Tank steering systems

The first really successful system was the British double differential design of 1924, which was copied by both the United States and Germany.

Systems using electric motors with variable speed controls have been tried on a number of occasions, but have not entered widespread service.

[1] The obvious disadvantage to this design is the cost and complexity of two drive trains, and the increased maintenance load that implies.

For tanks, considerable driver skill and constant adjustment are needed, even at the relatively low speeds seen on early designs.

A simple step up from the dual-drive concept is to use a single engine and split the power output into two transmissions.

This is actually an improvement over the dual drive solution, as it causes the entire tank to slow, not turn towards the loaded track.

[2] The downside to this approach is that high-power transmissions are failure-prone devices, especially in the World War II era.

[2] Dual-drive systems were widely used since the earliest days of tracked vehicles, including the Holt 75 tractor that saw widespread use in World War I.

Later versions introduced more gears to produce multiple turning radii, including reversing one track.

Further motion of the handle increasingly applied the brake to that track, allowing the turning circle to be adjusted.

Another advantage is outright simplicity; the steering system connects directly to the brake and nothing else, producing a very simple mechanical arrangement.

[2] Differential braking systems are essentially an epicyclic transmission with a single fixed gear ratio transmitted through idler pinions.

[2] The advantage to this design is that the brake is not being applied to slow the vehicle, it is simply clutching the second set of gears in or out.

This means it spins at a relatively narrow range of RPM compared to the main drive shaft.

When the steering clutch is engaged, the shaft spins one of the idler sets forward and the other backward, causing one track to speed up and the other to slow down.

[5] Double differentials were first used in experiments in France starting in 1921, and found on many heavy tanks of World War II, including the German Tigers.

The addition of a continuously variable output using a hydrostatic transmission was used on the Char B1, providing smooth changes in turning radius that eliminated the system's main drawback.

The low efficiency of this system meant it was not widely used, but rapid improvement in hydrodynamic fluid couplings made it common in the post-war era.

[5] This system was devised by Dr. H. E. Merritt, Director of Tank Design at Woolwich Arsenal, and manufactured by David Brown Ltd.

Unlike the full double differential, the other side is not sped up so the system is not fully regenerative, and since both idler sets spin in the same direction, it does not offer neutral steering.

The late-war state of Germany's economy, especially machining capability and supply of strong materials, meant that only small numbers of the complex double differential could be produced.

A comparable design was used on the French Saint-Chamond and in turn adapted to be fitted in one British Mark II heavy tank to compete against other transmission developments including a Mk II fitted with a British Westinghouse petrol-electric and the original prototype heavy tank "Mother" with a Daimler petrol-electric drive.

[9] While moderately successful, these early systems were large and extremely heavy; the one in the Saint-Chamond added five tons.

[10] Some of the first attempts at new electric transmissions were made by the British in the early-war period for the TOG1 heavy tank design (a petrol engine drove twin generators which powered one electric motor per track), and by the Germans as part of mid-war experiments in heavy tanks.

The most notable among the German efforts was the VK 4501 (P), a design by Porsche for a heavy tank which lost out to Henschel.

[11] The 100 or so chassis from this rejected design were converted to a self-propelled anti-tank gun, the Elefant (initially "Ferdinand") Panzerjäger.

In practice, the powertrain of the Porsche designs proved even less reliable than the traditional type, and by the late war era the supply of copper was too limited to consider using so much in drivetrain motors.