weed whacker question

Discussion in 'Friction Drive' started by dirtbikemike2435, Jul 6, 2011.

  1. dirtbikemike2435

    dirtbikemike2435 New Member

    i have found a 25cc homelite... and so i am wondering if a 2" bmx peg is too large, what type of speed to expect. also i dont mind pedalling at first:cool:



    whats your tire size ?
  3. professor

    professor Active Member

    From what I read, 2 inch is too big. Oh, and tire size doesn't matter. The tire is just a big roller between the drive and the ground.
  4. Basement_Modder

    Basement_Modder New Member

    Nonsense, he could use a 10" wheel if he wanted to. It would rob torque, but he could go fast eventually. A 2" bike peg is fine.

    Out of curiosity, what model is your motor? I have a homelite SX-135 I just rebuilt.
  5. dirtbikemike2435

    dirtbikemike2435 New Member

    im not sure, it was given from a friend and all i can tell is that its not old couple years or so
  6. srdavo

    srdavo Active Member

    Using a 2" bmx peg as a drive roller.......

    @ 7000 rpm's would get about 41 mph.

    You would have to pedal up to about 30 mph before the engine would try to pull you.

    Factor in wind resistance & your weight.... at this proposed speed, I doubt this engine would do much more than scream..... in agony.

    A 1" drive roller would get you into the low to mid 20 mph range & allow for your engine to work within its limits.
  7. Chalo

    Chalo Member

    The surface speed of the roller needs to correspond to a reasonably achievable surface speed for the bike. If you "gear" the motor too high with too big a roller, you'll burn up the centrifugal clutch and never get all the motor's power to the tire where you can use it.

    Weedwhacker motors typically make their maximum power at about 7000 rpm. The surface speed of a 2" roller at 7000 rpm is almost 42 miles per hour. One horsepower from a trimmer engine can't propel you that fast. The motor will bog down and the clutch will burn. The bike will never attain even its max torque speed, let alone the speed at which it makes its rated power.

    The surface speed of a 1.25" roller at 7000 rpm is 26mph. That's a realistic top speed for a one-point-something horsepower motor on a smooth running bike with an average sized rider.

  8. dirtbikemike2435

    dirtbikemike2435 New Member

    Thanks alot guys, is their any other ideas i can make a drive roller out of because i have yet to see a 1.25 bike peg, i have a 1.5 bike peg, also i have heard of people using a socket? any ideas
  9. Chalo

    Chalo Member

    I have seen knurled axle pegs about 1.25" in diameter, but they were the small kind with 3/8" nut threading in one end. The knurled part was welded on, so there is no reason to think they'd run particularly true, either.

    What does your motor have to attach to?
  10. dirtbikemike2435

    dirtbikemike2435 New Member

    its bolted onto a homemade piece of U channel, kinda like the staton
  11. Chalo

    Chalo Member

    I mean, what's the size and shape of the output shaft? How are you going to attach a roller, whether BMX peg, socket, or whatever?
  12. dirtbikemike2435

    dirtbikemike2435 New Member

    i welded a piece so that a 3/4 bolt can hold the peg to that adapter piece
  13. Doging7

    Doging7 New Member

    I am considering building a friction drive setup, powered by a 18 cc weed eater engine. I plan on gearing down the rpm with a simple sprocket set up. It would probably be around a 2:1 gear ratio. I have read that the range of weed whacker rpm is about 3000-12000. When you say 7000, is that about where the max power tapers off?
    I have also been thinking about what to use for a roller. The main reason i didn't want to build a friction drive was that i was very worried about shredding up tires. I have a hybrid bike ( tires skinnier than a mountain bike, but wider than a road bike) pretty smooth tires. Researching this seems to show that with smooth tires and well tuned setup, tire wear is a much smaller issue. I have used the Gear Rat caculator (somewhere one this site) and figured out that I should use a 1.65 inch to a 1 inch diameter roller. Thats based on my engine producing 12000 rpm WOT, then geared with a 2:1 ratio =6000rpm (then simple math i don't feel like explaining)= 26.8 MPH with a 1.65 inch roller or 17 mph with a 1 inch roller. So probably aiming at a 1.25-1.5 inch roller would be good, I am worried about the torque needed to climb steep hills.
    I'm also wondering if i should have a centrifugal clutch ( I have one, but there was not a stock one on the weed eater) or if i should rig up some kind of control to en-gauge the roller on the tire. Leaning towards the latter as it would be easier to adjust the pressure of the roller on the tire. I would use some skateboard bearings to reduce friction from the roller supports, requiring a little machining. I going to post more info on a separate thread if your interested, though this will probably be re-posted there.