What is Road Force? Diagnosing Tire Vibration with the Hunter Road Force Balancer

Many changes in the design of new vehicles highlight how important properly balanced and low Road Force tires are to ride quality. Advanced braking and stability systems, along with stricter fuel economy requirements, have led to the increased use of lower rolling resistance tires with a decreased contact patch. Suspension systems and tires also are getting stiffer. In addition, many modern tire/wheel assemblies can weigh as much as 80 lbs. as the customer demand grows for larger wheels in sizes of 20-inches and 22-inches. Using the Hunter Road Force Balancer (Fig. 8) will ensure that the tire/wheel assembly meets GM specifications prior to mounting the assembly back on the vehicle.


Fig. 8


In 2015, GM made the CH-51450-A Oscilloscope Diagnostic Kit (Pico Scope) an essential tool for all GM dealerships to help in diagnosing vibration conditions. The tool enables technicians to verify, measure and identify the type of vibration that a vehicle is producing. Once the area of the vibration is identified, the next step is to correct the issue. Any part that rotates can produce vibrations (such as tires, prop shafts, differentials, etc.), which makes diagnosis difficult. In some cases, the repair might require part replacement. But if the vibration is coming from the tire/wheel assemblies, the Hunter Road Force Balancer can help make the correct repair.


What Produces Road Force?


Previously, tire/wheel assemblies were measured for “static balance,” which allowed for weights to be placed on the inside and outside of the wheel flange/rim surface. While this is the first step in correcting a vibration condition, vehicles also need the Road Force checked. Road Force is a measurement of both sidewall stiffness and how much the assembly is “egg shaped.”


To understand the effects of radial force variation, imagine the tire as a collection of springs between the rim and the tire tread. If the “springs” are not of uniform stiffness, a varied force is exerted on the axle as the tire rotates and flexes. This force creates a vibration in the vehicle. (Fig. 9)


Fig. 9


The Hunter Road Force Balancer has the ability to measure this variation. By measuring, correcting and validating the tire/wheel assembly is within GM specifications for both static and Road Force, technicians can ensure that the tire/wheel assembly is corrected prior to mounting it back on the vehicle.


Excessive Road Force can be produced by incorrect tire mounting or improper bead seating to the rim. Both of these conditions will provide a low and/or high spot in that area of the tire. It can occur on either the inside or outside flange.


New Wheel Design


Many of today’s GM vehicles have a new flangeless wheel design (Fig. 10, #1) that has removed the machined flange (Fig. 10, #2) that previously allowed for the traditional “clip-on” weights. The flange was removed for esthetic reasons but has also driven the need for changes in balancers. Modern balancers need to account for this change by allowing wheels weights to be placed on the inside of the wheel.


Fig. 10


The previous method of “vectoring” or “Match Mounting” a tire cannot be used since the starting point is to measure the wheel runout using measurement arms. To address this concern, Hunter has developed the “180-Match Mount” process. (Fig. 11)


This process allows a more precise measurement and correction to the tire/wheel assembly by using the Road Force measurements to determine proper alignment of the tire to the wheel that produces the lowest Road Force.


Fig. 11


For more information on tire road force as well as additional tire and wheel diagnosis, refer to Bulletin #00-03-10-006M.


Detailed diagnostic steps for Tire and Wheel Vibration Analysis also can be found in the Service Information.


– Thanks to Chuck Berecz, Kent Woiak and Peter Joslyn



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