Hi everyone,
I am in the final stages of building a custom stainless steel frame for my 75. I have a VB&P Performance Plus package and offset HD trailing arms I will be putting on the car. What I am wondering is since I am building a new frame, is it beneficial to change any of the mounting points for the suspension (front or rear) and or differential? I have heard that the C3 has toe steer issues in the back? Raising the differential is another thing I have read about. I would like the handling to be as performance oriented as I can make it, using what I have. For example doing some autocross with it.
Thanks for the guidance and opinions!

 

Do you plan in driving this on the street a lot? As in, are you going to make this a driving car, but with the occasional autocross/road race weekend planned? Or is this gonna be used more often for road race, autocross, etc?

The reason I ask is, depending on your answer, you are going to be building to entirely different cars. For one thing, if you want in on autocross, you need to make sure you stay within a specific class. Take, for instance, the VBP kit you have. It has the dual mount rear spring, right? That puts you into a high class, right off the bat. Thanks to it, when I autocross, I end up having to give time to NEW Z06s. Obviously, I'm not at all competitive. But then, my car is mostly for driving, and the occasional bit of racing for fun. So I don't care about being competitive.

 

Lots of street driving with the occasional autocross. Same as you, I don't care about being competitive against Z06's. I just want to have fun with it. I don't really care about ride comfort though, so I would rather lean towards the autocross side of the equation. I don't care what class the car goes into, I just want it the best my particular example can be. Nothing more, nothing less. Yes it has the dual mount rear and front springs.

 

Well, then, in that case, one issue I have seen for myself is, when you lower the car, you will run into problems with the half shafts going level. You want them to be angled somewhat down. Raising the rear diff helps, but that was not an option for me, so I went with the solid disks in the back, instead of the bushings. Gives another inch, inch and a half. But for you, you sound like you could take on the job of notching the crossmember no problem...there is actually a nice thread up near the top someplace where a guy did just that, and posted pics. Check it out.

As for changing the mounting point on the suspension...I'd call and ask VBP first...as they are ALREADY doing that with the rear spring. They are mounting it on the cross member, instead of on the diff, like stock.

Past that, the other major issue is frame flex...which, if you are building your own tube frame...shouldn't be an issue, either. Post some pics!

 

Oh, and what size wheels and tires you planning on trying to run? That's another concern.

 

No disrespect intended, but if you went to all the trouble to do a custom frame, why are you keeping the stock suspension, given the many shortcomings it has? I got tired of bandaiding the stock suspension several years ago, so I modified a stock frame and put a C4 suspension on it. Absolutely no regrets or handling quirks. In hindsight I think it would have been even easier if I had started from scratch on the frame.

Just curious.

 

I don't nessesarily want to lower the car. We have lots of speed bumps around my area and I am also going to put a side exhaust, so I want the ground clearance. I am not using any part of the original frame, everything is made from 2"x4" Stainless HSS. So I won't be notching the crossmember as suggested. I will simply be repositioning the one I made. I have designed it so that there is a single crossmember welded in, with a stout bracket bolted to it, that the diff will bolt to. That way I don't have to dink around with droping that whole member just to unbolt the diff.

In which case, what should the approximate distance from the center of the diff yokes to the center of the wheel hub be to get the proper angle on the half shafts at ride height?

I am hoping to use 17" or 18" wheels. 8", 9" or 10" wide. Have not decided yet. I have the offset TA's.

I went with the VB&P package because it seemed like a good idea at the time. I needed a whole package, and I was not sure of any C4 packages, and the individual components seemed much more expensive at the time. The car was/is in awful shape (hence the new frame). Body and interior seem in a useable condition. So that is where I will save some money. I got it for $1500. I am young, it is a learning experience for me. It will never be a show car or an all out racer. Just fun for me

 

Two foot by four foot!!!! Wow, that car's gonna weigh a TON!!! Can't weight (pun) for the photers.

Regards from Down Under.

aussiejohn

 

Damn SHIFT key....lol

 

So I have decided to go with pretty much the standard location points, as best as I could measure, being a new frame and all. I will post some pics soon.

What is the deal with the rear toe for our style of vettes. Where should the diff sit relative to the wheels at ride height. ie what sort of angle if any should the half shafts be at? I have heard it is best for the U-joints if the half shafts are horizontal most of the time.

Opinions?

 

FACT: Level half-shafts aren't a goal, rather a negative side effect of dropping ride height(s) to lower CG and rear roll center (both worthy causes). Adverse toe-steer begins from the point where any further bump or squat puts the inner U's below the outer ones (relative to the chassis), resulting in a progressive shift towards toe out. Fail to setup with sufficient static toe in and the rear can actually go into toe out. Raising the diff in lowered chassis is done to relocate from what point along the rear toe-steer curve the suspension works to a better position, and minimize necessary static toe in (lessening scrub). Yes, there's toe-steer in droop too, but what is happeneing when the rear tires are loaded during cornering and acceleration takes precedence.

OPINION: If handling was the greater priority here, you'd lower your chassis and take a second and third look at rear suspension options other than the stock C2/C3 IRS. Suggest looking at what some of the aftermarket chassis people are doing, or at least considering what mods you can adopt. My $.02



TSW

 

"Raising the diff in lowered chassis is done to relocate from what point along the rear toe-steer curve the suspension works to a better position, and minimize necessary static toe in (lessening scrub)"

Approximately what should the difference in height be from the diff UJ's to the wheel UJ's?
Also does anybody have a graph of this toe-steer curve?
Also would relocating the gas tank lower in the frame to improve CG and rear roll center be worthwhile or is that a bad idea?

 

Tho there may well be a more accurate point for any one particular car which could possibly be found thru complex calculation and/or extensive testing, the default diff height setting should result in the inner U's being 1/2" above the outers at static ride heights. While you're at it, given the same applicable qualifications, the default setting for the camber struts is to lower them 1/2" below their stock (C3) location relative to the diff. For more on rear toe-steer, including a chart, see page 20-21 here...

http://www.corvettefaq.com/c3/vip.pdf

Bear in mind that any toe-steer chart is going to be strictly theoretical unless compliance at the TA busings is eliminated by replacing the rubber with sphericals or Johnny Joints (not a fan of poly at this location, nor at the camber struts).

It's common practice among racers and hardcore types to relocate the fuel cell (no mere gas tank) lower in the chassis, thus lowering the CG and shortening the rear roll moment arm. RC's themselves are the result of geometric function.

FYI, if you haven't already seen it, here's more important chassis setup info...

http://www.corvettefaq.com/c3/ChevyPower.pdf