Mitsubishi DSM Center Differentials
The Mitsubishi/DSM W5M33 AWD transmission is equipped with a center differential that splits power between the front and rear wheels, while allowing independent front and rear wheel speeds. Factory equipped with 2 spider gears the center differential is adequate for stock passenger car duties. Typically most performance enthusiasts have the stock center differential modified to accept 2 additional spider gears, as well as an upper torrington bearing to replace the stock thrust washer. While these modifications greatly increase the durability of the center differential for performance use, there are several other shortcomings that need addressed from a performance use standpoint.
1. Floating Cross Shaft - The cross shaft for the spider gears floats loose in the case. This allows the shaft to move radially, axially, and rotationally. The free floating cross shaft is an issue because it does not positively locate the spider gears and cross shaft. Under power the spider gears are forced outwards against the (thrust faces) walls of the center diff body. Since the cross shaft is allowed to move rotationally, the shaft does not stay perpendicular to the thrust face, or parallel to the gear bore, as shown in figure 1. Eventually metal to metal contact and galling can occur between the spider gear and the cross shaft, figures 2,3,and 4 show examples of this. It is important to note that this cross shaft and gear was removed from a 100k mile stock 91 AWD that was not raced nor abused.
Now if this isn't bad enough, the cross shaft is round, and the center diff body is soft. Over time the cross shaft digs into the body, shown in figures 5 and 6. In figure 7 we see the increase in rotational displacement of the cross shaft due to wear. The extra rotational play increases the possibility of metal to metal contact and galling.
2. Cross shaft is Loaded in Single Shear - In the factory configuration, and some 4 spider setups, 2 of the spider gears are not driven from the outside. This is occasionally referred to as 2 load point vs 4 load point. I have performed a simple FEA on a 2 load point cross shaft and a 4 load point cross shaft. The simulation applies a load equivalent to an engine that delivers 500 Ft*lbs, and in first gear. The 2 load point shaft has a peak stress of 224ksi right in the fillet between the short post and the body of the cross shaft. Keep in mind that most of the steels used in an application such as this would have a yield strength of about 100-150ksi. Imagine the stress during a 6500RPM clutch dump with a 700Ft*lb clutch!!
The FEA of the 4 load point looks much more promising, with a peak of 38ksi under the same conditions.
Occasionally the 4 spider center diff is made by modifying a stock cross shaft to make room for the extra gears, resulting in a 2 load point 4 spider. They typically fail as the above FEA predicts, and as illustrated in figure 10.
Enter the White Shed Speed 4-Spider.
At white shed speed we build a 4 spider center differential that addresses these shortcomings. We use a 4 load point cross shaft, and then we lock it in place so it stays perpendicular to the thrust faces. The cross shaft is an inexpensive OEM part, and is sourced from a DSM rear differential. In the rear differential this part is exposed to torque levels of up to 3.5 times that seen in the center differential. The rear differential cross shaft handles these loads without breakage, and without galling. The housing is machined in one operation on a CNC mill to ensure that the drive slots and thrust faces are "square". The end result is equal loading of all 4 spider gears, and a fixed cross shaft. To lock the cross shaft in place, the original drive slots are machined out to accommodate the lock keys, as seen in fig 11. A lock key is then made from hardened chrome-moly steel, and is visible in fig 12.
Fig 13 shows the lock key installed into the drive slot. Notice the square drive hole that the shaft is locked into - this greatly increases contact surface area, and reduces wear. The mating area of the shaft is shown in fig 14. Finally fig 15 shows the differential with the spider gears, keys, cross shaft, and thrust washers installed.
How does it hold up?
The prototype shown above has been in my personal car for 2 seasons of abuse. The car weighs 3300lbs, makes 650+whp, and has run 9.8@144. Each season I generally make it to 10 race events, with many passes on the street in between. I also use this car as my fair weather daily driver. Typically when making a pass the car wears Hoosier QTP tires. On the street and poorly prepped tracks the car does spin the front tires fairly aggressively which is extremely hard on the center differential. So far after 2 seasons there is virtually no wear on the thrust washers, and no wear or signs of galling on the gears or cross shaft. Other 4-spider center differentials I have had have been trashed after a partial season of use. Figs 16,17,18 illustrate the minimal wear experienced after 2 seasons.