This leads into the question: how to individually control the speed of the tracks? Transmissions in modern tanks are very complicated affairs. They make the transmissions in cars look as simple as wond-up rubber bands. The transmission in your car applies equal power to both drive tires. Without getting bogged down in the exceptions of certain FWD, AWD, limited slip & etc, the general rule of thumb is your car's transmission applies equal power to all drive wheels. If braking force is applied to one wheel, it effects all wheels with power. On a modern tank, the track's speed and direction (forward/reverse) can be individually controlled. The transmissions on earlier tanks were not as capable though. Most were similar to a skid steer transmission, i.e. a Bobcat. Most also had fixed turn radii. Some had as many as 6 to choose from. The driver would select the radius/direction and the tank would turn at a fixed rate in the chosen direction. Obviously, driving took more skill then vs. now.
From a scaled tank perspective, I viewed it as having 3 options. The first was to use an existing skid steer transmission. The challenge there would be to find a used one and within budget. A Borg Warner T-20 is a valid option. This transmission has been used in 6x6's like the Argo and Max. Finding a used one is a little difficult though.
The second option is to make my own. Not as complicated as it might appear on the surface. Of the three options, it is the least expensive, but not substantially so. Obviously, it's not a complicated affair, but does have a higher demand on the fabrication skills. Dave Manson has designed an excellent one he calls the Mantra; a combination skid steer/slip design.
The final option I considered was hydraulic, similar to a zero turn radius lawn mower. Potentially the most expensive of the three options, but only marginally so. The basics of a ZTR (zero turn radius) is the engine powers two variable displacement hydraulic pumps. The hydraulic pumps push the hydraulic fluid through the hydraulic orbital motors causing them to spin. The pumps have very simple controls on them that control the direction and speed that the hydraulic fluid moves. This intern controls the speed and direction of the hydraulic motors which are directly connected to the drive tires. Obviously this is a simplification of the process. Driving a ZTR is very easy. Each pump/drive wheel has a lever. Push it forward, and the wheel spins one way. Pull the lever backwards, the wheel spins in the other direction. The further you push in either direction, the faster the wheel spins. By adjusting the speed that the drive wheels are spinning, you can turn the mower. Just about perfect for a scale tank.
Hydraulics has a larger range of options as far as being able to control torque, final drive RPM's and greater maneuverability then the others. It also requires significantly less fabrication skills then a Mantra style transmission and marginally less skills then needed for adapting an existing skid steer transmission. Fortunately, I have been able to find a reputable retailer for surplus hydraulics.
Of the three, I like the hydraulic approach the best. If I have priced everything out correctly, it should be the least expensive. This is directly related to finding new surplus that matches my needs. If my fabrication skills were better, then the Mantra style transmission would be the least expensive. Hydraulics will give me better maneuverability then the other two options to include neutral steering. A neutral steer is when the tank spins in place; one track is going forward and the other in reverse. Also, the hydraulics approach does not require a lot of fabrication. While not exactly plug and play, it is the easiest of the three options to install. I still am on the prowl for a T-20 though. I have a used forklift engine that was given to me that would be a perfect match for the T-20. However, for this project I am going to use hydraulics and a 22 hp Kohler engine.
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