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Blue-S · 08-27-2009, 10:53 AM

Road force variation (radial force variation) is NOT about tire/wheel imbalance. Radial force variation occurs when there is a variation in the effective "spring rate" of the tire, as mounted on the wheel. Put more simply: you can balance a square wheel/tire, but it still won't roll smoothly.

Imagine that the tire construction has a "stiff spot" at one point in the rotation of the tire. This stiff spot may not necessarily correspond to a visible or measurable "high spot" in the tire itself, and would not necessarily correspond to a point of imbalance of the tire/wheel. What will happen when this "stiff spot" contacts the ground? The tire/wheel will be forced up, causing a thump that you can (potentially) feel as the tire rolls. The tire may look round, but it doesn't act round as it rolls with vehicle weight applied.

The Hunter GSP-series equipment (during force variation checks) will turn the tire/wheel slowly with a load applied to the tread via the roller, as it checks the center-to-center distance between the wheel centerline and the roller centerline. The result of the test is force variation in 1st, 2nd and 3rd order. The next step is to follow the machine prompts to reposition the tire on the wheel in a manner that reduces the force variation. Put simply, the machine helps you find the low spot on the wheel (via a wheel radial runout check) and directs you to place the "stiff spot" of the tire in the low spot of the wheel. The net result is reduced radial force variation and a smoother ride.

The rule of thumb is that for every 0.001" radial runout at the wheel's bead seat, you will have approximately 1 pound of force variation. Different vehicles have a different level of sensitivity to radial force variation, but high performance tires (and high performance vehicles) are generally more sensitive to radial force variation than a more typical passenger car. That is, you might feel a 15 pound radial force variation on a Boxster, but not on a minivan. The trick is to get the radial force variation down to a level that cannot be felt or noticed on that particular vehicle.

08-27-2009, 10:53 AM	#2
Blue-S Registered User Join Date: Jan 2009 Location: Corona, CA. Posts: 129	Rfv Road force variation (radial force variation) is NOT about tire/wheel imbalance. Radial force variation occurs when there is a variation in the effective "spring rate" of the tire, as mounted on the wheel. Put more simply: you can balance a square wheel/tire, but it still won't roll smoothly. Imagine that the tire construction has a "stiff spot" at one point in the rotation of the tire. This stiff spot may not necessarily correspond to a visible or measurable "high spot" in the tire itself, and would not necessarily correspond to a point of imbalance of the tire/wheel. What will happen when this "stiff spot" contacts the ground? The tire/wheel will be forced up, causing a thump that you can (potentially) feel as the tire rolls. The tire may look round, but it doesn't act round as it rolls with vehicle weight applied. The Hunter GSP-series equipment (during force variation checks) will turn the tire/wheel slowly with a load applied to the tread via the roller, as it checks the center-to-center distance between the wheel centerline and the roller centerline. The result of the test is force variation in 1st, 2nd and 3rd order. The next step is to follow the machine prompts to reposition the tire on the wheel in a manner that reduces the force variation. Put simply, the machine helps you find the low spot on the wheel (via a wheel radial runout check) and directs you to place the "stiff spot" of the tire in the low spot of the wheel. The net result is reduced radial force variation and a smoother ride. The rule of thumb is that for every 0.001" radial runout at the wheel's bead seat, you will have approximately 1 pound of force variation. Different vehicles have a different level of sensitivity to radial force variation, but high performance tires (and high performance vehicles) are generally more sensitive to radial force variation than a more typical passenger car. That is, you might feel a 15 pound radial force variation on a Boxster, but not on a minivan. The trick is to get the radial force variation down to a level that cannot be felt or noticed on that particular vehicle. __________________ Blue-S 2000 Boxster S 6-speed - Ocean Blue / Savanna Beige * 9x7 short shifter * Pedro's enthusiast mount * Carrera Ltw. wheels * Stebro bypass pipes * M030 coming soon! * Last edited by Blue-S; 08-27-2009 at 02:52 PM.