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Originally Posted by redpoint5
All I know is that our rudimentary steering system on the electric trike we made (2HP) was scary at 40MPH or above on a standard 1/4 mile oval running track. I've experienced both the under/oversteer problems mentioned above.
The other thing is we put brakes on the single small diameter rear wheel, to reduce complexity and cost of front brakes. Getting the car to pass inspection required a lot of shenanigans, such as reducing tire pressure to minimum, and scrubbing the front wheels side to side.
Trikes are a natural design concept when the goal is to make something as cheaply and as lightweight as possible, and lend themselves to an efficient teardrop shape aerodynamically. There's simply no reason I can think of to put the single wheel in front though. Even I got that one right as a high school kid.
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My trike was made with a deliberately flexible frame, lacked an anti-roll bar function, and had a high cg.
https://www.compositesworld.com/colu...mobile-history It was always very well behaved except on washboard gravel, where the rear suspension had some unplanned freedom, and the one time I accidentally ran it with toe-out. We are used to cars steering and handling easily because we usually start with well engineered parts. With a fresh sheet of paper approach, there are many more pitfalls for any number of wheels. Steering geometry is not a trivial problem at speed.
One easy way to think about the stability problem is to imagine a tilting table. At 45 deg, it simulates a 1g load in that direction on the road. The cg has to be low and centered enough that it never hangs over a line between the lowest two tires. You can plan for a bit less angle to the side, and with low power, much less to the rear. Racers like deltas because they accelerate out of corners better. Nissan recently did well with a near-trike at Le Mans.
Trikes oversteer or understeer mainly because all the weight transfer has to happen at one end. It as if one anti-roll bar was missing on a car. In a corner, the very rare diamond wheel pattern tends to put all the weight on one wheel, and none on the steering wheels, giving it a very evil reputation.
Rear wheel steering turns out to be quite stable if you just use the geometry of a regular vehicle running in reverse, but add a steering damper. The damper catches transient upsets, while the geometry still tends to self-center in a steady turn. On a delta or 4-wheeler, though, you can get trapped against a curb, unable to initiate a turn away from it.