Motorized Bicycle - Episode 013

[18 January 2007]

I have seen it asked a couple places why it is that most people who motorize bicycles use certain types. These types are either cheaper MTB's or newsboy-style cruisers. The short answer is because the type of bike that is best for pedaling is not the same type of bike that's good for motorization.

The first matter that one may encounter (that I encountered, anyway) is the assumption that one would want a bicycle that had the full-complement of 24 or 27 speeds, for those times when it was necessary to pedal. This sounds like a good idea, but it's generally not necessary. The reason is that you normally hardly spend any time pedaling at all. To prevent excessive wear on the clutch you only must pedal a few strokes to help the engine get the bike moving, and from then on you just steer and watch the scenery go by. Evan a smaller engine will put out at least two or three times as much power as a typical rider can, and that's enough to push the bike along at a top speed of 25 to 30 m.p.h., which is faster than most people can pedal a bicycle with "casual" gearing anyway. Gasoline-powered bicycle engines are great for getting around on a small amount of fuel, but they are lousy for getting exercise. An engine will usually only need help up the steepest of hills, and unless you live in a mountainous or otherwise extremely-hilly area, it just isn't often necessary.

The second matter is that a bicycle that is travelling at 25+ m.p.h. all the time tends to get quite a beating, even just with on-road use. There is a definite need for shock absorption of one kind or another--either mechanical suspension, or balloon tires, or both. And this is the other matter which contradicts efficient pedaling. In order to provide maximum pedaling efficiency, bicycles used for on-road riding generally have no suspension and run very narrow tires.

The usual reasons that people use cheaper MTB's is because these bicycles have full suspension--although the engine usually doen't benefit from this, but the rider does. The disadvantages of these bicycles is that they can be difficult to mount racks on, the suspension itself can be a maintenance problem, fitting effective fenders and a chainguard may be difficult and if you are particularly heavy, many cheaper MTB's suspensions will not be able to properly support your weight.

The reason that people use newsboy-style cruisers is because even they don't have true mechanical suspension, they can mount balloon tires. Other reasons are that these bikes are easy to mount racks on, they often come with full fenders and chainguards already mounted, and they generally aren't expensive. While you can get multi-speed hubs, most lower-cost examples are single-speed, but as above--as long as the engine is functioning, for most terrain you don't really need multiple pedaling gears.

One fact about the newsboy-cruisers is that they usually only have a rear coaster brake. I have seen a couple people note how daring this was--to ride a motorized bicycle that had only one brake, and a coaster brake at that--but if you're riding on fairly-flat terrain and in a rural or suburban environment, you don't really use your brakes much. If you let off the throttle, the drag of the clutch/engine will slow you down pretty quickly anyway. If you were riding in hilly terrain you would want a bicycle that had at least two brakes, and coaster brakes or roller brakes can overheat very rapidly if they are used for an extended period of time--but many places, a cruiser with ONLY a rear coaster brake would be more than enough.

I suppose people imagine what happens if the engine throttle gets stuck, and that they would have to brake to a stop fighting the engine. The engines have a kill switch which shorts the ignition and shuts the engine off pretty quickly anyway. ....Also I can speak at least for the 35cc Robin/Golden Eagle kit--that the engine can push the bicycle with quite a bit of force. I will have to try this "braking against the engine test" the next time I get it on a bicycle, but I am willing to bet that ordinary rim brakes would not grip hard enough to arrest the engine's torque at full throttle. I'd guess that hard braking might slow you from 25 m.p.h. down to perhaps 15 m.p.h. If the throttle got stuck, you'd have to shut down the engine first--either with the kill switch, or possibly by reaching back and puling off the spark plug wire (which is difficult to reach, but is left exposed).

There is also another matter with mounting this particular brand of engine kit on a RANS crank-forward bike--and I suspect may be an issue on a lot of recumbents as well. The engine has a built-in fuel tank, and (when mounted on the bicycle) the fuel tank's fill neck sticks out backwards on the rear side. The problem with this is that you have to tilt the engine & engine mount backwards, in order to provide seat clearance and to ensure good air circulation. Because of where the fuel fill neck is, you lost fuel tank capacity. I would guess I lost about 20% of the tank capacity because the engine had to be mounted leaning back. It would be much better to be able to mount the engine leaning forward; on upright bikes (MTB's or cruisers) that's easy to do but on most recumbents that isn't going to be possible at all.

So in conclusion: the RANS Fusion is overall a very good bicycle for pedaling, but it is a poor candidate for motorization with this type of engine setup. The riding position is very comfortable but makes it difficult to stand on the pedal to unweight the bike over major rough sections of road. It has 27 speeds, 26 of which aren't much use when the motor is running. The seat requires the engine to be mounted leaning back, which is not optimal. The frame is not particularly built well for pounding (hitting bumps all day long at 30 mph) and though it has full fenders available, the frame can't mount tires much wider than1.75 inches.
The RANS Fusion is a great bicycle and I'm happy I bought it, but there are other bicycles much better for this use.