Are the frame brackets, that the trailing arms attach to, the same over the C4 production run, or was there a change at any time? I have heard two different stories. Can someone clear this up for me?

Thanks.

 

AFAIK, they're the same for the the entire run....but WVZR-1 would be the man to give you the facts.

 

the only difference is in '88 onwards they dropped the bottom hole 1/2" to reduce anti squat, otherwise the same.
here is a pick of the 88 bracket on top of the 85 bracket.

 

Yes '87 and earlier THEN '88+ but I never knew it was a 'single' dimensional change of just the 'lower',

I'd think for a true comparison you would need to confirm the mounting points to the frame rail and then measure to confirm the difference. Standing 'side by side' if you will. Using the snapshot provided you sorta need to assume those are the same. Then there's the DRM brackets to consider. I had never seen the snapshot in this thread before. I don't know if I have both fabrications here or not.

I believe that the entire change was the brackets in this conversation and also the camber bar bracket that bolts to the differential.

OP -I recall conversations from many, many years ago where you asked similar questions. These brackets and camber brackets surface from time to time.

'blackozvett' - do you have both? If yes what is the 'center to center' spacing of the rod bolt? Is that your snapshot?

 

yes that is my photo - while I was doing research on anti-squat on the c4 I read a comment from an old thread that said the 88+ brackets had a lowered bolt hole to reduce anti-squat.
When I had to order a part from a vette wrecker in Melbourne AUS I asked him to compare the 88 brackets with an 85 and tell me if that was correct.
He rang back and sd the lower bolt hole was different by half an inch, I sd chuck a set in my order.
All other dimensions are the same, including the 3 bolt holes to the chassis.

 

Thank you!!

 

..... The early cars also have a hole in the side of the bracket where the parking/E-brake cable passes through .....

 

Yes - that's the JY identifier for early or late!!! You can see the mat in the snapshot through 1 of the 2 holes.

 

Thank you for the help/info, gentlemen. It's greatly appreciated.

WV, my apologies if I've brought up this topic before. I really need to organize things better in a notebook given the mish-mash of C4 suspension parts I have on my '69.

Just went out into the garage and looked at the bracket. It's the earlier model with the cable hole in it. Acceleration squat hasn't been an issue, given the '84 Z51 rear spring on the car, so I might just have to ponder if it's a worthwhile endeavor to try out a later bracket.

My main interest is in road course handling, not straight line issues/dynamics. I'd welcome input on what, if any, differences in handling or cornering geometry might arise from this. The only thing that pops into my head presently is perhaps a slightly less amount of wheelbase change with bump/droop with the angle change of the lower arm.

Thanks for any additional help.

 

Are you using an early c4 rear end in a c3 ?

The c4 (and the early c4 in particular) have lots of anti squat, about 115%. Reduced slightly in the 88+ with the lower bolt hole.
I have done some research on the subject recently as I have replaced my whole rear suspension prior to our new season that starts this month.

Apparently the high anti squat values built into the c4 (keeping in mind this car has only about 2 inches of compression at the shock) are to prevent the production built sportscar from bottoming out with 2 people, a full load of gear and a full load of fuel , particularly when it hits a bump or pothole at speed, and turning sideways into a tree resulting in nasty and expensive court cases !

Higher anti squat values are ok for drag racing, but not so good if your type of racing involves accelerating out of corners. this can also depend on grip and track temperature.

Have you mapped out your set up so you know where your 4 link intersecting points are at ? I dont know how the c3 chassis compares with the c4.

Here is a diagram I did of my intersecting points and with a 1" drop of both trailing arms at the bracket (I only ended up achieving 3/4" drop on the trailing arm bracket)
you can see how the intersecting point (IC) goes from above the neutral line to below the neutral line reducing anti squat.




If you want to know what effect this change has, I can tell you after March 18 - if I dont spear off sideways into a tree !

 

The concern there would be if the rear suspension bottomed on the bumpstops, which gives a sharp rise in rear wheel rate, resulting in oversteer. BUT, if it stays off the bumpstops then less anti-squat actually helps handling stay more linear. Anti-squat geometry effectively acts as a "geometric" spring when the car is accelerating, jacking up the rear ride height and making the suspension less compliant over bumps. It's not really ideal, and reducing it is generally desirable for handling and good traction on corner exits. Hence:

The DRM brackets are designed to reduce anti-squat more than either version of the factory brackets.

 

I knew you would be in soon

here is a comparison of the DRM bracket to early bracket, an extra half inch each way.
(sorry about the quality-off computer screen)

 

If my memory is still there. The DRM brackets are 15/16 separation. Every 1/16 counts.

 

its hard to pick where they have actually measured from in that picture, looks like its from inside the holes rather than centre to centre ?

 

I only use 'center/center' when doing bore dimensions and I have a pair of later GM(no hole) and they measure 105mm, it's fairly easy to do 'center/center' on these because the bolts are 'dog-point'.

105mm = 4.133"

That seems to make all of the dimensions mentioned questionable. Someone with an early might do 'center/center' on theirs (being on the car shouldn't create issues) and perhaps someone with known DRM could do the same.

 

I have a set of DRM brackets in a box someplace ... will see if I can find them.



Those dimensions came from an article written by the late Richard Newton, of 101 Projects fame. I never verified them & it would be good to do it.

We had a long post on these DRM brackets and anti squat years back, and it looked like they just moved the holes out an equal amount. If true that doesn't change the anti squat angle or percent anti squat, just the length of the virtual pivot arm.

 

So ... The DRM bracket is 5.2890 outside 4.3325 inside so 4.8108 hole center.

 

Scratch all that, I see what you're saying now. So the longer side-view swing arm (SVSA) actually just reduces the changes in anti-squat as the suspension moves.

However, the other important measurement on the comparison between stock and DRM brackets is the height of the two holes that mount the brackets to the frames. Are they the same for both brackets, or do the DRM brackets mount lower on the frame of the car (i.e., are the two holes higher on the bracket for the DRMs)? If the latter, then they are reducing anti-squat.

ETA: According to this post 10 years ago, the DRM brackets move the bottom hole further downward than the top hole moves upward. Any comparison of stock brackets to aftermarket brackets has to match the frame-mount holes to each other in order to properly see what changes are made to the arms' pivot points. Can anyone verify?

 

On my '88+ the bolt center is equidistant from the 'center' mount to the chassis rail. Where are yours in regards to that 'center' mounting point? I'd think equidistant.

I mentioned 'side by side' back in post#4

 

I will verify after lunch .... but

The DRM's have to be symmetrical, they have no markings left-right or up-down, so you can mount them four different ways.
They also have the mount holes centered in the bracket, again symmetrical, while the stock one are offset.
This is why I never put them in...

Old picture