MAC CAT 38cc chainsaw motor Razor Go Cart

[dmack]

I have read through a bunch of posts and I know there will be some challenges with this project but I still think it will work. I have a Razor electric go cart that I modified to run a much larger electric motor at 60 volts. It ran about 28 miles an hour which is bookin for a go cart that in only about 4 feet long.

I recently burnt out the controller and am tired of having to charge up 5 batteries every time I want to go out and ride around with my son. He also has a Razor go cart over volted to 36 volts.

I bought a 38cc Mac Cat chain saw from Craigslist for $20. It runs great and seems to have a lot of power for such a small engine. I stripped it almost completely out of it's case and it fits perfectly on the back of the razor with only a few modifications. The horizontal layout works nicely and I plan on mounting it right in the middle of the back of the cart for weight balance.

The issue with this is the same one most people encounter with chainsaw engines. The stock clutch.... It has a 3/8 stepped motor shaft with the gear for a cutting chain. After reading around on the web and staring at it with a shot of tequila in my hand my plan is to remove the stock clutch and attach a very small drive gear and then run a chain to a jack shaft to be able to adjust gear ratios and run a larger standard clutch. That will also allow me to put the motor right in the middle of the cart and be able to get everything to line up correctly.

My questions are about the gearing. The carts wheels are very small. About 5 inches ion diameter which will be beneficial with such a small engine. The motor likely runs about 200 rpm at idle and around 10,000 at full speed.

The upgraded electric motor I was using has an 11 tooth sprocket on it and I believe the axle has a 35 tooth sprocket on it. This is a smaller chain than a bicycle chain. The teeth are pretty tiny but I am hoping I can still use those numbers to get a baseline gear ratio to try to duplicate. 30 MPH max is the goal with enough torque to get the cart moving easily is the goal. Even at that speed the cart will be VERY twitchy at speed.

I need to do some math but anyone who would like to chime in on what sprocket to put on the motor, what size gear on the centrifugal clutch (5/8" diameter clutch) which will then have a jack shaft that goes to the 11 tooth gear which will use the original Razor chain to the 35 tooth gear on the axle. It will be a pain to have to re-engineer the stock sprocket set up so the jack shaft will need to be the solution to gear ratios and the new clutch set up.

i will post a few pictures soon of where I am on this project.

 

[dmack]

If the gear ratio calculator I found is correct the gear ratio I was running with the hi power electric set up was 3.18:1.

With 5 inch diameter wheels and an engine 5500 RPM would equal 25 MPH...

If the chain saw motor runs around 10,000 RPMs does that mean I need a 1.5:1 gear ratio?

 

[6StringMercenary]

Wish I could contribute more to the conversation but I will be watching this, as I'm hoping to do a similar bit of engineering and math. Thanks for posting and will watch to see how this plays out. Best to you!

 

[itsid]

If the gear ratio calculator I found is correct the gear ratio I was running with the hi power electric set up was 3.18:1.

With 5 inch diameter wheels and an engine 5500 RPM would equal 25 MPH...

If the chain saw motor runs around 10,000 RPMs does that mean I need a 1.5:1 gear ratio?

nope the other way around...

you DOUBLE the ratio if you double the rpm if you want to end up with the same top speed..

'sid

 

[jeeperjoel]

ran into a similar problem years ago mounting weedeater engines on bicycles and scooters, we were using spindles that ran against the tire, similar to a goped, the spindles were 3/4, to 1" around, but i wanted to step up to a chain drive setup, not doing exact math or tooth count the problem was that i'd need a large sprocket the size of the tire with the pinion sprocket at 3/4-1" to achieve this gear ratio, you're at an advantage in that your tires are small, the large sprocket you have will work, i could never get one large enough, but i'm not sure if you'll be able to find a small enough pinion... you might have to run a jackshaft with a 2nd gear reduction to get enough low end torque, electric motors have max torque through all rpm, but that chainsaw's not going to make power till it's spinnin pretty good

 

[dmack]

Part of the goals for this project is to keep the costs as small as possible. Anyone can take a bunch of parts to a machine shop and a couple hundred dollars later have a working cart. I want to see if it is possible with the items on hand and as little outside work as possible to make this work.

On that note I decided to continue using the original clutch. This is tricky as it is hard to adapt a sprocket that will work on the stock clutch due to it's design. I went to my local small engine repair place and they had an exact duplicate of the original clutch for only $5. I plan on using it on one end of the jack shaft using the original chain as the intermediate chain. I plan on grinding off the cutting blades and I got a master link to be able to make the chain a custom length for $2. For the jackshaft rather than buying one for $40 off the web I decided to use parts from the original electric motor that came with the Razor go cart. I disassembled the motor, hammered out the motor shaft and removed the bearings. The shaft has a notched end that works with the original sprocket that will allow me to change gear ratios as necessary. I also plan on cutting up the end covers of the motor to use to mount the bearings and jackshaft to the go cart. All I need to figure out now is how to attach this chain saw bell housing to the jackshaft. Likely pressing out the bearing, and using a sleeve to mate the shaft to the bell housing with a few welded spots TBD...

 

[firemanjim]

http://www.uscoolsports.com/mini_bike_gear_box_mini_bike_transmission_mini_cho_p/gear-700015.htm
See if that will fit/work for ya......

 

[dmack]

The cart lives!

Project update...

I took my first ride on the motorized razor cart today and it was epic. The project did deviate from the original premise however. I do believe the chain saw would have been an decent power source but the clutch and the 3/8" output shaft of the motor made it hard to work with. I wanted something bullet proof and the jimmy rigged set up that would have been necessary to make this work wasn't going to cut it.

So i bought a 47cc pocket bike motor off Ebay for $70 on sale. It was complete with carb, clutch and sprocket. I figured if it would make a pocket bike go 35 mph it would make the go cart go about the same and the readily available parts and gear ratios was very appealing. I also bought a used expansion chambered pipe for $10.

Assembly took 2 days. Required me cutting out some of the tubing at the rear of the cart and welding in 2 mounting bars drilled for the motor. I also removed an flipped the rear axle so the sprocket was on the other side which was necessary for this motor.

I cut a hole in the floor pan and had to cut the pipe in 2 places and weld it back together so it would fit the unique space and set up of this cart.

The result is spectacular. The 6 tooth sprocket on the motor with the stock razor sprocket on the axle has produced a fantastic gear ratio for this motor. It has great tork off the line and very quickly gets up to 30 mph. With the extremely short wheelbase it is scary fast. You can break the rear tires loose in the corners easily. The pipe gives it great top end and i have been hesitant to wind it all the way out in it's first couple test rides. Super, epic, awesome fun to ride. Better than I imagined. I will do a video of it soon.

 

[dmack]

pics

Cart pics

 

[landuse]

I wouldn't be using that seat bely if I wee you. Not unless you have a roll cage that you are putting on. You don't want to get stuck on that thing if you roll.

 

[dmack]

The center of gravity is so low on it that it doesn't flip it side slides. You can turn it so fast you fall off an I did drive it into a curb once which is why it has a seat belt.

 

[OzFab]

The center of gravity is so low on it that it doesn't flip it side slides.

...Until you hit a rock (or, in this case, a pebble) & it lifts...

You can turn it so fast you fall off an I did drive it into a curb once which is why it has a seat belt.

That's because you're sitting on the seat, not in the seat...

I recommend a better seat, something with a back &, possibly, sides & lose the seat belt

 

[dmack]

I like the idea of a deeper seat. Now that it is as fast as it is time to upgrade to better seat.