Can road force balancing really make a difference?
To answer the question of whether road force balancing is overkill, we should probably start by reviewing some basics. We balance tire/wheel assemblies primarily to eliminate, as much as possible, vibration. Vibration produces ride disturbances that are uncomfortable for the driver and passengers and may cause uneven tire wear.
Some vehicle vibrations are the result of normal operation; even the best engines, transmissions and drivelines naturally produce some vibration. Others are due to some type of abnormal condition such as engine or transmission problems, driveshaft and axle failures, suspension wear or failure, improper wheel alignment, brake wear or failure, wheel bearing wear, tire/wheel assembly imbalance, out-of-round tire or wheel, or the force variation of a tire.
That’s quite a list, but everything beyond the last three items is beyond the scope of our discussion, so let’s assume that we have eliminated all of the non-tire and wheel related issues.
All tires have some amount of static and dynamic imbalance. Whether it causes any problems is a function of the amount of imbalance, the wheel and tire combinations, and how the tire/wheel assemblies interact with the vehicle.
Static imbalance makes a tire/wheel assembly vibrate up and down. Dynamic imbalance causes the assembly to shimmy from side to side.
Since the introduction of the modern spin balancer, most tire/wheel assembly balancing equipment has been able to accurately diagnose and correct both dynamic and static imbalance. However, balancers can only fix balance problems. Even the most sophisticated balancers cannot eliminate vibrations from a bent wheel or an out-of-round or irregularly worn tire.
All tire manufacturers test tires at their manufacturing facilities for uniformity, including road force variation, conicity (the tendency to roll like a cone), ply-steer (the tendency to crab sidewise) and balance. Acceptable levels of uniformity vary among manufacturers, tire lines and the planned distribution channel for the tire.
Not surprisingly, the most uniform tires typically go to the automakers for OE fitment. How much variation is acceptable for independent tire dealers versus the discount clubs is probably something that is not discussed in public.
Road Force Balancers
So what does a road force balancer actually do? Road force balancers, in addition to performing a traditional spin balance, measure both the wheel and tire by pressing a large roller against the tread of the spinning tire. The roller applies 1,200 to 1,400 pounds of pressure to simulate the weight of the vehicle on the tire as it rolls down the road.
A computer in the machine, along with various sensors, determines variations of tire stiffness, radial runout and anything in the tire’s construction (such as inconsistencies in the belt package) that would prevent the tire from rolling smoothly when it is weighted by the car. By measuring both the wheel and tire, the road force balancer tells the technician where to move the tire around on the wheel until the effective high spot of the tire (when it is rolling on the car) matches the low spot of the wheel – a more sophisticated method of match mounting.
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