Overall...When these are running properly..are they as safe.

There is a type of hub currently used on some fixed gear(no freewheel, no coasting, cranks always turning when the bike is moving)bikes. This is the way the very old and current track bicycles work. Some freaks like me ride these kind of bikes around on the road too. The track bikes have no brakes except the resistance of your legs against the inertia of the bike moving when you want to slow down. I posted a picture of one of my fixies in my bike&bikehaulers post. Some people install a front brake for road riding. This type of hub is threaded for mounting a sprocket on both sides of the hub. Some hubs are threaded for fixed gears on both side and some are threaded for a free wheel on one side and a fixed gear on the other. To change gears you stop the bike, remove the rear wheel and turn it over and reinstall with the different sized cog now engaged. I thought about using one of these hubs on my motobike. Mount the motor cog on the left side.(the fixed gear threads onto the hub and is kept for unscrewing during legbraking with a reverse threaded lockring). A conventional freewheel single or multigear threaded on the right side. I tried a grubee supplied 44 tooth gear on one of my flipflop hubs. The hole in the center of the chainring is a larger diameter than the threaded gear mount on the hub so an adapter would have to be made. You could use the regular rubber spoke-grab system, but there would be no point in wasting a flipflop hub that way. Here is picture of a 1940s-50s flipflop hub on one of my fixed gear bikes. The two cogs have a different number of teeth. Change gear by flipping the wheel. These hubs work best on bikes with slotted rear dropouts so that the chain can be tensioned properly for both sized gears. Some hubs have giant wingnuts on the axles.



Some people who have single speed fixed gear bikes reverse the bottom bracket spindle and put their chainring and crank on the left side. If you hooked to motor drive chain to a freewheel on a rear hub you wouldn't be able to compression start the motor because the rear wheel would just coast without turning the hub gear.
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left side gear more thoughts

If you want to use a freewheel on the left side of a bike the ratchet would have to be the opposite of a regular freewheel, otherwise it would work for pedalling backward and freewheel going forward. There are leftside freewheels for BMX applications, but I think they are single speed.
Why not mill a strong what ever size sprocket you desire out of a BMX front sprocket for the motor drive side of the hub? some designs are very close to a motorized style.


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bmx chainwheel

I thought about doing just that. I will probablly use a disc brake rear hub instead of a flipflop hub. I was thinking of drilling slightly oversize holes in the sprocket to match the disc mounting holes on the hubflange. then line the oversize holes with rubber grommets or pieces cut from rubber tubing with the I.D to match the mounting bolt size and the O.D. to match the sprocket mounting hole. This would allow a little give and decrease the shock load on the mounting bolts. I have a nice heavy duty hub with sealed insert bearings I plan to use.