Most electric bicycles are built on an existing frame or a slightly modified one (usually to provide mounting points for the battery and motor). This is the approach we took with the compact tandem.

But why?

The problems with using existing bicycle frames for electric bicycles:

• No convenient mounting for the motor or batteries
• Motor and electronics exposed to the elements
• The medium quality frames are not as overbuilt as the cheap ones and aren’t up to the stresses of high performance ebikes, relegating most home builders to using very poorly built department store bikes

The lack of convenient mounting for both motor and batteries comes down to the frame being essentially two dimensional.  This problem is especially acute for the motor. When a motor is attached to a round pipe, the torque of pulling a chain tends to rotate it around the pipe, loosening the chain and causing all sorts of problems.  This can be overcome by mounting the motor in a location where there is a 3rd attachment point available. Three points are needed for stability– imagine a stool for example. Bicycles have two such locations, the triangle made by the chain and seat stays and seat tube or in the main triangle between the riders legs.  The rear end mounting exposes the motor to the abuse and the chain and seat stays are usually not parallel to the wheel, but rather at an angle, making mounting somewhat obnoxious. The in between the leg mounting is kind of scary because it’s quite easy to stick your leg in a spinning motor’s chain.  The batteries also have nowhere good to go except behind the seat, which is why so many Chinese ebikes have the rear end stretched out.  Not really the ideal position because their also tend to mount the motor on the rear, leading to a very unbalanced machine.

Let’s look at the other end of the two wheel spectrum: motorcycles.  Motorcycles have frames that are roughly like two bicycle frames with cross pieces holding them together.  This provides a convenient place to mount the gas tank and motor, which are both much heavier than on an electric bicycle (and more powerful).  The frames are also much stronger thanks to this design.  But, especially when built of big thick steel tubing, they’re heavy. Since they have humongous gas motors, it doesn’t matter very much.  Most motorcycles weigh over 300 lbs.  Forget picking it up.  At that weight, you just want something that can be heaved upright after it falls over.  I don’t want a bike that weighs that much, do you?

Since most electric bikes are already too heavy to casually lift (about the same weight as a bag of cement-50+lbs), throwing a bit more metal onto the frame shouldn’t really make any difference.

Also, most bicycles are of the upright diamond frame type, where the pedals are located in the middle of the frame. The width of the frame is limited by the bottom bracket width and the spacing between the pedals (there’s actually evidence that widening the distance between the pedals is good for your knees– search Google for Q-factor). On a recumbent, however, there is much more flexibility since the pedals are located at the front of the bicycle and we have room to play with on the rest of the frame.

So how about a motorcycle style frame but with the tubing sized for bicycles?  It’ll add a bit of weight, but hopefully make up for this deficiency with the benefits of secure storage and better weight distribution.  And who knows, maybe if it’s built out of aluminum and the rest of the parts are half decent, it’ll actually weigh less than many cheapo ebikes.  Let’s rock and roll!

Tags: Electric Bicycles, Ideas
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