2 Questions about my chain drive back mounted engine

Discussion in 'Rack Mounted Engines' started by tylerscool45, Aug 16, 2008.

  1. tylerscool45

    tylerscool45 New Member

    Well I have got all the things I need now to to attach the hardest part by welding it to the back of my bike which i will be doing by the middle of the week, over the wheel which i assume is under "rack" mounted. My 2 questions are

    1: Will a regular bike chain work for the chain saw engine?

    2. About how fast and how many mpg will a 38 cc 2 cycle chainsaw engine go with a 170 lb rider and a small mountain bike.

  2. srdavo

    srdavo Active Member

    yes...but you'll need to add a bicycle chain sprocket to the chainsaw.
    impossible to answer, yet.....
    What size sprockets are you using? (how many teeth?)
  3. tylerscool45

    tylerscool45 New Member

    2 things so the inside where the thing spins inside the chainsaw(sorry i dont know what its called) i have to buy a bicycle sprocket, or where i attach the chain to on the bike.

    2nd it has 6 sprockets the smallest is 14 the largest is 28. what do you recommend using for size?- and just so you know, you probably do but i am only using the back sprockets, as it is going to be attached to the back of the bike.

    if you have already read the first part i didn't understand before so i have to replace the part that spins with a small sprocket because if i do i will need a clutch, are you sure i need one and just won't work?
    Last edited: Aug 16, 2008
  4. srdavo

    srdavo Active Member

  5. tylerscool45

    tylerscool45 New Member

    thanks a lot for all the help srdavo me and my dad are going to work on it tommorow im only 15 so i can't weld myself very well. But i find it kind of funny that you did not use a throttle i just got one off the internet for 20 bucks arrived today looks like it will work, i had an option to do it like you did it but its probley not very safe.
  6. nathanl

    nathanl New Member


    Question for a chainsaw motor on back.

    This may be a dumb question, but, as I understand it:

    chainsaws drive on the right side, and drive clockwise

    I always see MB's done driven on the nondrive (left) side of the rear wheel

    so to rack mount it and have the chainline line up it either has to hang way off the side, or be turned around backwords so it drives on the left...

    which would make it drive the wheel backwards if you directly mounted a chain to it.

    Am I missing something? How do people get around that?

    I saw http://www.motoredbikes.com/attachment.php?attachmentid=1237&d=1189389855 ... I guess that works, just kind of.... ugly! ;)
    Last edited: Aug 19, 2008
  7. tylerscool45

    tylerscool45 New Member

    Well yes the one that the other guy has is ugly, but it works well if its not for the fact that he has no throttle. Weedeaters and engine kits all go one way which where you can put the engine in the frame. But most if not all chainsaws you cannot do that unless you put it upsidown. So adding to the back is the only option. I am doing it differently because instead of making another sprocket on the other side im basicly moving the chain from the front to the chainsaw. Ill show you what i mean

    ill post another one friday when it should be done

    Attached Files:

    Last edited by a moderator: Aug 20, 2008
  8. loquin

    loquin Active Member

    Well, the problem with the approach you sketched is that you don't have working peddles anymore, which makes your bike a motorcycle.

    One approach is to replace the cassette with two fixed sprockets, adding spacers as needed for alignment. The rear deraileur is also removed. Change gears (3) when peddling, with the front derailuer.

    Add a freewheel sprocket at the motor (or, if you can find one, a freewheel sprocket which fits where the cassette went.) When you're peddling, the sprocket up to the freewheel will also move, but, the drag should be very low.

    Next. Gearing. You will need to step down the engine RPM a LOT. A single reduction just isn't going to do it. You would need a sprocket at the axle that is nearly as big as the tire. If you take a look at Staton's information at his website, his gearboxes are 18.75 to 1 reduction. That's about right. Chainsaws can run at a very high RPM, 10 thousand, or more. You need to get the rear wheel spinning in the 400 RPM rate for the road. (400 RPM on a 26 inch wheel is about 31 MPH.)
    Last edited: Aug 20, 2008
  9. tylerscool45

    tylerscool45 New Member

    Thanks a lot for tell me that but i don't think 18.75 to 1 is a good gear ratio. I was told 13 or 14 would be a good ratio, is this correct cause im picking up the biggest rear sprocket i could find. What would happen if i did have a lower gear ratio? right now im not sure what it is
    Last edited: Aug 20, 2008
  10. HoughMade

    HoughMade Guest

    Couple of issues.

