Fuzzy math? Fuzzy is as fuzzy does...

Remember the power divider function? On a set of two or three drive axles the incoming power is most often split in half going into the first axle in the set. On a tandem drive this is equal between the two axles, or 50/50. On a tridem set if can be 50/25/25, since the power divider in the second axle splits the 50% it receives from the forward drive. In other cases, such as with planetary gearing, the power is divided equally between the three axles.

How can you tell if tri-drive axles split the torque 50/25/25 or 33/33/33? One way to discern is a GCW rating comparison between the tandem and tridem? A tandem group made up of two 23K. rated axles, versus a tridem group made using three of the same 23K rated axles. The tridem will have a higher GAWR (23K x 3 axles, or 69K) over a 46K rated tandem. But does the manufacturer permit additional GCW (Gross Combination Weight or pulling weight). If the answer is “yes”, then there is a real good chance it is truly an equal 1/3 split.

For example, Sisu axles are rated at GCW of 425K for tandems but increase to 625K for tridems. The P600 axles from Meritor tandem are 420K with 560K for tridems, using 5 planetary gears. There is a special way these suppliers split the gearing to achieve this increase in difference. Both split the power 33/33/33.

If the tridems share virtually the same GCW as tandems, then the torque is split 50/25/25 with the forward drive axle picking up half the power through the divider. Given this, the forward drive does the majority of the work, and wears tires much faster than the other outer ends.

All Dana, Meritor, and Detroit tridem axles are geared this way, 50/25/25 split. You will find the same GCW rating regardless of whether the configuration is tandem or tridem. The advantage of the tridem over a tandem in these configurations in GAWR, or the weight the vehicle can carry (not pull), in addition to improved traction.