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Effects of improper scrub radius on tire wear

Effects of improper scrub radius on tire wear

Effects of improper scrub radius on tire wear

(OP)
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~ !!

RE: Effects of improper scrub radius on tire wear

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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RE: Effects of improper scrub radius on tire wear

(OP)
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....

RE: Effects of improper scrub radius on tire wear

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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RE: Effects of improper scrub radius on tire wear

Are you also using the brakes harder?


Norm

RE: Effects of improper scrub radius on tire wear

(OP)
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 (!)

RE: Effects of improper scrub radius on tire wear

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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RE: Effects of improper scrub radius on tire wear

(OP)
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...

RE: Effects of improper scrub radius on tire wear

-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.

RE: Effects of improper scrub radius on tire wear

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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RE: Effects of improper scrub radius on tire wear

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

RE: Effects of improper scrub radius on tire wear

(OP)
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

RE: Effects of improper scrub radius on tire wear

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

RE: Effects of improper scrub radius on tire wear

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

RE: Effects of improper scrub radius on tire wear

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.

RE: Effects of improper scrub radius on tire wear

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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RE: Effects of improper scrub radius on tire wear

(OP)
Hello all,

Have been gathering info from a variety of sources and, after analysis of all opinions offered, come to the conclusion that:

A) I really must replace the factory caster/camber plates with adjustable versions in order to be able to control all the parameters I might need to change. Best choice from what I've seen are MaximumMotorsports - almost $200 a set but supposedly well worth it as opposed to some others.

B) Once A is done, set aligment as:

* -0.35 degree camber (as bad as the side that's "only" -1 deg is worn it would appear a very signif adjustment from stock -0.7 deg is required)

* .02 deg toe in L & R (have heard the less the better and the left side is already there - right is at .23)

* 2-3 deg caster or less - ideas?? (if steering "return" isn't very adversely affected it'd appear that too much could cause the inside outside 1" or greater to wear more excessively than norm as spindle turns toward extreme L-R - true?)

Once I get the plates installed am planning to put on a less expensive, similar tire to verify even wear with a tread guage before buying any of those awesome albeit pricey Neovas again.

BIF, the "steer ahead" (from the printout) is approx -1 deg when supposedly the spec is -0.5 deg...  any thoughts on a spec there? Will the plates help with this adjustment??

Very much welcome any and all feedback re this "apparently best consensus at the moment" solution....

T1

RE: Effects of improper scrub radius on tire wear

How do you set left and right toe independent of each other.

I would not reduce the castor. The only disadvantage I have ever seen with more than stock castor an any car I ever had was heavy steering.

Regards
Pat
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RE: Effects of improper scrub radius on tire wear



* 2-3 deg caster or less - ideas?? "

pretty daft. That'll give you negative castor under braking. Your call.

*.02 deg toe in L & R (have heard the less the better and the left side is already there - right is at .23)

"the less the better " Nonsense. There is an optimum.

BIF, the "steer ahead" (from the printout) is approx -1 deg when supposedly the spec is -0.5 deg...  any thoughts on a spec there? Will the plates help with this adjustment??

No, adjusting your toe will. Or at least that's what normal people do.

Cheers

Greg Locock


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RE: Effects of improper scrub radius on tire wear

Quote:

* 2-3 deg caster or less - ideas?? (if steering "return" isn't very adversely affected it'd appear that too much could cause the inside outside 1" or greater to wear more excessively than norm as spindle turns toward extreme L-R - true?)

What you're asking for is more commonly applicable to cars that use a SLA front suspension, and more than likely - those built earlier than about the mid 1980's.

Your stock caster spec is about 3 times higher at +7.1°, ±0.75°.  I strongly doubt that MM's or anybody else's caster-camber plates could give you the over 4° of adjustment that you're asking for.  There may not even be sufficient room at the tops of the strut towers to move things that far.  ±2° adjustment is something that you probably can get.

Non-zero caster shifts your camber as you steer the wheels, and when your caster setting is positive this camber shift helps oppose the adverse camber that chassis/body roll directly gives you.

BTW, the toe spec is +0.1°, ±0.2°


Norm

 

RE: Effects of improper scrub radius on tire wear

Pat - the notion of right and left toes has to include the idea that the steering wheel is fixed in the position that corresponds to driving straight ahead on a flat road.   I think having equal amounts of steering available in both directions is also a requirement.  I take it that way.

