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~ !!

 

[myt1]

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

 

[patprimmer]

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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[GregLocock]

* 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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[NormPeterson]

* 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

 

[NormPeterson]

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

 

[patprimmer]

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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[NormPeterson]

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

 

[GregLocock]

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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[myt1]

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 ;-) in a new pair of Advans.......

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

T1

 

[NormPeterson]

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

 

[myt1]

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 ;-) 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

 

[NormPeterson]

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

 

[patprimmer]

Norm

I guess you are saying that say one flat on the tie rod adjusting nut is not repeatable.

Regards
Pat
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[NormPeterson]

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

 

[myt1]

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

 

[Windjammer96]

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