weed whacker gear ratios

Discussion in 'Frame Mounted Engines' started by grayleon, Jun 8, 2008.

  1. grayleon

    grayleon New Member

    I have a 25cc weed whacker, and i want to run a chain directly from the driveshaft on the motor to a sprocket on the back wheel. i want to still be able to pedal the bike. what size sprocket should i put on the driveshaft, and what size should i put on the wheel?

    Last edited by a moderator: Jul 16, 2008

  2. stude13

    stude13 Active Member

    hi ive been wondering the same thing. something different----americanparagliding.com has a TINY TACH and hr. meter for 39.95 2 or 4 stroke. mitch
  3. HoughMade

    HoughMade Guest

    Well, let's run the math- I'll figure for a 25 mph speed on a 26" bike- adjust accordingly.

    A 26" bicycle covers 81.7" on one revolution or 6.8' (Pi x 26)

    A mile is 5,280', so in 1 mile the wheel does 776 revolutions (5,280 / 6.8)

    In 25 miles, the bike does 19,400 revolutions (776 x 25)

    At 25 mph, that would equate to 323 revolutions per minute, rpm, of the wheel (19,400 / 60)

    Now, the issue is to get the engine rpm reduced to 323 rpm to do 25 mph.

    Figure a 25cc 2 stroke weed wacker would do 7,000 rpm (may be more, maybe less- adjust as needed)

    To get 7,000 rpm to 323 rpm, divide 7,000 by 323 = 21.7

    Total gear reduction needs to be 21.7:1- so here is the issue, for a chain and sprocket system without an intermediate reduction, we have a problem. A 10 tooth engine sprocket would require a 217 tooth wheel sprocket....that pretty big.

    You would probably be better off with a friction drive. If you used a 1.25" engine roller with a 26" wheel, you have a reduction of 20.8:1 with nothing more needed.
  4. kengorman

    kengorman New Member

    Does anyone know if weed whackers have internal gear reduction or whether the shaft speed is the same as the engine speed?


  5. brendonv

    brendonv Member

    Yes whipper snippers are straight shafts, im currently getting ready to make a chain drive whipper snipper, mines a 31cc ryobi. Mine is going to be a 10T on the motor, going to a 50T then a 10T on the same shaft going to the wheel which will also be a 50T. So a final ratio of 25:1. You have to do a two stage reduction or ur going to have a sprocket the size of your wheel on the back. You could do a 20:1 reduction, so a 10T to a 40T to a 10T to a 50T. or a 30:1 so 10T to 50T to a 10T to a 60T. Any of those three would go pretty well. U will probably have speeds of about 20MPH.
  6. muddawg

    muddawg Member

    im a use what ya have type of guy...

    where are you guys gettin 10-12 and 48-50 tooth bike chain sprockets ???

    the best ive found are 16 tooth and 42 tooth

    lemme in on the secret guys or the vendor

    thanks mike

    oh and if ya aint looked at it yet check out the thread "chainsaws weedeaters and snowblowers oh my"