I have a 94 LT1 6 speed. It has been lowered by previous owner. About 3 weeks ago during a semi controlled slide I ran my right rear wheel up on a rounded curb. I could see I knocked something out of place. SO, I let my friend Joe take it to work. He is a lead mechanic for a local Tire Disounters. He installed new shocks, lined the car up, and balanced the wheels. Ever since the curb incident when I am off the throttle then back on the car dances. Hard to explain. Imagine shifting gears: the rear end squats and rises when you shift, right? Well when I shift the rear shifts right then centers as it squats, right then centers as I lift throttle. At 95 when I let off, it feels as if the rear is going to come unglued. Neither one of us can find bent parts, bad bushings, or anything broken.

Anybody have any clue?

I am heading to Deal's Gap in the Smokies in 4 weeks and would like to have the rear straight for those famous 318 curves in 11 miles.

 

EEESH! My 94 rear whips to the right when I hammer it in 1st or 2nd, but not when I lift off.

It looks to me like you will need to take the car to a Corvette specialist shop that can do 4 wheel alignments. They may be able to diagnose what is cracked or loose on a chassis jig. The ZFDOC beam plate is supposed to be the cure for the car dancing to the right under acceleration, so I would look at the attachment points for the C beam specifically.

Guess you'll have to drive the Dragon at a steady speed, never lift!

 

They all do this unless you go with 17x11's on the rear...

 

My guess is there is something about the rear toe that
is sub-optimal.

Any chance that you have or can get the printout
of the alignment specs? I believe that with modern
equipment, the before/after values are logged against
the work order.

If not, I would suggest that you gather the OEM
specs along with performance specs from places like
Guldstrand and Vette Brake Products (VBP).

Decide which numbers are most suitable for the majority
of your driving. The numbers for Solo II are going to be
different than something you'd want for a Silver State
run. Either of these would tend to be a bit harder on
tire life due to greater neg camber.

In my case, I prefer to have the rear able to lighten
up on trailing throttle. I like to think this helps me bring
the rear around in the cones. In order not to always
be catching the rear, I need to be gentle getting off the
throttle when the car is not travelling in a straight line.
There is lots of run-off room, so the penalty for getting
it wrong is only smoke and noise.

I would want more stability than this at speed on a
little known road with ditches and other obstacles. Even
if it means another alignment, I'd find out where matters
stand back there - cheap insurance.

.

 

17x11's are the only cure? I was going around a long right hander in second gear at around 75 MPH, RPM was right up there. The car felt nice and planted with my foot stuffed in the throttle but as soon as I let off the whole rear of the car felt like it was going to come out from underneath me.

Obviously, when letting off the gas the weight transfer shifts back to the front of the car, lightening the rear and making over-steer more likely when you've got the wheel turned a good amount. But I was rather surprised as the rear movement was violent. I have gotten used to giving the car corrective steering input going around corners because it likes to dance around but I've never experienced it this bad before.

Having been a good student of the C4 and Auto-X sections saved my ***. Just from reading about the proper way to drive these cars is a life saver. Had I not known better I would have stepped on the brakes in addition to backing off the throttle which would have put me in a slide for sure. When I thought my rear was going to come around I stuffed my foot back in the throttle to settle the car, and it worked, only then did I start braking. Good thing I had lots of room in front of me.

I just can't believe the dancing rear is a normal thing we all have to live with. I've already got beam plates. I wonder if those adjustable camber(?) rods would help any.

 

Jack one rear wheel at a time off the ground.
Hold the tire at the 3:00 o'clock and 9:00 o'clock position and try to aggressively move the wheel forward and back.

I'm betting the tie-rod (rod that attaches to the back of the center of the rear end housing and goes to each wheel to adjust rear toe) is broken internally where it attaches to the rear end. If I remember right, there is a plastic bushing inside the housing on the rear end.

Tom Piper

 

Interesting!!!

The classic "Bump & steer" is usually cured with the wider is better idea... But you seem to be having alittle different issue. I'm interested in hearing from the other guys what they think???????

 

IMO, bigger wheels & tires are the fix for throttle-on oversteer.

Trailing throttle oversteer is related to different forces and
changes in chassis attitude. The rear toe changes through
the range of wheel travel - I think this is by design and serves
to dish up a bit of intentional 'rear-steer'.

The magnitude of the rear-steer is affected by initial settings,
the amount of dive permitted (shock valving and springs), ride
height (stock or lowered: changes control arm positions) and
so on.

But in the spirit of the K.I.S.S. principal, my vote is to take
Tom Piper's advice and check the rear parts for unapparent
damage and maybe review the alignment specs.

.

 

It sounds like you broke the glued joint on the c-beam. Beam plates will fix this problem.

 

You have something bent or a bushing cracked/failed. Reinspect & take measurements, use a straight edge where you can.

I did a similar curb hit probably more than yours, you would be surprised what can get out of wack from a steering geometry point of view.

