The YZ450F swingarm (right) will fit on
the YZ250 two-stroke (left).
SWAPPING THE YZ250
SWINGARM FOR A YZ450F
I have read in the past that MXA
wishes Yamaha would upgrade the
rear suspension of the YZ250 two-
stroke with the rear-end parts from
the YZ450F. I am seeking MXA’s
expertise to determine what parts
from what year and size YZ-F would
be needed for the transformation.
I have access to a brand-new 2014
YZ450F chassis (sans engine) that
could be the donor. Will it just bolt
onto my 2012 Yamaha YZ250?
Since you have a donor YZ450F,
this is a simple swap. The best
way to do it, and we speak from
experience, is to place the YZ250
and YZ450F next to each other and
unbolt the swingarm, rear-brake
master cylinder and shock from both
bikes. Then, break the chain and
roll the YZ450F rear end (swingarm,
wheel, axles and all) over to the
YZ250 and bolt it on the bike. That’s
all there is to it.
However, if you tried to do this
with parts instead of a donor bike,
you would need a YZ450F hub,
25mm rear axle (in place of the
YZ250’s 22mm axle), wheel spacers
and brake caliper carrier.
Thanks to the donor bike, all of
these parts are handy. The stock
YZ250 shock will bolt on (and even
though the shock linkage has totally
different mounting, dimensions and
parts, the rising rate is identical to
the YZ250 two-stroke’s rising rate).
As far as braking goes, since you
took the rear brake system from the
YZ450F to the YZ250, it will be a
Now, we assume that you would
like to know if it is worth all of the
trouble in terms of weight, handling and performance. Here are the
(1) Axle. The rear axle will be
upsized from 22mm to 25mm. This
improves feedback from the rear of
the bike by making the swingarm
and rear wheel more unified.
( 2) Flex. The YZ450F swingarm
has a different “rigidity balance,”
which is a fancy term for swingarm
flex. The YZ450F swingarm has
more vertical and twist flex than
the YZ250 swingarm. How much
more flex? It is 4 percent less rigid
vertically and 5 percent less rigid
in twist. Conversely, it is 8 percent
more rigid horizontally.
( 3) Weight. The YZ450F swing-
arm is 350 grams lighter than the
YZ250 two-stroke swingarm. That is
12. 3 ounces (about 3/4 of a pound
lighter). But, there is more weight
savings to be gained (or is that lost?).
The YZ450F rear hub is 265 grams
lighter than the YZ250 hub. When
you add that 265 gram savings to the
swingarm’s 350-gram diet, the result
is a 1-pound, 5-ounce reduction in
( 4) Performance. You should be
able to feel the combination of the
stiffer rear axle, decreased unsprung
weight and flexier swingarm on
the track. How so? The lower the
unsprung weight, the quicker the
suspension reacts to bumps. The
improvement is most significant over
repetitious, high-frequency bumps
(like braking and acceleration bumps).
Why? The more weight you load
on the wheels, the more the shock
absorber has to struggle to bring the
mass to a stop. As the damper struggles with the weight, the suspension
packs, the wheels kick and the bike
becomes unstable. Shaving unsprung
weight lessens the suspension’s workload. It’s said that a 1-pound reduction in unsprung weight is equal to a
6-pound reduction in sprung weight.
Thus, Yamaha’s 1.3-pound weight
savings is the equivalent of saving 7
pounds on the whole bike. Plus, the
increased swingarm flex helps the
bike absorb the load while cornering,
with more feel as the bike bends
slightly more into turns.