vehicle pulling / drift

[rkeerthivasan_shar]

Hello tyre experts,

Thanks for your attention.

I would like to know an edge of information about car / vehicle pulling
due to contribution of tyres, particularly radials.

The tyre conicity & PRAT are the main culprit for this booming issue in
almost of the auto car makers.

The information about measurement method of conicity value, its marking
on tyre, positive & negative conicity will be much useful.

Experts, Kindly reply.

[CapriRacer]

Conicity (root word cone) is a tire property where a sideways force is generated by the tire and the force does not change direction when the rotation is reversed. Conicity will vary primarily based on the centering of the belt, but it is possible to wear conicity into a tire. The dimension is force (pounds) and it has a sign (plus and minus) depending on which way the force is pointing. So when a pair of positive tires are mounted, the conicity force points inwards for both sides. (I may have this backwards – I can never remember the sign convention!)

Ply steer is a tire property where a sideways force is generated by the tire and the force DOES change direction when the rotation is reversed. However many folks don’t use the term ply steer and use the overturning moment version of this force – P rat – instead. I like the term ply steer instead of P rat because it is easier to visualize, but I seem to be outvoted.

I forget what the P stands for (could be ply, but I don’t think so), but the rest is Residual Aligning Torque.

Ply steer (P rat) is pretty much a function of the overall tire design and varies very little from tire to tire in a production run.

With ply steer, all 4 tires are pushing the vehicle sideways in the same direction, so it’s somewhat important that this value be relatively low. This is not usually a problem. However, vehicle manufacturers seem to be making vehicles more and more sensitive and they have forced P rat specs on all their tire suppliers. (Can you tell I’m not a fan of the way they are doing this?)

BTW, in the US, tires have ply steer so the vehicle is pushed off the road – to the right. Sometimes vehicle manufacturers build in a little camber thrust to compensate for both the road crown and the ply steer.

Every vehicle has a level of sensitivity to these forces, and if the vector difference exceeds the threshold, the vehicle will pull.

For example, if a vehicle is sensitive to 10 pounds, then a Plus 7 and a Minus 4 (Vector difference 11) would exceed the threshold by 1 pound and the vehicle would have a very slight pull.

A Plus 20 and a Plus 5 (vector difference of 15) would exceed the threshold by 5 pounds and the pull would be stronger.

A Plus 20 and a Plus 15 (vector difference 5) would not exceed the threshold and there would not be a pull.

Don’t take this values as what is typical. If anything they may be a little small.

Ply steer is measured on a Force and Moment machine. There are a wide variety of types, but the latest is called the Flat Track 3 by MTS. I don’t know what these cost, but they are pretty pricey (over $3/4 million?)

Conicity is measured on a force variation machine. There are shop level machines (Hunter GSP9700) that are kind of crude, but still effective in diagnosing pull problems. A GSP9700 is a bit over $10,000.

But for quick and accurate results that a tire factory would need (or a research facility for that matter), then you’re going to exceed $1/2 million.

Vehicle manufacturers seem to fall into 3 different schools of thought about Conicity.

The Diamler School: Build conicity into the tire and make sure the value is always the same sign.

The GM/Ford School: Sort the tires into Plus and Minus and make sure a given vehicle gets one or the other.

The BMW School: Mark the side that points left (or right, I forget which it is!). This means the tire must look the same regardless of which side is out – so white sidewall tires are not used.

Since there is no industry standard, the conicity marks vary quite a bit, but most are water soluable and disappear before the tire reaches the consumer.

My personal opinion is that vehicle manufacturers have been trying to shift the burden about vehicle pull from their shoulders to tire manufacturers. The vehicles have been getting more and more sensitive, but there aren’t corresponding improvements in the alignments leaving the vehicle assembly plants. The published tolerances for alignment are too large by a factor of 2. As a result, many vehicles pull, and it’s always easy to blame the tire.

Luckily, there is an easy test and that’s to swap the front tires, left to right. If the pull changes direction, it’s the tires. If not, it’s the vehicle. (I’ve never heard of rear tires causing a pull.)

But most folks who buy a car don’t know what the problem was, just that the radial pull was fixed.