTB steering tends to feel to road-bike riders anyway),
then the feeling can be really heavy or "floppy", with a tendency to
pull into turns like a chopper. Your mileage may vary. Many people
like the feel of their converted rides as much or more than the
original. I tend not to, because I like really light steering.

In addition to changing the fore-aft weight distribution, there can
be geometry changes, notably head and seat tube angle changes,
depending on how your bike differs from the bike Xtracycle picked as
normative when they designed the thing. With 26" (mountain) bikes,
the changes are likely to be modest.

However, when you put an Xtracycle on a frame designed for 700c
('29"') wheels, the geometry changes are more substantial. The seat
and head angles are slackened, and the bottom bracket lowered. The
lower BB means it's easier to get a foot down, which is nice on a
laden bike -- you don't have to lean it when you stop and remain
seated. The slacker seat tube angle shifts more of your weight back
onto your butt and off your hands, which I also like for comfort, and
it also makes it easier to get a foot down (your hips are rotated
further back on the BB axis). But the slacker head angle exacerbates
the chopper steering feel by increasing "trail" (read about trail
here: http://tinyurl.com/oe6mq ; play with it here: http://
www.kogswell.com/geo.php ).

So, I think it's a good idea to contrive to reduce the trail on
Xtracycles generally, and on 700C/29" Xtracycles in particular. One
way to do it is to use smaller wheels than the frame was designed
for. This is easy with disc brakes. Another consequence of smaller
wheels is that the BB is further lowered, so there's a risk of pedal
strike if you overdo it. You can raise the bike back up part of the
way by running extra fat tires. And this is a good thing with a non-
mechanically suspended cargo bike anyway: air is no-fuss lightweight
suspension, and the only kind at all for the load on an Xtracycle.

Another approach is to start with a 26" bike designed for a
suspension fork, and use a rigid, short fork (e.g., Surly Instigator
with Surly 1x1 fork). This lowers the front end, effectively
steepening the head angle to lessen trail. Swap in a layback seatpost
and tall stem/bars to keep from pitching forward.