With a 2:59! Run what ya got! When I got the car some 9+ years ago, I never thought I'd be autocrossing. I figured I'd have a C5Z by now for track days...but a wife and two kids later, I'm autocrossing the ole 96 LT1, A4. It's still quite a blast.

 

Hello shakedown067,

Since the rear end is basically attached to the car by the bushings in the end of the batwing and the C-beam it would seem that your flex would be caused by either a possible loose bolt(s) on the c-beam or worn bushings on the batwing. There is the possibility that the c-beam is flexing or could have a crack in it. By design, I do not think you can eliminate all the flex without making some kind of mod to the setup. Maybe even solid bushings in the batwing.

I am wondering if the addition of a lower support from the bottom of the diff to the frame would help to keep it in place and alleviate the flex you are seeing.

See the attached pic for a simple fix. I know it is not a C4 frame but you get the idea. I have also seen where the c-beam gets additional gussets added to help stiffen it up to reduce flex.

Anyway, just some thoughts for you.

 

I'm not worried about eliminating it, unless it's not supposed to be there. Will have poly bushings installed in the batwing after this weekend's autocross. Just wasn't enough time to get it done before this weekend's event. May try to mount the camera where I can view the c-beam, but not sure if there is a good place for it.

Thanks for that pic! I need to save that for when I put my D36 under my truck...after I get a D44 for mine!

 

I'm not sure how much force it takes to crack the lattice on a rear, but I know first hand it can be done. Here's a pic of my old D36 rear after a day at the dragstrip years ago. I can recall hearing a metallic snap on one launch. I drove home and found this. To this day I don't know what caused it. Everything I checked was in order. I upgraded to a D44 and never had another problem.

 

TORQUE!

Ouch! Yeah a D44 is on my list of mods down the road.

 

Well man....this thread got to me and I couldn't help myself. I had to "geek out" and go vid mine to see what happens.

My vid shows about what I'd expect to see for flex/movement...not much. I've felt for a while that the C4 design is way better than the Caddy's design that I posted a vid of earlier, and I was right. The C-beam holds the Diff sound...when the C-beam is sound. See vid. I didn't edit it so the "high torque situations" occur at:
3:38 - 4:00 -I am in 1st gear going from no throttle to full throttle.
7:25 - 7:36 -I do a minor clutch dump in first gear, then do some mild drag type shifts to 3rd gear.
8:50 - 8:56 -I do a couple little clutch dumps and tire squeaks. Didn't want to go too crazy in my neighborhood and also didn't want to shake the camera loose.
9:55 > -I'm backing up a pretty steep driveway. Nothing major there, but the same thing in our Caddy and the diff would be flopping all over the place!



One additional note: IDK how much tq the OP is making, but my car is stock and I'm at 7000' elevation. So...330tq x .21 = 69. 330-69=261 ft lbs is about the max my car will make here.
X 2.68 (ZF first gear), x 3.45 (rear gear) = 2413 ft-lbs at the diff.

 

I'm new to corvettes but it sure seems like the stock rear spring would provide torsional support as well as damping. It's pretty cool to see the suspension work under load, great for diagnosing and tuning.

 

I have to agree with the stock spring giving support. But how much does it really support the batwing? We need someone else with coilovers to see and make that judgment!

 

THe weight of the car rests on the batwing bushings. The batwing rests on the spring. The spring rests on the knuckles...which are held up by the wheels & tires.

 

I have a idea that would solve the problem! But it's late, so I'll do a drawing of it tomorrow!

 

Another known problem

 

Here is my idea to support the batwing with coilovers. Use the old mounts on the batwing and weld brackets to the frame. Its a rough drawing but you get the idea.

 

Awesome stuff guys. I've got a stock LT1, only mods are hi-flow cats and 3" exhaust. Will see just how much torque she's running on saturday as I'm attending the Powertech Performance St Judes Dyno Event on saturday! After watching the vid, it certainly looks like the diff still moves, not as much as mine, but with the spring off, that might be the difference. Thanks for taking that video Tom!

I think that beam could actually be bolted to. I've got a couple of guys I can talk to about that. Will have to crawl under there and take a look. I like the concept of extra diff support with the spring removed.

I'm thinking about getting the c-beam plates first to see how the diff reacts...

I've got some dough set aside for a D44, so if she breaks, I'll just have to do it sooner than later.

 

I REALLY like the idea of the longitudal link that Colabear provided...
Thats ENDS the problem and could potentially extend the life of all the rear bushings by limiting the "roll" of the batwing.
Thats the goal, to stabilize the batwing because the entire suspension is bolted to that which is bolted to the frame. A stable batwing means the suspension would be truely free-floating and the spring and shocks would be absorbing the motion and loading instead of the unpredictable and unknown loading of hard parts tweaking around in unforseen ways...

