gear ratios for weed eater bike

Discussion in 'Rack Mounted Engines' started by keatonx, Feb 6, 2011.

  1. keatonx

    keatonx Member

    I'm building a rack mounted 16 inch weed eater bike with a 31cc reed valve MTD weed eater with tons of compression and power. I plan on driving this thing in the trails and maybe some small hills. I heard that the recommended gearing for 16 inch bikes is 14:1, but I only have the parts to have 4.5:1 on my chains and 3.5:1 in an old cordless drill gearbox i'm putting on my engine.
    I'm not completely sure how to add these, but i think it'll be less than 14:1. Will the powerful engine, the fact i'm a really short and light kid, and the drilled out muffler with no spark arrestor make up for the messed up gearing?
    Last edited: Feb 6, 2011

  2. srdavo

    srdavo Active Member

  3. keatonx

    keatonx Member

    Thanks! I thought you added, not multiplied, so I'm fine.
  4. 5-7HEAVEN

    5-7HEAVEN Guest

    You'll love it more on the trails if you gear it 20:1 or more.:detective:
  5. wheelbender6

    wheelbender6 Well-Known Member

    Weed eater exhaust can be painfully loud. You may be able to use an expansion chamber from a goped. Simply tearing out all the restriction in your stock exhaust may add a bit of power but it will definitely attract law enforcement.
  6. keatonx

    keatonx Member

    I don't have any good gearboxes for higher gearing, but i have 2 gears that do 7:1. However, i can't weld, so i don't know how to position it. that would be around 30:1, which will probably be painfully slow.
    Last edited: Feb 9, 2011
  7. keatonx

    keatonx Member

    I'll try the 15:1 and see how it works.
  8. 5-7HEAVEN

    5-7HEAVEN Guest

    If you're offroad, you will definitely appreciate the 30:1 gears. On the road, maybe 19:1.

    What you COULD do is set up the jackshafts so you could gear them from 3.5:1 up to 8.57:1. That way, you could swap gears for offroad and street.:idea:
  9. keatonx

    keatonx Member

    How would I set this up? Anyways, wouldn't 30:1 be really slow?
  10. 5-7HEAVEN

    5-7HEAVEN Guest

    Once you calculate the ratios and size sprockets you'll use at the jackshafts, it's a matter of removing and lengthening/shortening the chain(s) and swapping jackshaft sprockets to change gearing.

    If you'll be climbing offroad hills, you'll need 30:1 gears or better or the engine will bog. While testing and tuning my motorized bike, I leave it in 1st gear, which is 32.46:1. With a running start, I fly up the hill. If I start in 2nd gear, which is 20-something:1, my bike will bog.

    I have a shift kit with 8-speeds and shift kit. Top gear is about 14.32:1 @ 45mph.

    How are you getting 4.5:1 on your chains?
  11. keatonx

    keatonx Member

    I have a modified gearbox that does 3.5:1. That has a 9 tooth SBP sprocket that goes to a 40 tooth bicycle sprocket. It does 4.4444444:1.
  12. 5-7HEAVEN

    5-7HEAVEN Guest

    Well, you could have two rear wheels and sprockets, one with a larger sprocket for offroad.
  13. keatonx

    keatonx Member

    Maybe but it has pretty ghetto tires it would be hard to find wheels witth those tires.

    I checked my ratios and the drill gearbox+chain would only do about 13.5:1.

    I just really have no idea how to set up jackshafts, and i need to keep the pedals for legalty. So what I think I'm gonna do is quickly slap on the 13.5:1 with cable ties and duck tape,and see how it goes. THEN i will se what I'm gonna do.
  14. 5-7HEAVEN

    5-7HEAVEN Guest

    Good Idea!

    Yes, right it like itis and see if that's how you like it.
  15. keatonx

    keatonx Member

    My bike is actually frame mount now (wooden rack didn't work out so well), but the only difference in the drivetrain is the gearbox. The old one was from a cordless drill, and now it's from an angle grinder, so now the engine can be mounted with the crank parallel with the frame. It goes about 60kmh, and I'm adding a 3 speed pedal crank to the rear wheel, and a pocket bike clutch.
    The gears are tricky, because the derailler is for the right side, and the chain is now on the left. but Iwill get tons of speed and torque with the 3 speed.
  16. 5-7HEAVEN

    5-7HEAVEN Active Member


    I can't wait to see it run!