another-user

im pulling the diff out of my car this weekend. right now im trying to remove the halfshafts, but the diff isnt locking up, which is makeing things difficult to say the least.

the cars in gear but the rear end is spinning freely. thats not supposed to happen, is it?

weimer20

Man. trans?

BTAL

Still have your T/As and e-brakes?

another-user

yes. its a manual trans. still hooked up to the TAs, and the Ebrake is not set.

S489

maybe you could have someone stand on the brake pedal while you break all of the bolts loose . . .

stinger12

Are you just trying to unbolt everything and it is spinning on you? Just stick a screwdriver in-between the rotor fins and brake caliper - it will prevent everything from turning so can break the bolts loose.

weimer20

Hmmm, man trans, in gear, driveshaft in, half shafts in, and wheels turning? Boy that doesn't sound right to me.

another-user

excellent idea, if i still had the calipers on. ive been working between pulling the TAs and diff out.

another-user

i just went and checked something.

the car is in gear and the halfshafts are spinning freely, but the driveshaft is not moving at all.

weimer20

If the wheel is still on, get someone to hold onto a lugnut with a tire iron to counter the torque you are applying. They should break loose long before you are able to match the torque applied by the tire iron.

stinger12

If the calipers aren't on anymore, then just have someone stock a long heavy punch in the rotor fins and tell them to hold it while you break the bolts loose

weimer20

That does not strike me as something normal at all on a posi equipped car.

stinger12

Someone might have "tuned" your posi unit without springs. If you rotate one yoke, does the other yoke spin in the opposite direction? If so, then that is a characteristic of the springless posi unit. I just tuned my posi unit to Gary Ramadei's specs, and you can turn one yoke while the other yoke spins in the other direction - in other words, the driveshaft does not need to turn for the two yokes to turn, because the posi clutches are not locked and preloaded by the springs. When both wheels have the same amount of traction, the 10 tooth spiders prevent the yokes from turnign against each other and essentially lock up the diff and they tunr together.

another-user

ya... i dont know a whole lot about this stuff, but that sure as hell doesnt seem right.

no, they are turning together in the same direction.

S489

ditto stinger, or use long bar on studs

stinger12

On a stock posi unit, the springs preload the side clutches. There is no posi action until the yokes break the 70 ft/lb threshold to break the posi clutches loose, which then provides posi action. Removing the springs and shimming the clutches painstakingly does not lock up the diff. Essentially, the entire diff always has posi action and there is no pressure on the side clutches, When the tires spin at the same rate, the two 10 tooth spiders counteract eachother and the unit locks up like a solid axle unit, thus there really is no need for the springs as long as you shim the posi unit properly.

another-user

that sure sounds like whats going on here.

ill have to see what i can do about getting a tire on there and holding things still.

Lightweight 001

Which is also known as an open differential. Open differentials are not recommended for any big horsepower performance application or a Corvette. Here is some information I have posted on my website at www.duntovmotors.com which will explain the Corvette limited slip differential.

The Open Differential:

In an open differential, as long as the car is moving forward in a stable state, driven by the pinion gear (engine power), both tires are being driven, and the spider gears in the differential are stationary in relation to the side gears. Resistance torque from the tire patches is transferred to the differential by the stub axles. As long as this resistance is equal, the spider gears are stationary within the differential carrier. When this torque is unequal, the engine torque, through the pinion gear, drives only the wheel with the least resistance, which causes the spider gears to spin in relation to the speed differential.

When your car ‘turns in’ to a corner, resistance on the inside wheel is reduced because it has to roll a shorter distance due to the difference between the inside and outside turn radiuses. On the front axle this difference is accommodated by Ackerman steering geometry that causes the inside tire to turn more sharply than the outside tire.

In a turn taken at speed, the inside rear tire patch is reduced in size as the car’s center of gravity causes the car to lean. This lean plants the outside tire while lifting the inside, causing the engine torque to drive only the inside wheel, the one with the least resistance. In a high speed-high horsepower situation the inside tire tends to break traction. Bottom line: It turns in great, but you can only put the power down on the unloaded wheel and you end up accomplishing nothing but tattooing the racetrack with a single black stripe.

