peterc005

Is there a particular reason third gens have different front/rear wheel offsets/track widths?

Never had any other car that needed different front and rear wheels?

Was it because of the parts available at design time, a way to improve handling, for looks?

TTOP350

iTrader: (8)

I have heard of several different reasons but the one that sticks with me is, its supposed to help with groove wander..

RBob

iTrader: (1)

Scrub radius. Push the center line of the front tires out too far and the feedback is too strong. It doesn't take much either, just 1/4" changes can be felt.

RBob.

Night rider327

iTrader: (13)

To clear the factory disk brakes..
That's the ONLY reason I know of.

If you look at the early 3rd gens with 14" and 15" wheels and drum brakes.. All 4 wheels has the same backspacing/offset

When they went to rear disk brakes, the wheels went to 16" and diff offsets for front and rear.

TTOP350

iTrader: (8)

3rdgens had disc from 82-92
they went to bigger better rear disc in 89
16" wheels were avaliable in 84.
1LE brakes came in 88or so (iirc)
So i don't think it was the brakes/16" wheels

peterc005

Never hear of groove wander before.

GM made many changes to these cars over the years, so they had the opportunity to have matching front/rear tracks if needed, so I assume there is a reason they kept it this way.

I thought both the Delco and PBR rear disk setups were the same thickness.

Can't think of another car before or since with this type of setup?

SlickTrackGod

Someone messaged me to give a response here.

The closest response here is scrub radius. It was not to fix scrub as for wandering, it actually went towards making that worse. What the problem GM faced was hard parking lot steering effort and frame cracks near the steering box with the addition of the performance 16x8 wheels. The fatter tires strained these issues. By simply increasing scrub radius they help lessen this issue by allowing a more positive scrub just like pre-power steering cars used for years for parking lot maneuvers.

Dean

peterc005

Hi Dean, thanks for the response.

That does not make sense though because all third gens had the same track\offsets, but only the later years offered 16*8 wheels.

Also, if widening the front track for the scrub radius, why not increase the rear track too.

It's an unusual setup, one that GM never changed even though it would have been simpler to have four wheels the same.

I'm suspecting it's because GM wanted to reuse existing front suspension parts from other cars Eg. A arms, ball joints, drag link tie rod ends etc, and that this caused the front track to come out wider.

SlickTrackGod

You asked two questions combined as one. You asked about #1 different wheel offsets ( 16x8 performance wheels had 0offset fronts and 16mm offset rears.) #2 the brake hub offsets were different track widths front to rear.

GM engineers flat ****ed up #2. They uses a front suspension platform that was not married to the cars actual body width. Then #1 when they tried to make it into a performance platform they had to screw up the scrub width up front and baindaid the track width in the rear with a 16 mm to keep corner line track of the outside wheels in the same footprint path. This helped take out the tight under braking and snap oversteer these cars are still notorious for. These car stock just plain suck compared to a corrected geometry car. The GM engineers were still learning and were still using 70's technogy like my first post stated.

Your last post is digging for a buried treasure that doesn't exist. Your putting too much credit into all this.

MoJoe

iTrader: (5)

Dean,
What is your recommendation for "best geometry" for a street driven car? Anything at all? Or is street driven low performance enough not to bother?

Are any on the bolt-ons worth while? Taller ball joints? LCARBs? Lower the PHB?

(Hopefully this is an acceptible thread hijack.)

JamesC

iTrader: (5)

85 with the introduction of the IROC, IIRC.

JamesC

TTOP350

iTrader: (8)

Yup, i always think in terms of the birds.
The 84 anniversary TA had them. I think....

peterc005

Why did'nt GM just fit a wider rear end to allow four wheels the same?

//<86TA>\\

iTrader: (14)

The width of the rear is not the issue, as you can already use 4 of the same size wheels with the same backspacing front and back. Did you rear the previous posts by slicktrackgod? There are technical reasons why this was done on these cars well beyond the aesthetic ones.

sofakingdom

iTrader: (1)

I seriously doubt that it has ANYTHING WHATSOEVER to do with the actual "function" of the vehicle in any manner way shape form or fashion. Not "wander", not "scrub radius", not ANY of that.

