QuestionHow much differential pressure side to side will cause steering pull. This assumes that the tires are new.
Say the lhs is 25 and the rhs is 30 psi, would this be felt and if so which direction would the car steer ? Russell Pickersgill
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AnswerRussell,
The sensitivity of a vehicle to pull varies from vehicle to vehicle. Some are very insensitive, and some are very sensitive. So there isn't one answer to this.
Put another way, there is a threshold below which a given vehicle will not experience a pull, and above which it will - and this threshold varies quite a bit between vehicle models.
Plus there are several mechanisms going on at the same time that can be felt as a pull.
1) There is a tire property called conicity - root word "cone" - that is a sideways force that changes direction when the tire's rotation direction changes. It isn't that a single tire has "high" or "low" conicity - it is the difference between the front tires that make the different. If you have 2 tire pushing equally inward or equally outward, they cancel each other out. It's only when the difference is large that it becomes a problem - and this also gets into the "threshold" of the particular vehicle.
2) Steering alignment has a major affect. Because my expertise is in tires and not in alignment, I'm going to leave to others to explain what affect alignment has:
http://www.familycar.com/Alignment.htm
This is but one of several webpages devoted to explaining alignment. This one seems to have been extracted from a training manual from Hunter Engineering. Nevertheless, what you should get out of this is that pull is complex.
One of the questions that always comes up when people talk about a pull is: Is it the tire or is it the alignment. There's a simple test to help sort this out.
a) Swap the front tires side to side.
b) If the pull doesn't change at all, the pull is 100% in the vehicle.
c) If the pull completely changes direction, the tires are 100% the source.
d) But if the pull changes, but not completely, or even disappears, then the pull is caused by both the tires and the alignment.
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Now having set the stage, it's time to talk about drag. Drag can come from several sources - brakes, bearings, - and the topic you want to discuss - tire drag.
Another way to state "tire drag" is "rolling resistance" - and even different tires of the same size can have HUGE!!! differences. This is NOT a brand to brand issue - it's more a result of a technology triangle for the tread compound between treadwear, traction, and rolling resistance. Tires with great treadwear properties do not give great RR values - and the same thing applies to tires with high levels of grip.
But there are other factors that affect tire drag, such as inflation pressure. Pressure has a fairly large affect on rolling resistance and therefore drag - and the amount of this affect is proportional to the RR, which, of course varies from tire to tire - SO - I can't tell you where that threshold is between having a pull and not having a pull because it varies from vehicle to vehicle and tire to tire.
But 5 psi OUGHT to be perceptible. That's a 15% difference for normal passenger cars.
Direction? Rolling resistance is larger with lower inflation pressure, so the direction of the pull will be towards the lower pressure.
BTW, conicity is largely unaffected by inflation pressure - at least within the normal range of values - so it can be discounted when diagnosing a pull.
I hope that explains things.
