For the 2-wheel numbers, the stuntmen used Fiat 131's and also Ritmos. These cars are always standard production models. I checked them out personally and found that the only major modificaiton was the differential lock. The differential enables a car's drive wheels to go at different speeds in curves since the wheels have to make different trajectories when the car is steered around a bend. The differential acts as a torque divider which prefers to off-load the Rpm on the wheel that generates less friction. Drivers find themselves in this situation when they drive through puddles or ice: the wheel on the ice turns faster that the one on the dry surface. If the differential worked as normal during a 2-wheel number, the stuntman would have the wheel up in the air turning at high speed while the one touching the ground would be stopped. HOLER Togni has proved that you acn also drive on 2 wheels with a normal differential. The number is much herder because the drive wheel (the one in contact with the road) , exploiting the friction inside the differential, receives the same rotational force but turns a lot more slowly. Sufficient here to note that, in the past, the Fiat 131's used were always standard model. When the stunmen used cars from other manufactures, the carburettor was always a 2-float chamber model. Today, the carburettor is not modified because it can work properly even when the car is angled at 45°. The engine is not modified at all. The only change made is the exhaust pipe is detached so that the driver can hear what the engine is doing and also to add a bit more noise "music" to the show! Passenger compartment, bodywork and suspension are all standard. Then tyres (Pirelli) are inflated to a pressure 50% higher than normal. This prevents the left side rims from touching the asphalt. Naturally, it is the wheels and the shocks that are subjected to the highest level of stress. Every time the car hits the ground after a jump, the wheels are subjected to tremendous impact and the shocks are literally squeezed to their limit stops. Two-wheel driving can be done in any gear and even with automatic transmission. For flips and crashes the stuntmen use car that are to be scrapped. Preparation of these cars is extremely painstaking. The engine is not touched but the passenger campartment is lightened a great deal: the rear seat is removed, the passenger seat goes, all the interior cladding is removed. The doors are clamped shut to make sure they do not come open when the collision takes place. The back window is removed and sturdy abdominal and shoulder seatbelts are installed. In a word, risk is not something to be taken lightly but rather carefully prepared for.