As The Wheel Turns

Story by Pat Morrison//
May 1 2012

A smooth-running motorcycle isn’t only a matter of a vibration-free engine. It’s also about wheel balance. An unbalanced wheel can create unpleasant vibrations when riding, and if the imbalance is big enough, it can also induce a wobble. And improperly balanced wheels also have a tendency to accelerate tire wear.

This is why you must balance both wheels when you replace tires. It’s widely believed that balancing only the front wheel will suffice, and it’s true that an imbalanced front wheel is more noticeable than a rear wheel that is equally out of balance. But with both wheels properly balanced, ride quality is significantly improved; it literally must be felt to be believed.

A good way to differentiate wheel imbalance from engine vibration is to ride at about 100 km/h, pull in the clutch and hit the kill switch. (For safety’s sake, do this when there’s no surrounding traffic.) If you still feel shaking, it’s the wheels.

A wheel is imbalanced when there is more weight in one area along its circumference than elsewhere. The tire, the wheel and the wheel’s trueness can all contribute to the imbalance.

It’s rare that a tire is produced in perfect balance, and to compensate for this, tire manufacturers include a mark on the tire’s lightest point, which is meant to line up with the valve stem, generally the wheel’s heaviest point, thus reducing the amount of weight needed to balance it.

Wheels, too, are rarely produced perfectly balanced, and it’s very likely that there is more material in one area of a cast or forged wheel, even though it may be impossible to see.

Custom bike builders who chrome-plate cast wheels will often balance these wheels before the tires are mounted. This is because chrome plating includes copper, which is heavy and can greatly imbalance the wheel. The wheel is then balanced again with the tire mounted to compensate for normal imbalance.

There are two methods of balancing a wheel: static and dynamic. One is more precise than the other, though either way works well on a motorcycle.

Static balancing is often seen at road-race events, where race tuners mount wheels vertically to a special jig that allows the heaviest part of the wheel to spin to the bottom. The jig is a simple device that uses a precisely machined horizontal shaft that rests on low-friction bearings mounted atop two vertical arms. These bearings allow the wheel to spin more freely than it would on its own bearings, which use friction-inducing dust seals. The freer the wheel spins, the more accurate the balancing job will be. Through trial and error, the operator simply adds an amount of weight opposite the heaviest part of the wheel. The job is done when the wheel stops randomly in any position when spun. You can source one of these tire-balancing jigs for about $120 through motorcycle shops.

Another type of static balancer uses a bubble of air in liquid (much like a carpenter’s level) to indicate where weight is needed. The wheel sits horizontally on this device, through the centre of the hub, and the heavy side will tilt as indicated by the bubble. This is also a good way to balance wheels, with the added benefit that it is not affected by friction, though it is considerably more bulky and expensive than the bearing-type balancer described above.

Whereas static balancing averages out wheel imbalance (there may be more than one spot of imbalance, and it may not be centred along the centreline of the wheel), dynamic balancing pinpoints the location where additional weight is needed. This is done using electronic equipment.

The wider the wheel, the more likely it will need dynamic balancing. On a narrow 21-inch front wheel, there’s little chance that extra weight will be located far off the centreline; the rim just isn’t wide enough. On the other hand, a six-inch-wide rear supersport wheel can have extra weight toward the outside of the rim, which would place it farther off centre. This would have a greater effect on imbalance than the same amount of weight near the centre of the rim. This is why wheels balanced dynamically will often have some weight added to one side of the rim, and more weight added on the other side, but in another area of the wheel. The same wheel balanced statically would have weight added at only one location, and it would be placed somewhere between the two dynamic weight locations.

It is not absolutely necessary to balance your wheels dynamically, but if your bike has wide wheels and you spend a lot of time at high speeds (like on a racetrack during track days), it’s worth it to have your wheels dynamically balanced. You’ll also see benefits if you tour for long distances, as reduced vibration means increased comfort.
And don’t take it for granted that because you bought a brand-new bike, its wheels will be perfectly balanced. Even new machines are prone to improperly balanced wheels. So if you feel a shimmy in the handlebar or a shaking through the seat, have your local shop put those wheels on a balancing machine.

Technical articles are written purely as reference only and your motorcycle may require different procedures. You should be mechanically inclined to carry out your own maintenance and we recommend you contact your mechanic prior to performing any type of work on your bike.

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