Rear end is hopping in my coupe??

In my black A coupe (one in the pic) The rear end is hopping at speeds above 60-65 MPH. It is a 9" on a 4-bar setup with a panhard bar. I can see the top of the passenger side tire when I look out the back window while driving. anywhere under 50MPH I see it going nice and smooth around. When I get on the highway, I feel a vibration, and when I look back I can see the passenger side wheel hopping up and down. Its pretty easy to see. Its almost looks as if it is out of round. I cant see what the driver side is doing.
rear tires are bias ply Coker L-78's. I dont know what could be causing this. I switched rear tires side to side, and it still did it. I got them both re-balanced, and they are fine. I changed the pressure from 32PSI to 22PSI.

What could be going on??
Could the shaft be bent?? springs, shocks bad?

My panhard bar is Short (16" or so), but its parallel to the ground, and as far as I can tell nothing is binding in the suspensions range of motion.

Any ideas??

Thanks, ROB

 

Bad shock wouldn't contain a poorly balanced wheel, so that's a side issue. Jack up the rear and put it on stands, then put a rigid indicator up close to the wheel to see if it is true. Do the same with the tire. If no issue there, have the wheel dynamicly balanced. If it needs more than an ounce or so to balance, break it down and see if there is a defect inside or if the tube is folded(if used). While the wheel is off, check the axle flange for true with the same method above.

 

If all is well at speeds below 60 mph, then I'd suspect weak shocks, or shocks mounted in the wrong locations.

Are your shocks in front, or behind the axle? Or staggered (one in front, one behind)? Are they mounted straight up and down or angled? Angled which way?

 

They are Carerra (sp?) coilovers, mounted behind the axle tipped inward at 15 dergees or so.

ROB

 

I hate to throw parts at this thing. Any other advice? Ideas?

 

Sounds like throwing shocks at it might help, then bolt on those new (good quality) shocks.

 

See if you can find a set of radials to throw on temporarily. Might help find the problem.

 

Did you try what Scottybaccus recommended?

I think he's right on the money and you'll find either a bent rim or out of round tire.

 

Could be a number of things, but usually if you have an out of round tire, you will feel it also at slow speeds. I would check them however by jacking up the car and putting something close to the rim flange of the wheel first to see if the wheel is out some first, then at the tread. It could be that they were not balanced very well too.

We see some rims that really need to be balanced on the car, or lug centric by a computer balancer. That means that they need to use the lug nut centering device to mount it on the balancing machine instead of the cones inside the center of the wheel.

Let us know what you find...

Best way is to contact customer service manager Jess Hoodenpyle at [email protected] if you cannot figure it out.

All the best,

Corky Coker

 

Rob, find a shop that has a road force machine. Spin balancing wont tell you if the tire is out of round.

A road force machine pushes a dyno wheel up against the tire while it spins it and checks for run out. It measures the pressure tire pushes on the dyno wheel, if it is egg shaped it will push everytime the high spot comes around.
it also measures the run out of the wheel.

If there is a slight imperfection in the tire, and a slight imperfection in the wheel, they can spin the tire on the wheel and cancel the two imperfections against each other and correct it, if it is way out then the tire would have to be replaced( or the wheel if it is the culprit). Corky mentions the hub and lug centric topic too, after you Road Force the tire that would be your next step. If you have a "Discount tire Co." in WI, they always have those machines. Most big tire shops have that technology now. Especially anyone dealing with 22's and 24's

I did it on my Buick and eliminated a shimmy at 65

 

I have a similar problem......only mine's on the front.....no speedo, but at @ 2200 rpm (@ 60) the front end will bounce side to side, kinda a rocking motion one side will bounce then the other.....the rougher the road the worse it gets, nice smooth pavement doesn't do it.....had tires rebalanced as well.....no difference.
I suspect the shocks....

 

---Rob,
Change your shocks from side to side and see if the problem follows. Also check the balance of your brake drums.
---Mark

 

prep-H?

 

I swapped out my coilovers with some new ones off another uncomplete project. No change. Im going to find one of these Road Force Machines, and get to the bottom of this.

ROB

 

Typical symptom for rear tire(s) out of balance, or you have one with a lump.

 

I had exactly the same thing happening with my frontend. I tried new shocks , less toe in,different driveshaft angle, you name it, until I had them rebalanced. It showed that the weights on the rims were perfectly balanced but looking at the rotating thread surface of the brandnew driver´s side front tire ( Coker 5.60-15) made obvious it was out of round about 1/4 ". Bought a new one and the problem was solved.