The Locked Differential:

In a locked differential, the spider gears are locked to the side gears at all times, as if they are welded together. The differential’s only job is to equally split engine torque and redirect it to the rear wheels. You could never use a locked differential on a street driven car, as slow speed tight turns would break the axles in short order. You can use a locked differential on high speed racetracks if you can successfully accommodate the handling problems inherent with this setup.

With a locker, it doesn’t matter whether one tire is on a wet spot and the other on dry pavement; both wheels get equal torque all the time. When you get to a turn, the front tires don’t want to turn at all because the rear wheels are tracking equally and the only way the car is going to turn is for something to slip. Since nothing can slip inside the differential, either the inside rear tire must be induced to slip, or the front tires are going to slide, or there is going to be a combination of both.

Here is where a rear stabilizer will help. A rear stabilizer bar lifts the inside tire, which, along with the natural roll of the car on its suspension, causes the inside tire patch to further shrink which helps to induce slippage in the inside rear tire, which helps reduce front tire slippage (understeer).

The Corvette Positrac Differential:

The limited slip Corvette Positrac differential is an open differential with a spring loaded clutches that limit the speed differential between driven axles, thereby theoretically giving us the best of both worlds. It allows the car to turn in well, and the inside tire is discouraged from spinning wildly on corner exit. The clutches transfer the torque through the spider gears to help equalize the torque split.

There are three different preload spring sets available for the Eaton differentials, but only the one 200 lb. conical Belleville spring on the Dana. The higher the spring rate the more torque can be transferred across the spider gears. There are also two different clutch pack materials available for the Eaton.

On the Eaton differential, the 200 lb preload kit with steel clutch plates is the standard set up. It works just fine for a street car.

Eaton makes an optional carbon fiber clutch pack that uses 400 lb springs that works much better. You can’t get by with anything over 200 lbs of preload on a street car with steel clutch plates because low speed maneuvering (as in a parking lot) causes the clutches to ‘clunk’ and chatter. You can use the 400 lb. preload springs with the carbon fiber clutch discs on street driven cars because the natural lubrication (graphite) of the carbon fiber clutch discs allows smooth low speed operation. The 400 lb springs allow more transfer of torque, but with the carbon fiber discs there is another quality that enhances hook-up: When they get hot they expand and really hook up. We have used the 400 lb preload springs with the carbon fiber discs in racing with up to 700 hp, and it worked fine. They also work really well in street applications.

The 800 lb preload kit is a racing only set up, and is the greatest for hook up, but it may cause your car to push, but not nearly as much as would a locker. The choice of differential preload springs comes down to how much power you have, the size of the tire you run, how your car handles, how tight is the racetrack, etc. If you only have 400 horsepower, you probably don’t need 800 lb. springs. If you are ordering a racing differential from us and would rather have the carbon fiber clutch pack with 400 pound springs, just indicate that in the notes section of the check-out sheet. The change won’t affect the price.

To set up for a new racetrack, we used to station our crew chief at the most important turn at the track so he could observe the tire tracks on exit. If the inside was heavier than the outside, we put in heavier preload springs. At Atlanta, that is turn 7, the slowest turn on the track, and the one leading to the longest straight.

If your car is already pushing (understeer), using heavier differential clutch preload springs will make it worse. If you need the additional traction out of a corner that you will get with the 800 lb springs, you might want to address that push by one or more of the following: soften the front bar, soften the front springs, or try a rear sway bar. Remember, a little rear sway bar goes a long way. A stock rear bar may be all you need.

If you need a differential, or just some help, give me a call at 972-243-3838 or visit our website at www.duntovmotors.com.

another-user

well, that was about the most informative thing ive read yet.

ill find out whats going on when i crack the diff open next week.

stinger12