Darkshot

iTrader: (4)

I've had my Kumho 245/50/16 tires one for a couple months now, and the fronts are getting tad bit chewed up on the outer edges. I took it in under waranty and the alignment shop says they will do nothing for it because the alignment is correct. Huh? The alignment is obviously not correct if the tires are wearing funny.

I said "well can't you examine the tire wear and make adjustments to the alignment accordingly?" Isn't that what an alignment shop does? He wouldn't do it. I don't understand. What should I do?

Oh and please note that that the front end on this car is completely brand new except the steering box. All steering linkage, bearings, springs, struts, strut mounts, ball joints, etc etc etc.

I can post the spec sheet they gave me if that helps you.

adambros

iTrader: (2)

What I'd suspect is happening is that the tires are rubbing, or scraping -- on the fenders or fenderwells when making slow + sharp turns.

If you haven't replaced the factory steering box (edit : ahh, just re-read "completely brand new except the steering box".) than your stops are set for 225's, or smaller. Check the fenderwells for wear marks - a sure telltale sign.

Orr89RocZ

iTrader: (20)

i'm gettin the outer wear marks as well... rest of the tire is fine but the outer edges on both sides wear down. been doing that for years even after i get it aligned.

The rear end is fine and wears normally.

i dont get it but its just the way these cars work i guess

adambros

iTrader: (2)

wearing down is ok, he said "chewed up" thou . . . theres a big difference there.

Darkshot

iTrader: (4)

It's definitely not from rubbing. It's an alignment issue.

xplane

you like the corners? the tire bends under in sharp/fast turns and will do that, even if your not rubbing just doing a lot of sharp turns will cause it.

2kflhr

iTrader: (1)

Yep, if the alignment is correct (or so they claim) and all of the components are new I would suspect it is due to driving style. Hard cornering is hard on the outside edges.

ws6transam

Might I suggest something? As a long-term owner of the same third-gen for seventeen years, I have fiddled with alignment many times, and I've gone through I dont know, maybe seven, eight sets of tires. I always had feathering and chunking on the outer edges of my tires until I finally threw out the factory alignment settings and started having the camber set at between zero and -1 degrees. Tell your alignment shop to push the camber plates all the way in (You'll get about -0.5 degrees on a stock plate), use the 3/32 inch toe-in setting, and set the castor to +4.5 degrees. Your steering will be crisp, but perhaps a little flighty on uneven roads, and your outer tire wear problems will go away. Ever since running maximum negative camber from a stock camber plate, I haven't had any troubles with my tires and I get nice, even wear across the tire face. I've been set up that way for about ten years now.

sofakingdom

iTrader: (1)

It quite probably is.

The important thing to remember, is that the stock specs for these cars SUCK. One of the problems they create is burning off the outside edge of tires. Sounds sort of familiar, eh?? Those stupid specs also make the car wander like all hell, and understeer worse than what the car is really capable of. I have no clue why GM calls for them to be like that; all I can do is report the results.

Instead of setting it to the stock specs, have them set it up with +3° of caster and -.5° of camber on the left, and +3.5° caster and -1° camber on the right; and about .05" of toe-IN. Quite a bit different from the factory specs, which have very little caster and actually POSITIVE camber (tires leaned out at the top). Just that ONE piece of info right there tells you why your tires are getting smoked.

If they won't do it to the specs that YOU want it done to, take it to some other alignment shop that WILL. Don't bother trying to argue about "right", because it's entirely possible that they're actually already doing it "right" and that it's well within factory tolerances. Instead of that, just get them to do it the way YOU want it, which is the specs I just gave you.

And rotate your rear and front tires at the same time, BTW.

Orr89RocZ

iTrader: (20)

so how does this affect handling/steering feel?

sofakingdom

iTrader: (1)

Improves both (especially the feel)

Darkshot

iTrader: (4)

Thanks for the response. On the sheet they give me, toe is stated in terms of degrees and below the LEFT/RIGHT section there is just a "FRONT" section with "Cross Camber", "Cross Caster", and "Total Toe". Simply because I have to assume these guys are monkeys, what reference point can I use to convert your inches into degrees? Can I assume basic trig applies? And what should I tell them for the 3 additional FRONT values, are they simply some function of the LEFT/RIGHT specs?

