Got the car Dyno tuned yesterday and was finally able to do some hard pulls on the road. At about 50- 60mph under full throttle the car develops a SEVERE side to side wobble from the rear. It is an open G96-01 transaxle. I checked the rear toe and its about 1/64 toe in. Everything is tight and show no signs of wear. The car has 900 miles on it. Has anyone else experienced this? Rear bump steer on hard accel? Suggestions?
Severe rear end wobble on hard accel
- Thread starter Godspeed
- Start date
P.S. Its not a vibration like a bad axle,tire
etc. The car physically jumps side to side
etc. The car physically jumps side to side
It only happens on hard accel. It only happened once or twice before it was tuned because I was breaking in the engine and didn’t use full throttle.
Chris Kouba
Supporter
Put the rear end in the air and check your hubs. If there was weight on them without the axle stubs installed and properly torqued, there is a possibility they are shot. I forget exactly what the mechanism is but it's was the same on a FWD car I had. I had an issue on my GT where the rear end started wiggling under acceleration. I slowly limped it home and the right rear hub was shot.
After that, I'd do what Jac is recommending- clearly something is likely to be loose. If that's fruitless, find a cheap Go Pro knock off and get some footage of your wheelwells under braking and acceleration to see what's happening.
After that, I'd do what Jac is recommending- clearly something is likely to be loose. If that's fruitless, find a cheap Go Pro knock off and get some footage of your wheelwells under braking and acceleration to see what's happening.
Along with what has been shared above - Open differentials can misbehave under high horsepower applications. You may be experiencing torque being transferred unequally between left & right sides under both acceleration and deceleration. Tire inflation rates and traction / surface conditions will also be in play here…
Ian Anderson
Lifetime Supporter
I would start really simple first.
what tyre pressure? And same side to side?
Then put a tape measure around your tyres, are they the same size When inflated?
And check there is nothing stuck to the tread to cause it to loose traction…Labels, gaffer tape etc.
Ian
what tyre pressure? And same side to side?
Then put a tape measure around your tyres, are they the same size When inflated?
And check there is nothing stuck to the tread to cause it to loose traction…Labels, gaffer tape etc.
Ian
Everything is tight with no wear on any heim or hub. Checked tires 35 psi tread clean and within 1/8 inch circumference. Alignment is good. Leaning towards what Randy said. It feels like it’s clawing for traction side to side, but there is no noticeable wheel spin. Talking about surfaces I believe it’s only happened on concrete. I’ll have to do some more road research
Ian Anderson
Lifetime Supporter
35psi sounds high.
I used to run 22 front 26 rear.
Ian
I used to run 22 front 26 rear.
Ian
I’ll drop the pressures and try it.
Howard Jones
Supporter
First, I tried to see if you have mentioned it but I didn't see whether this is a GT40 or SLC. Second, 35 psi is way too high. I run my SLC at 26 cold going to 29-30 hot and my GT40 at 27 cold and going to about 28 street driven warm. 35 will be very nervous and bouncy with balance jumping back and forth from push to oversteer at will as well as very little overall grip. You are basically running the car on the center 1/3 of the tires in a straight line and then as the car loads up on a turn the light side REALLY has no grip as the loaded side improves somewhat momentarily.
This feels like really twitchy bad weird-ass shit is going on. You should be wondering which way the car will go next.
Try 28 all-around cold and drive it for 30 mins or so at legal highway speed then recheck. Do this on a moderate ambient temp day 70-80F. Reset to 28. I wouldn't run more than 30 hot on the street.
Also, run street camber settings on a street-driven car. 1/4 to 1/2 degree neg max. I run my GT40 at neg 3/4 and it wears out its street-type tires. That was from when it was used on the track. When it gets new tires I am going to 1/4 neg around.
Third, how much max power do you have and on what tires?
An important note: If you have an SLC and you are running a lot of power (more than 550hp) and good tires like Michelin sports then IMHO you are at the limit of the original C4 rear bearing IF you drive it hard often. Read the SLC rear hub thread elsewhere. I don't want to alarm you but have a VERY close look at the rear hubs if they are the C4 hubs. Not only the bearing run out and tightness but the hub housing itself and stub axle. Hub for cracks and stub axle for twist or bent. If you have the new type C7 (they are plenty strong) then make sure the big ass nut is tight. I torque mine to 200 PSI. There is a lot of variation in opinion on this torque number but my car has had no problems with the C7's @ 200 with many track days and about 1500 total track miles on slicks. Be sure to retorque after the first drive!!!!
Last thought. Are you sure you have an open diff? A warn church-type LSD can act really weird. If you do indeed have an open diff then you need an LSD IMHO. I personally really advocate for a Quaife-type TBD. Both my cars have them and they are magic. Both rear tires do EXACTLY the same thing no matter what you do with your right foot. Really friggin magic!!!!
This feels like really twitchy bad weird-ass shit is going on. You should be wondering which way the car will go next.
Try 28 all-around cold and drive it for 30 mins or so at legal highway speed then recheck. Do this on a moderate ambient temp day 70-80F. Reset to 28. I wouldn't run more than 30 hot on the street.
