Viva La Differential


Shown in the diagram are the internal components of one of the most highly stressed units in the automotive power train. The job of the differential in a rear-drive automobile is threefold:

Rotary motion from the transmission turns the input shaft and pinion gear of the differential. This meshes with the larger ring gear multiplying torque and converting the motion at right angles to the input shaft. The one-sided contact between ring and pinion creates forces that tend to separate them. These forces are taken by the pinion bearings and the output bearings which support the ring gear carrier. The older long-neck differential had a long pinion shaft with the bearings more widely spaced. This made the unit more durable and was popular with racers.

The axis of rotation of the pinion shaft and the ring gear do not intersect; the gears mesh somewhat below the axis of rotation of the ring gear. This design allows cars to be designed lower, without using smaller wheels, and goes back to the 1930s. Because the gear surfaces actually travel in different directions as they mesh, they slide under load. These are known as hypoid gears and require special lubricants with high pressure additives. The heat generated by this action can raise differential temperature to around 200 degrees at highway speeds, much higher under racing conditions.

The ring gear is bolted to the gear carrier, which conveys motion to the two rear half axles by two spider gears that mount within the carrier on a large pin. Captive between the spider gears are the side gears, which are connected to the output flanges through the sides of the ring gear carrier.

When the car is going in a straight line, the spider gears do not rotate on the supporting pin. In a turn, they revolve on the pin to accommodate different rates of rotation of the rear wheels. The reaction forces on these four gears pushes them outward against the gear carrier and the spacer discs. Tremendous frictional forces are generated, as these gears are not supported by bearings. Excessive wear of the spacers and gear surfaces causes increased clearances between gears. The gears mesh less completely, and the diminished gear contact surfaces are subjected to more stress. Gear material can chip off and entire teeth can break. These large pieces of metal can become jammed between other gear surfaces causing CATASTROPHIC FAILURE. This is one way ring gears break.

It is unusual for a differential to simply wear out from use. The problem is damaging bits of metal loose inside. The ring and pinion gear and the bearings are large pieces of machinery, reminiscent of a locomotive. The weak points in a standard differential are the spider and side gears. The little flakes of metal that come from gear surfaces wind up in the lube oil and get into the bearings. Regular oil changes are necessary, but are not enough. Some abrasive material may always be present. My tip is as follows: cement a large magnet onto the outside of the lower part of the rear cover of the differential. A magnet from an old speaker will do; use RTV silicone and hold it in place with tape while the cement sets. Large and small metal pieces will be held out of circulation, prolonging the life of the unit and preventing catastrophic failure. The rear cover is aluminum and will not inhibit the effects of the magnet.

Differentials are made with the assumption that the differences in rates of rotation of the inner and outer wheels in a turn will not be very great. This is not the case in autocrossing, where lifting and spinning of the inside drive wheel is common. Spider gears are under the greatest stress during these maneuvers. The spinning wheel gaining sudden traction and jolting the small gears can be damaging, especially if there is wear and excessive clearance between the gears. Limited slip differentials minimize the relative speed differences of inner and outer wheels by transferring torque more equally by use of friction clutches. The clutches themselves, however, are subject to wear under these same conditions.

The bottom line is that a differential with excessive wear can deteriorate more rapidly than one in good adjustment to start with.

So what can you do? Change the fluid regularly. Try a synthetic lube, which is more heat resistant. Try the magnet trick. If there are unusual noises, investigate. Grab the output flanges and check for side play. Visible movement is related to spider gear wear. Raise a rear wheel and spin it. A regular clunking sound could be damaged gear teeth.

At this point, the unit should be removed from the car. At least take off the rear cover. Inspection of the gears can tell you if the ring and pinion are o.k.; if they are damaged, special tools will be needed to properly adjust spacing of the new gears. Time to call in a pro or go with a rebuilt unit. Damaged spider gears can be replaced as a set with no special tools. They cost about $150., and are the same for all 1600, 2002 and 320i. Careful spinning of each input flange and feeling for roughness can assess bearing condition. Unless the unit was very low on lube, overheated or submerged in water, the bearings might be fine. Remove the gear carrier, and keep the spacers in the same places when reinstalling. Removal of the ring gear is necessary to get at the spider gears. Reinstallation requires heating the ring gear to expand it slightly to allow the bolt holes to line up. Boiling water in an old pan is usually adequate. Use loctite on the bolts. I have looked into several differentials. Some old units were assembled with no locking compound on these bolts; there were always a few loose ones. One backed out and jammed between the ring gear and case causing gear breakage. Once I heard of a case actually blowing open. In the units with loctite, no bolts came loose.

The factory manual provides adequate guidance for installation of new spider gears. The spacers come in 0.1mm size increments. With an assortment in hand, it is not hard to get minimum clearance; if you put on too many, the gears won't go in. Back off one size, and you will be correct.

The magnet works. I have found broken gear teeth stuck to the inside of the rear cover, along with a large quantity of metal flakes. Be sure and clean out the inside of the case with solvent to remove all metallic particles.

The Torsen limited slip design is a big improvement over spider gears. Worm gears mesh and slide, depending on load, providing variable limited slip action, and are relatively indestructible. Made for BMWs by Quaife engineering, the differential insert costs about $1300. It replaces the gear carrier; you supply the differential. This is about what a complete rebuilt limited slip might cost; however, for the enthusiast who is determined to cause stress to his drivetrain on a regular basis, the increased durability could pay off.

Please send questions or comments to: drautox@comcast.net

Last Modified December 25, 2000