Rat Rod Minibike scratch build

[mr.modified]

As if I don't have enough projects going at once as it is, I just came up with another one a few days ago. I have a few of these big, heavy, cast iron wisconsins that have been sitting around for awhile, but I couldn't think of anything to use them for. In the picture with both engines, the one on the left is a wisconsin model ACN, 6hp @ 3600 rpm. According to some research I did, they only made the acn in 1955. The one on the right with the sideways spark plug is a model AK. Made from 1939-1955. Producing an un-impressive 4.1hp @ 2400 rpm. They weigh about 87 lbs each, making them a dumb choice for a mini bike when a briggs flat head 5 horse weighs around 30 lbs. I figured I could weld up a mini bike frame pretty quick out of some junk i've got lying around, use up one of these wisconsins, and just call it a rat rod. I won't have to paint it or worry about performance that much. Just tack something together for the heck of it. The two orange engines are old two stroke jacobsen reel mower engines. I thought of using one of them, but their small size wouldn't make the project much of a head turner. I've got some rusty angle iron, some old bicycle parts, and a whole bunch of other junk thats not really good for anything else. Might as well make something out of that pile of junk.

 

[mr.modified]

I think the ACN would be the best choice for this project. Should be able to pull its own weight around better than the doggy 4hp AK. Here's a picture of the ACN next to an old 3hp briggs for size comparison. The model AK I have is a runner, but I've never had the ACN going. The first thing I did was check for spark. I had only a very weak spark every once in a while. These wisconsins have a magneto bolted on the side of the crank case instead of a flywheel magneto. I pulled the cover off the wico mag and cleaned the points up. Then I had spark. Next I found I didn't have compression, which usually means a stuck exhaust valve. I pulled the head off to find...a stuck exhaust valve. It wasn't froze up as bad as they usually are, with some pb blaster, I only had to tap on it a few times to get it working again. I cleaned the carbon off the head and piston. Looks like it might burn a little oil. I drained the oil, which was actually pretty clean. These engines have are splash lubrication, but use a pump to fill a trough that the dipper goes through. I guess the advantage is that they can pull the oil through a screen. They have 4 rings total instead of 3. 3 compression rings and an oil ring. Next I have to get the old muffler free. I'll have to take it slow if I want any threads left at the end. I'll get it running with the stock carb first, then I might make a dual carb manifold for the bike. I'm thinking old licence plates for fenders and square tubing exhaust pipe. Maybe some old wrenches somewhere. If anyone has any ideas, I'd be interested. The more unique this thing looks the better.

 

[Kenny1]

Can't wait to see what you come up with. That ACN is a good looking engine.

 

[mr.modified]

Last night I cleaned out the stock carb and got the old exhaust pipe loose with the help of a pipe wrench. I took a picture of how clean the float was in the carb when I first took the bowl off, but forgot my camera. For handle bars on this thing, I was thinking of using some chrome ones off of an old kawasaki snowmobile. Front forks and wheel will probably be from a bicycle. I'll rake the front end out slightly to keep the weight off of it. Brake will be a scrub brake. Instead of a clutch, I want to run a v belt from the engine to a jackshaft. Then from the jack shaft to the back wheel with #35 chain since thats what sprocket is already on the wheel I plan on using. I'll make a "suicide" shifter to engage the belt with a tensioner.

 

[mr.modified]

There's a pic of the carb and the engine showing the wico magneto. I set my front forks, the engine, and the back tire up just to get an idea of where everything needs to be. I put some red lines on the picture just to show about where I think the frame will be (roughly). Ground clearance on a DB-30 is about 5". I hope to have around 3" with this thing. Wheel base might be a few inches longer than a DB-30 which is about 3 ft long. I bought a 5/8" bolt for a rear axle today, so when I know the engine runs, I can start on the frame. I was going to test run the engine last night but ran out of time. I've got the carb back on, so all I have to do is fill it up with oil and bolt it to a wooden base that I used to have the model AK bolted to. It probably hasn't run in a good 20 years, maybe more.

 

[mr.modified]

I got the acn running finally. I don't have a gas tank on it, so I just filled up the float bowl to see if it knocked or anything. Sounds good, It revs up a lot quicker than my model AK does. I guess I'll have to get a drill bit big enough for my rear axle before I go any farther with the frame. When I get a gas tank rigged up to it, I'll get some video on here. With the rope start, you really get tired of pulling quick if it doesn't start right away.

 

[alexp]

looking cool :thumbsup: cant wait to see some progress. is that engine huge or does it just look like it :shrug: lol.

 

[mr.modified]

I guess that engine is about the size of an 8hp briggs flat head. The top of the head is 15 1/2'' high, not counting the plug. The main problem is how heavy they are. I should be starting on the frame tonight.

 

[oldfatguy]

an option might be to build something that is really geared down low for just brute power. think low speed trail machine that can climb the steepest hill or muscle it's way through deep mud or a swamp.

 

[mr.modified]

I've been thinking about how to gear it. I'll get the frame welded up, then figure out where everything will go. Last night I dropped the frame down like I show in the picture above with the red lines. Tonight i'm welding in the engine mounts. The engine base that is sitting on the frame is from a ventilated model AK wisconsin with the same bolt pattern as the ACN. I was thinking about what size chain I should use. The wheel I have has a #35 gear on it already, but #40 chain would look tougher with the big engine and heavy frame rails. I think I can fit everything into a 3 ft 5 inch wheelbase, give or take. I've already got a gas tank in mind. It's a cast aluminum tank from a 1960's lombard chainsaw. More pics soon.

