Hey guys,
I plan to drag race soon and I'm beefing up the drivetrain. I read somewhere a suggestion where they said to replace the cushions in your rear crossmember with solid aluminum disks. Does anyone do this? I am having my diff. rebuilt and bought new rubber front mounts but I'm wondering about the crossmember bushings. Ecklers has a disk that goes over the top of your bushings to stiffen them up. How stiff do you want the rearend? What do you guys suggest?
Thanks

 

If you're putting slicks on a C3 w/IRS, the solid locating kit will help, but you'd be better off removing the cross-member and replacing the rubber with solid bushings and then installing the plates. Don't know of any available source for these (if someone else does, please share), so you'll probably have to fab your own. The more abuse your Vette is going to see, the more important this will be.

While you're at it, it would be a real good idea to raise the diff in the chassis to help keep the U's from binding and to reduce any toe-out steer you might otherwise experience in extreme bump.

Just in case you're not aware, for drag racing you want to eleminate camber change in bump, so you need to lower your camber strut rods to parallel with the half shafts (or parallel with any alternate upper links). It takes about an inch to accomplish this. Don't trust eccentrics to hold their adjustment. Use camber lock plates and adjustable, true spherical strut rods.

You said you're beefing up, so I'll assume you know what you need to do to accomplish that.

 

the earlier editions of The Chevrolet Power Manual describe replacing the diff crossmember rubber bushings with solid steel. Recommended by The General.

Fabrication required (since nobody makes a kit).

also replace the front diff biscuit with a solid bushing (e.g. aluminum).

 

Wow guys, thanks for the info. This forum is great! I am finding that Corvette modification knowledge is highly specialized. I very much appreciate your sharing. Most muscle car or racing guys I know don't know the first thing about Vettes. They ARE unique in a really cool way.
I will definitely make some solid bushings. I work in a machine shop so it won't be a problem. I could easily make extras if anyone wants some. What would be the best material? Just regular soft plate steel? Wouldn't aluminum be better for weight? rpoL98 suggested aluminum to replace the front bushing. That was my next question. If I go solid on the crossmember I want to go soild for the front mount, right? That front mount gets all crushed when tight, how thick should the solid piece be in relation to the uncompressed new bushing? The same with the crossmember bushings, how thick should I go? Couldn't I reduce the angle of the half shafts by making all three bushings thicker? Is there a better way to lower your rearend? Do I need to keep the rear end at the same angle (front to back) if I lower it? Should I make the crossmember bushings tapered to accomodate a different angle? When we lower the rearend, aren't we increasing the angle on the front driveshaft where those tiny little u joints are?
As far as lowering my strut rods, should I modify the strut rod support bracket to lower the mounting points in the middle? Is that the best way to get them in line with the half shafts? If so, won't the length of the struts or half shafts have to change? Where can I read a copy of this Chevrolet Power Manual? It sounds good. Also Skunk, I'm not sure what you mean by toe-out steer. Is extreme bump when you launch and the car squats? I appologize for the dumb questions, I'm new.
Thanks guys!

 

Guldstrand made a kit, but I don't think it's available anymore.

Here's the thread with the dimensions. I'll see if I still have the pic to re-upload.

http://forums.corvetteforum.com/show....php?t=1601099

 

And here's the pic.

 

The Power Book recommends steel plate for replacing the rubber diff cross-member mounts. This one's still on my do list, so I can't get into much detail on the "how to". The solid front diff biscuit (haven't measured mine) that comes with many locating kits works good, but for drag racing you may want to add a traction bar designed for the C3. If you raise the diff in the chassis, you'll need to mod the front mount accordingly.

As for reducing the angle of your half shafts, you don't want them level with the ground sitting static. That will induce toe-out steer (more on that below) in bump (or squat) and rebound and could lead to bottoming the U's when you launch. The whole idea behind raising the diff is to actually increase that angle to avoid these issues when lower ride heights (suspension geometry wise) are used or when excessive bump is experienced from 100% load transfer to the rear axle. A stiffer rear spring can also help this issue by reducing suspension travel.

Another thing the regular hot-rodding guys don't know about Vettes is how to set pinion angle. Our diff doesn't move, so you're looking for an equal but opposite angle from that of the trans output CL, with about 2-3* being optimal if I understand U-joint dynamics correctly. You want / / not / \, and you certainly don't want these CL's concentric.

