View Single Post
Old 04-19-2014, 02:42 AM
Lightning Boy's Avatar
Lightning Boy Lightning Boy is offline
Motorized Bicycle Elite Member
Join Date: Apr 2014
Location: Michigan
Posts: 188
Lightbulb New here, with a DIY Chainsaw Bike

Hello fellow builder/riders! First, I'd like to thank everyone here at the forum for their helpful posts. This bike would have taken a lot longer without the tips I've found here. Deacon, thank you especially. (I hope this thread finds you in relatively good health and spirits.) Long story short, the information found here is a blessing for noobs like me.

Let's get down to business: I used the engine from an old chainsaw/ tree shaker? of unknown origin, and mounted the casing directly to scrap steel mounting brackets. The bike is my trusty old Huffy Diesel BMX from 1999 or so. The frame is more solid than most 20" bikes, and I wasn't easy on it as a teenager- so I trust it will hold up well. I wanted a drive that could bolt on and off without welds on the frame, so I would have the option of restoring it to stock in the future in case something went incredibly wrong. I'm a small framed fellow, maybe 140 if I'm lucky- so this mystery motor should have enough push to get me around at a good pace...

The brackets were just a matter of bending the steel and measuring/drilling holes for the axle and engine supports. The two beams on each side and the one coming off the seat are all connected by support brackets and the engine. The tensioner is a U-bolt through the saw handle and around the seat bracket. When I need more grip, I tighten the nuts. If you're brave enough, you can adjust it on the move with your right hand.

For the engine, I removed the clutch and bell, then cut small angled grooves in the clutch bell with a grinder for traction. Then, I reconstructed the clutch housing. After that, it was all nuts, bolts, locking washers, and loctite. I did have to tap the holes on the saw's casing a little larger, as I didn't trust the small threads and bolts under all the stress. It sits directly on the back wheel.

Once I got some forward motion from the engine, I figured it was a good idea to put some lights on this beast. The control panel box was recycled from a broken minnow aerator pump, and I power all lights from an 18v rechargeable drill battery mounted under the seat, on the frame. There are head, brake, tail, and L/R turn lights. The turn lights are on momentary switches, so you can blink them manually and not worry about leaving one on. The other two switches are just your standard on/off Single throw type, and they control the brake light and head light. When battery is connected, the tail/run light stays lit. All wires came from wall wart transformers I gutted for a separate electronics project. Pretty much everything I used was already laying around in the garage, with the exception of the old moped headlight from my Grandad's backyard junk pile. The tail/brake light was a broken tail/turn recycled from Dad's fishing boat. The turn signals are purple LED circles, mounted on each side of the back wheel on the mounting brackets. They're attached with Industrial Velcro. Just some extra stuff I had in the electronics tackle box that saved me from soldering up a few LED strips. I didn't need turn signals really, just wanted them for a nice touch. (Lights are off in this first pic, so you can see everything else.)

The first few rides, I was using lighter supports on the side brackets. They were made from an old car amplifier chassis and a reworked steel coin tray for a vending machine of some sort (little bro was making them at work, and he gave me one from the scrap box). That worked great, until the mounting tabs I drilled snapped clean off. Luckily, the rest of the engine frame kept me away from a disaster... After that, I decided to go the overkill route. The steel I ended up using is pretty stout stuff, with a plate twice as thick on the non-engine side to help with balance.

The tricky part was keeping both hand brakes, while still having a relatively comfortable throttle system. I ended up going with a gas pedal-like system, with a lawn mower throttle cable through the trigger of the saw. The cable goes through a hole in the bottom of the pedal plate, straight back around the outside of the saw to the trigger. There is a small spring connected to a bungee cord on the end of the top dropout tube to help bring the throttle back with the extra weight. The pedal was made from a random plate bracket (that looks similar to a hinge) and a solid support rod that already had the proper angled tabs for the right fit. I have no idea what they used to be, but turned out perfect for my purposes. Finding them was sheer dumb luck, and the pedal clears the front wheel nicely. I don't think I'll be needing full throttle through the turns anyway. I used coated down-rigging cable for the first couple days, but it breaks religiously... Should have went with the brake/throttle cable from the jump. There's a good reason everyone else is using them. The donor lawn mower with a bent shaft supplied the cable and muffler.

That muffler is a whole separate can of worms. It took a few tries to get that one right. This old saw has a torqued rectangular flange on the exhaust port for the stock spark arrestor and exhaust tip, and it wouldn't come off for anything. So, I just decided to bolt a soup can to it, and tap the bottom of the can for the muffler. That worked for about 2 days, until it sheared through the bottom around the mounting bolts. I cut away the sheared bottom, leaving a sleeve- then put a spam can over that with a hose clamp in the same fashion (to putt around until I came up with something better).Then, I bent up a lid/cap from some thin (yet much thicker than the soup can) steel plate to connect the sleeve of the remaining can (which I'd already formed to fit around the tire and clutch bell) with the muffler. I used the muffler bolts on the wall side and the hose clamp to secure it to the sleeve. It's a screwy looking thing, but it works well so far and doesn't vibrate. I've got a week or so of short daily trips without any problems now, but I spent a couple weekends working out the various weak points in my engineering. Now that the exhaust has stopped falling off, I might try a longer stroll in the near future.

Note: The plastic gas tank under the seat is only a reserve, not connected to the saw tank in any way. The lines you see only go under the seat so as to be unobtrusive while keeping the vent line and fuel filter intact. Just gives me a bit more distance, should I need it. It's secured by a bungee connected to my chain/lock on the seat post, and thick rubber bands found on veggies, with a foam pad between it and the seat post/cord hooks.

There's a small fender on the back made from a monster can, and one on the front from the doomed spam can. Put them on after getting caught in the rain on it and soaking my legs and hind parts. Surprisingly enough, it will run in the rain. Just stay away from the puddles! I'm sure that is helped by the aforementioned grooves I cut in the clutch bell for traction. Wouldn't work well with a smooth bell, I imagine.

It's not as pretty as it probably could be, but functionality first.

I've got a few more pics of it, but having a hard time getting them to upload. They're on the way soon, though.

Again, thanks for all the help! I'm sure there would have been much more time wasted and many lessons learned the hard way without the members of this inventive forum!

Attached Images
File Type: jpg image.jpg (930.8 KB, 354 views)

Last edited by Lightning Boy; 05-23-2014 at 11:12 PM.
Reply With Quote