For a specific selected gear, yes.
Because the two inputs are the steering input and the main drive. Both of these depend on the engine RPM, so the difference between them is literally just whatever gear has been selected on the transmission (plus whatever gear ratios the steering input has to go through, which except for the Tiger, tend to be always the same and non-changing).
Gotcha, cheers
I have another example, this time with the CD-850 transmission which is triple differential type. This is a diagram of the CD-850-5, I’ve highlighted the planetary gear set ups, including the two planetary differentials, for easier clarification.
In red are the two planetary differentials, green is the planetary gear set that provides the low and high gear ratios/ranges, and blue is the reverse planetary which provides the reverse gear ratio.
We can ignore the blue and green, and focus solely on the steering, which comes from the differential in the middle. Assuming 2800 input RPM, and that the torque converter is perfectly efficient, the differential receives 2791.79 RPM due to the initial gear ratios (22:34 and 31:20). This is a spur gear differential, and when the brakes aren’t applied on either side it rotates all together as a complete unit. On each side of the differential are 69 teeth gears, one for each side, which mesh with another 69 teeth gears, for 1:1. Finally we have a 109:31 gear ratio which goes into the sun gears of the planetary differentials, which gives us 793.99 RPM on those sun gears.
Whenever you apply one of the brakes on the steering differential (assume we apply them fully), one side is fully stopped, meaning the other side speeds up. This means the braked side receives 0 RPM on the planetary differential sun gear, and the other receives 1587.99.
No matter what gear is selected, those are the values you will get on those sun gears with the engine at 2800 RPM.
In effect, with triple differential steering, what happens is that instead of the steering input being connected and disconnected to the planetary differentials, it is always connected, and in straight line drive it provides equal RPM to both sides. When turning, RPM from one side is removed and added to the other side. But (assuming the brakes are fully applied), it is always the same RPM addition and subtraction. This means that at a specific engine speed, the difference between track speeds when turning is always the same. However because track speed is much higher when on a high gear, it means that the same difference makes up a smaller percentage, leading a wider turning radius on high gears than on low gears.
I notice the diagram is a cross drive, those have hydrostatic pumps and variable ratio right?
This is CD-850, specifically the CD-850-5 of an M48A2.
Besides being able to slip the brakes, the steering ratio is fixed for any specific gear. Each gear has one radius.
Nvm aha
Ah, I know what you mean.
Yes, very early versions of the CD-850, particularly the CD-850-3, provide this.
They do it by having a “split torque path”. In short, the output for the main drive goes through the torque converter, but the steering input actually bypasses the torque converter and is received from the engine. Here is a Russian diagram of the CD-850-3 from an M46, where you can see this.
Theoretically, this allows for infinitely variable steering ratios, because the engine output can be 2800 RPM which goes directly into steering the tank, however if the torque converter is still in converter phase, it’s RPM is much lower than whatever the engine is outputting. This means that for a fixed engine RPM, as the speed of the tank itself increases, so would the steering radius without the need for selecting gear ratios, because the output of the main drive would be “catching up” to the steering input until coupling phase is reached.
This isn’t the case with the CD-850-4 onwards, since they removed this split torque path, and made it so both steering and main drive inputs were taken from the torque converter’s turbine.
The reason for this is actually really simple: while it sounds good in theory, the torque converter that is used on these transmissions is really bad for this in practice. The reason these transmissions have such few gears is because they make heavy use of an extremely high torque multiplication ratio in order to minimize the amount of gears in the transmission. This specific torque converter has a stall torque multiplication of 4.3.
However, to do this these torque converters have very high stall speeds, and achieve coupling phase at very high RPM. This in turn actually made the steering of the M46 uncontrollable if the driver wasn’t careful, because for the vast majority of driving operation the steering input RPM would be substantially higher than the main drive RPM, and resulted in crashes during testing.
Ahhh I see, very interesting.
So basically the steering was very sensitive depending on engine conditions which made it easy to oversteer?
Yes.
