|Do you remember those cheap plastic yo-yo's they
used to hand out at children's birthday parties? They were almost
impossible to use. Only a skilled contortionist could wind them up
and down. Only the most adroit could even imagine Walking the Dog,
Round the World or Rocking the Cradle with one of those
If you wanted to do the difficult yoyo tricks, you
needed a Duncan Butterfly. It had some mass to it. It was easy to
wind and unwind. It flew straight and returned like it was on a
string (which it was). Why was the Duncan better than the dime-store
version? More moment of inertia. How does this apply to a motocross
bike? Read on.
QUESTION ONE: WHY'S THE BUTTERFLY EASIER TO USE?
The reason a heavier yo-yo handles better than a cheap one is
moment of inertia. With more moment of inertia, the Duncan yo-yo
accelerates slower, flings out with more control and, once in
motion, stays in motion. Additionally, a heavy yo-yo's weight is
positioned at a greater radius from its axis, further benefiting the
moment of inertia and increasing its ability to maintain angular
momentum (an object's resistance to rotating and rocking on its
QUESTION TWO: HOW ARE YO-YOS LIKE MOTORCYCLES?
There are many similarities between a yo-yo and a motorcycle. The
weight and size of the engine's flywheel determine how easy a
motorcycle is to ride. And, flywheel weight plays a role in
acceleration, throttle control, suspension action, braking and
QUESTION THREE: WHAT'S A FLYWHEEL?
A flywheel is a heavy plate of steel hidden behind the ignition
cover. Its purpose is to smooth out the irregular motion of the
moving parts inside the engine. It acts as both a counterbalancer
and a power constraint on the engine. In doing these tasks, it
creates a gyroscopic reverberation that changes the way the bike
QUESTION FOUR: HOW MUCH SHOULD A FLYWHEEL WEIGH?
Before a factory's engineers can choose the proper flywheel, they
have to consider seven factors:
- Overall engine weight: Lighter is always better.
- AC current: The size of the flywheel must be large enough to
house the magnets that are used to produce engine spark. But, at
the same time, small enough to make room for the stator windings
without creating a bulky engine case.
- Engine acceleration: An engine builds speed quicker with a
light flywheel and slower with a heavy flywheel.
- Inertia: A heavier flywheel carries speed further, but is also
harder to slow down than a light one.
- Flywheel effect: Every rotating component in the engine and
chassis creates its own spinning force (clutch basket, gears,
chain, wheels, and how the crank counterbalances piston weight and
stroke). The manufacturer needs to pick a flywheel weight that
best accommodates all the independent gyroscopic forces inside the
- Angular momentum: Believe it or not, too much flywheel weight
will make a bike hard to move around or corner.
- Intended use: The power characteristics and riding styles used
on a 60, 80, 125, 250, 300, 500 and 4stroke all require different
QUESTION FIVE: DOES MY BIKE HAVE THE BEST FLYWHEEL?
Most likely, but not always. Manufacturers do make mistakes. How?
Some try to make their fourstrokes snappier, quicker revving and
more two-stroke like by putting on the lightest flywheel possible.
This can often lead to horrendous stalling problems and explosive
acceleration (especially when the riding surface is loose or slick).
QUESTION SIX: DOES A HEAVY FLYWHEEL SLOW AN ENGINE DOWN?
Yes. Weight harnesses the rate at which the engine accelerates.
QUESTION SEVEN: DOES A HEAVY FLYWHEEL COST HORSEPOWER?
No. Heavy flywheels do not cost horsepower. They only change the
way the power is transferred to the ground.
QUESTION EIGHT: CAN A HEAVY FLYWHEEL MAKE A BIKE FASTER?
Yes. But only if the engine characteristics are too snappy,
powerful or explosive for the rider's style or track's terrain. A
bike with an engine that's too explosive will accelerate quicker
with a heavier flywheel weight.
Why? Because the weight smoothes and broadens the transfer of
power to the ground. The power is delivered more consistently and in
more of a conservative manner. The bike is more hooked up off the
line and less volatile on the course. QUESTION NINE: CAN A HEAVIER
FLYWHEEL CHANGE A BIKE'S HANDLING?
You bet. Toning down the abruptness of the power delivery makes
the chassis less susceptible to sudden bursts of acceleration,
excess wheelspin and engine stalling. Adding flywheel weight will
make a bike feel less twitchy. If the tires stay hooked up, with
less bounce or spin, the bike is easier to control.
QUESTION TEN: CAN A FLYWHEEL WEIGHT KEEP MY ENGINE FROM
Yes. Since an engine takes longer to come up to speed with a
heavier flywheel weight, it also takes longer to slow it down. The
stored energy of the added weight keeps the engine turning under
QUESTION 11: DO HEAVY FLYWHEELS MAKE A BIKE HARDER TO STOP?
No. Since the engine winds down slower, the transition from full
throttle to full stop is much smoother. Additionally, the rider can
use the brakes on a bike with more force, since the fear of stalling
the engine is reduced.
QUESTION 12: DOES IT HELP STARTS?
