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Discussion Starter · #1 ·
I took my car to a tire shop to have the front wheels re-balanced because the steering wheel began to shake only between 40 mph and 50 mph. At any other speed the shake was unnoticeable. The first thing the guys at the shop noticed was that both front tires were beginning to cup on the outer edge. There are only about 4,000 miles on these tires. So they did not re-balanced them but did rotate them – front to back and vice versa – and the shake disappeared. Now this car has all new stock shocks and springs, new front disc brake assemblies, new CPP 500 steering box, and all steering components are stock (tie rods, Pitman arm, etc) and are tight and show no wear. So, what is causing the cupping? I am beginning to think it’s the size of tires. They are 245/40ZR18 mounted on 18 X 8 wheels. I do not know the offset or backset of the wheels, but they aren’t even close to the fenders. With this size tires, does the alignment require different camber, caster, and/or toe-in settings than stock? Or could it be the stock shocks are inadequate for the larger tires. I don't think the tires are any heavier than the originals, though, since they're on aluminum rims.
Any thoughts and ideas are welcome!
 

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I would start by checking what the recomended wheel size the tire manufacturer shows for these size tires. Also verify the tire pressure is set correctly to their specs.

A lot of guys will change the suspension bushings to polyeurathane to ensure limited movement of suspension parts (especially with oversized tires). But I don't know if that is really an issue with what the tire shop saw. Offset wheels will only aggravate bearing life but again should not contribute to cupping of tires (unless they're worn).

Hope this helps.
 

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Cupping of tires is something that's always been a bit of a mystery to me.

For starters, I think that cupping can happen or not happen depending on the tires you buy. I.e., Brand A tires of a certain size will cup on a given vehicle, and Brand B's won't, even if the same size.

There's also no doubt that rotating your tires often will at least hide this kind of thing for quite a while. But if you're like many of us with a 55-57, you don't run the same wheel and tire on the front and back, so rotating is not an option if you have "bigs/littles".

I think that a lot of caster can also cup tires, but a lot of caster also stabilizes the vehicle, and makes it track straight, so there's both a reward and a penalty for doing that.

So I don't have any clear recommendations if you have the situation I described.
 

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stock shocks are not enough of a damper for those 18" wheels you need a good guality gas shock.
 

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Used to see cuooed tires alot but not so much anymore....It used to happen when tires were outta ballance and would wear uneven when outta line....:bowtieb:
:gba:
bowtie-trifive
 

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Cupping vs. shake?

I'm no expert, but I did once stay at a Holiday Inn Express. Formerly I haven't seen a relationship to cupping in the tread of a tire, and shake. I suppose anything is possible with a wider wheel/tire combo.

First, I would have a reliable tire shop spin the cupped-tire wheels on their balancer while I watched, to see just how far out, if any, the balance is. I'm presuming these upscale wheels do not utilize weights on the outer rim, and must be balanced with tape-on weights. I use a shop that does lots of aftermarket wheels with satisfaction.

Properly balanced, I'd re-install them on the front. If the problem persists, then I'd put some meaner front shocks on it (not for competition, just more aggressive), to see if that fixes the shake.

JMHO :anim_25:
 

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Cupping is a double edge sword. it's caused by out of balance or bad shocks...but is usually at a resonate frequency, your 40-50 mph only enforces that. The out of balance at that point is what causes the shocks to go bad and then the cupping becomes worse.
New tires and new, good quality shocks, hope you didn't buy any shocks from CPP.
 

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Discussion Starter · #10 ·
Thank you for all the input. I agree 100% with Rick L. The tread style/design on different tires of the same size can and will vary the type of wear pattern. I experimented with that on other cars and have proven that theory. I also agree that better quality gas shocks on the front would help, and I believe that is what I am going to do for starters. That and a new alignment.

Is it OK to put gas shocks on the front but not the rear – at least for awhile?
 
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