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Is this go-chair one of those infernal 3 or 5 wheel jobs? 4 wheelers should have no difficulty with stability on much steeper than 1:12. If the user grasps basic physics, is alert and can lean a bit they can greatly exceed 1:6.
A half inch step should work on 6" wheels, an inch would give the motor a run for its money.
Used mobility scooters are a lot less than 1000, but I presume the money has been spent.
NT