I'm just about through ANOTHER set of front tires with the outside edges almost completely out of tread while the centre and inside of the tire is still in excellent shape (although worn) Size is 245/50/16.
The first set on the IROC went down prematurely even though ALL of the suspension components had been replaced. I put it down to using stock alignment specs in a modified application with the car being lowered and an aggresive (but not crazy) driving style.
Now the same thing has happenned to my Coupe. This chassis is also fresh with new Intrax lowering springs, Del-A-Lum control arm busings, all new linkage and Spohn strut mounts.
I've added a small amount to the negative camber over what the OEM calls for in the hopes of eliminating this problem but I don't believe it has made much (if any) difference.
Is this a common problem with these cars and is there is special recipe that's been cooked up alignment-wise to solve this?
Any and all insights are appreciated. I have another set of tires on order and I'm going to check the existing alignment before I swap tires and have the car realigned. I've used two different shops by the way, both with excellent reputations and verifiable results.
Thanks.

 

Yeah the stock specs will do that... not sure what body cavity the factory guys had their head crammed up when they dreamed those up, but they're WRONG.

Shoot for as much positive caster as you can get, while maintaining about a half-degree split to the right; something around 4° L / 4.5° R is typical. Set the camber to about -0.5 L / -1.0 R. Toe should be inwards, very slightly, about .030 - .050".

 

That's pretty close to the last alignment with +4° L / +4.5° R caster but with -.5° left AND right camber. The right side seems to suffer the most so I can see how the additional half a degree would make a difference. As for toe, OEM specs out at +.15° left and right. Not sure how that translates into fractions of an inch however one thing I can comment on is that car left the shop with the steering wheel not quite centred and I'm suspicious of the toe setting. It was suggested that the tire wear I'm getting has as much to with the toe-in /out as it does with camber.
I will say that the vehicle drives and handles very well and offers no surprises when at speed.
I'm going to have it checked again before making any changes to see if perhaps something has shifted or was poorly adjusted.

 

well i have had a aggressive street alignment for about a year and havent had any outer tire wear like the crappy stock specs do.... vetruck gave me these alignment specs and the car handles alot better than stock specs and also it eliminates outer tire wear

+5 degrees caster on left side
+5.5 degrees caster on right side
-0.8 degrees camber on the Left side
-0.8 degrees camber on the right side
i told the guy 3/32" toe in and on the sheet it says, -0.05 degrees toe in on left side, and +0.05 degrees toe in on right side.

 

What toe did you end up with? The comparison using degrees versus fraction of an inch is dependent on tire diameter so comparisons are difficult. I do understand that toe out is expressed as a negative value (ie -0.17° is 1/16th toe out) or so says the internet.

 

it was confusing but i had 3/32 toe... the sheet said .05 and .05.... the shop that i went to said that i needed it in degrees and that their shop couldnt do fractions... so when i converted the numebers they came out to that.... needles to say im surprised that the common outer tire wear went away and the entire tire wears evenly now...

 

That's what I'm after.

 

but i believe i couldnt do it with stock strut mounts you can only move the stockers so far... i have j and m strut mounts and it made it possible to get those numbers.... they gave me WAY more adjustabilty than stock.... but yeah those are the numbers i would use if you want better handling yet still have even wearing tires... what were the factory people smoking with those stock specs.... i guess they wanted people to keep buying tires..

 

I have the Spohn pieces which look to have the room for greater adjustment.
I'll have to see what the shop says regarding the fractions of an inch/ decimal conversion and what they're capable of.
This tire thing is getting old.

 

you should have the adjustability now to get those numbers... the best thing to do would be to go to the alignment shop and see if they take the 3/32 toe number and if not ask them to convert that number in degrees... they should at least know how to do that considering they are an alignment shop afterall... but im overal happy with those specs..... i would think you would too...

 

The average alignment shop is not going to be able to accurately convert degrees into fractions of an inch. You actually need to do some trigonometry to get the fractions number from a degree reading.

Take your degree number, subtract from 90. The cosine of that multiplied by the radius of your tire is going to give you the inches in decimal that your tire moved in or out at the leading edge. Your toe angle should be set to the same number on both sides. So you multiply the product of your equation above times two to get the total toe. Convert decimal to fraction and presto you have your toe in a fractional measurement. I'd be surprised if the alignment shop knew what the tire diameter is on your car, much less how to figure out the distance from the y axis for that diameter (that is what you are measuring when you hit the COS button on your calculator)

The problem with inch measurements is that for a given inch measurement, you could have vastly different angles of toe. When we set toe we are looking for a given angle of the tires, not inches. The larger the tire, the less angle you get for a given inch measurement. Likewise for a smaller tire, that same fractional inch measurement will make for a sharper angle.

For reference, today I just worked out that one half turn of your tie rod adjuster will move your toe ROUGHLY .0707 per wheel with a 25.65" diameter tire. I ballparked the distance of the steering knuckle to the outer tread at 5.5". So one quarter turn on both sides from 0 deg toe is going to give you close to 1/16" total. .05 degrees toe is around .036" per side iirc. You should have equal toe on both sides. Dunno how you ended up with neg on one side and positive on the other

 

Oh, yes. Trigonometry. I remember it well.
That said, the info posted here is in fractions of an ich (aside from fastgta's +/- results from the alignment shop). GM specs +.15° L and R. The postive value being toe-in if I can believe what I read elsewhere. (I just verified that positive IS toe-in through GM).
So what are the degrees to shoot for? Based on your 25.65" tire and using your calculated values of 1/16th = .05° (approximate), then GM wants 3 times that .15° per side.
From the replies so far, (and thank you all) I'm at this point.
Castor and camber values seem to be straightforward.
The toe less so.
That's about 1/32" - 3/64" total.