Auggie

I had two problems with my Z28 when I went to wide tires. It pulled either to the left or to the right (i can't remember) and had tire wear on the outer edges. I went to a shop and all they do is frame and alingement. Here's where they set it.

Caster Driver side = +2* Pass. side = +3 1/4*

Camber Driver side = +5 min or .0835* Pass. side = -20 min or .334*

Toe set to 1/64" in.

I had this done maney years ago and just copied the figures off of the repair order. The tire wear has been perfect and the pull is gone.

Note: 1 min = .0167*

Auggie

sofakingdom

iTrader: (1)

Right, the "cross" caster & camber values are just derived from the #s from the individual wheels. Those tell you how much the car is biased toward one direction or the other; larger "cross" values make the car want to drift toward the side with the smaller #, which should be the left since most roads slope off to the right (due to the "crown" of the road for drainage) so you want to counteract that effect of the car being constantly forced to the right by gravity. The toe-in # I gave you corresponds to the "total toe", since it's a function of both wheels together.

I'm not sure about setting toe in degrees. I've always dealt with it in inches, or in feet per mile. No doubt it could be calculated with trig; I'm not positive what you'd use for the hypotenuse, but I'd guess it would be the wheel radius (8" or whatever). That works out to something about .15 - .2° per side, which kind of passes the smell test, if that's how they do it.

Out of curiosity, what specs do they say your car is set to now? IIRC the factory specs are something like +1° caster and +1° camber, with about a ½° split between the sides. Those will wear outer edges something fierce and make the car have a "vague center". The specs I gave you will eliminate the edge wear and give it a "strong center", and will make the front tires feel like they are more "into the road" in hard turns (make the car feel "looser", or at least less "tight").

Tidbit

Darkshot, The alignment they say they did probably is 'perfect'- what is perfect though? It is perfect to factory specs on a 235/55-16 tire and 7" rim- or at least good enough that no imperfect wear would show on a narrower higher profile tire. You no longer have that factory tire and rim on your 91 RS.You altered the tire and need to specify to them before hand that the tire is altered from factory and they need to go with 'your specs' rather than resorting to the alignment manual they have the gives a factory spec for your car with the original 235 tires. You can not assume they will know it has wider than factory tires or in other words you have the Z28 tires on it. You need to tell them you need specs for a Z28 and not an RS- even then, it still is not the greatest.


These cars do better with larger caster angles- however, with larger caster angles also come problems- less antidive and bumpsteer. We have adequate powersteering for 5 to 5.5 degrees caster but do not have the geometry from the factory in regards to tierods so bumpsteer kit needs to be added so the tierod shortens less as it moves up and back with the spindle in turn bumps to keep the ackerman angles tracking in sequence.I actually run 5* drivers side and-0.8* camber / 5.5* caster and -0.8* camber passenger side on my street car. This actualy decreases the feel of the powersteering and increases the effort yeilding more of a performance vavling feel to the car- The added caster increases stabilty at high speeds as well as gives the car an easier corner exit control- the wheel straightens itself. Disclaimer: Going this high on settings mandates you increase shock damper rates to a very good quality shock and strut, as well as you will need to increase the spring rate also to limit your roll movement to help aid in anti dive, limit suspension travel to decrease any potential bumpsteer (because of the arching up and back angle of the spindle steering arms), and limit camber gains that can play havoc on a cars twitchyness as the tire contact patch alters with suspension travel play. Note: the spring angle of rate effectiveness does not change with the strut caster angle, but the strut damper rate decreases in motion effectiveness with the greater angle apllied rearward with positive caster. This is effected even more as a car is lowered and the suspension articulation affects changes greater when the spans of the geometry decrease.

