There are a couple of advantages: First, we are cutting the torque forces in half, let's assume we are generating 1800# of torque. If we put that through a single diff, that diff has to be capable of handling 1800fp. If we run that same through two diffs, each one needs to be capable of handling 900fp, thereby reducing the stress on the components. As well, we are doubling the contact area for traction, one of the unknowns here is how much traction loss will there be on the aft wheels from loose salt thrown up from the front drive wheels. Using tandem axles reduces the radial loads on the tires and provides some redundancy. I suspect that if a forward tire were to give out, it would more than likely take the aft one with it (depending on how it failed), so redundancy is a little less of a benefit. Also, see my other post about the axle fairing and down force generation.
I am not familiar with the Gilmer belt drive, is it capable of 7,000-9,000 rpm?
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One thing I see that you need to take a good long look at as far as splitting the torque loads in dual rear axles. I work as a heavy truck tech. Twin screw drive is the norm in the field. We regularly see shattered u-joints, differentials, axles, and pinions. Every component in the drive train is rated to safely take all the torque that the engine can deliver under normal circumstances.
The problem comes when a wheel looses traction for an instant and then grabs again. The resulting shock load through the drive train can amount to several times the nominal torque being applied, and the weakest part is immediately identified. Sometimes multiple parts fail at the same instant.
You may be tempted to think that since the salt doesn't provide the same traction as pavement, this can't be a problem. In fact, this type of failure is far more common in the winter, because the limited traction makes it more likely for a wheel to slip and grab. We've had 3 towed in so far this week.
I'm not saying that your set up would be doomed to failure and won't work, just pointing out a potential failure mode.
One other thing to look at is the method of coupling the rear ends. In the pics of these set ups that I've looked at I only see 2 differentials. In the trucks there are 3, one in each rear end and one separating the input power between the two (called the power divider). The power divider is essential to proper operation because it allows the axle differentials to spin a slightly different speeds without binding up the drive train. Another relatively common failure that we see in the shop is when a driver locks in the power divider and takes off down the road. It usually burns up the diff or overheats and shatters the power divider in about 50-100 miles at highway speeds. I'd hate to think what 400MPH would do. This is because any time that you have 2 diffs they will be spinning at slightly different speeds unless the gears and tires are
absolutely identical. I find it very hard to believe that you can find 4 tires that are
absolutely identical in rollout diameter at all speeds. Normal manufacturing tolerances will inevitably produce some differences, which will translate into a very slight but significant difference in speeds between the axles, resulting in binding of the gears and excess drag, at best, catastrophic failure at worst without a power divider installed.
If you already have one in the design, forget that last paragraph.