You can mod your camber bracket, but why? An easy way to lower the camber strut inner links is to make a spacer plate to put between the diff and bracket and get longer bolts, or just buy a modified HD piece such as VB&P's #52000B (be sure and mention your CF membership for a 10% discount). That one has vertical slots, as opposed to the factory's horizontal ones, which allow you some flexability in locating the height of the camber strut inner links. Toss the eccentrics that come with it and fab camber lock plates for whatever final height(s) you need.

One small note: the holes for the bolts that mount this bracket to the diff are currently being manufactured oversized, so you'll probably want to bush them down if they haven't addressed this minor issue by the time you get yours...



The Power manuals are hard to find, but here's a link to a PDF of the Corvette Chassis section you can download. It's got a lot of good info any C3 owner can put to good use, but bear in mind that it's a road-race prep guide....

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

Bump and squat are essentially the same issue for what you're dealing with. When a C3 sits down on the rear suspension its track width changes. However, the front trailing arm link is fixed, so the direction the rear wheels point changes with track width. With the shafts level with the ground at static height, they are as wide apart as they can get. Any movement up or down from that static height will cause the track width to decrease, resulting in the rear wheels moving towards toe-out, which is an unstable condition. If you start with the shafts angled above level, then bump or squat will increase toe-in, which is a stable conditin, as the shfts become level. Hope that helps you visualize how that works. I just grasped it recently myself.

As the saying goes, "The only dumb questions are the ones you don't ask." There's a couple of guys here that love to chide others for not being as wise as they are, but for the most part you'll find lots of friendly help.

edit - I see I'm Batman has posted some cool info on the mounts. I want some!

 

Forgot to mention, the heim-jointed spherical camber strut rods you should get are themselves adjustable in length. This will compensate for any height change of the inner link points as well as replace the need for camber adjustment eccentrics. Lots of Corvette aftermarket places such as Guldstrand have them, and they usually come with a set of lock plates.

 

Hey Batman,
Thanks for the dimensions. That helps alot. I am wondering if I still need those locating plates over these or if these dimensions will make them fit tight enough to provide for correct locating? I have not taken my crossmember off yet as I ran into another problem. The shockmount. http://forums.corvetteforum.com/showthread.php?t=1789329

 

Skunk,
Thanks for all the information. Besides the camber changing and the track width getting wider, the wheels point in when the car squats? Wow. There is more going on here that I realized. I'm still trying to fully grasp how these cars act when the suspension moves. I guess you need to fully understand it if you want to take advantage and set it up perfect.
I made a spacer plate today as you suggested. It is 1" thick and will fit betwwen my differential and strut rod bracket. Is there a way to measure how close to parallel my struts are with the half shafts once this is back together? Will I want to alter the spacer thickness if it isn't exact? Or will a 1" lowering of the inner strut rod mounts bring things close enough to eliminate camber changes? Will I want a professional alignment after this? How do you set your camber? Is this modification only useful for drag racing? Will the car handle as well in corners when setup like this? What is the optimum camber setting for drag racing? Won't that setting be different now for me that my struts are parallel with the halfshafts and no longer have to allow for camber change during acceleration? Lots of questions, I know. I'm hungry for Vette knowledge.
Thanks.

 

Re-watched "Old Scholl Corvette Racer" recently which showed a clip from a camera pointed at a rear tire of Peter Klutt's C3 while he was hammering around The Glen, and yes, there's a lot going on in the C3 rear suspension.

Just making sure you've got the right picture on this... The rear wheels will only toe-in during bump (or squat) up until the shafts pass level with the ground. That's why you don't want to begin with them level in static, in which case the wheels have no where to go but towards toe-out in droop or squat.

Ideally, to keep the tire planted at the drag strip, you'd want equal length, parallel upper and lower links, which would produce no camber changes in squat, so that 1" spacer plate should put you pretty close to eleminating your camber gain. The down side is that the ideal drag setup isn't all that great for handling curves, because the camber on the outside tire will now head towards positive in roll, or as the car leans in turns. That VB&P bracket I mentioned allows you to easily eleminate camber gain when you get to the strip without being stuck with that compromised geometry the rest of the time. (The RoT handling setup for camber gain is apparently to have the inner strut links 1/2" lower than stock.)

Yes, absolutely get an alignment, preferably where you can watch. For any other type of driving than hardcore drag racing VB&P's recommendations are a great place to start, but if you're chasing the last tenth at the strip you'll want to set up differently. Let me know if you intend to go that direction, and I'll pass on some alignment stuff that I've found to be worth several tenths in the quarter.