More likely, it was because the K-member in the front had to be whatever size it had to be to accommodate the engine and the steering and various other things, most of which already existed in A NUMBER of other car lines and body styles; then the control arms, which already existed in A NUMBER of other car lines, were already whatever length they were; then the rotors, which also already existed in A NUMBER of vehicles, needed a spindle of whatever depth and were however thick; and when they got all done with that, the front track width was whatever it ended up as, and they called it good. And that's how the front track width was "chosen": by lack of "choosing" anything at all. That's how GM worked back then, and still does to a large extent today; nobody makes ANY kind of a "decision" at all, because then they might be taking a risk of some sort and might be subject to .... {shudder with horror} ... ACCOUNTABILITY. Anathema!!

Then, the production line they made the rear on, which also already existed in A NUMBER of other car lines, needed to be wider ANYWAY, so they made it as wide as whatever machinery they were using to build it at the time, could accommodate, and it seemed OK, so they called it good. Once again, no deliberate "choice" at all: they "chose" not to "choose" anything at all and merely accepted what fell out when they turned the machines on.

Knowing the manufacturing process in general as I do, and knowing what we know today about GM's crappy "decision"-making process from the 60s on, there are no possible technical, aesthetic, or any other kind of reasons having to do with THE CAR ITSELF that can account for this. It can't even be blamed on emissions or gas mileage like so many other inscrutable stupidities can. Instead, it was all about cost, CYA, design re-use, being cheeeep, laziness, not having to design something new, design by committee, using what was already on the shelf, fitting the production line, diffused responsibility, not incurring new costs, and possibly even, the amount of money it would have taken to "fix" something as trivial as ½" of track width, when ALOT of vehicles are 2" different OR MORE. Oh and of course, there was always the matter of COST.

A fun discussion for sure, but I'm pretty sure not one single word of what has been talked about here was ever uttered by anyone anywhere at any time for any reason in the place and at the time in question.

See my signature for a helpful mental discipline in determining the root cause of some bizarre phenomenon you are observing.

TTOP350

iTrader: (8)

http://en.wikipedia.org/wiki/Groove_wander
Cars with wide tires that have the same track width front to rear and having that tread pattern will pull the car laterally left or right more abruptly than cars with tires that are slightly off. Esp with those wide tires.

I'm not saying this is the reason but id bet its atleast part of the reason. Tire and suspension tech have come along way since then but most cars/trucks still have offset track width front to rear to lessen the effect.

peterc005

The reason I brought this up is I want to get a set of 17 *9 wheels with 0 offset and 275/40/17 tyres.

My concern is that because the front track is wider it could affect the handling with this configuration and potentially cause problems with clearance on the front guards.

@sofakingdom - your explanation seems the most plausible, but you'd have to figure that having different front and rear wheels would add costs and hassles too.

I guess unless someone knows a GM Design Engineer from the early 1980s I guess we will never know?

DynoDave43

iTrader: (11)

stealtht/a

iTrader: (1)

I think the truth might be somewhere in between some of these explanations, but I agree with sofas thinking that you don't want to overthink some of these design issues because they result from the fact that no one thought about them when the car was being built either. It would be nice to talk to a designer, but because of this issue, it may not yield any better answers.
However, if you don't get too excited or emotional about it, a fun discussion for the forum.

sofakingdom

iTrader: (1)

Indeed...


I would suspect that the answer to that, again involves the production machinery used to install them. Especially at Norwood, where the plant was like left over from ancient Rome, and they were stretching its useful lifetime just one more car model before it was SO obsolete they couldn't build anything even remotely modern on it and they had to bite the bullet, face up to the big bad union, and close it.


