| Re: Cupping of rear tires on the 85 Riviera
In some cases, certain tread designs are more sensitive to toe-in settings than other tread designs. When many "knobby"-treaded "all season" tires were used on GM vehicles in the middle 1980s, GM had to change the toe-in settings (at least on the front) to basically "zero" (whether measured in inches or degrees) AND also started recommending 6000 mile tire rotations to help combat the irregular tire wear of these particular tread design tires. In later years, the tire tread designs have been refined and are not quite so critical, but the toe-in settings have not changed from "zero" toe-in.
With every modern electronic alignment set-up, there are wheel sensors that are attached to EACH wheel, not just the front. There are too many cars which now have 4-wheel independent suspension for alignment shops to not be set up to check them! The computers will also provide a print-out of the initial and "adjusted" alignment measurement values, both for technician/shop documentation and also for the customer to have (if desired). In other words, you end up with a "four wheel alignment" whether you might need one or not.
It would seem that if the rear springs have "worn" or sagged enough that, with the engine not running, the rear of the car "droops" compared to when the engine is running, it should be a simple action to request that the alignment tech to the work with the engine running. This would ensure that correct "ride height" was maintained during the alignment proceedings! Usually, though, alignment values do not change very much within about an inch deviation from factory ride height specs, by observation.
ALSO, remember that all of the rear suspension pivot points are rubber-isolated with bushings just as the front suspension control arms are. With time, even with very low miles, the rubber will dry out and deteriorate, which can cause more issues with wheel alignment than a minor deviation of rear suspension ride height. Therefore, checking any of the rubber bushings in the rear suspension on components which might affect "rolling toe-in" settings should be advantageous to request. Just to be sure, you might firmly apply the parking brake and then lightly throttle into the engine, putting stress on the rear wheels with the locked brakes (NOT all four as it would be if you used the foot brake) and then look to see where any wheel/suspension deflections might be taking place--taking care to maintain observer, operator, and vehicle safety during such a scenario. Do such a check in "forward" and "reverse". If the bushings are allowing "too much give", it should be obvious, I suspect.
Typically, toe-in is a critical adjustment on an independent rear suspension vehicle, moreso than camber as radial tires are more critical of toe-in than camber being out of whack. Bias-ply style tires are critical of BOTH camber and toe-in.
In some cases, suspension designs have a built-in negative camber built into them. The Cadillac Catera is one such vehicle. They ALL exhibited negative camber on the rear suspension, even when new. The only adjustment was for toe-in, on the rear suspension. Checking the Holden forums and other similar sources revealed no "fix" for this negative camber situation, so it could be suspected to not be a real issue for rear tire wear on those vehicles.
So, make sure the rear control arm bushings are in good condition, the rear wheel toe-in is set to specs (even on a solid rear axle vehicle, there is a small amount of toe-in in the basic design of the rear axle housing, for a little stabilizing effect on the vehicle, independent of the front tires' toe-in setting), the rear shocks are still in good condition (no fluid leaks, air leaks of the air shock section, good damping action), and the rear tires are inflated to something akin to factory specs. If you need to purchase new tires, choosing a tread design that is more "ribbed" than "knobby" might be a help, too. Of course, tire rotation every second oil change can help.
Regards,
NTX5467 | 
3 Weeks Ago
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