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hi! wheel balancing is an issue that intrigues me as well, so I decided to do a little bit of research on this issue on this and this is what i came up with.
Wheel balancing is a must anytime tires are mounted on wheels. The wheel may also require rebalancing if the tire has been dismounted for repair.
Wheel balancing provides a smoother ride by minimizing tire bounce. This helps improve traction, steering control and extends the life of the tires. But no matter how carefully wheels and tires are balanced, they will eventually lose their balance. As the tread wears, the distribution of weight around the circumference of a tire changes altering the balance of the tire and wheel assembly. Eventually the tire may have to be rebalanced because only 1/4 ounce of imbalance can produce a noticeable vibration.
WHEEL BALANCE ISSUES
An out-of-balance tire and wheel will typically create a vibration or shake that become progressively worse as the vehicle's speed increases. The speed at which the vibration first becomes apparent will vary depending on the size and weight of the tires and wheels, the size and weight of the vehicle, the sensitivity of the steering and suspension, and the amount of imbalance. The vibration or shake usually starts in at 40 to 50 mph and increases in intensity as the speed increases.
WHY CHECK WHEEL BALANCE
The cure, of course, is to check the balance of all four wheels and tires, and rebalance as needed. But it's also important to remember that speed sensitive vibrations can also be caused by radial (vertical) or lateral (sideways) runout in a tire, wheel or hub. Loose, worn or damaged wheel bearings as well as certain kinds of tread wear can also cause vibrations. So too can an out-of-balance or out-of-phase rear wheel drive driveshaft (FWD shafts usually don't rotate fast enough to cause vibration problems).
THE WHEEL BALANCER
To accurately balance tires and wheels, an up-to-date and accurately calibrated spin balancer is needed that can achieve both static (at rest) and dynamic (in motion) balance. Old fashioned bubble wheel balancers could do a decent job of achieving static balance, but dynamic wheel balance can only be achieved with a spin balancer. This is especially important with today's larger, wider, heavier tire and rim packages, and absolutely essential for run-flat tires that have thicker, stiffer sidewalls.
Most wheel balancers today have self-calibrating electronics with accuracy to hundredth's of an ounce (or tenths of a gram). Graphical displays also make information easier to read and understand, and reduce the chance of making a mistake. Automatic data entry for wheel width and diameter on some balancers also saves time.
Most wheel balancers today operate a lower speeds. This helps extend motor life and reduces cycle times as well as risk to the operator. Older balancers typically had to spin a wheel fairly fast (about 500 rpm, or the equivalent of 55 to 60 mph) to generate a usable signal. But the more sensitive electronics in today's wheel balancers are able to pick up vibrations at much lower speeds (only 100 rpm, or 10 to 15 mph).
WHEEL BALANCING TECHNIQUES
One of the limitations of balancing tire and wheel assemblies off a vehicle is that repeatability can be an issue. In other words, you may not get the same results when you attempt to rebalance a wheel that has already been balanced. What has changed? It is not the tire or rim. What has changed is the geometry of the tire and wheel on the balancer.
The way that a wheel is mounted on a balancer will not only affect the accuracy of the balance job itself but also the repeatability of the balancing results. Worn mounting cones or shaft bearings are sometimes the problem. Using the wrong type of cone can also give inaccurate results. So too can dirt on the wheel or nicks in the wheel center hole. But another often overlooked cause is using the wrong mounting technique for the type of wheel.
The basic idea is to mount the wheel on the balancer the same way it is mounted on the vehicle. A pilot hole centric wheel (one where the center hole positions the wheel on the hub and prevents it from wobbling sideways when the lug nuts are removed) can be mounted on a balancer with a cone from the backside. But a lug centric wheel (one that does have some sideways movement when the lug nuts are removed) requires a different balancer mounting procedure. A lug centric wheel should be mounted with a cone from the backside and an adapter flange plate against the front side. The fingers on the flange plate must be properly positioned so they line up with the lug holes in the wheel. This is necessary to center the wheel on the WHEEL balancer shaft. If this is not done, the results will not be 100 percent accurate or repeatable.
Precision flange plate adapters are expensive and may only be offered as an extra cost option with a new wheel balancer. A set of flange plate adapters that covers most vehicle applications may run from $1300 up to almost $2000 depending on what you buy. But the improvement in balancing accuracy and repeatability can be well worth the investment. They can also prevent unnecessary comebacks and dissatisfied customers.
WHEEL & TIRE RUNOUT VIBRATIONS
Sometime wheels will still shake and vibrate even after they have been balanced. The problem is often excessive wheel runout or tire runout. Most tires should have less than .030 to .050 inch of runout. An out-of-round tire can produce harmonic vibrations that come and go at various speeds depending on how many "humps" are in the tire.
As a rule, most steel rims should have less than .050 inch of runout, or .040 inch of runout if the rims are aluminum alloy. Some trucks and SUVs can tolerate up to .060 inch of radial and lateral runout, but others can't handle any more than .030 inch of runout before vibrations become noticeable.
Runout problems can often be corrected by "match mounting" the tire on the wheel (rotating the tire so the tire high spot is over the rim low spot).
RADIAL FORCE VARIATION
Sometimes the problem is neither balance or runout. It is radial force variation (RFV). This is the amount of change in stiffness of the sidewall and footprint when a load is placed against a tire. Subtle differences in the position of the cords and belts in a tire's construction can create stiff spots that make the tire roll unevenly. The stiff spots act like runout to cause vibrations at various speeds.
Vibrations caused by RFV tend to appear at certain speeds, then disappear as the speed changes or increases (unlike vibrations caused by imbalance that usually get worse as the speed increases). In one test, a perfectly round wheel that was properly balanced experienced a vibration that appeared at around 50 mph but vanished at 70 mph. The vibration at 50 mph was caused by RFV in the tire, and produced as much side force as if the tire were out-of-round by .030 inches or out of balance by one and a half ounces.
Until recently, there was no easy way to detect let along measure RFV. All a tire dealer could do was switch tires until the vibration was reduced -- or the customer gave up and went away. Only the tire and vehicle manufacturers could afford the type of equipment that could detect and measure RFV. But a number of years ago, Hunter Engineering introduced a new balancer (the GSP 9700) that checks RFV as well as balance and runout. It detects RFV by placing a load against the tire as it rotates to simulate loaded driving conditions. If RFV is present, it shows you where it is and how to correct it. RFV can be countered by adding offsetting weights and/or rotating the tire on the rim. It also makes it easier to determine if a tire is responsible for a vibration problem or not. If there's no runout, no RFV and the tire is accurately balanced, the vibration is in the driveline or powertrain, not the wheels and tires.
I think that kind of answers all the questions that have been asked till now,including why wheels go out of balance despite sedate driving styles! It could just be that one landed up with rims that were badly balanced in the manufacturing process!! And then we come across tyre wear issues which depends on both the tyre and the surface it is driven in! | 
10th May 2009, 14:42
