Radial Force Variation
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Introduction
Tire vibrations can cause annoyance when driving a car. Particularly on smooth roads at constant speeds. The main cause of such vibrations is due to tire balance problems. Tires are made from a number of separate components, in order to achieve the requirements of comfort, performance etc. The part of the tire which goes into contact with the road is called the tread. This has a tread pattern to disperse water when it rains.
Actions Taken to Measure and Rectify Radial Force Variation
In the manufacturing process, this tread is applied as a slab of uncured rubber compound. If the tread joint is too big then this can cause a heavy spot on the tire, which leads to balance problems. There are other joints which also contribute to the problem, for example the ply joints.
This balance problem is easily overcome by applying small lead weights to the rim of the tire wheel in order to correct the imbalalnce.
However, with the introduction of radial tires, car manufacturers found that there were tire vibrations which could not be rectified by balancing. This was because the radial tire has a very flexible carcass and a very rigid steel belt. This construction leads to variations in stiffness around the tire, which showed itself as "out of round" when the tire was inflated.
To check for this problem, tire manufactures introduced a quality check at the end of the production process. The tire was inflated on a pair of metal flanges to represent the wheel and then rotated, measuring the variations in diameter, by means of a clock gauge. This measurement was called Radial Runout and the machine was referred to as a Uniformity Machine.
So, "Tire Uniformity" rather than "Balance" became the term used within the industry when discussing tire vibrations.
The Radial Runout could be rectified by grinding the tire with a fine buffing wheel, but this still did not resolve all the problems.
The vehicle manufactures worked with engineers at ASM in the USA and Hoffmann in Europe to develop a machine for measuring this type of vibration. Such machines were designed and built and were called Uniformity machines. They were introduced into the tire factories as a quality check at the end of the production process.
The Uniformity Machine constisted of a large diameter metal wheel to represent the road surface and a pair of flanges to inflate the tire and present it to the wheel under load. As the tire/wheel assembly rotated, sensitive electronic equipment measured the variations in forces transmitted to the wheel by the tire.
Radial Force Variation, abbreviated to RFV, measures the uniformity of a tire. It is measured on a Force Variation Machine and the results expressed in g/cm in Europe or inch/ounces in USA.
The selection limits where set by the Vehicle Manufactures. Tires outside limits were diverted into less sensitive markets,or eventually scrapped.
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