Certainly when you're setting toe using parallel strings as reference, what you measure is the individual toes and total toe is computed.  Any apparatus that measures directly off the wheels/tires will give you total toe, but not the individuals.  

I use strings, take a test drive, and still usually have to equally lengthen and shorten the tierods a flat or two on the hexes to get the steering wheel acceptably centered when driving straight ahead on reasonably flat roads.


Norm

RE: Effects of improper scrub radius on tire wear

My impression was that setting the steering wheel in the middle on a alignment machine does not reliably give straight ahead when driving and a road test is required.

I would have also thought that less than 1 deg would have little influence and would be less than backlash in the system.

Regards
Pat
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RE: Effects of improper scrub radius on tire wear

If you know total toe, but not the individuals and don't make some attempt at centering the steering wheel for straight, where would you start the process of setting either of the toes right?  In cases where the job started with replacing steering and suspension parts on only one side, you'd have a pretty good basis for starting there.  Otherwise not.

If a test drive is included, I'd expect it to be a somewhat iterative procedure.  What I don't know is how many shops - if any - would give the extra effort for a random customer.


Norm

RE: Effects of improper scrub radius on tire wear

The correct way of setting steering wheel on centre is to get symmetrical rack gain (rack motion per degree of steering wheel angle) around the straight ahead, which is typically assymetrical due to the alignment of the UJs in the column. This may result in a visual SWOC which is resolved with a piece of white tape, or in the old days moving the wheel on the spline.

Then you can worry about total toe.

 

Cheers

Greg Locock


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RE: Effects of improper scrub radius on tire wear

(OP)
Many thanks for the feedback...  looks like the puzzle is starting to come together. There seems to be a consensus that CC plates are a good idea and that MMs are one of the best choices.

Re the adjustment advice:

- Haven't seen any neg feedback re the proposed -.35 deg camber. Does that seem likely to help? Have heard on some of the more popular Mustang sites that I'd be an idiot to run less than -1 deg... "wouldn't help your problem and the handling would suffer". Any wisdom in those words??

- Looks like 3.5 deg caster would work well. It's within stock specs (4 deg max) and that should insure that steering feel and "return" are good. BTW, I wasn't proposing adding 3 deg to what I had - apparently I misspoke...

- Consensus is that setting toe toward the minimum range of stock specs would likely be best. The .02 figure (ea side) proposed is still somewhat less than the .00 stock min, from what I understand. It'd at least be a good place to start. Thanks, Greg, for the advice re setting steering rack to absolute center before adjusting toe - will be sure to discuss that with the alignment shop.

Thought I'd run the latest conclusions up the flagpole to see if you folks thought I was on the right track. Am hoping to get the CC plates soon and will test the suggested settings using lower cost tires before investing (that's the correct word re price, I assure you winky smile in a new pair of Advans.......

Again, thanks to all. Looking forward to seeing your replies.

T1

RE: Effects of improper scrub radius on tire wear

I've been thinking that the car in question is a 2005 or later because of the 18" wheels.  If it is an earlier model for which 18" wheels were part of a "tuner package", the later specs may be too aggressive for "normal driving".

You need to do a bit of interpretation with respect to what you read on the enthusiast sites.  About the site itself and its general"slant" and abouot the individual respondents.  Then compare to your own driving.  If you don't drive anywhere near autocross- or open tracking-hard, you don't want settings that are more or less optimized for that sort of driving.  

FWIW, on the V8/RWD car that I used to autocross I'd swap in about 2° worth of shims for the duration of events only from what was about a -0.75° street camber setting.  Toe was set such that it would go from slightly 'in' to about an equal amount 'out'.  Not quite optimum for either, but still better than optimizing either one and sacrificing everything from the other.  When my wife was still commonly driving that car, I ran its camber at about -0.25° to maybe -0.4° with 1/32" toe in.


Norm

 

RE: Effects of improper scrub radius on tire wear

(OP)
Hey Norm,

Thanks for the qwik reply....  and for the info. Re diff camber adjustments for racing/track -v- street, I understand that it'd be a good idea to mark the CC plates with the optimum settings for each so I can easily switch - perhaps w/o even taking it to an alignment shop. Think I could really get away with that method? If so that'd be another advantage to havin' the plates.

