Cure for rear end wiggles please

bcal · October 04, 2004, 07:14:54 AM

Hi all,
I've got a 39 Chev sports coupe ( Known in Aus as a sloper) and she suffers from the rear end wiggles. http://members.optusnet.com.au/bcalv/sloper/pics
It has a live axle on semi elliptic springs and the spring mounts fore and aft are at different heights with the front being 3 inches lower than the rear.
I believe that this can lead to rear end bump steer due to the axle moving forward on one side and rearward on the other. I reckon this is what's happening to my car.
The problem is how best to fix it. I could use longer shackles but 3 inches longer that what I already have (keep it clean

) would look 70's ridiculous.
Another idea I have is to reverse the rear eyes, which should be good for about 2 inches, and then lengthening the shackles just 1 inch. Would this work :?
Are there any other ways of eliminating this problem?

enjenjo · October 04, 2004, 09:31:07 AM

This shouldn't be a problem, this is in the range of what the factories are using. It's true the spring eye height can effect steering, but generally only when cornering. I've used spring eye height to fine tune cornering, but it doesn't have a real dramatic effect. The way your springs are set up causes oversteer, which is usually a good thing.

If your shackles go up from the spring, you can move the mount to below the spring, changing the effective mounting point without changing the height of the car.

I'd really be looking at something else, tires or spring bushings or similar causes for your problem. I did have a Chevy that did this, turned out the U bolts were loose, not much, but enough.

Bob Paulin · October 04, 2004, 02:46:13 PM

I'm going to ask a loaded question.....

You don't have a mix of bias and radial ply t(i/y)res, do you?

I once bought a '77 El Camino with radials in the front and bias in the back that was a handful to keep going straight.

I put a pair of radials on the rear, and it was a totally different car.

Sometimes, to get the "Bigs 'n' Littles" look, people will mix tire designs. It often results in a skittish car.

B.P.

unklian · October 04, 2004, 04:06:55 PM

Assuming the axle hasn't come loose from the springs,
it sounds like bushings,or tires,like posted above.

You probably have to completely remove the leafs from the car
to really inspect the bushings properly.

Also check the mounting points for cracks.

jaybee · October 04, 2004, 08:45:28 PM

Quote from: "Bob Paulin"I'm going to ask a loaded question.....

You don't have a mix of bias and radial ply t(i/y)res, do you?

I once bought a '77 El Camino with radials in the front and bias in the back that was a handful to keep going straight.

I put a pair of radials on the rear, and it was a totally different car.

Sometimes, to get the "Bigs 'n' Littles" look, people will mix tire designs. It often results in a skittish car.

B.P.

I did the same thing once. At the time I was reading "don't mix them on the same axle." It was OK up to a point, but when you started to lean on it the thing was REALLY SQUIRRELLY!

old32 · October 04, 2004, 08:48:11 PM

Hi bcal,

What hight lowering blocks are in the rear? Maybe try to get rid of them might help.

bcal · October 05, 2004, 03:03:52 AM

Thanx fellas.
I read the thing about the front and rear bushes being at different heights causes bump steer in Fred Puhn's How to Make Your Car Handle. He also talks about the roll steer thing as well.

Today I had a look at quite a few cars with leaf springs and found that most had the rear shackle mounted much higher than the front. A case in point is my Isuzu ute and it handles the bumps ok.
I'll replace the shackle rubbers with urethane and see how that goes.

Tyres are TA radials all round. 8" on the front and 11" on the back.
The wide tyres on the back combined with a rather wide track and shortish wheelbase probably don't help none.

Roy, It doesn't have lowering blocks but the diff bracket is rather tall. I not sure if lowering blocks can cause these problems.

Thanx again

Carps · October 05, 2004, 04:42:54 AM

Quote from: "bcal"Thanx fellas.Tyres are TA radials all round. 8" on the front and 11" on the back. The wide tyres on the back combined with a rather wide track and shortish wheelbase probably don't help none.

Moreso than the shackles ever will. Some of the problem could also be the difference between front and rear track width.

A panhard rod might be a big help here. Could also be your radial tyres. The difference between seting up for bias ply and radial ply tyres is uasally quite different and if not right can cause the car to wander when using radial tyres. Those here who have a late fifties or early sixties land yacht with radials on the original suspension will know what i mean.

bcal · October 05, 2004, 06:49:07 AM

Quote from: "Carps"
Moreso than the shackles ever will. Some of the problem could also be the difference between front and rear track width.

There is a difference of 4 inches between the front and rear track widths. This blows out to 6 inches if you measure from the outer edge of the tread. Not ideal but looks nice

QuoteA panhard rod might be a big help here.

Can you use one of these on leaf springs? Dunno but I think it would cause binding. Watts link may work?

QuoteCould also be your radial tyres. The difference between seting up for bias ply and radial ply tyres is uasally quite different and if not right can cause the car to wander when using radial tyres. Those here who have a late fifties or early sixties land yacht with radials on the original suspension will know what i mean.

She tracks quite nicely at highway speeds. Hands off the wheel and dead straight no worries. It's just when you hit a bump that things happen. Particularly hollows. The front goes over no probs but the back end gives a bit of a sideways kick. In the direction of the bump I think.
It's not real bad, nothing dangerous like but if I could at least reduce it then that would be nice.
Thanx Carps

tomslik · October 05, 2004, 07:31:39 AM

Quote from: "bcal"
Quote from: "Carps"
Moreso than the shackles ever will. Some of the problem could also be the difference between front and rear track width.
There is a difference of 4 inches between the front and rear track widths. This blows out to 6 inches if you measure from the outer edge of the tread. Not ideal but looks nice

QuoteA panhard rod might be a big help here.
Can you use one of these on leaf springs? Dunno but I think it would cause binding. Watts link may work?
QuoteCould also be your radial tyres. The difference between seting up for bias ply and radial ply tyres is uasally quite different and if not right can cause the car to wander when using radial tyres. Those here who have a late fifties or early sixties land yacht with radials on the original suspension will know what i mean.
She tracks quite nicely at highway speeds. Hands off the wheel and dead straight no worries. It's just when you hit a bump that things happen. Particularly hollows. The front goes over no probs but the back end gives a bit of a sideways kick. In the direction of the bump I think.
It's not real bad, nothing dangerous like but if I could at least reduce it then that would be nice.
Thanx Carps

"Can you use one of these on leaf springs? Dunno but I think it would cause binding. Watts link may work?"

yeah, GM does on the late model S-10's

carnut1100 · October 05, 2004, 09:51:23 AM

we had an old Holden ute that used to wander in the back when it hit bumps or when it was overloaded ( not that I would ever overload a ute of course

) and it turned out to be loose U bolts. We found it when we replaced the diff and ti was heaps better afterwards with them tightened down. It wouldn't move if you kicked at it, but it would under load.