Have questions about C4 Corvette suspensions for your Tri5? Ask them here!

[oceangoer]

Cheynut,

Thanks but that's Don's @ FOE Diffy setup down in Orange CA. He's behind on the website updates, I've already teased him too much about it.

Since we're on the rear suspension, there's a nagging item that's bugged me for many years, re the rear Toe-in assembly. And before I get into this, I did recently look over some other rear IRS toe in setups, and their definitely shorter rods

My question is

Why aren't the inner Toe-in Pivots in line with the inner camber Pivots ??


Guess I'll need to check the Toe-in change when the rear spindles are moved from min to max....

Michael...

[oceangoer]

chevynut,

Forgot,

re the rotated Vette Rack, Turn One has the hyd lines all made up and will install on rebuilds, I thing they sell them OTC

turnone-steering.com

Michael...

[chevynut]

My question is Why aren't the inner Toe-in Pivots in line with the inner camber Pivots ??

I can't answer that because I haven't studied the rear suspension that thoroughly. I'm sure it has to do with getting the right amount of toe change, or NO toe change with suspension travel. The halfshafts, camber rods, and toe rods all work together.

I do know that the camber arm brackets for the late suspensions are different than on the early suspensions. In '88 they moved the inner pivots down to change the suspension geometry (lowers the roll center and decreases negative camber gain), and the late brackets could be used on an early suspension to make that same change.

[NickP]

Nick, yours looks awesome too! Did Calvin use a BBC in that chassis? My SBC and LS engine dampers sit right smack over the top of that fitting and line. I thought Newman rotated the rack cylinder but I got to looking at his stuff recently and I noticed that one of the fittings is on top and the other was somehow moved to the bottom. I wonder how he did that.

BTW, I designed a rear toe bracket almost identical to yours and I don't remember seeing it before I designed it...great minds think alike, huh? I got one prototype from my laser guy to test.

Did you build the rear steer assembly from scratch? Mine will have to be narrowed so I can't really buy a kit. I'll have to get or make some shorter rods.

I had a Newman unit that I found on Ebay some time back. I drew it all up and made a dozen bracket pieces. It uses 1" DOM tube TIG welded together. I still have his casting piece. Calvin's 57 Nomad uses the stock cradle and the unit pictured is in a 57 2 door HT. Both are LS.

[55 Rescue Dog]

Just observing the stock set-up where the toe rods are very near the same plane as the axels, it would seem as you move them lower towards the camber rods you would see a bigger toe change through travel. Even more pronounced if the rear is narrowed. Modifying the frame, or raising the rear ride height would make it a non-issue, unless toe change is a good thing.

[chevynut]

it would seem as you move them lower towards the camber rods you would see a bigger toe change through travel. Even more pronounced if the rear is narrowed.

Keep in mind that the outboard end is also lowered as the inboard end is lowered. I'm not sure if there's a difference in the amount the inboard and outboard ends are moved on Newman's chassis. I think you'd have to lay it all out on paper or in CAD to see exactly what it does. Newman built hundreds of them and I've never read about any toe issues with his or other designs. Why don't you explain what the toe does with the stock setup? I don't see how you can simply say that narrowing a rearend makes it more pronounced without doing the analysis. It may actually make the geometry "better". Street Shop narrows the C4 rears up to 6".

Modifying the frame, or raising the rear ride height would make it a non-issue, unless toe change is a good thing.

Making c-notches in the frame to clear the toe rods weakens a frame that's already weak in the rear behind the humps. I built one frame where we extended the hump to allow the use of the stock toe rods, but he has to cut the trunk out to accommodate the modification. Raising the ride height negates much of the benefit of the C4 conversion by raising the CG, gives the car too much rake imo, and most guys looking for a performance chassis want the rear of the car as low as they can get it with at least 2-3" drop. A dropped toe adjuster, if properly designed, addresses all the issues.

[chevynut]

Here's an early C4 rearend drawing. Notice that the toe rod is horizontal and virtually parallel with the halfshaft, and above the halfshaft centerline. If the toe rod is lowered below the knuckle arm and the lower bolts are used as the inner pivot as Newman's kit does, it still sits essentially parallel with the halfshaft but sits on virtually the same centerline instead of above it. The inboard and outboard pivots are essentially just lowered and the angle doesn't change.