Takes a very good vette suspension guy to find it sometimes.

 

I would bet a paycheck that Tom is 100% right. This is a common problem with the C4. A cheap fix too!

~ Purp

 

It doesn't take much to cause the effect you are seeing. My car use to do something similar (on and off the gas), turned out to be a loose wheel bearing on the RR wheel. If it's not your wheel bearing my bet would be the tie-rod as well.

You say your buddy lined it up. Not saying you buddy doesn't know what he's doing but maybe the machine he used is not calibrated correctly. Get it on another alignment machine to see if it looks good.

To be honest, I gave up on having my car aligned on a machine. I now do it manually. I once had my wife's car aligned. She called me the next day saying it felt like she was driving with the brake stuck on. I checked the alignment and front toe was out 1/2". That was the worse case.

 

Give me an idea what beam plates and C beam are.


Thanks for the responses guys. I put the car on my lift at work and shook the hell out of each wheel with no play in the wheels...keep in mind I am 300 pounds and can press over 500. Joe (my alignment guy) is very good and he could find nothing. It is lowered and when I first got it, Joe lined it up and it was PERFECT. At 120 I could all but let go of the wheel and the car would track straight and true. But last night his manager asked if I had checked to make sure the spring had not started to split down the middle...sounds far fetched but I will check it.

Passengers can feel the car wiggle around. It moves enough that I have to make corrections with the wheel. If I make a mid corner correction (like increase/decrease throttle) the car actually scares me, and I LOVE the feel of having the rear slide out about 3 degrees.

 

The C Beam is an aluminum piece in the section of a C, that ties the transmission to the diff. Stock it is fastened by bolts only. The beam plate adds plates under the bolts and nuts that distributes the forces better.

Read about it and/or buy if from ZFDOC at : zfdoc C4 beamplate

Note, this fix will address a kick tio the right under hard acceleration. Most C4s have no drop throttle steer like you describe, and this may help but you may have something broken elsewhere in the rear.

I wouldn't attack the Smoky Mountains with the condition you have. If you can't lift off the throttle you can't hit the brakes, eh? I had an 84 BMW that kicked like this on drop throttle and I never had any confidence in the car on mountain roads. It's called evil handling, German engineering style.

 

Man I feel like a fool. I call the C beam "the frame for the tranny to the diff". Thanks for getting that straight. Could it be that the guy that put the tranny back in after replacing the clutch a few months ago did not tighten the bolts and the curb incident loosened everything a little more?

As far as the Smokies, braking does not affect it or at least not to the point that I am worried about it, just the torque of on/off/on throttle. I have been through Deal's Gap a hundred times on my 929, but never in a car. I don't think I will get over about 40 mph and the dancing is not really scary unitl you are over 65 in turns and 90 on the highway. Hopefully all will be fixed by then.

 

If the handling is unsettling on a level straight road, what might it do in an off camber decreasing radius turn, wet from a waterfall? There are a lot of things on those roads that can surprise you, and that's the real danger.

What you have is not a normal condition for a C4 and it could be really bad on those roads if you are trying to keep up with other Vettes. It's not a good place to loop the car.

Something I've been thinking about, has anyone in a Vette group in the Smokies ever encountered a group of 911 owners attacking the Dragon? Or do they just stay away.

 

I forgot to mention the back up plan. If I can't get the Vette straight I am driving my mom's 350Z....although it does not have that mean feel of the Vette, the Z is an absolutely wonderful backroad fighter. Thanks to newer technology and a more rigid chasis, I feel like a god behind the wheel of that thing. Very, very predictable and stable. Mom has had it two years and there is only 2100 miles on the odo...of which at least 800 are from me.

 

That's good, I hate to see anyone get hurt, or a good Vette go to the boneyard.

Like others have said, there probably is something broken, but it will be difficult to find. Even with beam plates that other thing might let go under load.

 

and have both kinds of skid marks to prove it.

 

The c-beam is attached to the trans and the diff. Each end is held on by only two bolts on each end. The C3 cars used a cross beam on the diff to hold it steady and firm against the frame. The C4s are not as steady with the c-beam design--only two bolts, they are long bolts, and they are located fairly close together, thus, the clamping force is only applied to a small area of the structure. GM helped out the situation by adding a thick adhesive to where the c-beam contacts the trans and diff. Application of this adhesive has been inconsistant to say the least. On my car, the upper c-beam contact point at the differential only got about one quarter of the area glued; on the bottom contact point it got a big glob, but it is all on the upper inside area.

Any, when you hit the curb, it is possible that you broke the adhesive patch and loosened the bolts. The beam plates are just oversized washers that help spread the clamping force of the bolts to a larger area, thus giving a better grip and less flex. Give it a good look and check the bolts. You can reglue the joints with a heavy urathane based glue like Liquid Nails.

Hope this helps.