Go visit a vehicle assembly plant and see how a factory builds cars. I guarantee you, it will look NOTHING LIKE a garage, a body shop, a tire store, etc. All manner of things that hobbyists and even service professionals never even have to think about, things that are essentially INVISIBLE and NON-EXISTENT once the vehicle leaves the plant, can be IMPOSSIBLE in the factory without MAJOR capital spending, if for example some large expensive machine is only just so wide and some junior engineering intern screws up and designs the track width a half inch too wide to roll through it, or the like. In a case like that it may well be cheeeeeeper and eeeeeeeeezier to use 2 different size wheels for a short production run, than to replace the machinery in the plant.

gt4373

peterc005

Yes, but the different front/rear tracks ran for the whole ten years which suggests it was a deliberate design. I don't think it could be due to groove wander because the different offsets in front/rear wheels means the four tyres would be square on the ground

Over those ten years there were at least three different rear axle assemblies and brake setups.

Major features like rear disks, T700, TPI, galvanised body, electric windows 1LE etc were added along the way so why not fix the track?

On the other hand, if it was a design feature, how come no other cars of that era got different front/rear tracks?

Maybe having more dish on the rear wheels was supposed to make them look fatter?

sofakingdom

iTrader: (1)

No it didn't.


Only 8 of the 11 (not 10) years.


It started in 1985, while they were still building cars in Norwood, in a plant that Abraham Lincoln probably stopped his train at (the tracks run right by it) and gave a stump speech when he was running for President.


Doesn't too much matter; if memory serves the rears have a "standard" offset wheel, it's the fronts that are the "weird" ones.


The visual difference between them is undetectable to virtually everyone's eye. Even people who know these cars well and own them, are tripped up by it. Also, don't forget, both Camaro and Firebird are affected identically; and given the other "styling" things that went on, it is inconceivable that the "stylists" would have come up with identical offsets for ALL 16" wheel styles on ALL cars of BOTH product lines in ALL years.


Only conclusion I can come to, is that in the archaic Bronze Age machinery in Norwood, they could only make the cars narrower than some limit set by building jeeps in WW2 or something, to roll through it; and the fronts had to be altered (narrower) to fit. Then since it would have been too expensive and confusing to use a different wheel arrangement at the Van Nuys plant and then try to mark them somehow and keep them straight after the sale, it was cheeeeeeper to make all of em the weird depth.

peterc005

I'm still not convinced about the tooling explanation. Third gens are not especially wide, it's not a truck or Hummer. Other cars of the same era are the same width.

Didn't know the rear track changed in 1985. Once again, it's sounds like a deliberate design change.

Maybe GM got a good deal from Borg-Warner for rear end that happened to be a bit narrower? Maybe from another car?

The Borg Warner rear ends came from Australia and used an identical rear end suspension setup, with coils lcas and a panhard bar. These were used on HQ series GM Holdens, which are mechanically very similar to third gens.

One clue is moving to the narrower rear in 1985, but changing back to a square track in 1993 for the fourth gen cars.

Stewie

Soaking, There are volumes of articles, studies, reports, columns, etc. written about the problems of GM administrative structure, culture, and decision making. And you were able to summarize the plethora of problems in one paragraph. I am impressed.

Stewie

Soaking, There huge stacks of articles, studies, reports, columns, etc. written about the problems of GM administrative structure, management, culture, and decision making. And you were able to summarize the plethora of problems in 2 sentences. That's impressive.

Stewie

"Sofakindom" sorry for misspelling. Couldn't edit for some reason.

sofakingdom

iTrader: (1)

Sure; but not any of the ones built in Norwood.

It didn't. Sorry if I left that impression. It was the wheel offset in the rear that changed in that year for all 16" cars.

Stewie, thanks for the comment. I had to do acoupla case studies on GM for the MBA I got shortly ago; while it has been obvious since the mid 70s that the American car companies had lost their way, actually reading the facts about what on at GM in those years was enough to make me sick to my stomach.

If you want a real good read, go here. http://en.wikipedia.org/wiki/Delphi_Automotive Gives a great snapshot of the culture inside the company over more recent times, after they "cleaned up their act".

InfernalVortex

iTrader: (4)

I think I agree with Dean on this....

It doesnt make sense that it was a parts bin decision, because there are other control arms that are wider. You can bolt in 1st gen control arms, but the ball joints are farther out and creates geometry problems. That would solve the track width issue, but they designed different ones for these cars.... albeit very similar ones.

