Tires: slight pull to the right on some road surfaces with unidirectional tires, alignment parameters, infiniti q45

Question

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Followup To

Question -
I have a '92 Infiniti Q45 which I dearly love.  It has 103K miles.  Front end repairs have included  rack and pinion replacement and strut replacement.  I keep it up to specs and have it aligned regularly at a shop that has its equipment tested monthly.  I also have my tires rotated and balanced about every 5000 miles.  

Since purchasing Falken unidirectional tires, now about 8,000 miles old, I have noted that the car tracks differently than it used to. On roads that are flat (not crowned) if the surface is very smooth, it will track straight.  If the surface is a slightly rough surface, it will pull slightly to the right so that my steering wheel is always turned slightly to the left, which is a little PITA on long trips but tolerable.  On a crowned road, the car always pulls slightly to the right.
The tires were rotated and re-balanced about 2000 miles ago and there is a slight vibration/shimmy above 65 MPH, as well as the continued pull slightly to the right.  I should also note that my tire is stock (215 x 65) and the model is Ziex ZE512.

Are unidirectional tires much more sensitive to road surfaces than a more conventional tread design?  Any thoughts about the problems I have described?

Thanks,
Ken

Answer -
Ken,

There are a few vehicle alignment parameters that can cause a vehicle to have a drift or a pull.  Every vehicle has a certain amount of sensitivity to these and it varies from vehicle to vehicle.  These parameters have tolerances, but some vehicles are more sensitive than the tolerances would have you believe, as most alignment tolerances are derived from what the factory can do, rather than what works for the vehicle.

But tires also have a role here.  Tires have a property called conicity (root word cone).  This property is a measure of the sideways tendancy of a tire.  Conicity can be both positive and negative (pushing left or pushing right).  It is the combination of the 2 front tires that influences how much push the tires generate.  Put another way, both tires pushing in (or out) the same amount causes no problem.  It's when the tires push the same direction that causes the problem.  Needless to say, the vehicle's sensitivity determines how much push is tolerable that determine whether the vehicle actually drifts or pulls.

Directional tires are no more prone to this than non-directional tires.  

But to determine if the pull is from the tires or the alignment, simply swap the front tires - left to right.  If the pull changes direction, it's the tires.  If the pull disappears, then it's a combination of tires and alignment.

So I would suggest doing this for a short period of time as the tire's direction of rotation would be wrong.  This won't cause any problems except that you would lose the advantage of the directionality - wet traction and snow traction.

Hope this helps.  


I will try that, Barry.

The other question is what are the causes of vibration/subtle shimmy at about 70MPH.  It is my understanding that the shimmy that develops about 55 MPH is from unbalalanced tires but what are the causes of a slight shimmy at higher speeds?

Thanks
Ken k

Answer
Ken,

The range for wheel end caused vibrations is 50 to 70 mph, but it can be outside this range as well.

If you've got a vibration on the same car as the pull, and the vibration has been gradually developing, that almost guarantees that alignment is the issue as the tire is developing irregular wear, which feels exactly like an unbalanced tire.