For pure functionality, the front-gas-engine layout is the best one for a gas-engine bicycle, IMO. Problem is, nobody out there does a kit just right.
The GEBE and the Staton kits both share the same problem--in that they take up the main place that it's convenient to carry cargo. What I've thought would be better would be a front-mount chain-drive engine setup with the chain on the right side of the wheel.
There would be a few advantages to this setup that no current gas kit offers-
.....This would mean that you could use a rear wheel (w/freewheel) on the front. When the engine was shut off or you were coasting down a hill, there would be NO drag from the engine at all, because the engine's drive-sprocket would just spin on the freewheel.
.....The engine could have a pull-start cord, and it would be easy to start while you were riding.
.....And finally, the engine wouldn't take up the rear rack space, leaving your bike's most-convenient cargo capacity for that use.
You would need a special fork to do this--but then again, a fork is an easy part to change on a bicycle, and so this engine kit could be made to very easily fit on a wide variety of bicycles. When putting this kit on a bike, you'd never have engine/frame fit problems, or chain/belt rub or idler problems. All that stuff would be built into the special fork, properly engineered.
The GEBE and Staton can be built on the front end of a bicycle, it's certainly been done more than a few times. The engine-drag-when-off problem is still there though, and the engine's controls and exhaust aren't really oriented well for that use however.
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