Tires: radial alignment/toe-in/XL Tires., radial alignment, alignment settings

Question
Hi Barry,

Thanks in advance for your help.  I have read your other answers, and I appreciate your expertise.  

My 1972 Buick Centurion convertible (455 engine/heavy duty suspension) originally came with bias-ply tires.  I am running radials on it now, although I am having some vibration problems--and I have checked out all mechanicial issues.  The remaining variables are alignment, balance, or tire type & pressure.  I have a few questions:


1) I have read that pre-1973 cars which came with bias-ply tires have different alignment settings, and the alignment needs to be changed when radials are used.  In particular, I read that if radials are used, the toe-in needs to be adjusted to zero (whereas bias-ply needed negative toe-in)--Is this true?  Are there any other alignment changes, in camber etc., that should be made to accommodate the radial?

2)When I went to get my new tires, the installer asked me to choose whether I wanted a "static balance" or a "dynamic balance."  I didn't know the difference, and he attempted to explain that the static balance was better because of the shape of my rims, which have a large back-space.  Is this true?  With regard to the dynamic balance, are there two types--one with tires off the car on a machine which spins them, and the other with tires on the car with a treadmill to account for brake/disk weight?  Which is best?

3)I currently have P-rated XL tires, as these were recommended by the Centurion-owners website due to the weight of the vehicle (4500 lbs).  The tires do seem to stabilize the vehicle laterally, but I wonder does the reinforced sidewall/load capacity of the XL tire itself produce greater vibration than an SL tire?  Please note that even though these are radials, I am still using the original recommended psi for my vehicle according to the owners manual: 30 psi back/26 front. (Some computers mistakenly list the rec. pressure at 28 back/24 front which was for the LeSabre..., and my understanding is that tire pressure with modern radials should not be less than 26..)

Thanks again for your input,

Ben  

Answer
Ben,

Like a great many things, there are a lot of myths about tires and you are encountering some of them.

1)  Radial tires are more sensitive to toe than bias ply tires are.  They are also less sensitive to camber.  I suspect that when radials were introduced, it was the camber that could be exploited to generate good side grip.  I recall a phrase - "Radial Tuned Suspension" - and I have to laugh, because I don't remember anyone actually identifying what it was that was different.  I suspect nothing, as I've looked at alignment specs and they don't seem all that different - and that brings me to what I think is the real issue:  

Toe.  

The factory alignment tolerances - not the target value, but the allowable range around that target value - are too wide by half.  My experience is that irregular wear (which may be the source of your vibration) is rarely encountered if the alignment is within the inner half of the tolerance.

2)  You need to find a different tire guy.  Dynamic balance includes static balance plus an additional feature which deals with side to side vibration.  So by definition, static balance is not better.

And you asked about on car balancing vs off car balancing - On car is better because it is also balancing the other rotating components, such as brake drums and rotors.  But because things have changed over the years, off car balancing is now the norm - and works just fine - except:

Everything has to work the way it does now, and I am not sure when some of these things became standard.  For example:  Hub piloting - using the center hole to guide the wheel into proper position - and then the concentricity of the lugs - also important.

But let's start by diagnosing the vibration.

1)  Do you feel it in the front or the rear (steering wheel or seat back)?  If you can't tell, have someone else drive the car and decide.  If he can't tell it's probably not the tires and rims (or any other wheel end component).

2)  Once you've decided which end it is, swap tires front to rear.  Did the vibration move, too?  If not, it's probably not the tires and wheels.

If you are left with tires and rims as the source, then you need to find someone with a Hunter GSP9700.

http://www.gsp9700.com/

This web site will explain what the machine does and in the upper left hand corner, there is a locator.

But you asked about XL tires.  That has nothing to do with vibration - round is round!

AND

You haven't identified the tire size you are using.  According to Tire Guides Old Timer's Guide, a 1972 Buick Centurion came with H78-15's with an option of J78-15's mounted on 6" rims, both inflated to 24 psi front / 28 psi rear.  This is probably where the computers you mentioned are getting their information.  Let's assume you are correct and use 26 / 30 psi.

You didn't tell me what tire size you are currently using, but I suspect the XL is a clue and the tire size is P235/75R15 XL.

OK, let's do some math:

At 26 and 30 psi, an H78-15 has a load carrying capacity of 1580 # / 1710 #, while a J78-15 is 1650# / 1790 #.

A P235/75R15 has a load carrying capacity of 1753# at 26 psi, and 1852 # at 29 psi, etc.  So the P235/75R15 has more load carrying capacity than both of the tires. It's not enough to worry about, but it is different.

So I've given you lots to do.  Good Luck