Rib Punching

June 15, 2010 at 4:28 am Leave a comment

It only hurts when I laugh! Except that it is not physical punishment that I’m writing about.

So it hurts alright- in the hip pocket, because rib punching robs a tyre of useful mileage, because one or more tread ribs has disappeared well before the others, making it un-roadworthy.

It might be a bit esoteric for the average reader, but this blog article is directed mainly at tyre men, who see many a tyre that they have to wonder about its unusual wear pattern.

Generally, it occurs in the intermediate rib of a light truck or truck tyre- that is, one in from the edge of the tread crown.

It’s caused by an unusual amount of movement of that rib, and that rib only, because the contact pressure is not evenly distributed across the crown of the tyre, even allowing for the fact that the contact pressure distributed across the (oval) footprint, is generally higher at the shoulder ribs anyway.

In the contact patch, movement of the tread elements occurs as the tyre rolls, steers, and brakes the vehicle. The familiar doughnut shape is distorted only in the patch, and the centre are of the patch distorts the most, because it has to accommodate the flat surface under it. “Tyre squirm” it is called, and the way to minimise it is to build a radial tyre, with steel belts under the tread.

So the tyre develops friction, slip angle for steering, traction fordriving,and in so doing, wears out.

However, when the tread design doesn’t evenly allocate the contact pressures, particularly if either contact pressure is high, or the tread pattern design is complex, then one rib or more may distort, move around more than its neighbours, and wear out faster in the process. Basically, that’s why as tyres for trucks get bigger, pressures higher, and speeds constantly high, the pattern design gets simpler.

For example, nearly all steer tyres on heavy trucks operate at 120 p.s.i., with a load of around 2800 kg to 3000 kg per tyre. So most have a rib design tread pattern, kept simple. The contact pressure is so high that water drainage from under the tyre is not normally a problem- it gets squeezed out from under it fast through the grooves or from the shoulders. Back further on the truck, there are more tyres, loads are around two thirds that, so pattern designs can be a bit more complex.

Taking this to the extreme, is the heavy aircraft tyre. Back in history, the tyre our company developed for the Fokker F28 ( the pocket rocket we used to call it) developed rib punching in the intermediate ribs as described earlier. Aircraft tyres are always subjected to heavy loads. (The aircraft designers never give the tyre designers enough room in the wings to stow the tyres that the tyre engineers would like to fit.)

So an ingenious test was devised. A new perspex plate was laid on the ground, and carborundum dust sprinkled on it. Carborundum is very sharp. The plane wheel was then rolled over the plate.
As the tyre contact patch distorted, the carborundum dust was pressed into the rubber, and stayed there, scratching its way across the perspex. Behold! We could see that the tread elements in the intermediate ribs were actually describing a circle as they moved, from the circular scratches. This excessive movement caused those intermediate ribs to wear out faster than the others. There was no other course but to destroy the tyre mould, and start again.

In the real world, this phenomenon occurs mostly when a series of tyre designs using the same basic pattern is either scaled up, or scaled down to accommodate a range of different tyre sizes with basically, a similar looking style; and one rib gets a bit too skinny to cope with the contact pressure. Or the customer uses the tyre in a manner that the tyre designer never contemplated!

Entry filed under: Blogroll, Funny, Tyre Industry, Tyre Technology. Tags: , , , , .

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