Yes and no. Heavier flywheel weights can make a big difference on
a concrete starting pad (or hard slippery dirt). But, on short,
fast, dirt starting lines (like in Supercross) a lighter flywheel
helps the bike jump out of the gate quicker, which allows the rider
to get his bars in front of the rider next to him.
The difference between these two concepts can be seen at
professional races. Four-strokes holeshot AMA Nationals (with their
longer starts and concrete pads), while two-strokes holeshot
Supercrosses (with their shorter starts and dirt pads).
QUESTION 13: CAN A FLYWHEEL WEIGHT IMPROVE MY SUSPENSION?
Yes. Every time a wheel leaves the ground, engine rpm increases.
The extra rev spins the rear wheel quicker, and when it hits the
ground it is harder to control. The wildly gyrating rear tire
bounces the bike around and works the suspension. With a flywheel
weight, the rear tire can stay hooked up accelerating over choppy
terrain. Over whoops, the tire stays closer to the ground and the
bike reacts more smoothly to every hit.
QUESTION 14: WHAT ABOUT THE MUD?
Just like on dry dusty dirt, the slicker the mud, the easier the
bike is to ride with more flywheel weight. Turning the throttle on
results in a more manageable power delivery and more knob bite. The
measured engine deceleration keeps the rider on the brakes much
longer and harder.
QUESTION 15: HOW CAN I TELL IF I NEED MORE WEIGHT?
Most bikes work fine with the stock flywheel weight, but there
are bikes that demand more flywheel weight. The classic examples are
the Suzuki RM250, Kawasaki KX125, Honda CR125, KTM 250SX and Yamaha
Not surprisingly, many factory riders run flywheel weights on
their race bikes. Why? Because they pump out as many as ten more
horsepower. The flywheel weight helps tame those ponies.
QUESTION 16: WHO SELLS WEIGHTS?
Steahly, Zip-Ty, Terry and DR.D offer bolt-on flywheel weights or
flywheels that have had more weight welded to them. The accessory
divisions at Yamaha, KTM and Kawasaki also carry heavier replacement
QUESTION 17: WELD ON OR BOLT ON?
It doesn't really matter. They both have pluses and minuses. A
rotor that is modified with a weld-on weight can fit in tighter
ignition cavities and is as reliable as a standard rotor. Weld-on
weights have to be sent away to have the work done (or suffer core
charges). Bolt-on weights are more affordable and seem easier to
install (but it isn't always true, because some bolt-on weights
QUESTION 18: DO BOLT-ON WEIGHTS FALL OFF?
When you bolt something on a wildly spinning crank that heats up
in excess of 300 degrees, there is always a risk of something going
wrong. Some bolt-on weights are only held on with two screws (others
mount under the center nut and are kept from spinning with set
screws). They can fall-off, but so can any other part of your bike
that is held on by a bolt.
QUESTION 19: DO THEY FIT THE SAME ON EACH BIKE?
Bikes with less room under the ignition cover or with an internal
or external rotor all take a different style of weight and mounting
procedure. Weights for internal rotors generally bolt or weld to the
exterior of the flywheel. On external rotors, the weights sometimes
fit beneath the flywheel. On some machines, it's necessary to space
the ignition cover outward to clear the weight.
QUESTION 20: HOW MUCH WEIGHT IS BEST FOR ME?
Most motocross riders end up running a 4 ounce or 8 ounce weight.
Enduro riders typically run 8 ounce to 12 ounce weights. Most MXA
test riders prefer the lightest flywheel weight that still offers
the benefits of the weight.
QUESTION 21: DOES THE PLACEMENT OF THE WEIGHT MATTER?
You bet! Four ounces on the inside of the flywheel is not the
same as four ounces on the outside of the flywheel. The farther the
mass is located away from the axis, the greater moment of inertia.
On most modern bikes, the exact position of the weight is controlled
more by the limited available space than by a desire to tune the
rotational mass. Only one aftermarket firm designs their weight by
measuring the moment of inertia.
QUESTION 22: HOW DO THEY MEASURE INERTIA?
World Four-Stroke and Veteran Champion Doug Dubach is a fulltime
test rider for Yamaha who just happens to own DR.D racing, which
sells flywheel weights. When Dubach set out to design flywheel
weights for DR.D Racing, he borrowed Yamaha's moment of inertia
How does it work? A flywheel, with or without weight, is placed
on a spinning platform. When the platform is wound up and released,
an indicator counts how many times and how long the platform unwound
and wound. Using a formula, the tester calibrates the exact moment
of inertia of the flywheel.
A flywheel with more inertia will take longer to wind and unwind,
but it will do it more times. Although the weight was picked by
using the kilogram-centimeter squared (kg-cm2) measure of the moment
of inertia, Dubach still advertises his weights by gram weights. A
weight is still the number that is most recognized by riders.
The stock YZ45OF flywheel weighs 469 grams and has a moment of
inertia of 3.6 kgcm2. Dubach offers his weld-on weight in 100, 150
or 203 grams (3.5, 5.3 & 7.2 ounces). Once welded to the stock
flywheel giving a total flywheel weight of 569, 619 or 672 grams
respectively-the moment of inertia is increased to 4.46 kgcm2, 5.27
kgcm2 and 5.98 kg-cm2.