Motor placement conundrum

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Jacob
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Last seen: 11 years 8 months ago
Joined: Saturday, January 2, 2010 - 06:21
Points: 11
Motor placement conundrum

Hi there.

I am a bit stumped about motor placement. The motor is a used Etek that I am mounting on an old Suzuki GN-125. Since I don’t have much to spend I have been fabricating a mount out of 3/16 X 1 1/4 X 1 1/4 angle iron from the local big box hardware store. I am mounting to the swing arm in order to avoid issues with chain tension/slack and to avoid a second set of sprockets and chains.

Below is a picture of the bike; the four marked spots are where I was going to place the motor (#1 and #2) and where I may place the motor (#3A & #3B). I preferred spot #1 since it was mounted inside the frame to the swing arm, but it didn’t work because there wasn’t enough clearance (the circles are not to scale). Spot #2 had the motor outside the frame (I cut off the kick stand and foot peg holder outlined in pink since this photo was taken). The problem there was that the motor kept hitting the shock absorber and didn’t leave enough room for the rear sprocket.

My next proposed positions may be a bit radical. The first placement, 3A, is on a cantilever attached to the swing arm with the motor “floating” within the area the old engine went. However, I wanted to mount the batteries there and would be forced to move them to a “saddle bag” position around the back wheels. Another alternative is to rotate the cantilever 90 degrees and mount the motor above the frame. In either position, the motor would bob up and down as the swing arm moves and have plenty of cooling.

My questions are these:

  1. Is it really OK to run the Etek counter-clockwise in order to keep it within the centerline of the bike? I have read posts that it is OK and not OK and the Etek manual doesn’t say anything about it.
  2. Does anyone have experience with the mounting positions (3A or 3B) I am looking at?
  3. Does anyone have experience converting a Suzuki GN-125?
  4. Should I not worry so much about adding an additional sprocket on a lever attached to a spring to maintain tension on the chain? (How do ICE motorcycles avoid slack when the swing arm moves?)
  5. Is there a large efficiency loss if I add an intermediary set of sprockets on a small axle? It has occurred to me that if I mount the motor to the frame where the old ICE engine went, and put an axle as far forward as possible on the swing arm, the slack on the chain from the motor to the intermediary axle would be minimized and the tension on the chain from the intermediary axle to the rear wheel would be constant. It would, however, add a level of complexity and cost to the whole setup that I prefer to avoid.

Btw, as an experiment to test the strength of the steel angle I am using, I secured a four foot length of it to a fence post, mounted the motor to it, added three ten pound weights to it (hanging by a rope) and let it stay there for a day. The two lag screws that I was using to attach it to the fence post had begun to let go of the wood, but the steel and 3/8 nuts and bolts were fine. This gave me some confidence in using this metal, even considering the torsion and other physical forces that will be acting on it when riding. I am using washers and lock washers where appropriate and may pay to have the steel welded, too.

I look forward to your comments and feedback.

Peace,

Jacob

reikiman
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Joined: Sunday, November 19, 2006 - 17:52
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Re: Motor placement conundrum

Might I suggest getting a copy of The Secrets of El Ninja ...?

Position 3A has the disadvantage of being the ideal location for batteries. On the other hand a rule of thumb I was told long ago is to place the motor where the engine goes. That is if the output shaft of the motor is in the same place as the original engine output shaft then the geometry is the same as stock.

That said the other positions have their advantages.

Jacob
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Last seen: 11 years 8 months ago
Joined: Saturday, January 2, 2010 - 06:21
Points: 11
Re: Motor placement conundrum

Thanks for the reply reikiman. Good suggestion and I do have TSEN, and other good books. Unfortunately, the GN-125 is much smaller than the Ninja and other sport bikes, that many folks have written about. I have been considering getting another frame, or stretching this one. More likely I will go w/ 3A. 3B scares me a bit because of the chain's exposure, even though I could enclose it, something moving that fast near my back is frightening. The more I consider "saddle bag" batteries, the more I like them. Perhaps I could even engineer a way to make them removable for ease in charging.

BTW, I am now fairly sure that the Etek doesn't mind counter-clockwise rotation.

I'll keep the forum posted on progress.

Be well,

Jacob

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