If the positve and negative values are just typos, this should be +.05° per side. Sounds reasonable too however neither value approaches GM's spec.

Are you saying +.05° total is your target?

If the above are the values you all are using, I can see how the factory spec is WAY out.
.15° per side is vastly different than .05° total. Or even .05° per side.

 

Oh, yes. Trigonometry. I remember it well.
That said, the info posted here is in fractions of an ich (aside from fastgta's +/- results from the alignment shop). GM specs +.15° L and R. The postive value being toe-in if I can believe what I read elsewhere. (I just verified that positive IS toe-in through GM).
So what are the degrees to shoot for? Based on your 25.65" tire and using your calculated values of 1/16th = .05° (approximate), then GM wants 3 times that .15° per side.
From the replies so far, (and thank you all) I'm at this point.
Castor and camber values seem to be straightforward.
The toe less so.
That's about 1/32" - 3/64" total.

If the positve and negative values are just typos, this should be +.05° per side. Sounds reasonable too however neither value approaches GM's spec.

Are you saying +.05° total is your target?

If the above are the values you all are using, I can see how the factory spec is WAY out.
.15° per side is vastly different than .05° total. Or even .05° per side.
I will say that the tire wear I'm getting does not appear to be caused from excessive toe-in as whouold result in a saw toothed tread. Mine is distinctly on the extreme outside of both tires with the right getting it more than the left.

 

Ignore the GM spec. It is WRONG.

I suspect it may have been produced by someone who was bribed by a tire mfr who shall remain nameless but who also invented a brand-new oddball size exclusively for these cars, to guarantee that these cars would need these "special" tires every 5000 miles, and who probably also demanded a kickback from said mfr based on every replacement set sold.

The specs that I gave you and that 88 gave you are basically identical, subject to how far rearward you can get the top of your struts to go. Most cars won't go past 4½° on the right, meaning that's about all you can give it. If it'll go farther, like his which has aftermarket strut mount adjuster plates, then more is good, up to the point that the tire is SO FAR rearward that it rubs the fender well in near-full-lock turns.

 

I notice left/right are not all identical. Is that because of unequal weight distribution especially from driver? Would equalizing the weight in some way change the specs to be symmetrical or are there other factors?

 

Crown in the road.... tends to make cars drift right. The specs tend to offset that.

 

The degree readings are per side, not added to each other.

My rough calculation is that .05 degrees is .036" per side which in total would be just a little over 1/16" total. The number might actually be slightly less (maybe 1/16" even?) because I am ignoring scrub radius.

If you ran .15 degrees of toe in you would burn up your tires very quickly and the car would push like a snow plow.

 

[QUOTE=sofakingdom;4963090]Ignore the GM spec. It is WRONG.QUOTE]

That much I understand. As I mentioned earlier and the point of this thread in the first place.
That said, using my Spohn strut mounts, achieving the best caster and camber shouldn't be difficult. The values are also easily understood. At LEAST +5° left and right caster and somewhat less than -1° left and right camber (ignoring the variance for road crown).
To clarify the toe adjustment, and going through all the degree vs. fractions BS I'm looking for +0.05° left and right (postive being toe-IN) which should more or less equate to 1/16".
Yes, no, maybe so?
At again, thanks to all for your patience and contributions.

 

At not to have Sofa blow a blood vessel, .15 degree per side IS the GM spec. (And yes I understand it's wrong).
For the record.



Look at camber settings!! +1° !!
The GM engineer and tire manufacturer must have had a good time with that one.

 

Yeah that's what I mean about the GM spec. It's just STUPID WRONG. It's almost as though somebody designed it SPECIFICALLY TO EAT TIRES. I honestly don't think that they could have done THAT good a job of screwing it up, purely by accident. There HAD TO BE some kind of intent.

So, basically, when it comes to alignment on these cars, close the SM, and do what makes sense instead. Which is what I did (yes I have a copy of the Helm's of course) after the tires that came on the car lasted about 5000 miles and the first set of tires I bought for it only lasted about 8000 miles and the car drove like crap for every inch of the way. I took one look at those specs and realized that they were .... messed up; and furthermore, they EXACTLY accounted IN EVERY DETAIL for the destruction I witnessed on my ex-tires. So I've been having mine aligned as I described up there for the last 25 years, and tires have been lasting about an ordinary mileage, which is, about 20% longer than the tread life "warranty".

IMO 1/16" is about right on the toe-in, for "total" toe. Roughly 1/32" each side. It's not just super critical though. OTOH you can also use a bit more negative camber and a bit more toe, which will cause a bit more scrubbing but also a bit more of a positive "centered" feel. Alot of this stuff isn't about "it has to be just like this or else"; rather, it's a form of TUNING, where you have the freedom to alter it, within reason and accepted principles of the physical world, to suit youru personal situation and tastes.

 

How much, if any, would these specs change with non-stock tire sizes?

 

None. It's all about keeping the wheels pointed in the right direction. Tire size doesn't really enter into the picture.

 

Not exactly sure how my last alignment ended up mangling my tires other than a) I didn't alter the factory spec enough as I requested -.5° camber L and R when it's suggested that .8° - 1° is better (and I can believe it looking at the tread wear) and b) the toe-in is a ridiculous .15° positive for BOTH sides giving .30° total.
Can you say goodbye tires?