VegasJen

o.k. so i've been looking at the rear of my '81 and trying to figure out what the real differences are between it and the "revolutionary" c-4 suspension. the biggest differences i can see are 1) the third member uses a torque arm to mount the front instead of to the rear crossmember of the frame. what is the purpose of a torque arm anyway? 2) instead of a single pivot point on the trailing arm as with the c-3, the c-4 uses two pick up points to the frame and two on the bearing carrier. why is this an advantage? 3) there are what appear to be tow or strut bars off the rear of the diff cover to the bearing carrier on a c-4. i can see no other major differences. are there any other major differences and why are the differences i listed above so much better than what our c-3s have?

Ganey

The primary gain in handling in C4 was the tires. See 77 Facts on my website. :cool:
I'll let Dave (ddecart) tell you about sus..

Jason Staley

Well, you've hit most of it. The biggest difference is they went from a trailing arm arrangment to the two parrellel links & the toe end bar which goes to the center section. This changed fixed the toe out problem that occurs on C3 rear suspension. The C3's rear suspension's toe-in is affected as the wheel moves up and down. If the 1/2 shaft is parallel to the ground and the wheel moves up or down, it moves toward the center of the car in an arc. This causes both wheels to toe out in a corner because one wheel moves up and the other moves down, but the forward pivot points on the trailing arms are fixed in place relative to the chassis. The C4 suspension fixed this by using two parrellel links which allow the tire to move up and down, but they don't control the toe-in of the wheel. That is where the toe-in link that goes to the center section comes into play.

However, there is a pretty good fix for the C3 suspension. I've done it, and it makes the car more predictable in the corner. It is covered in great detail in the VIP (vette improvement program). You can find it by doing a search on this site for VIP. It's been on here several times. Here are the basics, what you do is make sure the inner u-joints on the 1/2 shafts are at least 1/2" higher than the outer u-joints next to the wheels. This way as the wheel moves up in a corner (the outside wheel) it actually moves out and thus it toes in instead of out. The inside wheel will toe out but that has little effect because it is lightly loaded, and it actually points out but in the right direction this time. The main problem with this fix is the car will probably sit too high by the time you get this measurement. To lower the car back down, here's where the fun really begins. :crazy: :crazy: You have to move the DIFFERENTIAL & CROSSMEMBER UP!!! relative to the frame. This required me to remove the factory rubber crossmember bushings, flatten the metal "bushing retainers" on the frame, and fabricate some polyurethane bushings (I didn't want to hard mount the x-member as described in the VIP) to move the diff & x-member up 5/8 of an inch up in the chassis with no increase in gear noise from the rearend. My corvette is lowered, so if your is fairly stock you might not have to move the x-member up - haven't given this much thought since mine is lowered.

Regretfully, I don't have any pictures, but if you want I can sketch up some drawings to help you better visuallize what I did. :cheers:

shawn_cake

The best fix is to use the Guldstrand 5-link rear suspension. I really costs big $$$ - but it really works. I have this setup on both my BBC and SBC vettes - if you like to corner in a C3, this product is a good investment..


[Modified by shawn_cake, 12:05 PM 1/27/2002]

flynhi

Hi Jason, I read the Greenwood article very differently and actually talked to John about it several years ago ( he was very friendly and cooperative). As I read it the problem was with camber and was due to the inner attachment point for the strut rod being too high. Early production models for this design (63-67) had a bracket that had a very high attachment point, C3s (68-80) had a different design to lower the attachment point to reduce camber change in a turn. Greenwood suggested adding a 1/2" alum plate to lower the C3 bracket further. Vette Brakes took Greenwood's idea and fabricated a "Smart Strut" bracket to allow a lower attachment point with some adjustability.
Greenwood also does not recommend setting the outer u joint higher than the inner due to other geometry problems. So I don't lower the rear beyond the point where the half shaft is parallel to the ground.
Also see Norvals recent thread on the 5 link mod for C3 suspensions.
All that being said, I think there's more improvement to be had from 17" alum wheels and performance tires.

ddecart

Thanks Ganey. Ummm..... :D

It has been a LONG time since I've looked at a C4 suspension. Does anyone have a link to a pic or diagram of one? Lemme poke around and see what I can find....

At work I should be able to dig up some old C3 test data and compare that to some C4 data as well.

Jason Staley

flynhi,

I agree with you that the camber issue is probably the worse thing about a stock C3's suspension. I didn't fabricate this part of my mods, but was lazy and bought Vette Brake's "Smart Struts" which resulted in a big improvement. I also added 17" alum wheels and modern perf. tires. All of these mods really help bring the old C3 suspension up to modern standards - or at least a whole lot more predictable. I haven't had it at the track yet, but I would like to do some solo events and/or low speed autocross just to see how it does.

