Weed Eater Friction Drive Bicycle

Hi

Ok, thanks. It would be a fair amount of work to machine a drum, shaft and roller. I'm guessing you're going to have a steel shaft that threads into the drum and then the roller will be attached to the shaft in some way like with set screws? The roller will probably be knurled steel.

Hope he's reasonable.

You and me both. That is why I am encouraging him to somehow use the existing drum and braize some sort of adaptor onto it. If things get too expensive, I am just going to go direct drive onto the wheel and forget the clutch.
 
Yes, I went direct drive on my first friction drive build. I lifted the U channel off the wheel with a clutch cable in order to disengage it. I must say, the centrifugal clutch is like magic after the hassle of my first build.

Maybe the problem with using the existing drum is getting the shaft centered on it?

I had a centrifugal clutch drum that had enough room so you could thread a nut onto the end of the shaft inside the drum to hold the shaft.
 
I'll look around. There may be posts and pictures here, still. I also had a blog that documented it but it's inactive now. Maybe I can find it...

I've attached a pic of the bike with the Robin 33cc 4 stroke engine with the centrifugal clutch on it. I think the pix of the older version are on another, older computer I have stored. (I think I built it in 2009.) I'll go look for it later today and post anything I can find.

It was a Ryobi 26cc 2 stroke weed eater engine on a Rans Tailwind recumbent bike (the same one in the picture). I found a BMX footpeg of about 1" diameter that happened to have the correct threading and threaded the foot peg to the crank shaft end. I got over 200 mpg with that set up!
 

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direct drive build

I found an old blog post with some pix! I forgot I had a 'suicide crotch clutch' at first. I'll see if there's anything else useful...

Here's the text I wrote to go with the pix:


The rest of this post is about my efforts to make a friction drive motorized bicycle using a Giant 26" mountain bike and a Ryobi 31cc 2 stroke motor that is commonly used in lawn equipment. This isn't a "How To" post since I'm no expert. It's just my effort to share the fun I had with this project.

I decided to see if I could do it first without welding and without spending any money, if possible. My idea was to use stuff around the house and only spend money if I had to.

Just to be clear, a friction drive motorized bicycle uses a 'drive spindle' attached to the crankshaft of a small engine to drive the wheel of a bicycle using only friction between the spindle and the bicycle tire. The engine spins the spindle and the spindle is brought into contact with the rubber tire on the bike and that drives the wheel.

As far as I can tell, this is the easiest, least complicated and cheapest way to approach this project. But, it appears to me that you give up a lot because of the slippage that occurs as the spindle tries to spin the wheel. This is most obvious going up hills. So, I suspect that the chain-driven, direct drive of the 2 stroke kit would be much more efficient and powerful.

So, if you're interested in learning from my experience, read on!

I have several bicycles including a Giant (aka 'Rincon') size 16 mountain bike with standard, 26" wheels and knobby tires. I decided it was best-suited for motorization since it's the beefiest bike I have.

I took the engine from my Ryobi 410R cultivator. The engine had a housing around a centrifugal clutch. The housing has a square tip that drives the rototiller tines via a square cable a lot like a speedometer cable. I couldn't see any way to leave the housing on so I had to remove it. I couldn't tell what was holding it to the end of the crankshaft. I now think it was a left-hand threaded bolt that went into the end of the crankshaft. But, I couldn't see what was going on so I ended up drilling out the head of the bolt. This left the shaft of the bolt in the crank end and I still haven't been able to drill out the shaft. At the moment, it doesn't matter since I got the housing off and don't need the threaded crank end (if that's what it was).

Once the housing was off, I put a pipe wrench on the centrifugal clutch and spun it off the threaded crankshaft in the normal manner (counter-clockwise) while holding the shaft from spinning by putting a screwdriver blade between two fan blades on the flywheel. The crankshaft diameter is 3/8" and has a 24 threads per inch thread on it about 1/2 way down the shaft.

I went to a local bike shop and bought a knurled, BMX foot peg for $5 that happens to have the same size hole and thread pitch. That was an amazingly lucky find. Before I found that piece, I simply didn't know how I was going to get a drive spindle.

I initially removed a steel sleeve and washer that go on the crank and take up the space between the flywheel and the back side of the clutch. The flywheel is held on by the sleeve and washer pressing against the flywheel on the inside and the clutch on the outside. By leaving the sleeve and washer off, there was nothing holding the flywheel in place; the flywheel managed to spin itself loose on the crank and sheared the flywheel key.

To make matters worse, believe it or not, the flywheel key was CAST into the flywheel. This meant I had to buy a new flywheel to get the key. Instead, I filed out a groove where the key casting had been and then ground down a square steel nut and made my own flywheel slot and key. This seems to work fine so far. But, this could cause problems since the key is supposed to be made from soft metal that will shear before it hurts the engine. So, I will get a proper key and replace it.


I needed some way to mount the engine to the rear end of the bike and still be able to lift the spindle off the tire when I wanted to stop. I decided to mount a pivot point behind the seat and rock the entire engine up and down like a teeter totter. The bike has 2 nuts brazed to the frame for mounting a rack so I started there and mocked up a support bracket from wood in order to get all the dimensions. I ripped some 1" x 1" strips and cut them to size and drilled them out and mounted them to the bike to get a feel for how things might work out best.

Once I could see that I could make a sturdy frame using the brazed nuts on the bike's frame, I switched to aluminum bars and made the bracket for the pivot point. Aluminum is easy to work with and I was able to cut it with a hack saw and drill and bolt it in place. Once I had the frame for the pivot point, I brazed (not quite welding) and bolted a 1/2" aluminum tube to a steel bracket mounted perpendicular to the frame. This became the pivot point and I used a large, 1/2" steel bolt for the pivot.

