Cashmo

I'm looking for a 1 way or 1.5 way limited slip for a 1968 Vette used in autocrossing. The flywheel/clutch is so light we're having issues with the rear end locking up to much under deceleration with the stock type posi. Does anyone sell such a thing for C3's?

Thanks,
Jeff

Bob3700

Jeff,

Please describe the handling problem that you are having under decelleration?

How do you believe that the diff is contributing to the problem.

bob

Solid LT1

Eaton style posi units are spring loaded just like C4 Vette units. Rear lock-up problems under braking would be caused by too light of a front spring rate soft compression dampening in my opinion. There are 3 spring packs for a Eaton style posi 200lbs/400lbs/800lbs of breakaway force. Stock C3 diffs have 200lb springs. Try Toms differentails for posi spring packs.

Twin_Turbo

C4 vette units are not spring loaded like the eatons
80-82 Dana 44


C4 super Dana 44


no springs!!!

gkull

From what you are saying...... i might be wrong, but i would be looking at weight x-fer by not having enough front spring/sway bars and you need to be looking at brake bias by using smaller diameter fronts than rear tires

69autoXr

Have you talked to Raftracer? I know he used a very light clutch and flywheel combo (15 lbs total) in his 72 BSP car, but of course the diff needs to remain stock for SP. Wonder if he encountered the same problems ever? Some of the others have mentioned springs and weight transfer; with his stiff setup I don't know if there is any weight transfer!

Chris

Cashmo

While possibly a contributing factor, I'm not sure brake bias is the main issue here. The problem is that while on the brakes or even just off throttle, if you turn the wheel the rear end wants to come around. Lifting mid slalom was a scary experience. In one instance, doing 65mph or so on the rev limiter, I was really hard on the brakes in a straight line setting up for a 180 turn. All of a sudden it got really quiet and I could hear the corner workers talking. Since this is a relatively loud car I knew something wasn't quite right. I let off the brakes and the motor caught and lit up again.

Any thoughts of trail braking were out the window in this car. I had to really move my brake points back, brake in a straight line and roll into the throttle when I turned the steering wheel.

SCCA Street Prepared rules state in part...

"P. Limited slip differentials are permitted. This permits locked differentials, either by design, welding, or mechanical means. Differential
cases, internal differential parts, and axle stubs may be machined
as required for clearance and installation to the extent that
material may only be removed, not added, and the exterior of the
case may not be altered in any way. This machining may serve
no other purpose."

From talking to others in the class you can swap in any LSD (Torsen, etc.) as long as it fits in the stock case. Gear ratios can only be changed through update/backdate allowances. Corvette (‘68-’82) is the line in the rule book so any C3 parts can be swapped. FWIW, I believe the spring rates are 1200 lbs front and 600 rear.

The owner has a brake bias adjuster sitting at home that he'll install. However, I'm thinking that a 1 way LSD that works under acceleration but acts like an open diff under decel would be a better solution. A 1.5 way diff that gives you a little engine braking under decel would be the ultimate. I just don't know if such a solution exists for C3's.

I believe Danny might have actually owned this car at one time. He'd like to see a heavier clutch/flywheel arrangement on the car. There's a lot of shuddering going on getting out of first that puts stress on all the rear u-joints. I think Danny just drives around the problem on his car too.

Thanks for the replies. The car was a hoot to drive and I learned a lot this weekend.

Jeff

69autoXr

Interesting...maybe I won't put in that auto flexplate and multi-disc clutch.

gkull

Jeff, I can appreciate what you’re saying if your rear is now acting like a solid axle vehicle. I’m a part time 125 shifter kart racer. They have a floating solid rear axle. You can only hard brake in a straight line. Or else it will loop the kart faster than you can react.

Having a light or heavy flywheel is not the Vettes problem. My rear street tires for many years have been 315/35/17 on 11 inch wheels. I can hit my brakes hard up to 140 mph going into a turn and hear or sense one or the other rear tires locking. I just glace up at the rear view mirror and see a blue haze of smoke and a big single 315 black stripe appearing. I have never in my memory ever had the car loop it’s self under braking since I went to quality tires 20+ years ago. This is including road racing in pouring down rain.

Trail braking is my normal mode of setting up coming into a turn. I have bigger CI higher compression motors with the lightest weight cranks/dampers/lighter flywheels. So my compression braking is beyond most Vettes

I’m now using a 22# flywheel 4.11 rear end with the $700 toms differentials polished posi unit. 85W-140 mobil syn gear oil without posi lube additive, but I do have a metal additive that lowers friction and heat.

69autoXr

This car indeed has a 15 lb flywheel and clutch combined weight, we're talking instant engine braking here. GKull, your 22 lb flywheel probably still has a 25 lb pressure plate behind it.