The other thing is that as far as i know, the 15's all use the same offset. This is a 16" wheel specific issue.

Which takes me back to what Dean was saying about the stresses of parking lot turning on 245/50/16's, which were super super huge by 1985 standards.

peterc005

Maybe the simple answer is that it was a cosmetic change.

That is, the different front/rear track didn't matter, but that 16" wheels were dished out to fill the wheel well better to improve looks?

Large corporates like GM eventually get so big and bureaucratic that the whole thing becomes dysfunctional.

One problem was that the big car companies have a hierarchy where once staff make it to the "executive" level all of the benefits, perks and prestige start to flow. Making it as an "executive" is similar to being "made" in the mafia, with special training courses etc. Staff are obsessed with making it as "executive", not necessarily doing the right thing. Key to making it as an "executive" is never embarrassing the people above you which leads to poor decision making.

Colt

Cost savings is the reason. Gm most likely got a sweet deal on certain length rear axle tubes (or axles) after the front end design was completed.

Everything on these cars is horribly engineered unfortunately. The front suspension design is pathetic, the torque arm has issues, the chassis are as stiff as a slug, the brakes are a joke, the steering uses a box when everything else went to a rack, and the exhaust routing is ridiculous.

sofakingdom is right. I have spoken with someone who was at GM during the years the 3rd gen was designed. Those were bad times for GM. The were losing money hand over fist and they fired all their senior engineers. So, GM's engineering B-team designed these cars.

I don't think I have heard of any car coming from the factory with a 36mm sway-bar. This would be what is called a 'band-aid'.

peterc005

I'm still not quite sure about the design argument.

GM sold 4m of these cars and seems to have gone to some trouble to ensure commonality of parts with other GM cars. The fact that the model ran for ten years without major changes suggests the design was pretty sound.

When I bought the Camaro I was surprised about the steering box, but the fact that it's rare to do a rack and pinion conversion suggests it works ok.

My guess is that GM got a deal on rear axles that were a fraction too narrow, and then compensated for this with deeper dish wheels.

TTOP350

iTrader: (8)

But the deep dish wheels go on the front..

petrolhead

So what is the best fix for all this. I know these cars are primitive is design, but they are fun to drive. I've read about scrub, bump steer, lca's and all that, whats the best bang for the buck to make them better? That's what I'm all about!

dlipka

I think that there is a much simpler reason for the differences in the wheels. When they went to 16" wheels, they also went to wider wheels. Front wheels need to be equally balanced around the steering pivot point. In other words, on each front wheel the amount of tread on the inside of the pivot point must be equal to the amount of tread on the outside.

If the front wheels are not set up this way, then braking on surfaces that have unequal grip (such as having one wheel on the road and one wheel on the gravel) will cause the steering to turn in the direction towards the biased side with more grip.

Therefore, GM would have set the 'new', wider, 16" front wheels up in this fashion. After doing this, they probably realised that there was insufficient guard clearance to run wheels with the same offset at the rear without fouling the body. Therefore they needed to move the wheels inboard to prevent body interference. Hence the differnet offsets.

sofakingdom

iTrader: (1)

Exactly...

They're the "normal" ones. They have ZERO offset, like older wheels.

The "weird" ones are on the rear.

See my signature for help when observing an unexplained phenomenon, and you're coming up with various proposed "explanations", and you want to eliminate the ones that don't account for the situation while zeroing in on ones that do.

Which in this case, pretty much eliminates ANYTHING "technical" about steering or suspension design, as well as "aesthetics". Leaving only...

Fireblood83

That's right! 16'' wheels were used for the first time on the 1984 15th Anniversary Trans Am and then on the 1985 IROC-Z

nosajwols

iTrader: (1)

THIS!

The rears are the oddballs not the fronts! The fronts will work on all four corners and have zero offset. When on the back (the fronts) they widen the track a bit in the rear but still fit in the fenders.

When using GTA rims the dish is noticeably deeper on the front and shallower on the rear, this actually looks really odd (looks like the rears have a smaller rim). Looks better with fronts on all four. IROC rims hide the offset better. So looks are not why (the way they did it looks stupid)!

Cost wise it complicates assembly and adds cost to have different rims front to back, specially considering the tire is exactly the same. This is one reason why almost every car that has the same size tire front and rear has the same size rim! They would not add cost and assembly complexity just for ***** and giggles. So cost of the rims is out (the way they did it is MORE expensive)!

This leaves only three options:

1) It was done for handling or driveability reasons (grove track, cornering, parking lots, etc.). Oddball offset was cheaper than a new narrower rearend.

2) It was done to fit on the assembly line--rear track width (still cost savings but it would have to offset the extra cost of the rims and complexity of assembly noted above). Again, cheaper than a new narrow rear.

3) Maybe under testing under extreme rear suspension compression the standard offset rim's tire touched the fender lips--due to the panhard bar rear suspension (when going to the 245s). The odd rear offset moved the tires in a little. Again, cheaper than a new narrower rear...

InfernalVortex

iTrader: (4)

Can we assume 0 offset is "normal" for 16x8 wheels? Nothing else ran 16x8 wheels back then. The Corvette and the F-bodies were the first mass produced cars to use 16" wheels I think?

TEDSgrad

iTrader: (4)

Good 'splain of da lip, dlip. You mean those new shiny 18" x 11 wheels are heavy and imbalanced? Who would'a thunk it!
Somebody 'round here preaches 17 x 8, and I agree (unless you want to get creative w/backspacing and lengthen the a-arms).

nosajwols

iTrader: (1)

Zero offset was the "norm" at one time for NA, 60s, 70s and rear drive 80s. When most makers went FWD in the 80s they started changing the offset to what we see today and eventually phased it into rear drive cars (like the fourth gen).... There were some excpetions in the 80s like the C4, etc.

Our rears are neither old rear dirve zero offset nor are they FWD offset. Our fronts are zero offset like typical NA rear drive cars of the vintage.

The rim diameter has nothing to do with the offset other than 16" and larger rims got popular at the same time the FWD offset got popular.

SlickTrackGod

Oh wow. I thought I explained everything well in my post. I guess people can not visualize the problems. I started by saying basically the same thing Sofa said many posts later-that is how the base. That is how the base platform originated. THEN, in 1985 when they sought better performance the createdthe offset wheels in the now wider track. Go re- read what iI wrote

SlickTrackGod

I'm on my phone, i all explain later in very simple layman's terms when I get to a computer

sofakingdom

iTrader: (1)

And it may very well all be perfectly true, accurate, and factual; I have no doubt.

But I DO have doubt, LOTS, that ANY of it will be related in ANY manner way shape form or fashion HOWEVER SLIGHT to why GM did what they did.

Oh BTW I screwed up up there somewhere; got the front and rear offsets confused, as to which were the "normal" and which were the "special" ones. I apologize, I'll try to proofread myself better in the future. Pretty sure the rears are std offset (zero, i.e. the flange plane is in the center of the rim) and the fronts are the "abby someone" ones, offset inwards.

Lonnie P

You have to realize, whatever the reason, GM went to a good deal of effort to control the rear track width. They were not as concerned with the front, other than clearing the tie rods.


Cars with aluminum rear drum brakes had no wheel spacers...
Cars with iron drums also had a rear spacer (approx. 1/8" thick) to compensate for the thinner drum material. They were worried about spacing down to this level.


Cars with 1LE brakes were wider in front, but the front wheels were the same.

Joe Tag

iTrader: (4)

Abby Normal....Are you saying that I put an abnormal brain into a seven and a half foot long, fifty-four inch wide GORILLA?


I don't know that many people can appreciate the Young Frankenstein reference Sofa.

sofakingdom

iTrader: (1)

Well I hope that those who "get it", enjoy it.

One of those movies that will be remembered for a long time.

SlickTrackGod

On the first day, God created Earth... Too far back?

60's...nothing really handled except the corvette

70's things progressed a little further, but still, nothing touched the Corvette

The G-body and the S10 truck share the same basic lower front suspension components. The cars were coming off an era that switched from Bias ply tires to radials. Cars were having still a little bit of tracking problems, and lots of suspension geometry problems in articulation. Proof in the aftermarket support programs of modern day has changed dramatically how the 60's and 79's modern touring cars now handle. Nothing is the same anymore. Geometry points have moved substantially and everyone has adopted the SLA front suspension (Short/Long Arm).

At that time, GM took a control arm and a steering platform that was already in place of the G-body and put it into the S10 truck AND the new generation 3rd gen Camaro and Firebird. The new platform is wider then the G-body or the S10, so the stretched the steering geometry in order to make it fit the wider chassis platform. This is the inherent problem we face with lack of Ackerman, Was it a problem? Not so much on a skinny 14x7 or 15x7 60 series radial tire of the time. More so was the learned problem of vehicle straight line tracking. The Vettes were already doing a wider rear track with much fatter rear tires-SO- hence some idiot engineer said lets put it all together with the rear just a tad wider (they made the housings purposely at 55.250") So as someone on here stated, they purposely fit the "narrow but stretched" front suspension with a little wider rear suspension track in order to hopefully reduce tread wander for straight line tracking. The car is tooted as a Performance car, but is not rivalling the Corvette so it is married with other conventional GM parts in the 1982 start of the 3rd gen.

Fast forward to 1985- they release the IROC camaro. The car now legitimately has shown in previous years to be a viable "platform" of track width vs wheelbase utilizing those wider front suspension pivot points- However the steering geometry still sucks for ackerman (to this day!!!, Might I add) Through learning the platforms weaknesses and strengths (mainly weaknesses) from 82-84, then in 85 placing fat IROC wheels onto the car during prototype testing before production, They realize they have an issue with several things. First being the steering effort and the cracks in the frame that develop under the steering box- they come up with the bandaid fix the "wonderbar", Secondly- the wheel to be a 0 offset will clear the tierod up front- Great!. Third- the wider scrub radius helps with parking lot manuveuring of the steering effrot while at a stand still- Great again, the little old ladies driving them will not complain), but 4th- did anyone notice how the car "climbs on the suspension geometry when the tires are at full lock trying to turn sharp? Yep, Ackerman went worse. Tell the customers "its a sports car, not your average grocery getter, it will happen with fat tires ma'am"...good enough for production.

Meanwhile, the car's front and rear roll centers have been out of wack from the get go, but this has not been a concern with a car on previosly with 7" wide tires... But NOW, it has Corvette Gatorbacks!!!! 8" wide ones...ooouuuhhhh. The result is the severely cantered roll axis, combined with a wider rear track makes for ...AND HERE"S THE KICKER I STATED ABOVE PEOPLE...a car that is already tight into snap oversteer into an EVEN WORSE ANIMAL. In order to get the car a little more responsive to turn in under performance braking, THEY HAD TO NARROW THE REAR TRACK using a +16mm offset wheel. The Gatorbacks did not have straight line tracking issues, they also pulled a very decent sustained lateral G on a skid pad by having the front and rear tires take the same exact footprint path with the outside tires. It reduced the push into snap oversteer, but ackerman sucked for tire wear issues if used spirited.

This effect today is further pronouced by the proven testing of ultra wide grippy r tires on cars like Pablo (who will be upset I mention him but I give a rats ***. I state facts, Ive seen his car in person and know for fact it scrubs the **** out of those wide front tires and wears them out much faster then the rears, He is practically full throttle shoving them through hairpin corners. Ackerman is that bad and gets worse the wider the front track is....as well as straight line braking wander. Hang on to the steering wheel folks.

And that is your history as to why the 3rd gen has different offsets. PERIOD. Goodnight

Dean

SlickTrackGod

Oh yeah, Proof of all this is when GM went to the 4th gen the retained the same rear end geometry COMPLETELY, yet married an entirely new developed front suspension platform with SLA and a Rack steering. The rear axle was widened to match the new more proper front suspension geometry that was now able to house a much better 17" factory wheel that had substantial positive offsets like the Vette.

peterc005

@SlickTrackGod - great post and the best explanation so far.