High speed Instability with bushing type change

[TUF-427]

Hi ,
I have a cobra replica which is fitted with a triangulated link live axle 9in rear similar to a LH-Lx style torana factory setup.
A few differences are the lower trailing arm is a lot shorter at 370mm ,
the top arms are 275 mm at approx 45 deg I think.
I used to run the factory rubber style press in bushes in boxed factory style arms with varying success,as it would push the bushes out of the arms,so I would tack them in and it would push the rubber out of the bush sleeve!
I also tried various urathane bushes with limited sucess also.
I use the car for sprint days at QR etc and also a bit of drag racing.
I fabricated tubular upper and lower trailing arms to take 2 piece (shackle style 75 - 90 duro approx ) urathane bushes and only have a HD stock style rubber bushes in the diff eyelets.
The upper arms are adjustable.
The Car is extremely unstable in the rear now at high speed and hard acelleration and is hard to keep in a straight line above 140ish,with any twitch of the steering wheel inducing rear float.
The front end(Uc torana) is reasonably direct with good turn in ,was stable before the bush/arm change..
Springs are 600lb front 190 Rear,Koni orange all round,27 mm swaybar front only.
Tyres run at the track are Proxes RA 1

Any help appreciated as Im considering chucking all the old stuff back in.....

Peter..

[Wojtek]

Hi Peter,
A clue may be the length of your lower trailing arms, and the fact that your bushes are sliding out, indicating excessive lateral loading on the bushes. Applying various designs of bushes does not take away the fact that there is an inherent problem with the suspension design as in the form of a four link live axle with the upper arms positioned at an angle sharing both lateral and longitudinal loads. I would also suggest that you check the chassis pick-up points for (anti)squat geometry. A good way to learn about the suspension geometry is to build a small model - paddle pop sticks are good for trailing arms.

Wojtek.