Here is a video my friend made at our meeting of the Chesapeake Area
Metalworking Society in Maryland USA. Our discussion did not come
through so you don't need to turn on the sound.
It is a differential variable speed control mechanism mostly copied
from others' designs. I built it to help me understand how they work.
There is a variable belt drive of limited range and a fixed ratio chain
drive connected to a differential which compares them and provides
Differential speed controls have an extremely wide range of ratios since they can cross zero.
The video shows what is going on pretty well. The wheel drive part
got finished only a couple hours before I left for the meeting. It
shows better than looking at the drive sprocket that is sort of buried.
The neutral here is a bit odd since it is not disengaged but an infinitely
The only thing I think is new is the skate wheel differential. I had no suitable gears for the differential and thought it would be quieter at
Pressure on the skate wheels is not constant. The idea was to use
them as a true disengaged neutral but that happened at at one of
the high speed ends of the range and not near low so a better way
is needed to make them disengage for that to be useful.
Gears would carry torque better. The limit would be the belt.