Another benefit to these mods, is that the car seems to ride better too. :) :)

kaiserbud

Hey Guys - I searched for Vette Improvement Program and found one article - no alternate web page was noted....can you provide further info?

Jason - Flynhi - you guys have 17" wheels - I wanna see !




[Modified by kaiserbud, 11:00 PM 1/20/2002]

VegasJen

thanks for all your input. i had a suspicion that the rear tow rods were to keep the rear wheels in the direction of travel since the c-4 essentially doubles the amount of bushing and therefore available deflection in the trailing arm assemblies. the list of suspension mods so far fairly extensive but the "smart struts" were the next major change for me. to date the mods are: 1) all poly bushings 2) 460# front springs 3) 1.125" front bar 4) 5/8 rear bar 6) koni adjustables 7) front "spreader" bar 8)'93 base coupe saw blades with firehawk sv50 all around. the difference is phenominal, of course, that doesn't mean i'm done! :smash: next on the list is "smart struts" and then front transverse leaf spring :D then it'll just take another 200hp and maybe a 6-speed and some new brakes......

flynhi

Jason,
You've really put together a nice package - the way GM should have sold it originally.

Clutchdust reminded me to mention the front spreader. If you plan to autocross, you must include this mod. It really stiffens the front end.

Can you post a pic of your PS wheels with Nittos?

Jason Staley

Thanks flynhi for the advise, but I'm one step ahead of you. :) I've already bought a front spreader bar, but haven't got it installed yet - too many projects :jester :jester .

I've also heard that it will stiffening the front of the car. I also plan on welding the extra gussets in the frame and welding up the frame as described in the Chevy Power Service Manual (I think that's the name of it) - probably won't be able to get to all the areas since the body is still on the car. They say that welding up the chassis helps stiffen it - makes since. Hopefully, not only will it handle even better, but I can get rid of my last few little annoying squeeks :blueangel: .

By the way, here's a link to my page and a pic of my car. :cheers: http://temp.corvetteforum.net/c3/greatwhite/

Jason Staley

kaiserbud,

just found a link to the VIP article and some other suspension articles:
http://www.corvettefaq.com/susp.asp

the VIP article is at the very bottom of the page. You can download it in PDF format - it's pretty long.

flynhi

Jason,
You're lookin good!
Do you have a link to the Chevy Power reference re frame stiffeners?
Thanks,Will

Jason Staley

Here's the link:
http://www.corvettefaq.com/misc.asp

It's listed as Chevy Power Book for Corvette . It's a pretty big PDF file - about 8 megs.

ddecart

Hey guys. I've foudn only a couple poor pics of a C4 suspension, not really enough to get a good visualization of what's going on. If I saw it correctly, the trailing arm was replaced with 2 trailing links. The camber rod is still there. The halfshaft is still a loaded member of the suspension.

What the dual links will do is a couple of things. One, they will change the side view swing arm of the rear suspension. Cool, huh? :D Ok. Look at a side view of the C3 suspension. The forward end of the trailing arm IS the pivot point. Not so in the C4. The intersection of lines drawn forward from the links is the effective pivot point. This will affect anti-squat or anti-lift in acceleration and braking most likely (depends on where that point is exactly). The links will also allow a more true up/down movement of the knuckle . Th knuckle can move up and down instead of rotating in a curve. I think this should alter the toe and camber curves to some extent.

I'll see if I can find some kinematic and compliance data for the suspensions. That's where the real answer lies.

clutchdust: One note on "doubling the amount of bushing and therefore available deflection" Not quite so. The C3 suspensionn has a pretty sizeable bushing and the bushings on the C4 are smaller. What's important is the design of the bushing, the material, and how it reacts under load. 2 bushings, even if they were identical in size to another single bushing could together be stiffer or softer than the single bushing. It depends very much on the design and the loading that those bushings see.

Dave

VegasJen

thanks to all who responded. although it seems the c-4 suspension was a marked improvement over the production c-3, it appears the aftermarket has effectively created do-able solutions to the c-3 shortcommings.
to the effects of the spreader bar, i just got mine for christmas and i haven't autocrossed it yet but the seat of the pants feeling--i sure thought it felt tighter. maybe it's just my brain feeling like the money was well spent but i sure do think it tightened up the car. as some of you have said, i also plan to straight weld the frame (only the bottom, since i'm not pulling the body) in addition to some of the other things i still have to do.
one other question no one tackled yet: what is the purpose of a torque arm as in the c-4 and late model camaros that go from the tranny to the diff?