B arry's T ire Tech

This is a series of articles on the technical aspects of tires, their care and usage.

My primary purpose in these articles is to help people understand tires and thereby reduce the risks we all face every day.

..........and since tires is just about the only thing I know about..........

Please drop me a note if you have a topic you want to see:

Barry@BarrysTireTech.com
Vehicle Drift and Pulls and Tire Conicity:

What you should get from this page:

  • A drift or a pull will only hurt the car if the problem is alignment.
    • Misalignment could cause a tire to wear quickly and/or unevenly.
  • If your vehicle has a drift or a pull, swap the front tires, left - right.
    • If the problem remains, then the problem is not the tires. It's in the vehicle - typically alignment or a draggy brake.
    • If the problem disappears, then there is a problem with both the vehicle and the tires. I suggest dealing with it now rather than later when you rotate tires. These things don't fix themselves.
      • To fix a drift/pull that has disappeared:
        • Step 1) Get an alignment. My opinion is that the published alignment specs are too wide - by half. See my page on misalignment on how to assure you're going to get a good alignment here.
        • Step 2) The result of the alignment should be that the drift/pull should remain - albeit at a lower level. If you can live with that, Good! You're done.
        • Optional Step 3) If you still have a drift/pull that you can't live with, follow the procedure for a 100% tire problem below.
    • If the drift/pull completely changes direction, then the problem is 100% the tires.
      • Swap one of the fronts with one of the rears.
        • If the problem remains, then the problem tire is still on the front - the one you didn't move.
        • If the problem goes away, then the problem tire is now on the rear - the one you did move. EXCEPT it's possible there was a problem with the pairing.
          • Swap the rear tires for the tires on the front.
            • If the drift/pull doesn't come back, then you've properly paired all 4 tires.
            • If the drift/pull comes back, then you know which tire needs to be replaced.
            • (See below for a fuller explanation)

Tire Conicity:

The root word is "cone". It's a tire property where the tire is exerting a sideways force when rolling. The force can be pointed inwards towards the vehicle (negative) or outwards, away from the vehicle (positive) - and of course, zero!

This can only be measured when the tire is loaded and is usually expressed as a force (pounds or newtons). Tire manufacturers use a TUG (Tire Uniformity Grader) machine to measure conicity. This happens while the machine is also measuring uniformity (Uniformity is discussed here!).

It can also be measured at a tire dealership by using a Hunter RoadForce Machine or something similar.

The problem is NOT excessively high values!

What causes a tire related drift or pull is the vector sum of BOTH front tires: So if both front tires are high in the same direction, there is no pull. Same with 2 low values (provided they are the same direction!)

But if the front tires are High/Low, or even 2 lows in opposite directions, you could get a drift/pull.

So it is possible to fix a drift/pull by simply swapping tires around until you get a combination such that both tires on a given axle are close in value.

While this isn't usually a problem, some vehicles are somewhat sensitive to conicity. Just be aware that your vehicle might be one of those.

HOWEVER!

For practical purposes, conicity can not be corrected. So either the tires have to be properly paired, or the offending tire has to be replaced.

Please note: A drift or a pull does not cause damage to the car unless it's the alignment that is the problem. A misalignment can cause a tire to wear quickly and/or unevenly, resulting in a high conicity value. That is usually the case where the drift/pull disappears when you swap front tires.

Tire manufacturers are aware of the conicity problem. It can be somewhat difficult to fix, so many tire manufacturers are careful to sort tires into positive and negative stacks before they go into the warehouse, and only ship tires of the same direction to a given dealership.

So what causes conicitiy?

Belt Centering

This is the one most commonly pointed out. Given how tires are made, it is somewhat understandable that improvements in consistency in manufacturing - especially new machinery - would improve conicity.

Variations in tread rubber distribution in manufacturing:

Remember the image at the top of the page? (and to the Right - the top one!)

Well, if the tread rubber applied on top of the belt isn't even side to side, that will cause the tire to be slightly conical. That's because the inflated tire tries to even out all the stresses and that means making the belt the same side to side (assuming the belt is properly centered.)

It only takes a small difference.

To the lower right is a photo of the business end of a rubber extruder. This type of machine is commonly used for extruding the tread rubber - and I hope it is obvious that getting the die opening even side to side may be a bit difficult.

There are other ways of making the tread - and each way has its advantages and disadvantages. And that's where designing new equipment comes in.

That also means that it is possible to wear conicity into a tire. In fact, it used to be somewhat common to grind the tread of a new tire in such a way to get smaller conicity values. That same principle would apply to wear. (They don't grind tires much nowadays!)



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