Home made 30cc Ryobi friction drive


New Member
Local time
7:10 PM
Jun 1, 2008

Greetings fellow "motored cyclists". I just wanted to do a quick post on how I built my bike. Maybe some of my ideas would help somebody else out. I started with a Univega 18 speed mountain bike and an old 25cc motor of unknown origin. I used a metal spindle at first but found that it was way too hard on the tire so I changed over to "Bow stops" commonly used on boat trailers to catch the bow of the boat when reeling it up onto the trailer. My 1st attempt was with a plain rubber bow stop but I found out rather quickly that the rubber on rubber wasnt such a great idea since the bow stop got munched after only 3 trips around the block. Next I tried using a "thermalized rubber" ( more like soft plastic if you ask me) and this too got munched rather quickly. It was the Polyurethane Bow stop that finally did it since the urethane is much more durable than the rubber it doesn't get chewed up and is relatively easy on the tire tread. Keep in mind that by the time I figured this out I was on my 3rd motor ( the first two ended up shearing the key off on the flywheel) I was using a 30cc Ryobi weed whacker motor at this point. What I finally did was to take the clutch, pull starter etc. off and then weld an extension onto the crankshaft and the using a high speed grinder I cut off the pawls for the pull starter ( since I was going to a "bump start" they were no longer needed for that purpose) Next I took a piece of 3/16 X3/4" flat bar and popped 3 holes in it, one dead center to clear the crankshaft and two others the same distance as the studs that were left after grinding the pawls off the flywheel. ( I only ground the heads off enough to pry the pawls themselves off the flywheel which left about 1/8" sticking out) After drilling the holes in the proper location I just slid the flatbar over the crankshaft and aligned the other two holes with the pawl studs and then plug weld the flatbar to the studs. It was at this point that I learned about the "Curie effect" and had to buy motor # 4. ( for those that don't know, the "Curie effect" is something that happens to magnets when they get too hot. It causes the magnets to lose all magnetism) I considered just buying a new flywheel but unfortunately they don't sell them separately and it was cheaper to just go buy a new weed whacker. Once the flatbar was plug welded to the pawl studs (and quickly quenched) I ran a small bead arond the crankshaft to the flatbar and quenched it also. This basically eliminated the worry about shearing the key off of the fywheel regardless how much load was put on the motor. After making a couple washers with holes drilled on 90* centers and match drilling the bowstop all the way thru I used some 8-32 allthread to make 4 studs that, after welding them to the washer on one end I pushed thru the bow stop holes and fastened with nuts on the end closest to the motor, on the oppsosite end of the bow stop I welded a 3/8-16 nut in the center of the washer. once assembled the bow stop with its new drive axles (8-32 studs welded to the washer on the near end and fastened with nuts on the far end) and threaded it onto the extended crankshaft and the drive roller was set to go. Next I used some 1/2" plate to make the motor mount using the 4 existing holes in the motor frame which i drilled and tapped to 1/4-20 threads On the side opposite the motor I used a roller bearing mounted in a piece of 3/8" plate to help support the "driveshaft". I affixed the motor with a 1/4-20 x5 1/2" long bolt to the existing frame mounts just below the seat post so it could pivot up and down off the tire and also be removed easily. next I used a "twist shift" gear shift handle with the spring removed as my throttle handle and added a heavier throttle return spring to the stock carb. The throttle cable was attached to the throttle lever on the carb by inserting a 4-40 machine screw thru the existing holes in the throttle lever and looped around the stud and then crimped to the cable itself which formed a small loop at the end ot the cable and slipped over the stud and was held in place by a nylock nut left loose enough for the cable to move freely on the stud. I mounted the kill switch to the handlebar gooseneck with a hose clamp tack welded to an angle bracket. Next I used some 1/2" square steel tubing with a handle on one end and a hole on the other end, after bolting the end to the motor mount I bent the tubing so that it would catch just under the seat and was easily maneuvered in order to engage/disengage the motor from the rear tire. I simply reach down and push the lever down and towards the frame and it stays there with the drive unit disengaged until I am ready to go again ( I have to pedal maybe 3 cranks and then i drop the motor back down and away i go) After expirementing with a spring taken from an old window frame I found the right tension and tack welded a nut to the center post of the bike frame and another to the outboard ( right side opposite the motor) bottom side of the motor mount and connected the spring to these. Thus far my best top speed is 25 mph ( as determined by my Girlfriend pacing me on a ride in her car) but I think with some fine tuning I can get perhaps 33mph out of it. One final note:I forgot to take pictures of a custom made "cowl induction" type setup mounted to the original motor cover and I must say that it does a phenomenal job of cooling the motor. basically all it is is a 3 sided box made of 16 ga steel with a cover over it and the front and one side left open that catches the air and forces it thru the stock air vent and into the cylinder head region. I ran some road tests using a thermometer stuck down in the cooling fins of the head and found that it kept the motor about 60* cooler after 2 identical trips around the neighborhood with the cowl induction setup installed. I first made the trip without it on and took the temp reading (270*) then I let the motor cool for an hour and went for a ride again on the same route and the temp read 210*. Hope somebody can use this info for themselves in the future.



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I am the 1st to reply?!

Everyone else is too upset regarding the EPA engine status I guess.

Well Squid this is very clean and simple set up, well done.

I hope to finish my project this weekend (it is also powered by a Ryobi engine), although it is not quite as clean and simple as yours.
Put on a larger wheel for more speed. Larger drive wheel=speed.
Good information thanks!
Nice, I love homemade motorized bikes!
Anyone can buy a kit and motorize, but it takes a true tinkerer to make his own from a weed trimmer!

props to you squidman!
Yes... very sweet.

Maybe you could weld some type of handle on that box so you can tote it around from one bike to another, even if the engine's hot. It'd make carrying upstairs a lot easier to control, also!

Excellent work, tho. And nice tip on the polyurethane bow stop!!