Rack Mounted Bike Engine Project

INTRODUCTION

I've started a new project, a rack mounted engine on a bike frame.  While I have money to buy specialized parts for this build, my mission is to get it done using commonly available items, and my own limited engineering/building skills.  I have an okay shop with a few tools to help me out, including a lathe, MIG welder, drill press, and assorted hand tools.

I know that not everyone has these items, so one could question my "mission" about commonly available items.  Oh well.  I might have to resort to using those items on occasion, but I'm still trying to keep it "real" and affordable.

OVERVIEW

I have a MTN TEK mountain bike that will be the host for this project.  This is the same bike I used with the push trailer (see earlier post).  This will be a belt drive to keep it simple and quiet.  The engine (Greyhound 2.5 hp from Harbor Freight) will be mounted to a platform attached to the frame of the host bike.  It'll have a lever actuated idler pulley as a form of clutch.  The clutch lever will be mounted on the handle bars, along with a throttle lever.  The belt will be a standard Goodyear or equivalent automotive 3L belt.  After running some numbers, I'm planning on about a 14:1 direct ratio.  At 3600 rpms, that should move the bike at close to 20 mph.  I think the engine has enough power for that and is good enough for starters.

LARGE DRIVEN PULLEY

With a 1.5" drive pulley on the engine at 14:1, I'd need a 21" driven pulley on the bike. To purchase a large enough pulley (a.k.a sheave) would be hundreds of dollars.  But I figured I could make one without too much trouble.  A bike rim might work, but the closest size is 20".  That's a ratio of 13.3:1 - probably still okay.  I needed a 20" bike rim that I could mount to my 26" rim as a large driven pulley for the belt drive.  The local classifieds at ksl.com had plenty of used bikes for cheap.  I bought one for $5 and disassembled it, cannibalizing all the usable parts.

When I got to the wheels, I removed the tire, tube, and cut the spokes out of the front one.

I struggled to find ways to attach this "rim-pulley" to my rear 26" rim.  I had a number of ideas, but finally decided to drill holes around the edge of one side of the rim-pulley, in-line with the spokes on my bike's 26" rim.

<=== Here's how it looked after the first hole (see the side wall of the rim at the far left of the photo):

Before I went any further, I wanted to test fit the "rim-pulley" for clearance.  There is less than 1/16" between the pulley and the bike frame - good enough!

I completed the remaining holes, nine in all.  This gives me an attachment point on every other spoke on the left side of the MTN TEK wheel.

Why every other spoke?  Not because of science or engineering, since I don't really have any.  Just seems like with that many, it "otta" hold.

Then I took a Phillips pan head screw and cut a slit in the shaft of it with a hack saw.  I kept a nut on it, threaded up to the head so that if I buggered up the threads, removing the nut could straighten them out.

Once the slit was cut, I used a belt sander to open up the slit wide enough for a spoke to nest into it.

The screws goes through the side holes I drilled in the "rim-pulley," then intersect with a spokes on the 26" rim on the host bike.  The nut threads on the back side and holds it all together. I may need to use nylon lock nuts or Loctite to keep 'em on long term.

It takes a while to cut these screws and widen the slit, probably about 5-6 minutes a piece.  I thought about using split bolt connectors but they were too expensive at four bucks a piece on the low end.  I might try using a Dremel to carve the slits in these screws to see if it speeds the process up.  I've made a total of three so far, which will allow me to mount the "rim-pulley" to the host wheel for alignment.

MOUNTING THE "RIM-PULLEY"

Placing the bike upside down, I disconnected the rear rim and slipped the "rim-pulley" next to the host rim.  I then placed the first of the three Phillips pan head screws through the opening in the side of the rim-pulley and aligned the spoke in the slot.










Here it is with the nut pinching the spoke against the rim of the pulley.  I'm not tightening this down real tight because it appears to stress the spoke.

And here's how it looks with mounting complete.  It's centered to within 1/8" total variation.  With some adjustment, I might be able to get it closer.  To find the high side, I cranked the pedals to get the wheel spinning then put a Sharpie marker close to the pulley such that only the high side would hit the Sharpie, thus marking the pulley.  I then stopped the wheel, found the Sharpie mark, and moved the pulley in the opposite direction from my mark.  And while I have a dial indicator (for use with the lathe), I don't think I'll need that level of precision.