    Chain saws normally have a pitch of 3/8" and bicycle chains have a pitch of 1/2" so you will need to attach a sprocket to the saw clutch of 1/2" pitch. You can'r mix a 3/8" sprocket with a 1/2" chain.

    Second- you need more reduction. Even if you could get a 6 tooth 1/2" pich sprocket (goog luck), that means that for a reduction of 13 to 1 (your number), you would need a rear sprocket with 78 teeth....good luck again, but as Lou said, 13 to 1 is not enough. 18 to 20 to one is more like it. That means a rear sprocket of at least 108 teeth...getting close to the size of the wheel itself.

    This does not mean it cannot be done. The links above show it can be. However, if you are going to use a chain drive, you need a jackshaft or gearbox to drop the RPM more. A friction drive is probably the simplest way to use a chainsaw engine.

    Finally, I don't get where you get this ratio of 13 or 14 to one. With that ration, you can count on no low end power and, if you can keep it running, too much top end, but likely you will not have enough torque to get to the power band so it will be unsatisfying all around.
  11. tylerscool45

    tylerscool45 New Member

    1st ya i know i already got a 14 tooth sprocket which i am welding on tonight
    2nd will a 90 tooth work
    3rd i dont know if a can get a jackshaft or gearbox if i can't will it still work
    Im not using friction drive btw
  12. nathanl

    nathanl New Member

    26 inch wheel with a 9000 rpm motor driving a 14 tooth cog to a 90 tooth cog on the wheel gives you a topspeed of over 100mph. It wouldn't really work. Check out http://www.motoredbikes.com/showpost.php?p=87814&postcount=1 It has a gear calculator that gives you an idea of what you need to get in the 30mph redline gear area.
  13. tylerscool45

    tylerscool45 New Member

    So what would happen at 2 to 1 gear ratio? Thats what i want to know
    Last edited: Aug 20, 2008
  14. nathanl

    nathanl New Member

    2:1? The numbers you were asking about before were 6:1... neither would work. You'd just damage your chainsaw engine/clutch.
  15. tylerscool45

    tylerscool45 New Member

    You mean it would be broken, Chainsaws are meant to be at that high rpms.
  16. HoughMade

    HoughMade Guest

    Yes they are, but what everyone is telling you is that they run way, way, way, way, way too fast without reducing the crankshaft speed of the engine by about 18 to 20 times for use on a bicycle. You want the engine running at its natural sweet spot, which is at a high rpm over 7,000 in this case, maybe 10,000, while reducing the speed with sprockets or gears enough get adequate torque to the wheel.

    Think about this- take any manual transmission car, put it in the highest gear, 5th or 6th, and try to leave from a stop. Doesn't work well and most times will kill the engine. Now imagine a car with even more gears, 10 or 11, and trying to start from a stop in 11th gear- that is what you are asking that chainsaw engine to do at a ratio of 6 to 1, let alone 2 to 1- that would be a theoretical 16th gear. Won't work.
  17. tylerscool45

    tylerscool45 New Member

    so 18 to 1 would be the sweet spot cause i can get up to 21 to 1 or how bout 17-1 its a 22 inch wheel btw
    Last edited: Aug 21, 2008
  18. loquin

    loquin Active Member

    18:1 with a 26 inch wheel results in apx. speed of 43 MPH at 10000 rpm

    the ratio of 22 to 26 is 0.846

    0.846 times 43 mph is 36 MPH

    Here's the calculation that you need to use with your calculator:

    Speed = RPM / reduction ratio * wheel diameter / 336

    10000 / 18 * 22 / 336 = 36.3 MPH for 18:1, 10K RPM, 22 inch wheel


    10000 / 17 * 22 / 336 = 38.5 MPH for 17:1, 10K RPM, 22 inch wheel
  19. Luka

    Luka Member


    If you put what I would call a 'half'-jack' on the seatpost downpipe...

    A half jack being a simple post with two gears on it. One larger than the other, spinning freely, but welded/bolted to each other...

    Then you could run a chain from a smaller gear at the chainsaw, to the larger gear on the half-jack. Then run another chain from the smaller gear at the half-jack to the largest gear at your crank.

    This way, you could reduce the gearing twice. Plus you would retain full shifting on the rear.

    If I did this, (and I may. Using an old husqvarna chainsaw), I would put a freewheel crank on, so that i wouldn't have to keep pedaling when the engine is running.

    I have drawn on your own illustration, what I mean.

    You would eliminate the chain going down to the back wheel.

    Attached Files: