Morning. Alignment question: My 73' is pulling to the right and i had the alignment checked..The front end needs realignment..no problem..the mechanic said the front alignment must be performed in conjunction with the rear alignment...again..no prob..but, when he checked the rear tires range of motion he was able to make major camber changes without loosening a single bolt. He said he couldn't fix it, and I need to take it to a corvette specialist. Any guess at what could be the problem?

 

Rear end camber is adjusted by the struts - either by adjustable struts or non-adjustable struts (by bending).
Rear end toe in/out is adjusted by adding or removing shims at the trailing arms. Sounds like your mechanic didn't want to tackle the nasty job of adding/removing shims.

 

The stub axels could be missing their c-clips and are free to move out as much as they want, pinion shaft could be worn, strut rod cams loose. You'll have to get under there yourself to see what's moving but I think you'll see the stub axels moving in & out alot.

Warning: Highly controversial material follows.... The half shafts act as the other strut rods for maintaining camber. In other words, you can have solid strut rods and still have a major camber problem if the c-clips are missing.

All the strut rods are adjustable, the factory ones are mounted to cam bolts at the inboard end, if you have a bent one replace it.

 

What do the strut rod bushings look like? Might be time for new bushings.

 

In the interests of being accurate- the camber won't change if the c-clips are removed or fall off. The camber will change only if the yokes wear.

 

In the interest of being even more accurate, and at the risk of long drawn out debates. missing clips will allow the camber to change.

What your front end guy did, and what all front end and alignment shops do, is check for looseness in all the suspension components before attempting to align it.

So he unloaded the frt suspension and checked ball joints by manually manipulating the wheel to look for looseness. Looseness in the ball joints manifests itself as camber change. The top and/or bottom of the wheel able to move in or out. He did the same on the back and found that by hand he could move the top of the wheel in or out. This is uncontrolled camber. He further found it to be in a component outside his ability to fix, meaning it likely wasn't a bushing, but rather a wheel bearing or an upper control arm (half shaft) issue.

There is a Yutube video posted here that illustrates on a race course at speed a wheel without circlips coming out so far it over centres and flops to the outside, but in that same video you can see the camber of the wheel changing almost constantly until those 2 times it moves out far enough to flop out. The constant change is what your front end guy is referring to. He is noting an ability for the camber to change on it's own and as such there is no point in trying to adjust it to any spec.

The two most likely causes are worn yokes on the diff or loose wheel bearings.

Steve g

 

Before you can properly set alignment on ANY car, the condition of the moving parts in said car's suspension must be such that it will hold an alignment. If your alignment tech told you that you should see a 'Corvette Specialist' he was likely indicating that your rear wheel bearings were worn out. as Steve and others also mentioned, the inner yokes of the rear half-shafts ('axles' in layman's terms) are held in lateral position by c-clips that keep them from moving around inside the differential. If these are missing or otherwise compromised the rear wheels can then move in and out around an arc whose center is the outer strut rod mount. You can easily check the bearings yourself by jacking up the rear of the car, grabbing each rear tire at 12 o'clock and 6 o'clock, and jiggling the tire. If you are getting more than about 1/8" of movement the bearings need replacing. If you unbolt the strut rods at either end and can move the wheel in/out any noticeable amount you have some kind of issue with the aforementioned c-clips and the differential case will have to be opened up to see and eventually correct the problem. Either one will be time-consuming and expensive to correct...how expensive will be determined by how much ability you have to do the work yourself. Welcome to the wild, wacky, wonderful world of C3 ownership, and Happy New Year!!!

 

Alot of shops won't touch rear alignment because either it's too much work for them or they don't know what to do. In addition, if you have original TA bushings and shims in the car, forget about it. The bushings will be worn out and have too much slop and the bolts/nuts holding the shims won't be able to be loosened. The only way to fix it is to take out the TA's, put new bushings in and replace the shims with S/S ones - and load up the TA bolts with anti-seeze.

 

NEVER bend a strut rod to adjust camber. The stock strut rods are adjusted via cam bolts at the inboard ends.

 

Do you have a link or the keywords to search for? I'd like to see this. I've known guys who ran without the clips and never had an issue.

 

Bending the camber struts instead of adjusting them properly is a Bubba speciality.

As for running without the clips...

 

Whoa..this is excellent info..I'll take a look this weekend and get back to you. Thanks to all!!!

 

He has a lot of problmes under there. I don't think the severe camber change resulted from the c-clip, as they say it broke the c-clip. Those c-clips were never intented to hold under a lot of loading.

First, the tire wanders all over, which makes me wonder how much air he has in it and what type it is.

Second, part of the holding of the suspension has been defeated by the setup. The camber rods are Heim spherical joints and offer no resistance to the trailing arm moving. The stock bushings act as a spring to keep a certain position and the loading of the rubber bushing resists the arm moving away from the camber angle it is set at.

From the way it flops in there, i bet either he has spherical joints at the front TA mounting or the bushings up front are shot.

Third, he is going far beyond what is considered normal if he is pulling over 1 G of lateral acceleration. Normal side loading was considered to be .78 G so to exceed it this much means he needs more work to control it.


I'd still like to see the video where the stub axle actually comes out of the housing. That is a long way to move a suspension and I find that hard to believe, unless they have it sitting in Heim joints and didn't take adequate measures to keep all under control.

 

Here we go again.

The heim joint has no give in it asnd as such asllows less camber change, not more.

Watch the top of the tire in the times other than when the wheel flips out. It is moving constantly. Low air pressure would make the bottom move around relative to the road surface, but the top is what's moving. It is because the yoke is moving out of the diff. The half shaft is the upper control arm.

Absolutely the clip was intended to hold that load. Someone on here posted an engineering chart which showed the load a clip of that size will carry.

The yoke moves out as far as it ca on the two instances that it gets so fat the wheel reaches an overcentre point and flips right out.

The clip may have broken the first time, but it was broken or missing the second time.

If you research this issue on the forum you will come across the GM documentation that describes the suspension design and how the lip was intended to hold the wheel in place.

Steve g

 

Thank you Steve. There having been so much misunderstanding and disinformation about, and denial of this being an issue, despite overwhelming evidence IMCO, I'm not sure I'd have gone to the trouble myself...

 

Don't look at the tire, look at the trailing arm (or rear sway bar arm) and the inner u-joint. This is where you see it move. The tire distortion at speed can and will continue all around the rim, not just at the lower point where it contacts the ground.

As to the Heim joint, if all points of control were positive, yes, it will hold better and more solid. However, in the event of a failure, you will have only two points holding (front of TA and the camber strut rod) and it will flop around like a well-oiled hinge.

The original bushings were engineered to support the rear in the event of a u-joint failure. It won't hold well, but it will hold somewhat and that is enough to safely stop the car. This resistance also helps contain camber changes.

With spherical joints, if the u-joint breaks, the tire will just plain lay over until it hits something. There is no resistance and there is complete failure. This is what is happening. When the third link (halfshaft, stub, clip) is broken, there is nothing to resist the camber changes at all.

I don't have to research the forum as I have seen enough data on the design of the rear to know that first, this application is far beyond the design limits. This is something most forget. When the IRS was initially conceived for Corvette, the typical IRS of the time used either an upper and lower a-arm and sometimes a light locating arm forward or the suspension used something like a heavy locating (trailing) arm and two lighter links across as upper and lower.

The issue with either is the geometry to keep all links from binding the axle when moving is hard to manufacture and keep in alignment and still allow aligning of the rear end when needed. As such, most used either a type of slider so the splines slid in and out or they used the huge rubber donuts like with Lotus and some Hewland transaxles.

The problem for Corvette was these solutions didn't work with the torque and horsepower that Corvettes were capable of and where the designers envisioned them going. They were too light of duty.

However, the use of the halfshaft and stub as the upper link was a workable solution given the parameters. Remember, this was designed for a car making 0.80 G max and using a tire patch that was roughly 5 inches wide and 4 inches long. Odds are the tire would simply break away before exceeding the design limits. This was a much heavier duty design. Still, it is not unlimited.

While it seems not that big of a deal, a 3300 lb car will generate 2640 lbs of lateral force at 0.80 G and 3300 lbs at 1.0 G so there is about 660 lbs of force difference.

Because all of this is far beyond what this setup was designed for, guys like John and Bert Greenwood designed their own 5 link rear and began using it in the early '70s. In the late '60s, Chevrolet had redesigned the IRS for all its mid-engine design prototypes as they were envisioning much higher G forces than the 1963 design could take. In the late '70s, Chevrolet designed their own 5 link to replace the IRS in the upcoming C4. This is one reason they were about to brag about 1.0 G with the 1984 Corvette. You could achieve 1 G in a C3 with the right tires, however, it would either distort (rubber bushings flexing past their limits) or break (like this one). The 5 link was designed to take 1 G without breaking.

Under a stock rear and with the stock parameters, you do not need that clip in most driving. However, the car in the video has a modified rear suspension that generates much more stress on the clip and all components and has much better tires than designed for and so you get issues you wouldn't normally get.


Still, the stub never flopped out of the rear end. It pulled out a long way, but never came out.

 

I agree, that tire is wandering all over the place, the car must be a "handful", to say the least. FWIW, the tire looks like a "drag radial" to me.....

 

This is a key point. The car was exceeding 1G side load vs street driving which is limited to around .78. I've watched the video so many times my eyes are crossed, but I can only see possibly three very short intervals where the yoke moves away from the diff. It is true that the tire does distort throughout the video but that's not related only to yoke movement.

 

I have never read any engineering documentation about the purported issue of slip joint binding as being the driving force behind the design other than through forum discussion. I am somewhat skeptical for a number of reasons. Slip joints are used in drivelines to this day of considerably more torque without binding. If that were to happen on a driveshaft in a conventional rear wheel drive vehicle the suspension locks up solid. Large trucks put upwards of 2000 lb ft through slip joints without issue. While torque is multiplied through the diff it is also divided in two to go to the wheels resulting in an only moderate increase over driveshaft load. Additionally, previous discussions about this very thing suggests that the solution would have been the ball and trunion joints used on frt wheel drive cars, suggesting that was a later development. They were around for years before the IRS came into production and were used in production oldsmobiles in 1966.

My contention is that the design is the result of a cost issue rather than a engineering limitation. Fewer parts and faster assembly trumps the best engineering solutions most of the time. Look at the McPherson strut suspension used in most cars today. A modern technology of vastly inferior function used everywhere solely because of cost.

The GM documentation states that the design puts 19% of potential load on the outward movement of the shaft. The potential load is not a negative number and as such GM had anticipated that under normal circumstances there would be an outward load of the shaft. Obviously in order for there to be an outward load of up to 19% of potential load there had to be 0 loading in the inward direction. Any load on the positive side moves the yoke off the pin. And this seems to be where everyone gets hung up. It doesn't matter if there is only 1 lb of force in an outward direction, that is enough to pull the yoke off the pin and thereby change the camber.

There is much about the no clip stationary yoke theory that remains unexplained or even illogical. I don't believe that the yoke sits solidly against the pin until there is a sufficient force to move it all the way to it's extreme outward limit of travel to the point where the spring force on the wheel reaches an over centre point and flops to the outside without ever moving a lesser amount in other circumstances. That seems illogical to me. Secondly, if you believe that the yoke sits against the pin except under extreme conditions without camber changing then you would also have to believe that the condition of the bushings in the strut rod is unimportant as well. In order for there to be a constant inward load of the half shaft there would have to be an equally constant outward load on the strut rod. A bushing with the rubber missing and a quarter inch of play would therefore have the same effect as the quarter inch of slop in the yoke, none. The looseness in the bushing would simply be taken up by the outward force and wouldn't move. Why does everyone unanimously agree that strut bushings should be checked when there is instability in the rear? If the upper shaft can't move out the lower can't move in either (load not changing direction being the theory). The people that believe camber won't be fluctuating with a loose yoke must also believe that loose bushings in the strut rod will also not result in camber fluctuation and therefore can not be the cause of instability in the rear.

When looking at the video keep in mind that the yoke movement is at the centre line of the wheel and as such the small amount of movement is amplified when viewed at the outer circumference of the wheel. Also the trailing arm looseness will cause movement at the frt and rear of the wheel, not the top and bottom. It does not provide any anchor of vertical movement. If it did you could not change camber without the use of wedge shaped shims in the ta.

Steve g

 

When I pull my RE apart, I notice one of the strut rods were bent. I figured WTF I have only 300 hp. Then I read of clown mechanics bending them for adjustment. I ended up putting VBP heym HD rods on and haven't turned back yet.