The car came with the stock 17" Cobra wheels...  About 2 yrs ago my son found a set of nearly perfect Saleen 18's (similar vintage to my '97) that would allow me to put big enough tires out back to get decent traction with the 4:10 gears. The Advans help enormously in that regard, as well.

Glad to hear you don't think it's nuts to run less than -0.5 deg camber if that's what it takes to make the tires live on the street. Looks like the value I settled on, -0.35, is pretty close to what you were using so I'm becoming convinced it's worth a try...  The Mustangers that were wailing and gnashing teeth about less than -1 deg probably do spend more time at the track - or drivin' like they're there - BTDT, but not all the time  winky smile  The most curious thing about those recommendations is that they all blamed my wear on too much toe-in. Never seen toe issues do that this badly before, esp when it's within the adj spec range for this car (according to the align printout - approx 2.5 deg total total toe).

Looks like the next step is getting CC plates & some less expensive tires to test it all on since I'm starting to get comfortable w/ the proposed align specs.  Will let you folks know how this turns out if interested - please feel free to offer more quality advice on the issue....

AllTheBest,
Roy

RE: Effects of improper scrub radius on tire wear

Once you've established the CC plate locations, a "go/no-go" gauge or similar hard stop limit on adjustment would be more repeatable than marks.  I don't offhand know of a similarly simple approach for making toe swaps equally repeatable.

I hope you meant 0.25° toe . . .


Norm

RE: Effects of improper scrub radius on tire wear

The accuracy within which you can stay within "one flat" each time you change the setting can be reasonably consistent.

But over time, the setting will tend to "creep" one way or the other if you don't have some fixed reference.  Just turning the nut one flat (or however many it takes) is otherwise done with respect to the previous setting rather than the original setting, so any errors that are individually within acceptable tolerance can accumulate until the total "adjustment" isn't putting things where you think they are.

If your adjustments are done at the competition site in order to minimize street driving on the aggressive settings, it is unlikely that facilities to make toe adjustments easier will be present.  So yes, I am suggesting that repeatability is open to question when you're laying on the ground with poor sight lines past your arms to see the tie rod ends and their jam nuts, and poor leverage to get things tightened.

Anybody who is hardcore enough about autocrossing or track days to be considering separate sets of alignment parameters is not just an occasional participant.  I'd think you'd be looking at a minimum of a dozen events/season, times two adjustments per.


Norm

RE: Effects of improper scrub radius on tire wear

(OP)
Norm,

Thanks much for the suggestions re CC plate adjustment repeatability. Contrary to appearances I'm not the type to run this car at the track many times a season. It's 13 yr old and I've ~never~ done so before...  OTOH, these Advans are the closest thing to the perf of slicks (and still survive a rain shower) that I've ever seen so I find myself contemplating track time.

From what I've heard it's advisable to crank in 1.5 to mebbe even 2 deg neg camber when doing such crazy things...  worth it to me even on an occasional basis. Since the CC plates allow diff adjustments for diff uses it made sense to at least attempt to use that feature a few times before I'd take it to the track. BIF, my thought was to make those changes  at/near home right before leaving home rather than attempt it "in the field" & vice-versa. Thinking about leaving the toe same for both if possible.

Indeed the 2.5 toe figure was a typo / brain fade. It says it's currently at .25 total - have heard that around .02 / side or .04 is the apparent ideal for this chassis. Do you have any thoughts on that?

Thanks for your time,
Roy

RE: Effects of improper scrub radius on tire wear

Please excuse if this has already been covered, I really haven't read all the posts.

From what I gather you have installed Saleen wheels on a Mustang. The Sahleen Mustangs do have their struts modified to reset camber to -1.0 degrees, after repositioning upper strut mount inwards 1".From past experience roadracing Mustangs for many years, I can say negative scrub can be a good thing, and has nothing to do with tire wear. Chances are, IF your toe is close, your problem is in bumpsteer and/or improper ackerman if you do a lot of track events.

IF you have lowered the car and/or changed caster, your bump steer envelope has changed.

We like a bit of negative scrub which quickens turn response.

Too increase positive scrub, you must move the upper strut mount inwards, slot the upper shock mount at the knuckle, and re set camber. If you are loading heavily, then the slot needs to be welded back to a round hole to keep it from slipping. I believe Steeda sells an offset bolt for this purpose but have never tried or have confidence in them.

Good luck! Have fun!

Bob Hahn
 

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