Technical AV8 is UNSTABLE Any ideas?

So let me start with a little history of this car. I built it in 99 got it on the road and got all the bugs out by 2000. It has always been dependable and fast. I've driven it to Bonneville and Vegas several times going speeds over 80 for long stretches comfortably. Last year driving up to Tahoe i hit some bumps on 80 east and the rear end felt loose slowed down made it home safe gave it an inspection found my rear panard bar bracket loose and thought that must have been it.. But it wasnt .. didn't improve Actually since last summer it has gotten more unstable as time goes on. Which leads me to think something is worn or has failed. With every inspection i find a small thing to fix but never THE thing.
The best way to describe the problem is if something sends the car up, when it comes down it's anybody's guess where she'll go.And with the condition of these freeways thats a scary deal. Lets cross some of the obvious off the list. Caster and Camber? good.. checked multiple times. Rearend location? Parallel to front i beam and centered in frame and no movement. Drag link? solid. Tie rod ends? good. I am considering king pin and bushing ware. But again when i shake the tires i dont see any movement there either. Tires? swapped all four and didnt change a thing am I forgetting anything obvious? Any help would greatly be appreciated...

 

Long shot. ... cracked frame. Flexing when under load. It will feel like the rear end has a mind of its own.

 

Have you inspected the frame itelf?

 

You didn't describe your front axle setup. Do you have a panhard bar to the frame on the front end?

 

Borrow a set of tires & wheels from another car and see how it drives with them on. I suspect you broke a belt on one or more tires when you hit whatever it was.

Come to the darkside and run radials...

 

I didn't see shocks listed in the "OBVIOUS".

 

Bad rear shock? would that affect my tracking? I've poked and pried on all cross members. nothing there, swapped all four tires again no help. dont see any cracks in frame..front panard is solid...

 

It could if one has gone bad completely.

 

A bad shock wont change the steering feel / direction. It will just keep bouncing. I would agree with a crack in the frame. You are describing a bump-steer situation.

Can you remove the springs so you can simulate the suspension travel - like while on jacks or something?

 

Okay well that's on the top of the list for Sat morning...stupid job. They pay me just enough to keep coming back on Monday

 

By the way unless your king pins are about fall out, you can not check by moving the tires. You need to jack it up and watch for movement. I agree with the tires and possible cracked frame. Gary

 

Well as it turns out there's a local parking lot with a large sway in it for drainage and when i cross it even at low speed 5mph it'll do it i just need an outside set of eyes to help

 

My money's on a broken bead in a tire. When you switched the tires around did you go front to rear on the same side? Sometimes it's not easy to see tire tread separation with the eyes. Let the air out of each tire and "read" the tread pattern. If that doesn't show it ...try over inflating... a bit and look for a distortion in the tread pattern. Don't blow the tire off of the rim though!! A major malfunction, like a cracked or broken component, would be easy to locate. I don't think that's what's happening here. It's the little stuff that's hard to find.

 

If you have one of those Go-Pro cameras, you could zip-tie it to the frame to record what the tires are doing.

Even tho this car in the video below is not hamb friendly, it give you an idea of the major forces involved during suspension movement

https://www.youtube.com/watch?v=FIFZMa1pyX0

 

I didn't switch the tires around. i put on an entirely different set of wheels and tires. Again no improvement.

 

Watched the youtube clip and it was eye opening. I'm running a solid rear axle but i'm sure those same laws apply.

 

Do you have a mixture of bias ply and radial tires?

 

What am I? A crazy person? hehe... No. Bias plys all the way around.

 

What is the supension setup on this car? I am having a hard time wrapping my head around your description of what the car is doing exactly.... "if something sends the car up, when it comes down it's anybody's guess where she'll go"...

Are you talking about when the suspension is traveling up, then down it gets squirrely?

You mentioned the rear end felt loose..... how about the shocks. If one is dampening and the other one isn't (both at same end of the car) it can make a car feel "loose" and can make it want to change directions on you. You also mentioned hitting some bumps just before it started this.. look at the shocks close... any bent? Pull them off and compress and extend them... do they work the same? A bent shock could be binding up and stopping in its travel on compression (down stroke) yet extend out fine on rebound stroke.

Leaf springs?? Look close at the leaves along their length.... may have a broken one causing one side to be weaker than the other.

Did you check toe in? Could have bent the tie rod tube changing that.

Need mo info !!

 

This is whats baffling me it all looks good. I've even had a couple of guys who know tons more than I do a walk around cant seem to locate it. I guess this weekend I'm gonna put her up on a lift. check the frame for any cracks check the front leaf spring pack and the shocks as well. Thanks for all the input. I do appreciate it. Hopefully I'll find something. Keep you posted

 

Pull the shocks out and bench test them.

 

check your leafs in your spring pack...my $.02... are you running friction/ oil shocks? if you busted one of those just right it could be like having a solid rod between your frame and axle...you wouldn't have to pull them to know, just go push down on your rear quarters.

 

I once had a '36 pickup with bad shocks that would swap ends on a rough road. If shocks weren't important, Detroit wouldn't put them on cars in the first place.

 

Steering box mount cracked or something allowing flex? And another vote for checking the toe in.

In the UK as part of the annual inspection (pre 60 cars became exempt a couple of years ago tho) someone sits in the drivers seat and 'rocks' the steering wheel while the examiner is underneath inspecting all the components and mounts for play / flex etc. ie it's a two man job to do the inspection. So that's a second use of the fairer ***. The first? Operating the pedal while brake bleeding

Chris

 

Maybe broken eye bolt.

 

Bump when it occured could have been just a coincidence. What are you running for a center carrier. If its a Posi or limited slip could be you've got a clutch/spring engagement issue.

 

Possibly broken spring centering bolt , allowing small amount of movement of rear axle.?

 

I snapped the end clean off a rear shock on a highway run one day, hit a big bulge in the concrete. Next thing I know the rear driver's side of the car is bouncing around like a ball every time I hit a small bump.

Up she'd go, way up, the down, and bounce, bounce, bounce, we went, down the highway. Kids in the back seat thought it was funny, I was fighting to keep the car on the highway, it was going all over the place! Every time the rear went up, the front on the opposite side would nose dive, bump steering the car all over the road. (4 wheel independent suspension)

I had to slow right down and we crawled into a service station where I unbolted the broken shock and got the mechanic to weld the end back on. Once that was fixed, she was back to her usual self on the road.

Good luck with the fix,

Glen.

 

are we talking about traditional cross spring front and rear?If so, after you have tried everything else,remove[or at least disconnect 1 end]of your front and rear panhard bars.If it does'nt cure it,hook them back up,your not out much time.If it does work let us know and I'll explain why,but I'm too tired today to spend hours explaining why it can't possibly be the problem.try it

 

When you check that look at the hair pin mounts at the frame, and all the spring shackles

Martyn