User Guide

12
13
tire companies began making tires specifi-
cally for mountain bikes, the new tires were
made to fit the 26” rims. So the evolution of
the mountain bike tire size was like that of
wheel sizes; a selection made by what was
available at the moment.
The limits to wheel size
The modern bicycle configuration places
the rider’s hands almost directly above the
axle of the front wheel. Between these two
points there resides the headset, or steering
bearings, and the frame’s head tube which
holds those bearings. Attached above this is
the stem. Below the steering bearings, the
fork crown requires space, as does tire clear-
ance for suspension fork travel. As wheel
size increases, the room for the components
decreases. If the wheel is too big, the imme-
diate result is a handlebar that cannot be
lowered to the desired position (Fig. A2).
An overly large front wheel can also create
problems with the front-center dimension,
the distance from the bottom bracket to
the front axle. Front-center effects steering
quickness, handling, and stability. The tire
contact patch is directly under the front axle,
regardless of the wheel size. A longer front-
center adds stability, but slows the steering
response of the bike somewhat. A short front
center does the opposite, and if paired with a
large wheel may allow toe-clip overlap, where
the rider’s foot contacts the front wheel dur-
ing a sharp, slow speed turn.
The diameter of the rear wheel dictates the
chainstay length, important because chain-
stay length is a factor in bottom bracket
rigidity. Also, the location of the rear tire
contact patch, relative to the rider’s center of
gravity, effects traction on steep terrain.
As a result of ‘crowding’, larger wheels
only work on larger bikes. On a smaller bike,
the head tube would have to be shortened
so much that the frame would lose torsional
rigidity. Even worse, the rider’s hands would
be lifted such that they could not achieve the
proper position. The good news is that small-
er people already had a low center of gravity
to begin with; its just those bicycling sky-
scrapers who were too far above their front
axles.
Large wheels provide a suspension effect
Imagine a skateboard wheel running over
a series of 1” bumps. Since the skateboard
wheel has a radius of only an inch, each time
the wheel contacts a bump it has to move
vertically 1 inch in just 1 inch of horizontal
motion (Fig. A3). This is an abrupt change
with drastic effects on the rider’s motion.
With a 20” BMX wheel, that same 1 inch of
vertical motion happens over about 4 inches
of forward motion. This turns what was a
radical bump into a mild annoyance. Now
ride over our bump on a 26” wheel, and
you have almost 6” of horizontal distance
to spread out the impact, and the bump is
hardly noticeable. With a larger wheel, not
only does the bump disturb your riding less,
but it also has less effect on your speed.
The suspension effect of a larger wheel
is somewhat similar to that of a suspension
fork, where your front wheel is moving up
and down while your frame (and your body)
take a smoother line, and with less loss of
speed. Its also similar to the effect of running
your tires at lower pressure, letting the tire
casing flex to absorb the bumps. However,
both a suspension fork and tire flex require
you to tune the springs to get the effect.
Wheel size is always at work. On the Fisher
29ers you’ll notice the big wheel effect in two
ways; The bike will ride smoother and more
comfortably. And the bike will be slightly fast-
er, especially on broken or rough ground.
Fig. A2
29 inch
26 inch
Fig. A3