Nico71 - the substractor works exactly as you described it. Moreover, it is possible to use two motors of different power, and so I used much stronger motor to drive, and weaker to steer. It's a good solution, because subtractor works best when steering motor works at smaller speed that the driving motor - so you can use weak motor with additional gear reduction.
Anio - the front blade is used to let the tank dig itself into the ground. It's a common defensive tactic, to dig tanks down so that only the turret is above the ground, and while other tanks need infantry to dig them, the S Tank (Stridsvagn 103 is the full name) can dig itself. So yes, it has the same purpose as the excavator's blade.
As for tank drifting, it should be possible for most of the tanks that use hydrostatic transmission (which is standard in modern tanks), I believe it's easier for smaller tanks with small treads span. But then again, Swedish army shows its creativity, and some soldiers made a tank drifting with the Leopard 2 tank, I believe they drove it on a deeply frozen lake:
I don't think my French is fluent. I've been taking extensive French lessons for 4 years in my college school, but it was like 7 years ago, and I didn't speak much French since then, so I have rather good understanding of ce que vous dit, mais je ne pense pas que je suis un bon parleur aujourd'hui
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Merci beaucoup to you all for your kind interest, any questions are welcome.