No matter what kind of unconventional design makes you even think of rear wheel steering, forget it. Now. Forever.
There is no stable position and as soon as you apply any steering input it wants to go that way forever and fast. Over correcting and speed wobbles are the norm.
Ditto.
Big time.
I get asked about this a lot and have come up with a simple exercise for anyone to understand how rear wheel steering is unstable compared to front wheel steering:
1. Take a wrapping paper tube and hold it with your thumb and forefinger about 3" from one end. Let it hang. This is
stable. To move the tube to the right, we move our hand to the right and lower part of the tube follows. When we stop moving, the whole tube comes to rest. This is called
positive stability.
2. Now hold the tube in the middle. The tube can be at any angle and moving left and right has no stability effect, positive or negative. This is called
neutral stability.
3. Now place your hand under the tube and try not to move. Without constant correction, the tube falls over. This is
static instability. If we constantly make little motions to keep the tube upright, we have created
artificial static stability however the system is still unstable without power and constant control input.
4. Now try "turning". To move the tube right, we must move our hand left, then right very quickly. To stop a turn, we have to move our hand out in front of the CG of the tube to stop the momentum, then back very quickly to stabilize the tube in one spot. This is
dynamic instability, the situation that Andy Green found himself in.
The Thrust SSC team thought they had tested this by building a Mini Cooper with rear wheel steering. While it demonstrated stability at low speeds, this stability was caused by the damping forces of the steering system and the tires. This damping increases linearly with speed, while the inherent dynamic instability forces increase with the square of the speed.
If they had driven the Mini to 150+ mph, it would have swapped ends just like a forklift or a shopping cart pushed backwards. Try the shopping cart. Easy at walking speed, run with it around some corners and be ready to let go!
Read Andy's account in Noble's book; it is well described and shows a big disconnect between the designers' intent and reality. Andy has been quoted on many occaisions that no one, ever, should make an LSR car with rear wheel steering. Playing with the cardboard tube and shopping carts can help you understand why.
