Nothing new after this weekend. Of course I forgot to watch the motor during the dyno event on Saturday for any excessive movement, but I was having fun with my 2year old son. So hope to schedule some time on a lift within the next couple of weeks to look things over. Next event isn't till June, so I've got some time to hopefully get her back into shape (man I'm tired of saying that). Hoping new c-beam, beam plates, and batwing bushings do the trick and I don't have something that's cracked...not that I don't want to do a D44 swap, but I wasn't quite planning on it so soon.

 

From an engineering perspective I would say that everything is fine and working normally. The C channel is flexing as designed.

The old saying if it can't shake it will break certainly applies.

And the other pictured showed a failure.

Since it is being consistant you could get the measure of the angle and preload it down so when you load it and it twists up the driveshaft and all universal joints are straight. Reminds me of the old days...lol.

 

I don't agree. Did you see the difference in the OP's video and mine?

 

Well if the C channel is made out of 6061-T6 and no one has heated it enough to destroy the heat treating or has welded on it then it will flex under the amount of the load presented.

In order to determine the allowable flex you can do all the math. You can't compare one video to another unless you first did some engineering to determine both HP's and the coefficient of drag on the tires and on the street/strip pavement of both runs etc etc.

I say normal.

 

Will be interesting to find out. But while some movement may be normal, maybe more so with coilovers, this seems to be excessive. Hope to get it up in the air for some good inspecting later this week to late next week. Crossing my fingers for sooner than later.

 

Great thread! My .02$ something is not controlling the pinion. that is the job of the C beam. Im bettin you will find a prob with the beam, the mount on the 3rd member or the bolted joint. The tork in the lower gears and strain is limited by the traction. Looks like your hooking real nice. Beam plates are a option, I was also thinking about cutting up a spare beam and using it to weld some tapered reenforcement to the stock beam to move the flex foreword and distribute it over a broader area. The way the spring is mounted to the bat wing could add resistance to horizontal pinion movement but there is nothing outboard to resist rotation. So I dont think the coil over set up has reduced any pinion control. I do think your flirting with a major failure with that much movement.
edit;
btw if the stamped sheet C beam is 6061 treated to T1 welding to reinforce should be fine, just shape the parts to aviod stress risers. Its a very common alloy, I have welded a lot of it.
I miss spoke here^ 6061 T6 is very weldable but reverts to 6061 it can be solution treated and will age harden

 

Something may be wrong, but I believe the C channel is going to flex a certain amount and the flex I see looks about normal for the size and shape of the C channel.

You can replace that C channel with some other structure. However you will then need to do the engineering down the line as each component fails and has to be stronger to handle the past down load.

 

I'd bet it is T6. Therefore you really cannot weld on it or even get it that hot reletively without changing the strength. Not a little change a lot.

 

I would expect that much pinion movement on a leaf spring car. I would think it is too much for a truck arm or even a 4 link car. It looks to me like way to much for a IRS.

"I don't agree. Did you see the difference in the OP's video and mine?"
ya what he said^

 

O.K. I already did some math earlier in the thread, did you notice it? We both had our cars on chassis dyno's this past weekend, and we both have video's posted on diff movement on this thread. He made 270/296, I made 278/312. I don't think that "tire drag" is a meaningful contributor here. His video is showing movement in 2nd gear. So:
OP's car:
330 tq, x 1.63 2nd gear x his rear gear 2.59 = 1393 max tq in his vid to cause that movement.

My car:
330 (conservative based on dyno results)x .79 (for my elevation) = 260 peak tq x 2.68 (my first gear) x 3.45 (my rear gear) = 2410 max tq, which results in basically zero diff movement in my video.

So, once he shifts to second, I'm putting over 1000 ft-lbs out my diff in my vid compared to his...but his diff is moving substantially more than mine. Why?

The C-beam is responsible for the nose of the diff.

 

That is some good real world engineering.

The math I was talking about is the deflection of the C channel. Once you do that math then you will know the allowable deflection for the load being placed on it.

Answer might be max allowable = .5" Then you will get closer to finding out the real problem.

 

It would be nice to know what some of these parts are made out of. T6 is pretty stiff and hard. I have not handled one of these parts so I couldnt guess how hard/soft it may be.

 

Just FYI, I was in 1st for the entire run.

 

I would agree that if we are working with 6061 T6 .5 of deflection over the length of the beam would be sustainable. But It looks like all the flex is centralized right at the pinion to beam joint.

ed This is a d36 not a 44 iirc the pinion is about 1" shorter than a 44 makeing it even harder to control.

 

Maybe the OP could send the video to GK and ask him what he thinks.

Give him all the info, maybe he will say normal or oops somethings up.

 

That is beyond what I know how to do. Can you, with your engineering?

I can observe some vids though and see a substantial difference between two cars that are putting down essentially the same real world torque.

Copy the first gear only, Shakedown!

 

OK I will play.

Take a look at the C channel, what do you see. You really have two aluminum plates one on the top and one on the bottom. They are connected with sort of a make shift truss.

However my bet is you could twist it rather easily and that it is not designed to stop or eliminate twisting, ie clockwise or CC. But it is designed for the top plate to be put in tension and the lower plate to be put in compression. So it would be a fairly easy engineering problem if you did not get to crazy.

You would simply figure out the size of the top plate and the exact Al. material and heat treat and figure out the tensile strength. Do the same for the compression strength of the bottom plate.

Looking at the C channel I believe this is what the engineers had in mind note that the bolts holding the C channel in place are large and have washers and are in the same horizontal line going from front to back.

How was the C channel manufactured. Cast, Forged, Fabricated, shot peaned, then bent and then heat treated? How it is or was manufactured will also give you some insight to what they had in mind.

If would be fun to have the engineer of record that did all the long hand calculations chime in and explain exactly what they thought about while designing and specing the production of the part and what type of QC they had. How critical of a part is it to the strength and stability of the vette.

It is a light weight stiffiner, that is all. It is not a super strong part that can take a ton of force and not change shape. It will move. They, the engineers did not have too much cad/cam feedback engineering back in 1984 where they could actually see the C channel move in all ways with sensors attached and capture the data and replay it and watch it move dynamically. Not sure about that but I think that was still a few years off. So it was probably Engineered both in theory and practically.

Maybe he has a bad C channel maybe not, my point is in order to really determine it you would have to get the acceptable specs.

 

Huh. How can we obtain those "acceptable specs"? Of course that would be ideal, but I don't know that we can do that. Can we?

OR, swap the C-beam and see if the diff moves less.

I agree with your first two paragraphs above.....I wrote basically the same thing to try to reduce "lore", a while back, right HERE.

I also agree that it would be awesome to have some engineers speak up about a bunch of the C4's features. I don't see them around on this forum though. Unfortunately. Could answer lots of questions.

 

If someone has his contact info, I'll be more than happy to share the videos I've taken. Three in all now (two autocross events and two dyno runs). Will need to get this straightened out before my next run in June! Well, at least replace what I can afford for now.

 

Just the latest videos.

Dyno day



Another autocross