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Old 01-06-2012, 02:09 PM
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maurtis maurtis is offline
Motorized Bicycle Elite Member
Join Date: Dec 2011
Location: Kyle, TX
Posts: 707
Default Mongoose XR-75 MTB Weed Eater Friction Drive

Hi all!

I have posted a few times, but figured now would be a good time to formally introduce myself! I stumbled upon these forums while looking at different ways to upgrade my son's Power Wheels, oddly enough. The more I read, the more I yearned to motorize a bike! I gave up on motorcycling when my son was born, so thought a motorized bike would be a fun way to putter around the neighborhood without the dangers of traditional motorcycling (on the open road).

I really wanted to go with a 66cc China Girl, but since I currently have a cheapo Mongoose XR-75 mountain bike from Walmart whose frame does not allow for one, my plan for my first build was to get a cruiser style bike. I still want to do this, but plans for my initial build changed when I found a 30cc Ryobi for free on Craigslist. I figured the price was right, so friction build here I come!

The biggest issue for me is that I currently only have access to hand tools, jig saw, Dremel, drill, sockets, etc. I would LOVE to get bigger tools some day, table saw, drill press, scroll saw, welder (!). So this build was all done by hand in my garage with my brother-in-law and I, using hand tools, and supplies from Home Depot. Mainly 1/8" aluminum bar stock. I would have preferred steel, but too hard for me to work with using the equipment at hand. Even though I tried to make things rigid, there is still more flex in the system than I would like. But, it was my first build and I learned a lot for next time...

Since the motor I got was from a pole saw and not a string trimmer, I was thinking the clutch might hold up to the stresses better. Mainly I wanted to leave the clutch on to be able to idle without lifting the motor (or just leaving it to stall). But after seeing it in action and finding that the motor still stalls at idle if the motor is not lifted, I probably should have just not even bothered with the clutch, lol. I think I just need to tweak the idle speed to get the clutch to disengage...

So, that being said, on to the pictures!

For my drive spindle, I wanted to used a knurled bike peg. Since I could not find any locally, and was too impatient to order one, I went with a 3/4" drive socket. I made some grooves in it with my Dremel, but against a standard mountain bike tire, it just slips and slips and slips... Even with it slipping, I still got a surprising amount of torque and speed. The 3/4" socket has an outer diameter of about 1". I mounted the socket to the clutch housing by grinding the center part of the housing square to tightly press fit into the socket drive hole, then filled it with JB Weld, up to the opening where the clutch torx screw goes.

I wanted an easy way to have a bearing on the other side of the drive spindle, and someone else on the forum (sorry, I forgot who it was, but thanks!) mentioned putting a bearing inside then socket instead. So I found a bearing that fit well at a local bearing store, tapped it in with a hammer and a block of wood, and used more JB Weld to fill the gaps between the socket and also creating a lip to help hold the bearing in place, just in case.

While the main motor pivot point is a piece of aluminum that I bent in my vise to a U shape, I secured it to the frame with a U bolt. I found that it would pivot a bit when the motor was on it, so I bent up some additional support pieces and attached them above and below the U bolt, and bolted those pieces to a support bracket, just below the front of the seat.

Last edited by maurtis; 07-20-2012 at 01:43 AM.
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