A simple cross brace left to right in front of the diff, then mfg the link to attach WITH the C-beam in place...its done. IMO of course...
Thats the lightest and simplest solution to this problem IF there is not something thats currently broken back there thats causing the diff roll.

 

I think that you should vid your C-beam. I really think that is where your movement is coming from -either from bending of the beam (meaning the beam is "bent" and not strong in that direction anymore) or poor attachment to the diff for what ever reason.

Well, if your car is a stock LT1, then we are putting similar tq to the rear. You are "at sea level", 340 tq (yours is 10 more than my '92) x your 3.07 1st gear, x your 2.59 rear = 2703 lb ft at the axle shafts. So with your gearing and elevation, you've got 300 more lbs of tq at the diff than I do, which is something...but not enough to cause the differences that we're seeing in these vids.

I don't like the idea of trying to support the diff housing from the spring perches. Why? Two reasons:
1. It doesn't solve any of the problems that the Batwing and the C-beam haven't already solved -if functioning as designed.
2. B/c the spring perches are only about 3-4" away from the batwing, as they relate to the centerline of the axle (the axis of our toque), and only about 6" apart on the longitudinal axis. That provides terrible "leverage" against the torque that the diff housing imparts on the car, and I feel would likely break the rear diff cover. The batwing already provides a ~3' lever arm on the longitudinal axis, and the C-beam is a good design as it provides a nice LONG ~8'? lever-arm to deal with the torque that the diff housing wants to exert on the car. It's a great design...I just thing the OP's got something wrong with his torque arm system (the C-beam). Interested to see what comes next!

 

I have to go along with this, the movement does not appear to be the type of movement associated with bat wing mounts.

This sounds feasible to me but I doubt that a stock LT-1 could deflect the c-beam that much.

 

I mentioned several posts ago about "all of the heims" and I was told I was foolish! If the C-beam is secure and we've seen the response of the suspension with stock camber rods (LCA) and the transverse spring there's only 2 explanations and I mentioned replacing the after-market "heims" with stock components and do the drive again.

The LCA's (camber rods) should be easily swapped to check and note improvement if any! If there's none and the C-beam is secure then there's the obvious!

 

This breakage occurred with my old engine which IIRC was only making about 370 ft/lbs at the wheels at the time.

 

I guess you're talking about me..but I don't recall saying that you were foolish. What I did say was;
"I'd inspect the item who's job is to prevent that motion. I'd inspect the C-beam."

It is not the job of the control arms secure to stabilize the diff. It just is not. That is done by the C-beam and the bat wing. Sorry if you felt insulted, but I hope that the OP spend his time inspecting the parts that are the most influential on the problem at hand. That would be the C-beam...not the suspension.

 

bottom line, Seeing that diff roll as much as this one does, means that :

A. C-beam is loosely attached and allows that much travel at one end or the other

B. the C-beam is able to twist and flex that much (which aluminum is quite capable of doing since its an "active metal" that has high rebounding ability, aircraft wings for example)

C. the entire system, engine, trans, C-beam, diff moves as torque is redirected from good tire bite on road surface. If that C-bean rolls 1/4" at the diff end, 6ft away that would be 2" at the trans (haven't done the calc, just a guess).Not likely but possible.

D. there is minimal travel associated in ALL joints and connections involved in the suspension support system that individually would never allow this much diff roll BUT collectively DO provide this amount. 1mm is 1mm until you multiply it by 10...then its 10mm. Several mimimally "loose" spots added together.

It would be interesting to see how much of this travel vanishes with even the slightest wheel spin/slippage in corners and on hard acceleration. My bet is a BUNCH if not ALL. BUT because the OP has created a more solid rear suspension and now gets superior traction and power transmission from crankshaft to the road...the torque is no longer absorbed by the tires/road contact. Its now seen in the suspension where the STRESS will appear at some point in hard parts where before the wear was 99% in the tires...Just a theory from an engineering perspective.
A common problem is engineering is that when strengthening one part you essentially weaken another...or force stress to another location.

In any case, beam twist, loose joints, whatever it is, stress is being applied to hard parts and THAT WILL result in a failure someday...that is why I like the idea of the link from diff to frame with bushings that would stop the for/aft roll just as the batwing end bushings limit the left/right movement of the diff. The batwing was/is designed to tolerate high verticle loading by the lattice design, not so much twisting. Its not going to take these new stresses for long...Like the boom on a crane...up/down is fine. Almost limitless loading capabilities. L/R its fragile. Sideloading is a major contributor of boom failure.

In any case, its great to see good discussion and debate. Thats where ideas come from, not bickering and criticisms.
I congratulate the OP on the best and most exciting topic for discussion that I've seen in a very long time !