There are too many Black tumblers. So i made yellow version tumbler 
(In fact, i didn't have sufficient black bricks :-)) 
Anyway, my yellow tumbler has a steering system, rear shock absorber and 3 state transmission.

3 state transmission is made using 1st generation lego power function system, as you know, 
the system has a disadvantage that the power function motor runs only one speed.
 Because the remote controller has just on/off switch. So, the motor can not run various speed. 
(But, next Generation lego power function, solve the problem. LEGO made new controller can be control various speed) 

I made 3 state transmission, using a differential gear and two lego XL motors. 
It¡¯s a really simple and easy structure, but effective system. 
The transmission has 3state. First state is 2 motors run at reverse direction but wheels rocked. 
Second State is 1motor runs then wheels run at half of maximum speed. 
Third State is 2 motor runs at same direction the wheels run at maximum speed.

Enjoy watching~