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Tuesday, September 4, 2007

First design

This is how BEMAP, Mk I, is envisioned at the moment: It's made using a .3DS model from 3DM called 3DM-Bike, found on Google's search engine. The actual site the model is from is 3DM3, specifically their free models page. Unfortunately there's no author or info to credit the actual model. I modified it a bit in Lightwave 8.5's Modeler so it would look more like the bike I will eventually be modifying (Columbia 2005 26" Comfort Bike, women's style). I also had to rescale it, since the original had 72-foot diameter wheels. :-) It's just a basic representation, and doesnt' include batteries, etc, or even the connecting pieces between the front fork and the drive frame.

This is a temporary physical mockup of it, using the testbed bike I got from a friend (thanks!), along with a part of the frame and drivetrain from an old rather-worse-for-wear Murray Biotech bike a coworker gave me about 11 years ago, which is my first organ donor for the project. Obviously an actual working version won't be zip-tied together (though I might put some on just for the wierdness of the look). It also won't have the pedal cranks on it.

Your first question is probably along the lines of "why not just bolt the wheel into the front fork?" The answer is that the front fork isn't as wide as the rear fork, and thus won't hold the ex-rear wheel with sprocket set, which is needed to receive the power from the motor at the other end of the chain. Also, I still need a place to mount that other end of the chain and it's sprocket assembly and bearings. What better place than the pedal bearings, already welded nicely to a set of frames that have just the length I needed for the chain, wheel mounts, mounts for derailers, brakes, etc., that are all already made for a rear wheel and drivetrain?

So, as you can see, I've basically cut the entire rear frame off the old Murray, and mounted it onto the front fork of the testbed bike so that the axle mounting point is as close as I can get it to the original fork's axle mounting point, which lets me keep the turning control as close as I can get to the original. The motor is mounted on a frame just behind the pedal bearing/sprockets, and would have a reduction gear assembly between it and the pedal gear.

Credit goes to LongTimeLurker over at THEMAnime forums for inspiring the idea to do the front-wheel drivetrain in this way, during an IM session. If we hadn't been conversing, I wouldn't have thought of this, and would still be trying to hook the motor up to the pedal drivetrain, with all of it's potential hazards.

Of course, it's possible to just alter the original front fork to hold a rear wheel, instead, but where is the fun in THAT? No, seriously, I considered it, but I don't have the metalworking tools to do it right, without seriously compromising the strength of the fork, and risking the wheel coming off during a ride when it breaks.

I wouldn't mind *replacing* the front fork with a newly-made-for-the-purpose wider fork that *would* hold the rearwheel, but again, I don't have the capabilities to make one. And I'm not sure I can get the orignal one off the Comfort Bike to do it, either. That would still leave me with needing to make a set of mounting points for the drivetrain and motor, as well as the derailers.

The "whole rear frame" idea gives me all those mounting points, plus a place to mount the motor controller, heatsinks, etc. Even batteries would probably fit on it, but I want those in back to balance weight (in the middle frame is a possibility, depending on the batteries).

So, helpful readers, criticize away, and either help me figure out a better way, and exactly why you think it would be, or help me figure out the details to do it this way. :-) Hop to it!

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