I was reading the thread about seeding and remembered thinking about this before.
My approach would be to make a unit that would mount down in the center under the axle and attached to a cross mounted frame between the final drives. There are two places where sizes of pulleys can be varied according to the speed wanted. That can be calulated out and I am not including those calculations in this message. You might want different speeds for different applications. It would consist of a pair of shafts held in a pair of front and rear mounting plates about 8" apart probably using ag ball bearings (common as fleas on most combines) on all 4 points. One shaft would be belt driven by a decent sized "Vee" belt from a pulley on the regular PTO to a pulley on that shaft. (Multiple thinner belts could also be used). That would be the driving shaft. It would have a flat belt pulley between the mounting plates. For this discussion lets say it might be 3" wide x about 4 or 5 inches in diameter. On the other shaft mounted between the plates would be a tire and wheel about like a wheel barrow tire or the like that would friction drive from the flat pulley. Perhaps a flatter faced tire would be better. There would then be a PTO stub shaft mounted on the back end of the same shaft that the tire is mounted on. If you desired one could also be mounted on the front of that shaft but about the only use that pops to mind would be a snow blower and if one were used the friction drive would need to be kept dry.
Such a drive would easily power a seeder and many other implements. I have no idea what the maximum possible output from such a drive would be but as a long time #$%& CUB owner I'll bet it will handle anything a CUB engine could throw at it.
Contact pressure could be controlled somewhat by the amount of air in the tire.
There is a company that makes a very heavy 6' or 7'+ bush-hog for large tractors that uses a special friction drive tire not only to power the mower but to turn a 90 degree change of direction with the power without a gearbox.
I sure hope this is making sense to somebody besides me...
I just toss this out as another possibility with including too many specifics.
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