The Engineering (and Benefits) of Tyre Uniformity
A tyre is a lay-up of fabric layers, rubber strips, and steel wire. These individual components are assembled on a collapsible drum, inflated to the toroidal shape we are familiar with, and vulcanised in a mould. As each layer is applied, dependent on the component and its purpose, some overlap may be required to maintain the integrity of the tyre casing under pressure. You would call this a ‘join’.
Sometimes these ‘joins’ can be seen as a slight bulge in the wall of the tyre late in the day, when the sunlight is at an acute angle to the wall. They are essential to the make-up of the tyre, and are not a fault in the construction. However, they can have an effect on the ride quality of the tyre if they are too large, or are too close together.
Think of a tyre as a series of springs, like you see on the Rovers currently on Mars. As the tyre revolves, each spring is compressed in turn. However, if one spring is ‘stronger’ than the rest, when that spring hits the ground, it will cause the axle to lift, which, if it is at a speed which harmonises with the spring rate of the suspension, can cause a vibration. The tyre may be perfectly round, the rim may be perfectly round, the assembly may be perfectly balanced, and yet there is still a vibration due to ‘non-uniformity’.
This causes car designers quite a bit of angst, particularly if the vibration occurs at a speed that the car is commonly driven.
The tyre manufacturers have sophisticated machines which can measure this ‘non-uniformity’ and, by grinding away a miniscule amount of rubber from the tread at the ‘stiff spot’, can correct for it. Strangely enough, they can take a round tyre and grind it slightly out of round, so that it behaves as though it was round. The degree of uniformity required is specified by the car designers and their N.V.H. (Noise, Vibration and Harshness) engineers.
Rather than do this after the tyre is built, the tyre and production quality control engineers spend much effort in sequencing the tyre building process so that the cause of the non-uniformity is reduced in the first place – by accurate control of the component dimensions and staggered placement of the joins of controlled overlap to minimise their effect. This is what you pay for when you buy a good quality tyre. ‘No Vibrations!’