Need Gear Ratio Help

Discussion in 'Rack Mounted Engines' started by bryant, Sep 10, 2009.

  1. bryant

    bryant Member

    I am New. This is first post after the introduce yourself post. I sure hope I am posting in the right place so if not let me know. I have a 2.5 hp Tecumseh (4 cycle) engine I plan to mount on a rack over the back wheel of my mountain bike. I have used jpilots gear ratio calculator and have come up with 10 teeth on the engine shaft(centrifical clutch)and a sproket on the wheel of 41 teeth. Bicycle wheel is 26 inches. This appears to be about optimal for speed and climb. I weigh 225 pounds. Never having done this before does this sound ok. It does in jpilots calculator but I really don't know the engine max rpm so I just used the figure 2000 rpm. This engine came off a sears lawn edger and I can't find much info, especially what the engine max rpm is. If anyone has any suggestions please respond.
     

  2. srdavo

    srdavo Active Member

    I'm guessing that engine will run close to 4000 rpms.

    I think you'll need at least a 10:1 gear reduction, in order for it to pull you from a stop & up hills. And you should top out around 30 on flat ground.

    That's a lot of torque on a normal bicycle wheel. I mangled one, BAD.... with my 2.5 hp Harbor Freight Greyhound.
     
  3. bryant

    bryant Member

    Ok Dave, Thats the type info I need. Do you think a jackshaft arrangement would work out?
     
  4. srdavo

    srdavo Active Member

    yes... a jackshaft would work fine.

    Question: Did your engine have a gearbox on it, in it's previous life as an edger?

    here's an unfinished thread, about my trial & error, four stroke trike build.
    http://www.motoredbikes.com/showthread.php?t=17839
     
  5. bryant

    bryant Member

    No gearbox on the edger. Only had a belt pulley that went to another pulley of the same size at the blade end. They used a lever arrangement to lift the blade up and down which also would tension the belt so the blade would turn. That trike is really a cool machine. I enjoyed looking it over and reading the comments. My knowledge of gearing is in its infancy so I am now trying to figure out a combo from the 10 tooth or 12 tooth centrifical clutch to the first gear on the jackshaft and then from the next gear on the jackshaft to the wheel sprocket which I guess will still be around 41 tooth or + or - of that. I think the jpilot gear calculator may work for me but I would appreciate any suggestions along these lines. I assume I would also put in the max rpm(about3600 or 4000) and get max a mph readout. Listen to me. A complete amateur in these matters and there I go trying to sound like I know something. Anyone listening please correct me because thats the way I will learn. Thanks
     
  6. srdavo

    srdavo Active Member

    hahahaha.... this reminds me of my first lesson, here, in gear ratios....
    (totally off topic.... but amusing!!)
    http://www.motoredbikes.com/showthread.php?t=4175

    As for the edger's design.... a belt drive with a slip clutch to the jackshaft... then go chain drive to the wheel.

    the driven pulley would need to be twice as big as the drive pulley... = 2:1
    then a 10 tooth sprocket to a 50 tooth on the wheel...=5:1
    totals 10:1

    there are a lot of combos to get ya there.
     
  7. bryant

    bryant Member

    Thanks Dave, Read your "gear and ratio 101" lesson. If I can absorb the lessons in that piece then I should do ok. I appreciate you helping me. Tom
     
  8. srdavo

    srdavo Active Member

  9. bryant

    bryant Member

    Dave,I couldn't have asked for more good info than you have supplied. I am still going to mount the2 1/2 hp on a bike but for now I am just itching to get a bike going ASAP and enjoy a little fun riding and get some motorized bike experience before it gets to cold around here to really enjoy it. I ordered a DAX 80cc yesterday so in about a week I should be getting on the road. I looked at a bunch of the reviews and even though there were some good vendors I felt DAX gave a lot in spare parts and the build instructions. Me beginng tobe so darn old things don'tcome easy as they once did so I feel comfortable going this way first.
     
  10. scokes

    scokes Member

    I just wish it would stop raining, got stuck in the middle of a gusher Friday afternoon. Spent thirty minutes twiddling my thumbs and still ended up riding home in the middle of it. This weekend looks like it will be beauty though. If you have any questions, we're here to help. Good choice of motors. I too am riding an 80CC. Just remember, if the police ask, it's only 49cc. AR law says, over 50cc, needs to be licensed. "shhhh"

    Have fun.
     
  11. bryant

    bryant Member

    Yeah the rain is beginning to be a b...h. I guess wet pavement could be a problem. You are right on Ar law for motorized bikes. I looked it up the other day and it sure does state 50cc but in another section I found a reference to 2hp. Anyway I plan to carry the printed out version that says 50cc any have the 50cc already circled in red ink so maybe I can pull it off if I get stopped. A copy of the engine kit showing the 49cc engine stappled to the copy of the Ar law should do the trick.(I hope) Thanks, Tom
     
  12. scokes

    scokes Member

    Well, I work next to the PD here, and my step-dad is a Lt. here. My chinese kit has no markings on it whatsoever, yours probably won't either. So unless they plan on pulling it apart and putting a micrometer on it,,,, Don't ask, don't tell. From my experience, they will mostly sit back and admire it.
     
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