It's not a bike, it's not a car with low ground clearance and front wheel drive. It's tanks.
Tanks have low ability in overcoming obstacles, poor mobility in rough terrain, a significant speed loss on the trampoline.
What is the law of contradictions works on the tanks. Where should not be - they fly. Where the tank needs to fly, he falls down and gets stuck.
https://youtu.be/VRwYfFgNwg8
When the tank climbs the mountain, there is no inclusion of low transmission, it is just slow going, not changing the load on the engine.
Perhaps the reasons are as follows:
Bad ability to overcome obstacles and problems of interaction between the wheels of tanks and the surface.
https://youtu.be/Vc-xXmm2Svk