Effects of improper scrub radius on tire wear

[myt1]

Am trying to solve a problem with abnormal tire wear that seems to be stumping everyone... Have found my favorite sticky tires ever but after only 12k miles they've worn dangerously only on the inside 1" or so on both sides and nobody seems to know why... Tire wear is normal otherwise on front and no problems with the rears. Looks for all the world like excessive negative camber but it (and all other alignment params) are within spec (-1 deg on camber). Only other factor is that I installed these tires on Saleen 18" rims (which, since it's stock on those cars shouldn't cause problems) and that may have changed my wheel offset. Is it possible that the wheel offset of those Saleen rims has altered my scrub radius adversely to the point where these Neova Advans won't live as long as they should? Or do you think I should look elsewhere??

Any and all suggestions welcome... I just want to figure out a way to make these tires last as long as they ~should~ !!

 

[patprimmer]

The phrases favourite sticky tyres and abnormal tyre wear tend to go hand in hand.

Why would extra scrub only wear the inside unless it is pulling more toe out under dynamic conditions.

Is the toe, camber and castor set to spec. -1 Deg camber should give slightly faster wear rate on the inside edge.

Are the suspension bushes worn and causing variations from static settings when moving.

Regards
Pat
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[myt1]

You're quite right that the better tires grip the more they get driven hard... It's "because you can" when you have them (!) Still, the T/A KDs I used before on the stock rims were similarly soft but wore evenly (well, mebbe just a bit more on the ~outside~ edge if I really drove the hell out of it).

Don't know if scrub radius can cause my issue - hence the post. OTOH, given the fact that the Saleen wheels are the only other variable and that different offset (from stock) can cause scrub radius issues I'm wondering if there's a common thread here. Esp since the alignment shop checked the front end for both component wear (virtually none) and the usual camber, caster, toe & are all nearly dead-on the stock settings (which worked before the new wheels & tires).

Have posed the Saleen specific part of this equation to their owner's group - if I find an answer there I'll be happy to share the solution. Am thinking that perhaps since Saleen Mustangs were lowered the wheel offset might have been calculated for their specific ride height and suspension mods....

 

[patprimmer]

Wider track via more offset has the effect of softening the springs and lowering the ride height.

I did not say that sticky tyres encourage harder driving (although I don't disagree as a generalisation). Sticky tyres tend to have a softer compound and maybe a thinner tread depth to maintain tread block stability.

Regards
Pat
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[GregLocock]

scrub radius has no first order effect on tire wear.

Cheers

Greg Locock


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[NormPeterson]

Are you also using the brakes harder?


Norm

 

[myt1]

Many thanks to all who have replied. And humblest apologies if I've inadvertently said or done anything that violated site protocol... Certainly hope not.

Am not driving the car any harder (including braking) than I ever have. Matter of fact, since all the other variables I could identify appeared to be the same ~except for~ the new tires and diff wheels, it appeared the answer lie there... Neova Advans are indeed soft and are a "maximum performance summer tire" just as the T/A KDs I once used (those wore evenly - on original rims) so I'd sorta crossed that one off for now. Wheel offset seemed to be the only other potential issue. I'm nearly positive that they have a diff (positive?) offset since the tires now protrude out of the wheel well more so that seemed like a good place to look. But evidently scrub radius is unlikely to cause excessive wear on the inside 1" of the tire so apparently I should keep looking.

Anyone ever had to solve an issue like this before? Guess I could go back to the T/As to see what happened then but that sounds like an expensive experiment (!)

 

[patprimmer]

Gred said there are no first order effects, not that there are no indirect effects like changes to compliance due to changes of loads being applied.

Regards
Pat
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[patprimmer]

due to the tread centre being further outside the king pin axis.

Regards
Pat
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[GregLocock]

Yup, so what I'd do is cure it the way you usually would, ie change the static camber and/or toe. No drama. In an OEM development program static toe settings are finalised /after/ the final tire submission.

Cheers

Greg Locock


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[myt1]

Not sure if direct / indirect effects of scrub radius could cause the excessive inside wear I've seen or not... but it'd be nice to be enlightened. OTOH as is so often the case, perhaps it's time to go with the "if it walks like a duck & quacks like a duck - you've got yourself a duck" philosophy here.