Conclusion- If the car is still fairly stock supsension wise (lets say KYB struts and prokit springs- Average street enthusiet upgrade) then just set the specs a tad higher than the Z28 specs. I would suggest keeping the alignment to about 3.5/4 positive caster (drivers side/pass side) and -.02 to -.06 range of camber (shooting for the -.04 mark) on both sides. Now based on bushings used and such as well as tie rod qulity and type I would generally suggest keeping that also to factory specs in about the 3/32 toe-in range (this allows for thrust setback of the wheels under road load and the bushing deflection)

Darkshot

iTrader: (4)

Here are some pictures and the alignment sheet.

ccowner4life

What would the specs be for P215-65-R15? I feel like I need an alignment. Not sure exactly though; the steering is loose on left turns but tight on right turns. Sorry for taking your thread but had a question about a different tire size and didnt want to make too many threads.

Any help and I read all of the info and its great to know just not sure about my size tires. I am have two barely used 205-60-R15s I am putting on the front they are alittle smaller in width than the others I mentioned. Would that have any effect on the steering? just curious

Thanks for the info

Tidbit

ccowner- check on the home page of this website under FAQ and it gives stock alignment specs.
Smaller tires will have no consequential effect on drivability and steering other than a safety and speed issue for the size of car and the correct tire size and contach patch to safely control the weight of the car in emergency situations (aka panick braking and turning to avoid a collision)

Darkshot- upon first glance at your picture I can clearly see you have a strut issue. Get new shocks and struts. you are suffering from excessive suspension movement and camber changes as a result under forces of corner loads with in adequate struts. Your tires have cups on the edges- a dead giveaway. I am surprized the alignment shop did not tell you this.

Darkshot

iTrader: (4)

Umm, the entire suspension is brand new. Struts, springs, strut mounts included. IROC replacement springs and Koni Reds on the toughest setting with big sway bars and poly bushings. My suspension is very stiff for a daily driver.

hgffrank

Looking at that big close up picture of the tire, that looks exactly like a rub.

And just for info, I had Kuhmo 711's 245/50/16 on my 90 Firebird, and they didnt rub going forward, but when backing up, the flex of the suspension was just enough that the tire rubbed on the inner fender.


And just a bit of advice, lose the Kuhmo's, they are garbage. I have access to an alignment rack and am a certified technician, and I can tell you that no matter what I did to my alignment, those damn garbage Kuhmo's never wore properly and always wore out prematurely. I switched to Avon tech M550's and my tire wear was never messed up again. tire quality make a big difference in wear characteristics, and Kuhmo is a low quality tire(I dont care what anyone else says, "I" think they are garbage. I gave my Kuhmo 711's two tries and after the second set of tires became garbage, I vowed I would never buy Kuhmo again.

84z28350

i would just double check to make sure that your not hitting anything...


I know with mine, if i do just the right thing during a turn i can get my LH (and only LH ) tire to drag on the screw inside the fender...

Darkshot

iTrader: (4)

Okay, they say they can use custom specs but cannot be responsible for them. Thats fine with me, however they measure toe in degrees. I just need to figure out for-sure the proper conversion method for toe-in. Anybody have any ideas? I do tend to be slightly agressive around corners.

bilms01

Toe setting in degrees is the angle using centerline or thrust angle as a reference. The hypotenuse should be the theoretical line drawn by the centerline of the wheel/tire assy

sofakingdom

iTrader: (1)

Use what I suggested; about .2° inward on each side (which I believe is negative, but I'm not sure of that)

Alot of these cars will not go much past 4° of caster with the camber also where it needs to be, because the slots aren't long enough. 3 & 3.5° is about as much as you can reliably count on being able to get, with the camber also set right.

As an aside, I've been using the specs I gave you, for 20 some-odd years on the car I have; probably 8 or 10 sets of tires at least. They've worked fine with every kind of wheel and tire combo I've ever had on it, including the original 215/65-15 Goodyear POS things that came on it, 235/55-15 Pirelli P6, 235/60-15 Michelin XGT-V, 245/50-16 Michelin XGT Pilot, and now 255/45-17 Pirelli PZero.