Re: [DML] Re: Exceptional steering rack rebuild???

["Bruce Benson" <delornut@xxxxxxxxxxxx>]

David, I can't argue with what you say here. It's probably the rule that
most tire shops like Goodyear and Firestone subscribe to when dealing with
the likes of everyday front engine cars. Typically with a front engine car
vibrations caused by run out will disappear after about 40MPH. Vibrations at
speeds higher than that are more than likely imbalance providing suspension
problems have been ruled out. The DeLorean, however, with it's light front
end is much more susceptible to even minor imbalance or runout. In my case
the vibrations from run out continued past 60MPH. The shop I deal with is a
high performance tire, wheel and alignment facility with vast experience in
tuning out things like vibrations. Your comments had me wondering if I might
have missed something so I re-read a portion of the Jim Horner
High-Performance Wheel and Tire Hand Book. He maintains that the ultimate
balance is a full dynamic balance on machine followed by an on the car fine
tuning because machine balance doesn't take into account the other rotating
parts. You're right, once this is done one needs to keep track of the
relationship between the wheel and the hub when removing the wheel. Run out
exceeding .060" is considered too much. Since I'm keeping track of the wheel
to rotor position because of the on car balancing then shaving .080 from the
high side of the tire doesn't seem to be extreme when the result is no
vibrations. I've spent hours trying to move a tire around the rim to
eliminate run out with little success. The tire should be mounted with the
stiffest part of the tire, it's high spot, next to the smallest area of the
rim's diameter, its low spot. That's where the valve stem hole is drilled.
In some cases moving the tire 180 degrees will solve problems but I've found
the DeLorean ( mine at least ) is very unforgiving of any imbalance or run
out.

Bruce Benson




> Runnout can be lateral or vertical as you point out. If it is the
> wheel (rim) there is little that can be done. If it is not too bad a
> wheel shop can sometimes bend or machine it out. If it is the tire
> sometimes remounting it can help. I do not like "shaving" to correct
> it. If the tire was properly made (and shaved) at the factory it
> should never be necessary in the "field". I also do not like or
> recomend dynamic balancing "On The Car". If you should ever have to
> take the wheel off you will lose the balance unless you index it back
> on in the exact same position. Tires can be balanced very accuratly
> with a dynamic balancer off the car. If you have a problem with the
> rotating assembly of the car (the rotor or hub) it should be corrected
> not "balanced out". BTW to balance ON CAR assumes that the bearings
> are perfect and are not causing runnout. That is not a good way to do
> it because every bearing will have some runnout and play. Bottom line
> is to have all 4 wheels dynamicaly balanced off-car. If it takes too
> much weight to balance or they are wobbling in and out or up and down
> on the machine find out what is wrong. Do not try to "shave" tires or
> balance a bent rim or a defective tire. Shaving tires is best done on
> heavy trucks with recaps, not cars. I jack the tire up off the ground
> and put a small piece of 2X4 next to the tire but not touching. Now
> spin the tire by hand and watch if the gap between the wood and the
> tire changes. It should not change more than 1/16 of an inch. Same for
> up and down. If it does move a lot check the rim. If the rim is bent
> the tire will not run true. If the rim is NOT bent the tire is
> defective. Another point to make. All new tires come with a mark to
> indicate where to put the valve stem in relation to mounting the tire
> onto the rim. If the shop does not follow that mark it can take an
> excessive amount of weight to balance the tire.
> David Teitelbaum
> vin 10757






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