Adjusting A Differential
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How I Tightened my Rear End
-or-
Adjusting a Spitfire Differential
Alum. No, not really. It was done with $5 spherical washers.

by Steve Schultz Illinois Sports Owners Association

Before discussing how the job was accomplished, it might be useful to review a few more basic things. First, two reasons to rebuild: 1.) It leaked oil very badly, and 2.) it was very noisy. Not the standard differential 'whine', more gear clatter, especially when going into reverse from a stop, or cornering at normal speeds. When the car was jacked off the floor, there seemed to be excessive free play when turning the rear wheels-one could rock one tire without movement in the other. After rebuilding the entire rear end-hubs, bearings, axles, U-joints, nothing seemed to resolve the problem. There were no real performance problems, however. So once again, a Triumph owner with nothing to do.

The differential was partially disassembled at the 2002 Transmission clinic, and incorrectly diagnosed excessive wear as the cause of the free play. We noticed that the small gears (see parts 7&8 in the drawing) had excessive free play. Especially problematic were the side gears (8) which could move nearly 0.1" in and out. They could easily be twisted off axis as well. The smaller pinion gears (7) appeared tight on the shaft, but moved in & out quite a bit-maybe 0.03". At the time, we determined to discontinue the rebuild and go on to other Triumph adventures. (A second-hand differential was purchased and installed into the car and driven last Summer.)

In thinking through the design and function of this wonderful device, it seemed reasonable that a few small shims appropriately installed could remedy the situation. The rationale being simple: The side gears (8) actually are centered by splines in the axle stubs, and shouldn't really wear much against the diff cage. Next, because both side and pinion gears are conical (tapered), free play will increase dramatically if a gap is allowed to develop between them. But more importantly, none of these gears actually turn during normal operation, so wear should be a minimum even in a Triumph! More on that latter in the Design & Operation section. So, off to the Victoria British catalog, and Bingo! Replacement washers available. Sort of a 'one size fits all' strategy, since all that's available for my car were 0.041" thickness, but that's a start. Now before starting out on a differential rebuild, it becomes immediately obvious that there's a lot more written cautions about this assembly than any other. And that's where the famous "Stagmeisterspreader" comes in. Joe made a special press (or puller, as it might be) to attach to the top casing of the differential to allow assembly of the cage back into the housing. The tool is an absolute "must use" due to the very tight (no clearance) fit. So with little to loose, and knowing the transmission clinic was upon us for 2003, disassembly started at home.

Removing the cage from the housing was easily done with a drift and light hammer taps. All parts were carefully marked and bagged to keep left/right sides separated. We'll need to know the total spacer stack-up (parts 12) to be able to set appropriate crown/pinion free play on assembly. Unfortunately, to remove cross pin (5), the crown gear first needs to be removed from the cage. That was actually a difficult operation since the attachment bolts were lok-tighted in. The gear orientation was marked on the cage to guarantee reassemble as the original. By the way, treat the crown/pinion with respect-no heat or rough handling here. Victoria Brits sells the set for $190, and they are matched. I don't know of other sources, except in complete assemblies. So a nicked tooth is about a $200 problem.

Once the crown was removed, the cross pin easily tapped out, and all four smaller gears could readily be removed (7&8). Sure enough, the washers were well worn, confirming suspicions. As it turned out, the two new washers (.041", parts 9) were added and combined with the thinner of the original set to produce an assembly with what felt like appropriate gear play. Application of the "Stagmeisterspreader", and Bingo! (again), the assembly was complete. Fortunately, crown/pinion free play was good. Add new oil seals and we're done. Considering the differential did not whine when last used, this all seems to make complete sense. So, as soon as the snow melts (?) and the car comes out of storage, we'll find out more.

Differential Design & Operation. Why is the gear mechanism so complicated? Actually, it's quite simple, and very ingenious. I believe it took the auto industry quite some time to perfect the concept, and since then all cars use a similar design (although hydraulics have modified a few things recently). The key benefit provided by the differential is to allow the tires to rotate at slightly different speeds when cornering, when both wheels are still firmly connected to an engine (transmission) operating at a constant speed. It also makes bumps a bit easier to absorb. The first cars had a simple crown/pinion gear attached to a solid shaft between both tires. That works well when going straight, but as the vehicle turns, the inside wheel needs to turn slower since the arc is a shorter distance compared to the outside wheel. Not only is handling poor, but both wheels slip on the pavement (The outside wheel goes too slow, the inside too fast). That makes for bad tire wear. The next step was to split the axle, and power only one wheel. Not such a good idea either. Kinda like one leg longer than the other.

The differential design solves these problems at minimum cost. Here's how: The crown/pinion gear assembly always rotate with the speed and direction of the transmission shaft, as they are directly connected to it, except for the gear ratio to the crown. The differential cage is bolted to the crown gear, so it also rotates at that speed. Cross pin (5) is captive in the cage so it always rotates with the speed of the transmission, as geared down by the crown/pinion ratio. Under normal, straight ahead operation, both wheels rotate in the same direction and at the same RPM. Because gears 8 are splined to the axle shafts, they also rotate with the same direction and speed as the wheels. However, the two smaller pinion gears (7) can not rotate with respect to each other on the cross pin shaft since they are meshed into opposite sides of the side gears. So what've we got? No relative movement among any of the smaller gears INSIDE the cage. Power is transmitted from the transmission to the crown/pinion, into the cage and out to the wheels. It works like a solid rear axle shaft with equivalent power to each rear wheel and we're happy.

Now, to understand the operation during cornering, think of yourself as a bug stuck on the rim of a tire. When you look at the other tire while traveling straight ahead, the other tire travels at the same speed as you, so it appears to not turn at all. (I'd guess, the bug would think of it as the whole world is turning, but not him/her/it). Then as the car turns, the inside wheel must slow, and the outside wheel must increase RPM. Again, from the bugs perspective, it looks like one wheel goes backwards while the other goes forward. (If this is difficult to understand, now is a good time to get a beer, as I did when writing) So during cornering, even though both wheels generally go forward, one travels slightly backwards with respect to the other. Back to the drawing. Side gears (8) can easily move in opposite directions because they ARE meshed on opposite sides by small gears (7). So even though the cage rotates at the transmission speed, which generally turns the wheels, the smaller gears inside the cage rotate at an RPM of the DIFFERENCE between the wheels. Power is divided appropriately, and again, everybody is happy.

So what happens when the car is jacked off the floor and a rear wheel is turned by hand? Only the differential gears can turn, and the cage remains stationary. That in affect requires them to turn in opposite directions. What happens when the car's stuck in snow, and one wheel spins? As the transmission RPM is increased without sufficient drag on one wheel, the cage turns only that side gear and the other is allowed to remain stationary. Because this is the extreme and unusual condition of operation, it is also the only time that significant wear is placed on the shims (9) and is the likely cause of failure in my differential. By knowing which shim was badly worn, we can even determine which tire was stuck. Be reminded that as cage RPM increases, centrifugal forces press gears 7 hard into the shims, and are likely to wear quickly. In other words, don't spin tires when stuck in snow, or reserve the Stagmeisterspreader.

Thanks again to Bill and Sherri Pyle for hosting the clinic, as well as all the helpful people that made it great fun.

Well, I'm done with the beer now. I wonder why this mechanism is called a differential?
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