Stock Car Rearends - Lock 'Em Up

When your converted race car came off an assembly line in Detroit, both drive wheels didn't pull all the time. Even with a Positrac, Detroit Locker, or some other limited-slip differential, the drive wheels do not always rotate in sync. Remember, limited-slip differential means limited slip, not no slip.

In a short-track race car, an OE limited slip won't hold up to the rigors of Saturday night racing. More often, the use of a limited slip is illegal at local short tracks. If they let you lock the axles together some other way, why can't you use a limited slip? Probably because of the expense involved, as race-duty limited-slip units can get quite expensive.

For these applications, the spool is preferred when locking up the rearend. Do not expect to find one of these in the wife's sewing basket. In this case, the spool replaces the spider gears, the crosspin, and the ring gear carrier. The spool is one piece, not unlike a saucer with a tube through the middle of it. There is enough flange to hold the ring gear. The tube part is splined inside to hold the axles and machined on the outside to fit the bearings. There is a substantial weight savings here.

In the rearend, the weight of the rotating parts is important for two reasons. First, any weight in the rearend is unsprung weight. This is weight that is not supported by the springs. Therefore, this weight is not controlled by the springs and shocks. In other words, if the suspension were to be cut loose from the rearend, the car could be lifted up and the rearend parts, wheels and all, would still be on the ground. Now the unsprung weight is controlled by the track. It inputs forces into the car through the unsprung weight. The springs and shocks have the job of trying to control the car. The lower the unsprung weight, the easier the job is for the springs and shocks.

In addition to the unsprung weight, there is the weight of the rotating assembly. This weight must use some engine power to accelerate and braking power to decelerate. The more braking force used, the more engine power required to accelerate again.

When it comes to keeping weight down, the spool is one of the best tools to use when locking the rearend. There are spools that have been subject to various machining processes to reduce the weight even further. In this case, less is more. Less weight costs more money. Performance is improved, but the wallet gets thinner.

Enter the mini-spool. The mini-spool eliminates the four spider gears. It fits inside the OE carrier, thus, it's a little bit lighter than the OE assembly but heavier than a spool. It does the job of locking the axles together well. In the Ford rearend, the mini-spool is a one-piece part. It has internal splines for the axles and a hole for the crosspin. The mini-spool for the Chevy rear is more complex. It consists of several parts that replace the spider gears with steel blocks. This assembly takes a while to put together along with some careful fitting. See "Lock It Up," July '03 Stock Car Racing, page 62, for details on this procedure.

The redneck way to lock a rearend is known as the electric method. Electricity is applied to a metal stick that then melts around the spider gears. The more correct term is welding, which has a long history of being the method to lock rearends. The process sounds crude, but it is quite effective and has been used for many years. It can be done with the entire rearend in the car, in the case of the Chevy or any car with a rearend that has a removable back cover. The axles can be removed, and the carrier assembly caps can be taken out so the carrier can be removed. Watch for any shims and note their position for reassembly. Still, it is best to remove the rearend from the car. This is because you have a better position when doing the welding on the bench instead of under the car. The drawbacks to welding the spider gears are the lack of weight reduction and the possibility of weld spatter contaminating the rearend housing and warping the carrier, but both wheels pull together.

To weld the spider gears, some preparation is necessary. Of course, you should drain the lubricant and clean the entire part. Begin by brushing naphtha on the part and wiping it to keep it as clean as possible. Next, to get it really clean, spray brake cleaner all over and let it drain. Do this until it drains clean. Use brake cleaner in the red can, not the green can. The red can does not burn, but the contents of the green can makes you wish for a package of hot dogs and a long stick. Once the spider gear area is clean, allow it to rest until it dries thoroughly. If you start to weld and you feel a stinging sensation in your nose, stop and let the brake cleaner dry some more. The brake cleaner fumes are bad for you. Getting the area really clean increases your chances of applying a good weld.

Look at the spider gear area and decide where to weld. Too much heat for too long will warp the carrier. When this happens, there will be a tight spot as you rotate the pinion around the ring gear. To avoid that fate, weld one spider gear, rotate to the other side, and weld another. Don't just weld-stop, look around and admire your race car, and let the weld area cool before you go back to welding. Following this procedure will ensure that you don't warp the carrier.

When welding, weld spider gears to spider gears first. When all the gears are welded to each other on both sides of the carrier, weld the spider gears to the carrier, again going from one side to the other.

The actual welding can be done with an arc or MIG welder. A MIG welder is preferable because it is cleaner and, in this case, it's easier to see what you have done. With arc welding there are small pieces of flux you won't get out; however, they will possibly come loose later. This flux is glass. Think of what glass powder does when it goes through the gears.

At this point, you have locked the rearend of your race car by one of several means. Reassemble everything, making sure it is clean. It seems to go without saying, but don't forget to clean around the pinion bearing behind the carrier. This is an area where weld spatter, slag, or trash can remain hidden. Rotate the pinion several times. Do this with the open face of the differential down while spraying a cleaner into the bearing area.

If you use a silicon type of sealer instead of a gasket, use it sparingly. When the cover is tightened, the silicone squeezed on the outside is about the same on the inside. Don't overdo it.

To recap: a spool is best; a mini-spool is good; a welded spider gear unit will work.