Mower Math

There is a pulley on my mower below the battery below the seat that drives the differential / transaxle.  It would be nice to keep this part but nicer to drive it directly from an electric motor rather than via the complicated arrangement of belts and pulleys that go all the way to the 14 HP gas engine in the front.

This differential is just forward / neutral / reverse and had no gear shifting; that is all done by the belts in an unusual arrangement using “variable pulleys”. The biggest problem was finding out the gearing ratio of the transaxle.  Was going to put it up on blocks, (somehow) get it running and (somehow) count the revolutions of the wheels and the main pulley.  Maybe use a video camera.  This all seemed complicated and even dangerous, so I had another look at doing this without the motor running, and without dismantling the whole rear end.

Turns out I could prop one of the pulleys that maintained tension on the belt with a piece of pipe I cut, then putting the mower in drive and then turning the pulley it would inch forward.  Pieces of tape on the pulley and the tire let me keep track of the number of revolutions.  Turns out spinning the input pulley to the transaxle 21 times turns the 17.5 inch tires once.  This comes out to a little  over two inches (2.618) of forward progress of the mower for each spin of the transaxle input.

The big question is: what sort of motor would I need to drive this directly?  Electric motors tend to be 3,000 to 5,000 R.P.M.s.  Would that be too fast, or too slow?

For 3,600 R.P.M. (for instance) times 60 gives 216,000 R.P.H (revolutions per hour). At 2.618 inches per revolution, we would go over half a million inches in that hour (565,488 to be more precise).  There are 5,280 x 12 inches in a mile. which gives 8.925 M.P.H.   which is pretty reasonable.   So I can directly hook an electric motor directly to this transaxle and get a reasonable speed.  I like this idea because it eliminates all the belts and pulleys, but keeping the transaxle keeps things like the brakes and the back wheel arrangement the same.

While researching this I learned that people race riding mowers.  Might be a new hobby in my future.

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