New rear end and new traction issues!

[SmittyTime]

The end of last season my G friend grenaded the 12 bolt in her 10 second Chevelle. She decided on a Fab housing, Moser 9" rear which is a "bolt in" for the chevelle. I set to installing it in the Chevelle and we managed to get one last outing the end of the season. First two passes the car launched hard then unloaded the tires and lost traction. The third pass, I set the shocks to the lowest setting and it launched hardest it ever has, wheels up with her best 60 ft to date.
Since breaking it out this season, she has been unable to get the car to launch without breaking traction as the rear susp unloads.
Here's my question. Since the fab housing is much larger than the original 12 bolt housing, it stands to reason that the upper arm are at a higher angle. Is it possible that because of the repositioned CG we are experiencing the traction isssues now? If so, how can I possibly overcome that when it is "non adjustable"? Prior to diff change, car was deadly consistant and nothing but the rear has been changed since this issue. Any thoughts or suggestions??

Smitty
72 Chevelle (hers)
72 Nova (his)

[CDR Performance]

Do you have video of the car launching this year?

And yes the 9" is larger and creates issues in the swap.

[supernova]

I can see the issues in changing to the bigger rear end. I my opinion you should put ajustable control arms on it. I would bet that the pinion angle has changed because of the new rear end and needs the be ajusted with new ajustable control arms. ( I would put uppers and lowers on it )
I think the pinion angle would have been the only thing that changed with the rear end swap because of the new bracket locations.

Just my thoughts.........

[SmittyTime]

Thanks for the responses guys! The car is already set up with upper and lower arms and I set pinion angle after install. The car hits the tires real hard off the line but as the car "settles" the tires break free. My concern is the CG. ?? Just bumping up the upper mounts 2" on my software shows a radical change in the center of gravity. You know the deal. Fix one problem, create 3 more..............

Smitty'
72 Chevelle (hers)
72 Nova (his)

[Mike Peters]

Thanks for the responses guys! The car is already set up with upper and lower arms and I set pinion angle after install. The car hits the tires real hard off the line but as the car "settles" the tires break free. My concern is the CG. ?? Just bumping up the upper mounts 2" on my software shows a radical change in the center of gravity. You know the deal. Fix one problem, create 3 more..............

Smitty'
72 Chevelle (hers)
72 Nova (his)

I don't think you mean C/G (center of gravity)........I think you mean I/C (instant center). Yes, the short I/C could cause too much separation and therefore unload the tires. Once the extra separation occurs and tops out the shock travel, the tires will follow the same direction as the rear of the car. The description of the hooking and then unloading would seem to indicate this is indeed what is occuring. Better shocks with stiffer rebound settings would control this from happening.

[SmittyTime]

My bad. IC is EXACTLY what I am refering to, not CG. She's got a good set of adjustable shocks on there right now. The car seems real stiff as it is but I suppose dialing up on them can't hurt at this point. I had them turned all the way down only because the rear seemed exceptionally stiff. Going to try that and get some video at the same time this weekenend. Thanks for the ideas, etc.

Smitty
72 Chevelle (hers)
72 Nova (his)

[Mike Peters]

She's got a good set of adjustable shocks on there right now.

Not trying to dispute you but, what brand are you calling a "good set"?

The car seems real stiff as it is but I suppose dialing up on them can't hurt at this point. I had them turned all the way down only because the rear seemed exceptionally stiff.

The forces the car can create are greater than what you can create with the ol' "push down on the car" testing method in case that's what' you're using to determine the stiffness. If the rear suspension is truly stiff and you're on the lightest setting but, still having problems, unbolt the shocks, remove the springs, and check for bind during travel of the rear end housing.

Going to try that and get some video at the same time this weekenend. Thanks for the ideas, etc.

Good luck and keep us posted

Smitty
72 Chevelle (hers)
72 Nova (his)

[SmittyTime]

I appeciate the extra thoughts, so it's all good! She has a set of QA1, 12 way adjustables on there. (stocker stars??) I did pull the shocks as you mentioned, last week at the track and saw no binding issues. Great minds think alike and all that nonsense........ I'm convinced at this point its an IC issue but am unsure as to how to overcome it. Stay tuned........ oh, the joys of racing!!!!!

Smitty
72 Chevelle (hers)
72Nova (his)

[Mike Peters]

[quote="SmittyTime"]She has a set of QA1, 12 way adjustables on there. (stocker stars??)

Not trying to burst your bubble but, QA1's aren't always held in the highest regard in the food chain of shock brands. Doesn't mean you can't hope though....

I'm convinced at this point its an IC issue but am unsure as to how to overcome it.

I agree. It sounds like an I/C issue and you'll over come it by controlling the rate of separation with the shocks. If the QA1's won't do it, you'll need to upgrade or send them off to get them re-valved.


What slicks are you running and what pressure? That's another factor you can experiment with too. Controlling sidewall flex can help eliminate the energy transmitted to the car.