Also notice that the halfshaft is almost level, just barely lower at the outboard end. The strut rod points down and is about the same length as the halfshaft. As the suspension is compressed, it gains negative camber and the knuckle pivots around the outer u-joint. I'm not sure what the toe does, but since the stock toe rod is attached above the knuckle arm and longer than the halfshaft, I would think it would cause a slight toe in as the suspension compresses.

With the toe rod lowered along the halfshaft centerline, it looks to me like there would be little or no toe change as the suspension is compressed. I don't see how shortening the halfshaft and toe rod affects any of that.

What narrowing of the rearend does is steepen the angle of the lower strut rod, thereby shortening the distance to the instant center of the halfshaft and the strut rod. To correct that, all one would have to do is lower the inboard pivot point of the strut rod slightly as was done with the 88 and later suspensions. A narrowed late suspension probably still has the instant center further out than an early suspension, but I'd have to lay it out to confirm it. I already had some custom inner strut rod brackets cut.

[chevynut]

Here's an early C4 rear...note the inner strut rod brackets:




And here's a late C4 rear...the inner strut rod pivot is noticeably lower:

[rockytopper R.I.P 5-13-2017]

What size wheels and or tires and what wheel offset works with the C4 setup in a trifive?

[chevynut]

What size wheels and or tires and what wheel offset works with the C4 setup in a trifive?

Front or rear? Early or late C4?

[rockytopper R.I.P 5-13-2017]

Front or rear? Early or late C4?

either and both front and back.

[Custer55]

What size wheels and or tires and what wheel offset works with the C4 setup in a trifive?

I have been running the stock 90 Corvette wheels and tires on mine for a while now with no clearance issues at all. I have the late C4 set up. Fronts are 9.5 x 17 wheels with 56mm offset, Rears are 17 x 9.5 wheels with 56mm offset plus a 3/16" spacer which makes it a 51mm offset with the spacer. Tires all the way around are 275/40/17 (stock size for the 90 Vette) I think a 285 tire would be fine in back. In the front I will probably eventually go to a 255 tire just to lower the front of the car a bit. Not sure if this would work on any C4 conversion but it is working on mine.
Brian

[chevynut]

I have the late C4 set up. Fronts are 9.5 x 17 wheels with 56mm offset, Rears are 17 x 9.5 wheels with 56mm offset plus a 3/16" spacer which makes it a 51mm offset with the spacer. Tires all the way around are 275/40/17:

Custer per my calculations your front tires are 68.4" across and I'm surprised they don't rub. Most guys start having problems at around or a little over 68" but that's with stock suspension geometry and steering. The c4 setup turns differently and not as far. Yours is a 55 so you have more room too. Have you turned sharply with the suspension compressed like driving into a steep driveway? How much did you drop the front from stock?

Maybe 68" to 68.5" works fine with the c4 setup since yours seems to work. How close are you tires to the frame at full lock?

[chevynut]

What size wheels and or tires and what wheel offset works with the C4 setup in a trifive?

In the rear a 275 tire works without mini tubs with the limiter being the tubs at 46.5" wide. You actually have about 12.5" to work with in the wheel well. You might squeeze a 285 in there but it would be pretty tight. Once you minitub it the dog bones become the limiter and you can go 295 or possibly 305 depending on how tight you want the clearances. You can go really close to the dogbone. To go any bigger you'd have to narrow the rearend which you can't do much without narrowing the frame. I think you can get away on the big end with a 55 nomad with the big wheel openings.

[Custer55]

Custer per my calculations your front tires are 68.4" across and I'm surprised they don't rub. Most guys start having problems at around or a little over 68" but that's with stock suspension geometry and steering. The c4 setup turns differently and not as far. Yours is a 55 so you have more room too. Have you turned sharply with the suspension compressed like driving into a steep driveway? How much did you drop the front from stock?

Maybe 68" to 68.5" works fine with the c4 setup since yours seems to work. How close are you tires to the frame at full lock?

My suspension drop is about 3" The frame is 4" off the ground at the kick up at the firewall. I try not to go to fast in and out of drive ways and angle through them if there steep. At full lock there isn't much clearance to the tire at the fender or the frame, maybe 1/4" to 1/2", but like I said I have not had anything rubbing to this point. The C4 suspension design may be the reason it works as I had rubbing issues with the stock suspension with McGaughys 2" drop spindles at the top of the fender opening with 235 tires on the front on sharp turns at 15 to 20 mph.
Brian