On Sat, 04 Aug 2007 12:33:57 -0500, Tim McNamara
wrote:

In article ,
"Tom \"Johnny Sunset\" Sherman" ""sunsetss0003\"@invalida .com"
wrote:

See http://www2.trekbikes.com/Bikes/2007/mountain/69er.html.

Is the bigger front wheel to better roll over obstacles, or is it
just a demented fashion statement? Conversely, does the smaller rear
wheel provide any real advantage?

Back in the day, Charlie Cunningham built MTBs with a smaller rear
wheel- IIRC 20" rear and 26" front. Many dirt motorbikes have a smaller
rear wheel. I have no idea why. Maybe Carl Fogel does, he used to ride
trials and the like.

Dear Tim,

Sorry, but we fooled you.

Don't feel bad, since 9 out of 10 trials riders will mistakenly insist
that their front tires are bigger.

After all, trials machines use 21-inch front rims and 18-inch rear
rims, so the front tire must be 3 inches taller, right?

Nope.

Look at this 2007 GasGas TXT 300cc, which has the standard 21 x 2.75
front tire and 18 x 4.00 rear ti

http://i16.tinypic.com/4pl4yhk.jpg

You can't tell the front tire from the rear with a yardstick on a
trials machine, particularly if the rider's weight is squashing the
lightly inflated tires.

For practical purposes, the front and rear tires are the same height.
In fact, the height of the tread blocks on a particular model of tire
has more effect on the tire height than whether it's a "21" front or
"18" rear tire.

On the front, the narrow 21-inch rim mounts a skinny 2.75 tire. The
combination is tall enough to roll over obstacles easily, but still
light enough to steer, bounce, and pop up in tricky places. The rim is
strong enough because the leading tire takes less impact, carries less
weight, and has more suspension.

On the rear, the wide 18-inch rim mounts a 4.00 tire, which is so
thick in cross-section that it ends up just as tall and willing to
roll over obstacles as the dainty front tire.

The 4-inch width gives traction for the engine, the huge increase in
cross-section gives more pneumatic suspension at the same 4~6 psi
(think 700x38 versus 700x21), and the massive rim and tire withstand
~400 pounds of rider and machine slamming into waist-high rock ledges.

Since the rear tire steers a gentler curve than the front tire, the
enormous increase in weight doesn't hurt handling.

That's why the original 19-inch front and rear rims were replaced.
They were fine for pavement and okay for fairly smooth off-road
riding, but too heavy and clumsy for the front and not heavy and
strong enough for the rear when riders began bouncing over fallen logs
and big rocks.

Cheers,

Carl Fogel