The steel bolt was part of a 4x4 steel mounting bracket for fence posts I had left over. It comes in a U shape with a tapped hole in the bottom with the large, steel bolt in it so you can adjust the height of the 4x4. It was perfect for my purposes. I straightened the U bracket out so it was flat and used the threaded hole to hold the bracket with the long end of the steel bolt going through the 1/2" aluminum tube for my pivot point.

Then, I put a handle (part of an old garage door opener) on the front end of the steel bracket and a large rectangle of 1/2" plywood on the other end. I mounted the engine to the plywood after cutting a hole in the plywood for the crankshaft and flywheel. This way, I could move the plywood up and down like a teeter totter using the handle.

I braced the plywood with some L shaped steel that had been the rails for the old garage door opener. I cut it with a hack saw and drilled it and bolted it to the plywood to stiffen it. The throttle cable was only long enough to reach to the garage door opener handle so I mounted it there on the handle with a kill switch. At this point, my only problem was finding a suitable spindle.


As I mentioned earlier, I got VERY lucky. I read on the web about someone using a BMX foot peg as a spindle because it's the right diameter (about 1"), made of steel (won't wear out) and knurled (good traction). I went to our local bike shop and found a foot peg in a box of old parts for $5. It turned out to be an exact fit for the crank shaft! It had a 3/8" hole and a threaded end with 24 TPI threads that screwed directly on to the crank. I found that it would stay in place if I used blue Lock Tite and tightened it a lot.

The only complaint I have is that the spindle sticks out too far from the engine. I couldn't see any way around that so I used spacers on the plywood frame to move the engine back from the tire to get the spindle to meet the tire somewhere near the center. If I could mill metal, I'd mill an aluminum spindle to my needs.

Once I had all the pieces, it was just a case of trial and error to get it mounted and working. I got it working after about 2 days but found that the frame wasn't stiff enough and the rear of the plywood would move away from the bike as the power came on. I added some additional support straps from aluminum and steel until the frame was stiffer. After that, I found that I could hook a bungee cord on the rear of the frame on the opposite side of the engine and this would hold the frame down and inward, against the pull of the engine under power. This helped a lot.

I also found the knobby rear tire wasn't the best for the spindle to grab on to. The rear tire was also cracked and, basically, shot. So, I bought a new, slicker 'street' tire for $15 for the rear wheel and that helped a lot. I probably should buy a new front tire, too, since it's also shot.

Eventually, I got all the bugs worked out and started down the street for some power, speed and mileage tests. Overall, I was very pleased. The top speed is about 25 mph on flat ground and 35+ down hills. It's plenty fast. It has trouble pulling up hills, probably due to slippage of the spindle and a lack of torque in such a small engine. If you pedal a little, though, it helps A LOT and you can go up most hills easily.


I added an electronic bike computer ($10 at Fred Meyer) and carefully measured mileage and gas consumption and found that it gets 98 miles per gallon on flat ground at moderate speeds. The stock fuel tank holds only 2.5 cups which is enough for about 15 miles range. I'm carrying a cup of reserve fuel (enough for 6 miles) in the front bag but I'd like to add a larger fuel tank. I may also move the engine forward a couple of inches to get it closer to the pivot point. I suspect this would help all aspects of performance.

The 'suicide clutch' works well in most cases but can be clumsy since it takes one hand to hold the handle and that leaves only one hand on the handle bars. I'd like to operate the pivot with something like a clutch handle and cable from a motorcycle so that both hands would be on the handle bars at all times.

I also understand friction drives don't do well in the rain. That could be a problem. I haven't tested that yet.

I've also read on the web that several people think you need to have the spindle supported at both ends. I could see that would help and I might consider that. I'd need a long, metal U bar and a high speed bearing for the spindle.

Overall, it's one of the best projects I've undertaken. It was a lot of fun to build and engineer and a LOT of fun to ride! If you like this kind of stuff, I'd definitely say GO FOR IT!
 

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Thanks again for that. I appreciate it. It seems like you got all the bugs worked out prety nicely. If I had to do something like that I would try and find some way to lock the "suicide clutch" into place when I have engaged it, and then unlock it when I need to stop.
 
I found a couple of pix of the old direct drive on the recumbent. I also found the following info I wrote. I see it was back in 2008.

As of 7-23-08 I switched to a 26.2cc Ryobi 4 stroke engine on the recumbent and ran a mileage test getting 231 miles per gallon. The 4 stroke engine is MUCH quieter and has a little more torque for going up hills but isn't quite as fast as the 31cc 2 stroke Ryobi engine. For now, I'm keeping the 4 stroke because it's quieter and cleaner and gets about the same gas mileage as the 2 stroke.

8-12-08 UPDATE: I continue to ride the recumbent with the 25cc 4 stroke engine and love it. I've had no problems with it and I have well over 300 miles on it. I put a smaller, 1" drive spindle on, replacing the former 1.3" spindle and routinely get over 250 MPG. The smaller spindle also helps with hill climbing. Theoretically, it reduces the top speed but I don't care about that since it still goes much faster than I need. I find it's best to travel about 15 mph. I've ordered another used 25cc 4 stroke Ryobi engine and expect to receive it soon. When it arrives, I'll put it in the mountain bike and do some testing.
 

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Thanks. I would love to get a 4 stroke engine, but they are so expensive here. You guys are fortunate that you can get used engines relatively cheaply and easily over in the States. Here in South Africa, even if it is second hand it is expensive. I just don't think the market here is as flooded with engines as it is there.
 
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