By John Copeland
One of the easiest ways to make your kart more comfortable, more “friendly” to drive, and overall faster is to accurately balancethe wheels. It’s something that big-car racers have known for years. And while their wheels are larger and heavier than ours, kart wheels spin faster, because of the small diameter, and so balance is just as important. Whether you run dirt or pavement, 2 cycles or 4-stroke, properly balanced wheels can make your machine faster and a lot more pleasant to drive.
There are a variety of wheel balancers on the market these days, but nearly all of them use a shaft to simulate the spindle. That shaft is mounted between a pair of very low-drag precision bearings. The whole thing is then suspended in some sort of support framework. The idea is pretty simple; the tire and rim, mounted on a front wheel hub, slip onto the shaft and the heavy side of the combination rotates to the bottom. As you add weight to the high side, the light side of the rim, the entire assembly becomes more balanced. The goal is to get the wheel/tire assembly perfectly balanced so that it will sit motionless regardless of what rotational position it is in.
That part of wheel balancing is easy. And you know that doing things right, I mean REALLY right, is never easy. Ideally you should not locate all the weight on a single point on the rim. That’s because there is a difference between “static” balance and “dynamic” balance. Static balance is the condition in which the wheel/tire combination will sit stationary, regardless of rotational orientation, when mounted on a free-turning shaft. This is easy enough to achieve with a simple shaft balancer and is not disturbed by all the added balancing weight being in a single location on the rim. Dynamic balance, however, is the state of equilibrium in which centrifugal forces on a rotating wheel and tire do not produce any oscillating forces on the axis of that rotating mass. Single-point weighting of the wheel/tire combination can, when the wheel rotates, create unequally distributed centrifugal forces that can make you wonder if you forgot to balance the wheels at all! Since all we really care about is how the wheels behave when they are spinning on the track, dynamic balance is the ultimate goal here.
Years ago car tires were static balanced on a bubble balancer. The mechanic just set the wheel and tire horizontally on thebalancer and hammered weights onto the rim at the point that put the bubble in the center. Today’s sophisticated tire dealers mount each wheel on a machine that spins it up to 50 or 60 MPH and indicates where on BOTH THE INSIDE AND THE OUTSIDE of the rim the weight needs to be added. Unfortunately, the relatively low mass of kart wheels and tires has made the accuracy of these machines somewhat unreliable for our use. But we can take a lesson from the basic concept. If we want our wheels and tires to be dynamically balanced, we need to divide the weight that we add between the inner and outer rims. A little more work, but easy enough.
Still looking for more? Want to approach perfect dynamic balance? Try taking the weight on the outside rim half and splitting it. Move half of it clockwise around the rim about 20 degrees. Move the other half counterclockwise about 20 degrees. Now do the same with the weight on the inside rim half. Because you have moved the weights away from the “lightest” part of the wheel, you will need to increase the amount of weight at each location a little bit but, once you get it right, you will be as close to dynamic balance as you can get without a spin balancer.
Is it worth it? Can you feel the result of all that time and trouble? Only you can be the judge of that. But if you haven’t made every effort to achieve dynamic balance of your wheels and tires, well, your kart just isn’t as good as it could be. And if you are the kind of racer who can’t stop looking for that little bit extra in performance, if you’re always trying to find ways to make your kart just a little bit better than is was at the last race, why not give dynamic balance a try?
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