What size roller?

Discussion in 'Friction Drive' started by Jake.B, May 16, 2012.

  1. Jake.B

    Jake.B New Member

    Hey guys, i have a build i'm going to start later this summer. I'm trying to get everything squared away. My main problem is the correct roller size on the drive shaft. I have an 80cc chainsaw i'm mounting to my 20 inch bike. The full weight including the motor, my self, bike, and extra weight for mounting is probably around 200 pounds give or take 10 pounds. I want mainly speed, a little torque just so it doesn't take me forever to hit top speed. I want to accomplish the best gearing possible. I understand its trial and error, but i figure the better i start out with the easier it will be to achieve. It will be pedal start, no clutch so i don't need torque for take off. Its all flat land so no hill climb torque needs to be factored in. At first i was thinking 1 3/8 but then i was suggested 2 inches? what do you guys think will be the best size?

  2. wbuttry

    wbuttry Member

    Well jake in cali it is illeagel to run on street and to you are 14 you dont have a drivers licences yet either. Most people wont help u much till you are older and we all know you will be on the street with it as soon as your mom and dad turns theyre heads any way. 80cc is way to big plus it is illeagel to put on a bicycle your best bet is save up money and get you a good kit motor and drive kit in stead of american engineering it . it will last a heck of a lot longer and be more fun but really u need to wait a few yrs on it .they can be a hand full if you are not carefull....
  3. Jake.B

    Jake.B New Member

    I understand, its not for the gov't owned street. My grandfather owns 3,200 acres there are many paved roads and different spots to speed around. I would not take it on a gov't owned street. Im more into the building and tuning the project than speeding around illegaly on the road.

    Im not the average 14 year old, im bigger and smarter that most of my peers. I can understand thinking "hes just a kid, dosent know what hes doing" i get that alot. Im sure 99.9 % of you guys on the fourm know you stuff. I understand the guys who ride on the rode illegaly are the ones who ruin it for everyone. Im not that guy i know how much these MB mean to alot of you, and im not willing to ruin it for them all. Now, with that being said i think i got my roller worked out. Im still open to any suggestions.
  4. Happy Valley

    Happy Valley Active Member

    That's a pretty mature attitude for someone regardless of their age in the second post and I commend it, frankly wish it were more common across the age spectrum.
    In answer to your first post question given the facts you've stated, I think your idea of somewhere between 1 1/4 and 1 1/2 would be in the ballpark. Much larger and you'll have little low end torque and have to pedal a direct drive like crazy to get to started and up to speed.
  5. loquin

    loquin Active Member

    Chain saw motors often have a pretty high RPM - what is the rated max RPM on the chain saw you're using?

    I wouldn't even think of two inches. At 10000 rpm, you're looking at a potential top end of 60 MPH, assuming the chain saw can put out 6-7 HP, which is certainly possible for an 80cc saw motor. NO bicycle is safe at that speed - the bearings, wheels, tires, etc. could easily fail... or your steering could get the 'wobblies' and, at that speed, any sort of issue could be fatal.

    The other issue with a motor of that size, is that 6-7 HP could easily cause the roller to slip on the tire, which will grind off your tire... It'll be difficult to keep enough roller pressure.

    The formula to calculate the maximum possible * speed is:

    V = W x D / 336.1
    where V = max speed, W is the maximum roller RPM, and D is the roller diameter in inches.

    Rearranging that equation to solve for roller diameter:
    D = V x 336.1 / W

    So, assuming 40 MPH, and 10000 RPM:
    D = 40 * 336.1 / 10000 = 1.34" , or, just slightly over 1-3/8", which falls right in the middle of Happy Valley's range, above.

    Finally, for safety's sake, make sure you have more than one brake - a front brake is a real necessity. At speed, it's just too easy to lock the rear brake if you have to make an emergency stop, and on pavement, the rubber can grind away in one stop... (I've done it!)

    * The maximum actual speed also depends on the tires you're using, tire pressure, how much power is available, road surface, rider size/weight, your riding stance, wind speed and direction, even air temperature and altitude...
  6. 2stroker

    2stroker Banned

    I understand that. my 14 year old brother just built his own go kart then he welded a trailer for his canoe outta scrap steel together so he could pull it with his go kart to the lake through the woods. Then he turned around and built his own mini bike with 5 horse 2 stroke again from scrap steel. I think your capable of building a kit. But i think you would have more reliability outta one you bought. Its fun building your own. But i now its alot funnier to ride one for 50miles and not have to worry about your "american engineering" flopping.( no offense) I have a couple homemade kits my self. They worked good. for a little while. then i was walking. Post pics when your finished im interested in seeing your creation!! Good Luck and i hope ya do better than i did!
    Last edited: May 17, 2012
  7. Jake.B

    Jake.B New Member

    Yeah, i found the specs for the engine and it said
    Max operating rpm 6000-6500? I thought that was pretty low, considering its a 2-stroke
    Chainsaw. Im going to plan this build out and figure everything
    Out perfectly before building. Once I get the saw i can use a tachometer and figure out the rpm. 60mph is way to fast i wanted 45 max. ill probably use a 1 5/8 roller if the rpm is around 10,000. Thanks for the help ill post pictures and keep posted on the build. Ill start with the chainsaw. Im picking it up saturday, what do you guys use as rollers? I was considering a socket and find some kind of
    Rubber boot to epoxy over the roller.
  8. loquin

    loquin Active Member