A spindle drive is a primitive type of transmission. A rod, referred to as a spindle, is attached to the output end of and engine. This rod then comes in direct contact with a tire.
There are several limitations to this design. The spindle-tire interface is prone to inefficiency and slippage since the contact area is very limited. Water of any sort on the tire will render a spindle drive unusable until it dries.
Simplicity. The greatest advantage to a spindle driven transmission is simplicity. It is because of this simplicity that spindle driven scooters are the generally the least expensive scooters available.
Low maintenance. Spindle drives also require no lubrication and minimal maintenance.
Wear. Spindles cause excessive wear on the tire to which they are connected and require constant re-adjustment in order to maintain an optimal pressure on a tire's surface. the black magic spindle is an aftermarket spindle that has TONS of grip, but it also wears your tire down alot faster than a stock or knurled ADA spindle.
Sensitivity. stock Spindles cannot grip the tire on water. but if you have a black magic spindle you can ride on water and you can ride on packed dirt at minimal speeds of coarse.
Pressure stress. In order to maintain an efficient contact, excessive stress must be put on the spindle, and therefore, the engine. It is not uncommon to bend or break a crankshaft on a spindle drive. 3rd bearing supports are sold to help remedy this problem.