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 axis).

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 cornering.

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 handles.

QUESTION FOUR: HOW MUCH SHOULD A FLYWHEEL WEIGH?

Before a factory's engineers can choose the proper flywheel, they have to consider seven factors:

1. Overall engine weight: Lighter is always better.
2. 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.
3. Engine acceleration: An engine builds speed quicker with a light flywheel and slower with a heavy flywheel.
4. Inertia: A heavier flywheel carries speed further, but is also harder to slow down than a light one.
5. 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 motorcycle.
6. Angular momentum: Believe it or not, too much flywheel weight will make a bike hard to move around or corner.
7. Intended use: The power characteristics and riding styles used on a 60, 80, 125, 250, 300, 500 and 4stroke all require different flywheel mass.

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 STALLING?

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 hard braking.

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 YZ450F.

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 flywheels.

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 require drilling).

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 tester.

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.

Reprinted from MXA June 2003.

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