we asked each test rider what he liked and what he
disliked. Then we set out to fix the flaws. Remember, our
goal was not to ruin the offroad, trail-riding and GNCC-style racing capabilities of the FX, but to enhance the
overall package with an eye toward motocross. Here is
what we did and why.
Q: WHY DID WE CHANGE THE REAR WHEEL?
A: The YZ250FX comes with an 18-inch rear wheel
shod with a Dunlop AT- 81 tire. We wanted to switch to a
19-inch rear wheel—not because the 18-inch won’t work
on a motocross track, but because we place our trust
in the tires we know best. Most test riders wanted the
19-inch Bridgestone 403/404 tires that come stock on the
There are tricks to swapping to a 19-inch rear wheel.
First, it is possible to transplant the complete rear wheel,
axle, wheel spacers, brake carrier and axle blocks off
of the YZ250F onto the FX; however, if you don’t have
a donor YZ250F, this will be very expensive. Since the
YZ250FX has a 22mm rear axle and the YZ250F has a
25mm axle, you can’t make a tit-for-tat swap (without
buying an axle, axle blocks, wheel spacers and rear
brake disc carrier). The best solution is to buy a 19-inch
rim, longer spokes (YZ250 two-stroke spokes work) and
have the big hoop laced onto the stock FX rear hub.
Q: WHAT DID WE DO WITH THE GEARING?
A: The MXA wrecking crew has turned several KTM
300XC bikes into 300SX motocross bikes, and the
wide-ratio, six-speed gearbox was always a problem,
which is why we always say that it’s best to start with
a 250SX and bolt the 300cc engine kit on it. Motocross
demands a close-ratio, slam-bam-thank-you-ma’am set of
gears. It is hard to get six wide gears to mimic five
narrow gears, and the 2015 Yamaha YZ250FX’s
six-speeder was no different.
First, second and third gears are much lower than on
the YZ250F, while fourth, fifth and sixth are higher. In
offroad riding, this provides a decent balance between
the need to go slow in the tight stuff and fast over open
ground; however, when racing the YZ250FX in motocross,
first gear was useless. Second gear was lower than
optimum and very short. Third was the best all-around
gear, while fourth was a big reach, and the FX tended to
fall off the pipe on the upshift. If you could pull fourth,
then you could pull fifth. The only American motocross
track that could feasibly need the FX’s sixth gear is the
long start at Glen Helen.
With some math, we figured out that if we geared the
FX up by going from the stock 13/51 gear ratio to a taller
13/50 combo, we could get the exact second- and third-gear ratios of a YZ250F. This gearing was great out of
turns and down short straights, but it made the jump from
third to fourth even more iffy. Since the MXA test riders
wanted to grab fourth without having to say a few Hail
Marys in hopes that the FX wouldn’t bog, we went to
13/52 gearing. By lowering the overall gearing, we made
second gear even shorter, moved third into the sweet
spot, and made it easier to grab fourth on fast straights.
The philosophy: MXA didn’t want to ruin the cross-country
capabilities of the YZ250FX; thus, the changes we made were
with an eye toward keeping its offroad credibility.