There's some more discussion here:

http://forums.corvetteforum.com/show....php?t=1632678

Bob3700

Cashmo,

You post stated:
________________________________________ _____________
"In one instance, doing 65mph or so on the rev limiter, I was really hard on the brakes in a straight line setting up for a 180 turn. All of a sudden it got really quiet and I could hear the corner workers talking. Since this is a relatively loud car I knew something wasn't quite right. I let off the brakes and the motor caught and lit up again."

"Any thoughts of trail braking were out the window in this car. I had to really move my brake points back, brake in a straight line and roll into the throttle when I turned the steering wheel."
________________________________________ ______________

I personally don't see how your problem of locking the rear brakes up is a differential problem. It sounds like a brake bias problem.

There is a difference between trail braking and max effort braking. You don't stand on the brakes and turn the wheel. The car will swap ends as the rear brakes lock up. Once all your max braking is completed and the car is stable, you can continue to drag the brake while turning into the corner. To me, that is trail braking.

You can balance you car to achieve "trailing throttle oversteer". Not that is a very fast way to get through some tight corners. Carry a bit more speed into the corner and brake hard. When you reach the turn-in point under braking, you turn in. The rear starts to slide out and you catch it with the throttle and slide/accelerate through the corner. You don't have to wait for the front tires to bite to begin accelerating. That might not be possible on a tight solom course, but on actual race track it works quite well.

The light flywheel and clutch assy is what every roadracer is running. Makes for a quick reving engine and max compression braking. Am not sure what "Too light" is in that reguard. Can't see that as a problem either.

I would be installing the brake bias cockpit control.

Just my .02.

Bob

gkull

What it really comes down to is not just the weight of flywheel/clutch assembly. It is the total weight from the damper with everything rotating inbetween to the weight of the rear tires and wheels.


I'm not sure how much my Tom's posi unit holds the rear tires together under off powered compression braking. I do know that I brake hard in a straight line and ease off as i turn in or down shift. I have never experienced uncontrollable 360 degree looping in my Vette. GoKarting was another learning courve

Cashmo

Unfortunately it also happens without using the brakes. Just lifting with the steering wheel off center in an offset or slalom brings the rear end around, sometimes rather violently. I know the technology exists to alleviate the problem, I just don't know if there's an off the shelf product available for C3's.

Thanks,
Jeff

69autoXr

Yes of course, but all else being equal, the lighter clutch/flywheel combo is going to experience faster (in this case severe!) engine braking.

Bob3700

Jeff,

Man, if the rear end is swapping when you turn the wheel and you are not on the brakes, that is just pure oversteer and can be corrected with a much larger front stab bar or a smaller rear.

I know that when you get into the short courses, you set the car up to eliminate understeer (which is what the cars are designed to do). You want to be able to throw the rear end around. On faster courses, the setup has to change cause of the speed. A little understeer is desirable on the fast tracks, it keeps you alive and your car in one piece.

Try disconnecting the rear stab bar and see how it reacts. You can also adjust tire pressures to provide more rear grip. Increase tire pressure at the end you want to slide ( in your case , the front). You could also try to put more fuel in the rear (adding wt on the rear will help also).

I still don't think you have a diff problem. It is in the handling setup of the chassis. Work with that first.

HTHs

Bob

gkull

I just can not buy that engine braking (that is letting off the gas - not down shifting and over speeding the engine) would cause the rear end to come around on anything, but the most poorly setup or miss matched suspension.

Here is my case. I have driven 360 sprint cars. they have lightened everything to the point of where you have no flywheel, no clutch, no transmission just a direct drive shaft. You push start the car. coming into a turn with 15-1 compression you let off the gass and it doesn't pitch side ways until you hit the brakes. Which is a single disk near the differential for the solid axle. There is no posi unit.

The little post about 1200 pound front springs and 600 rear would make me wonder about how it works or in this case doesn't work. I have a co-worker who owns a historic big block road racing vette, This is a severe duty tack car with 496 ci and the stiffest set of front springs he even carries in the hauler are something like 800 inch pound. One of the last local races he was using some sub 700 pound front springs, something like 680's

RedVettrcr - runs very potent low 400 ci motors road racing and I'm pretty sure that his car is very stable even with his super light multi disk clutches

JackDidley

Just for the record, I rebuilt my C4 posi last spring. It is spring loaded. A conical washer (I forget the technical name) loads each side. they have to be compressed on order to install the spiders. I had to rig a home made tool together because no one local had the J tool.

Twin_Turbo

yes, the waved washer but not springs like in an eaton:

bashcraft

Conical washers (usually refferred to as Belleville washers) are considered to be springs.

Twin_Turbo

man, going into semantics here or what "Eaton style posi units are spring loaded just like C4 Vette units" Look at the eaton above, what do you see there? geezzz