Also, run street camber settings on a street-driven car. 1/4 to 1/2 degree neg max. I run my GT40 at neg 3/4 and it wears out its street-type tires. That was from when it was used on the track. When it gets new tires I am going to 1/4 neg around.
Third, how much max power do you have and on what tires?
An important note: If you have an SLC and you are running a lot of power (more than 550hp) and good tires like Michelin sports then IMHO you are at the limit of the original C4 rear bearing IF you drive it hard often. Read the SLC rear hub thread elsewhere. I don't want to alarm you but have a VERY close look at the rear hubs if they are the C4 hubs. Not only the bearing run out and tightness but the hub housing itself and stub axle. Hub for cracks and stub axle for twist or bent. If you have the new type C7 (they are plenty strong) then make sure the big ass nut is tight. I torque mine to 200 PSI. There is a lot of variation in opinion on this torque number but my car has had no problems with the C7's @ 200 with many track days and about 1500 total track miles on slicks. Be sure to retorque after the first drive!!!!
Last thought. Are you sure you have an open diff? A warn church-type LSD can act really weird. If you do indeed have an open diff then you need an LSD IMHO. I personally really advocate for a Quaife-type TBD. Both my cars have them and they are magic. Both rear tires do EXACTLY the same thing no matter what you do with your right foot. Really friggin magic!!!!
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Howard,
Do you track yours at those pressures? Your and my "track usage" are totally different, but I run > 40 psi (cold) all the way around. Less if the wing is off.
Neil
Supporter
I run right at 40 psi in my rear tires but that is at the Bonneville Salt Flats. The tires are 11" tread width Hoosier race tires and the transaxle is a Porsche G50. The launch is clean with no evidence of wobble or shake. I even leave a pair of black tire marks on the white salt.
Howard Jones
Supporter
Frank, my car would be undrivable at those pressures. I am using R7 Hoosiers. 285.30.18F, 345.30.19R. Generally speaking, COLD tire pressures should start at about 1% of total vehicle weight. 26 PSI on a 2600-pound car. This is a good rule of thumb when someone asks "what should I run for tire pressure?"
When I was doing tech at track On a typical 2400lb Cobra on street tires, for example, I would usually tell them to run 25-26psi cold knowing they would go to about 28-29 hot for a novice driver on a moderate air temp day, 80F ish, in full sun. This is a safe, pretty close to best performance pressure in that case for most street tires
Rich, Check CV joint bolt tightness also!
When I was doing tech at track On a typical 2400lb Cobra on street tires, for example, I would usually tell them to run 25-26psi cold knowing they would go to about 28-29 hot for a novice driver on a moderate air temp day, 80F ish, in full sun. This is a safe, pretty close to best performance pressure in that case for most street tires
Rich, Check CV joint bolt tightness also!
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Howard,
We've been running enough downforce to take off the road reflectors (literally 100s of them in 10 minutes). But with the bump springs and now less downforce, that's not happening. I'll try less pressure and see how it does and how that effects our temps. We are running real street tires, but I get that anything close to what we've been doing would be a nightmare with R7s or A7s.
Thanks,
Frank
We've been running enough downforce to take off the road reflectors (literally 100s of them in 10 minutes). But with the bump springs and now less downforce, that's not happening. I'll try less pressure and see how it does and how that effects our temps. We are running real street tires, but I get that anything close to what we've been doing would be a nightmare with R7s or A7s.
Thanks,
Frank
I received advise from a gentleman that’s intimately involved with the development and manufacturing of the SLC. I was advised that the spacers that are held in double shear arrangement for the rear toe links may be installed incorrectly. Sure enough I have to larger spacer at the top and the smaller spacer on the bottom. I’ll fixing that and doing an alignment this weekend. Stay posted.
Howard Jones
Supporter
Initial setup. The toe link arm should be parallel to the upper a-arm as seen from the rear. Then measure toe change and make minor changes in the stack of the washers until you get the least possible change to toe as the suspension travels through its range up and down, I then install spacers to achieve this alinement angle for best toe stability.
Make measurements with the spring out and the car sitting on a block under the chassis to achieve normal ride height. Support corner suspension assembly weight with a jack under the lower a-arm, jack up and down with jack through the suspension travel range as you measure toe at the full bump, full droop, and at normal ride height. Set final toe to achieve no toe out at any point and about 1/16inch per side toe in at the point of max toe in. You can refine this to as close to no rear toe as possible but do not allow ANY toe out no matter what you do.
Here is an early on picture of my car when I finished my first setup. Note that the toe link arm is sorta in the middle of the upper and lower a-arm angles. Your car will be different but the concept is the same.
Make measurements with the spring out and the car sitting on a block under the chassis to achieve normal ride height. Support corner suspension assembly weight with a jack under the lower a-arm, jack up and down with jack through the suspension travel range as you measure toe at the full bump, full droop, and at normal ride height. Set final toe to achieve no toe out at any point and about 1/16inch per side toe in at the point of max toe in. You can refine this to as close to no rear toe as possible but do not allow ANY toe out no matter what you do.
Here is an early on picture of my car when I finished my first setup. Note that the toe link arm is sorta in the middle of the upper and lower a-arm angles. Your car will be different but the concept is the same.
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