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The Tail Moves Violently Side to Side Above 100 MPH

Posted: Tue Sep 14, 2021 12:05 am
by LS14gezr
‘71 Nova, back half ladder bar, Koni single adjustable 12 bolt, 10.50 big tire car. Old race car I’m bringing back. Nicely done in it’s day but needs attention now. It just started recently to vigorously shake the tail side to side. Not up and down. Balanced the tires. Then I found the forward rod end of the diagonal bar to not have any torque on the thru bolt and the rear thru bolt to have about 30 ft/lbs. Tightened those two and checked the others. Figured that was it but no. No change.

It’s been suggested to me that I may have a shock going bad. That might explain the uncontrolled side to side motion at speed. Have any of you experienced a shock go bad and how did the car react?

I’m considering just replacing all the rod ends in the rear suspension but I’d like to know WHY. Besides old age that is. How do you properly inspect rod ends?

No vibration in the steering wheel. The car tracks straight until about 900’ and then the side to side sets in. As the speed continues to increase the more pronounced the side to side movement becomes until it becomes a question of safety for myself and my competitor.

I would welcome anyone opinions getting me started in the right direction.
Thanks in advance.

Re: The Tail Moves Violently Side to Side Above 100 MPH

Posted: Tue Sep 14, 2021 4:24 pm
by miniv8
How is the panhard bar set up?
I'd pull the shocks out and see if everything is moving freely and not binding because of some geometry error.
Check the welds on the axle tubes going into the 12Bolt.

Re: The Tail Moves Violently Side to Side Above 100 MPH

Posted: Tue Sep 14, 2021 11:14 pm
by LS14gezr
miniv8 wrote: Tue Sep 14, 2021 4:24 pm How is the panhard bar set up?
I'd pull the shocks out and see if everything is moving freely and not binding because of some geometry error.
Check the welds on the axle tubes going into the 12Bolt.
No panhard bar. Diagonal link.
That’s a good suggestion, checking the axle tube welds. Will do!

Question to anybody - What are the symptoms of a worn rod end? How to test for it?

Re: The Tail Moves Violently Side to Side Above 100 MPH

Posted: Thu Sep 16, 2021 9:31 am
by lefty o
radial tire in front, and bias ply in back?

Re: The Tail Moves Violently Side to Side Above 100 MPH

Posted: Fri Sep 17, 2021 9:40 pm
by LS14gezr
No. Bias front, radial rear, but the car was sashaying with bias slicks too. I’m going to the track next weekend. I’ll update then.

Re: The Tail Moves Violently Side to Side Above 100 MPH

Posted: Sat Sep 18, 2021 7:29 pm
by dannobee
Get under there and put your hands on every component and really try to shake everything. Rod ends usually clunk around before they really get worn, but check anyway.

Are you sure that you're working on the right end of the car? Loose front suspension components can do some wicked stuff too. And check the front toe to make sure it's not toed out. Have you ever checked/adjusted the front bump steer?

Re: The Tail Moves Violently Side to Side Above 100 MPH

Posted: Mon Sep 20, 2021 7:34 am
by Rick!
Mount a GoPro underneath the rear with a view of all the suspension components and make a run.
It should take one run to see what's going on.
How old are the slicks? What did they look like on the balancer, significantly out of round? Flat spotted from sitting?
If they were just bubble balanced, then you still can't discount them as not being a contributor to the issue.
From your description, you may have a lateral stiffness issue that is getting excited. Since the tires and wheels are furthest from the center of the car, the lateral forces they can impart to the car will be magnified by the time they travel through the suspension to the frame.
Have you just jacked up the car, set the axle on jackstands and spun it up with somebody watching each rear wheel?
Maybe find a racing buddy with a spare set of slicks with wheels that fit your car and make a pass.