 

[alexp]

nice. cant wait to see some more pics :thumbsup:

 

[Confused1985]

Another mb for the freak bike thread! Can't wait to see it running
Keep up the good work

 

[mr.modified]

I welded up some extra support where the rear axle goes. In the picture it's just tacked together. I'm going to make spacers out of cast iron pipe drilled out to fit over the 5/8 axle. I set the back tire on and blocked the frame up on some 2x4's. Thats the first time I actually set the engine on the frame. This thing is gonna be a tank! I set the seat and forks roughly where I think their going to go. Your arms are going to be about straight out once I get the handle bars on there. I also have a pic of how I was planning on making the drive train. Dual v-belt drive back to a jackshaft. Then from the jackshaft, I'll use number 40 chain to the back wheel. 35 chain would look too small. I'll gear it on the low side. I don't think your gonna want to get going too fast with it. I was originally going to use a scrub brake, but I'll wait until later to figure that out. Another problem is that the engine happens to be right where the foot pegs should be. I think I can work around this, maybe make some guards around the pulley and starter cup so you can put your feet in front of them. I've been thinking about all kinds of ideas for how the exhaust will be set up. I'll have to get the frame welded up to see where everything is.

 

[mr.modified]

I've been thinking about those front forks. I think they will hold up, but I guess I better not weld them to the frame. I'll make some sort of plates on the side and bolt them on. Just in case the bearings go bad or something and I can't get parts for it. Not that you couldn't cut them off and re-weld new stuff on, but it would make it a messy job. It would be neat to make a springer front end. Then I'd have a "full" suspension with the springs in the seat.

 

[mr.modified]

I finished the frame and bolted the engine down. Ended up welding the front end in anyway. If I have to replace the front end, I'll just weld the new parts to the front. It's heavy, but not quite as bulky to move around as I thought it would be. There's plenty of room to remove the head while the engine is still in the frame if nessesary. I still have to weld up the mount for the handle bars. I don't think i'm going to use the bicycle seat. I was thinking of making up a traditional minibike seat using some seat foam from a mercury sable and cover it with some red vinyl from an old napa creeper. I think it would sit a little lower and look cleaner with the frame. There's a picture of one of my gas tank options, the cast aluminum chainsaw tank. The tubes on the bottom are part of the old chainsaw handle and could be changed around. I could also use the stock tank or weld one up. The stock tank almost would fit back in it's stock location in front of the engine, but the cap is too close to the frame and it would stick out the right side of the bike too much, in the way of your leg.
Forgot to add that the wheel base ended up at about 3' 5''. The frame is about 10 3/4'' high where the seat will be.

 

[mr.modified]

Handle bars finally mounted. Handle bar brackets/clamps are from a 1980 colt cosmo moped. Bars are from 78-79 Kawasaki snowmobile. Next priority is probably to make the kick stand so I don't have to keep setting it on a 4x4 block. Still a lot to do, but at least it looks like something now.

 

[mr.modified]

Rat rod mini bike - YouTube

Started on the exhaust. The pipe you see in the video is going to be a lakes pipe cut out. The rest of the exhaust system will be under the seat. This is the first time I ran the engine while it was in the frame. It smokes a little when you step on it. Idles nice and smooth after re-cleaning the carb. :doah: Also started on the seat. I have to weld up a plate to hold the bolts from turning when you bolt the seat down. There's going to be three studs sticking through the plywood bottom.

 

[buckeye]

I like the way she sounds. Looking good.

 

[mr.modified]

Finished up the seat last night and bolted it on today. Ruined an industral stapler in the process trying to make it work with the wrong size staples... Also got the foot pegs on. I used hooks from a tow rope and welded them to some brackets which are made from the same steel as the lower frame. Got the front forks painted black. I'm going to make a mini vw beetle muffler with two 3/4" tail pipes. The muffler will be bolted about mid way in the rear frame of the bike. Also I have to make the flange and cap to block off the lakes pipe. This week I hope to get the chain hooked up. Since I am both slightly cheap and lazy, I decided to stick with #35 chain to avoid changing the sprocket on the back wheel. I'm also going to use a single belt to drive it. If anything doesn't work out, It will be fairly simple to change it later.

 

[mr.modified]

Muffler about half way done. The inlet is about 1" pipe, cut from the frame of the bicycle that provided the front forks. Inside the muffler, there are five holes in the top side of the inlet pipe. Also the end of the pipe is not sealed to the muffler. Once the exhaust gases are in the muffler, they flow toward some square areas which I cut out of the tail pipes. The square cut out spot on each tail pipe faces away from the center inlet pipe. It will be interesting to see what it sounds like. In the second picture, you can see about where the muffler will be on the bike. The area in red shows roughly how the muffler will be connected to the pipe on the engine. I was thinking of using a slightly smaller pipe from the engine, and sleeving in into the pipe from the muffler. The muffler will have some tabs welded on it and will be bolted to the frame on some rubber mounts. This way, engine vibration wouldn't break anything. It would be better if I could find some good flex pipe in the size I need. We'll see what happens.