For whatever reason this particular brand/type of tires are acting as if the camber is significantly out of kilter so it'd likely make sense that's the variable to change. Had planned on getting aftermarket camber plates anyway as mine is fixed (evidently becoming the norm... sigh) so it'd be simple to test that concept. Anyone got a Scientific Wild Guess where they'd start w/ camber settings given my wear pattern and the stock -1 degree norm: +1 to +2 deg camber - or more?? Wear on that inside 1" is pretty severe compared to rest of the tire...

 

[GregLocock]

change your camber by 0.3 degrees positive. The handwavy explanation for that number is ugly.

Cheers

Greg Locock


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[LionelHutz]

-1 degree can be a lot of camber depending on the width and profile of the tire. Toe can change the wear pattern as well. I'm running 275/30/19 (~11") front rubber and -1 degree was eating up the inside edge something terrible (it looked sliced off at about a 30 degree angle). The tires were old and needed replaced when I got the car though so I had it aligned and moved it back to -0.3 degrees and the wear is much better on the new rubber.

FYI, good "max performance summer" rubber isn't expected to last more than about 20k miles.

 

[patprimmer]

My swag is -0.5 deg is normally a good starting point for modern low profile wheel and tyre combination.

It also depends on tyre pressure tyre construction and driving style, and some of the above I expect is why the auto manufactures track test with the specified tyre before setting factory specs.

Greg has a LOT more experience than I do at this so his advice is more qualified.

Regards
Pat
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[hemipanter]

I have not read all the responses so I may repeat what has already been said. I do not think the scrub distance has changed so much with your wheels, it would then appear directly to the fenders on the car.
As for scrubb and its impact on suspension, so it has no impact as long A-arm system is designed as a parallelogram, (other than material load), but now is usually the upper A-arm at an angle and scrub together with cambercompensation produces a leverage effect on the coilovers, and thereby transforming the Mr.
Scrub also has a lever effect around the SAI, and toe-in should be more in agreement with the increased scrub distance, not to produce toe-out.
Goran

 

[myt1]

Thanks to all taking time to respond - your answers have given me some confidence there's a logical solution to this problem.... Re specific camber suggestions, does "change your camber by 0.3 degrees positive" (someone else mentioned -0.3, too) mean set it to +.03 or change it by that much positive from the stock -1 deg, Greg? I'm betting the former but not putting any money on it (!)

"Handwavy explanation" - now there's a term I have to remember - being in I.T. (there's a reason that spells "it") it'll likely come in handy some day...

BIF, thanks for the detailed explanation of scrub radius effects (or not) on tire wear. Between that and the offset not likely having changed much by Saleen wheels that factor is looking like a dead end hypothesis.

AllTheBest,
Roy

 

[NormPeterson]

Move it in the positive direction by 0.3°, ending up at -0.7° if you're at -1° now. You likely wouldn't want a +0.3° actual setting, as if you drive with any enthusiasm whasoever it would be the front tire outside shoulders taking a beating instead.

FWIW, -0.75° is the factory preferred camber setting, though there is ±0.75° tolerance from there.

You're one of about three people I know of just this week with the same problem with this specific chassis. At least two of you are at -1° camber or just a tiny bit more negative.

Toe? Condition of struts (dampers)?


Norm

 

[NormPeterson]

Incidentally, I think the offset on the 18 x 9 Saleen wheel is +36mm, or only 9mm different from that of the 18 x 9.5 GT500 wheel and the 17 x 8 wheel, or 14mm different from the 18 x 8.5 non-GT500 18" wheel.


Norm

 

[WhizzMan]

I've had a similar problem with a FWD drive car, McPherson style. Things improved with a little toe-out on the front. Personally, I'd be happy for 12K miles for a set of tires on that car.

 

[GregLocock]

Yes you can trade camber for toe, as a solution. They don't have quite the same effect, but without knowing a great deal more I'd say that altering camber is less likely to upset the steering feel.

Cheers

Greg Locock


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