 

great answer, there were even some things I didn't know. Now I also know why the dragvette setup is not all that good for street use and why I need to eliminate the springs in my rear diff, so that movement of the axle is minimum. Looks like I have some work ahead of me.

Thanks

 

I have the dragvette 6 link kit on mine along with VBdual mount spring setup. I have a urethane front mount and I made aluminum discs for the rear mount. The car handles great, I keep up with C5 and C6s at the autoX and I have about .1 degree of camber change through the arc of travel. Yes it might not be the ultimate for road racing and I might be able to squeeze a little more on a road course with different suspension but for a car tha tgets drag raced street driven and autoX it is a hard system to beat.

Here you can see some of thesuspension and the aluminum discs. I have since gone to a slightly longer spring and have had it aligned and the shafts are now more parallel to the ground.

 

For now I'm planning on primarily drag racing. The Dragvette setup is interesting, but for now I guess I'd like to just strengthen and fine tune the standard setup.
Is there an ideal angle that the half shafts should be at when at extreme bump? Can I set my ride height with my spring bolts to try to achive that angle at extreme bump? I was thinking of putting my video camera under there and launching hard and seeing how it looks, heh heh. Right now I have a soft rear spring. A local Vette place had suggested that adjustable or drag race shocks could allow the car to squat less at launch, even with a soft spring. Does that sound right? Are these available for our cars?
If you have any suggestions for my camber or suspension settings or anything for straight drag racing, that would be great. I have my whole rear drivetrain and suspension torn apart right now. I'm painting and fixing and replacing old parts with performance stuff as much as I can afford to. I bought a ton of new parts. I was going to bring that up in another thread and see what you guys think about my choices and what my weak link might me. Being all ripped apart, this seems like a great time to go ahead and set this thing up nice. I appreciate all the advise. Thanks!

 

Since, in an ideal world, you want to have your half shaft angles not go beyond level with the ground in squat to avoid that toe-out steer situation, that's your real target for determining rear ride height for the strip. You may not be able to achieve this in the real world, but it should be worth doing as much as possible to minimize this characteristic. You'll have to test to find out how much travel from static to full squat you have to know how far above that point your static height ought to be. In any event, stiffening the rear spring will reduce this distance without effecting weight transfer, and raising the diff in the chassis may help avoid surrendering a low ride height.

A common misconception about dampers (or shocks) is that they effect the amount of weight transfer. This is incorrect. They effect the rate of transfer, not the amount. For the drag strip, you want front shocks with very soft (or fast) rebound settings to allow weight to come off of the front as quickly as possible so it can head to the rear. Stiffening bump in the rear as a crutch to reduce squat works to a point, but at the cost of slowing down weight transfer onto the rear.

During the first moments of shock loading drag slicks can actually induce droop in the rear suspension before weight transfers and squat begins, so rear camber probably needs to be zero at static ride height with your anti-camber gain geometry keeping things straight in that plane. Toe-in should be set so that it never goes beyond zero to toe-out at any point in normal suspension travel. The stiffer that rear spring, the less toe-in you'll have to run, resulting in less rolling resistance down the track. As for the ultimate number, you'll have to test to find that for your specific car.

The last thing to dial in will be front alignment, which is all about eleminating tire scrub. Get some video or pics of your car under full accelleration in race trim about half way down the quarter. From that you can estimate the front ride height the car is at during most of the pass. Have your alignment set up at that height. Camber should be zero with front toe at zero for solid, poly or delrin control arm busings. With rubber bushings, you need about 1/16th toe-in to compensate for their compliance.

You will find having your own tramel bar and knowing how to use it to check toe to be an invaluable tool to have at your disposal. They don't cost very much, or you can easily make your own. Here's a great link that goes over using one... http://members.shaw.ca/rjsill/tech.htm

I've been away from the drag strip over on the road courses for a number of years, so someone else who's current on drag shocks will have to fill you in as to what's available nowadays.

I could be making this stuff up as I go just to see who believes it, so don't anyone hesitate to jump in here if I've messed this all up. I'd rather be corrected than leave disinformation hanging...

edit - For anyone inadvertantly taking this out of context, this is drag strip info. Setting up your C3 in this type of manner will compromise handling for anyother type of driving.

 

Those parts should be all you need. I haven't had mine back on the road since the install, but it's getting very close. I'll give a report when it's running again.