Friction drive gear/roller ratios

Discussion in 'Friction Drive' started by Happy Valley, Jul 27, 2008.

  1. Happy Valley

    Happy Valley Active Member

    Help me understand something. For example, would it be correct to say the ratio is 26/1 with a 26" bike wheel and a 1" friction roller?

    For some reason, with a friction drive set-up I'm having a hard time getting my head around this issue. I was talking with Dave Staton about a friction drive set-up with the Robin EHO35 and asked him which was harder on the engine, smaller or larger rollers. He said the larger the roller the more it worked the engine.

    I'm trying to understand the torque/hp elements for this set-up and the variables like roller size that will effect the way I use it.

    Thanks for any insights.
     

  2. 5-7HEAVEN

    5-7HEAVEN Guest

    Hi Happy,

    Discounting slippage, bicycle A has a 1" roller, which spins a 26" tire at a 26:1 ratio. Bicycle B has a 2" roller/26" tire with a 13:1 ratio. What Dave meant is that bicycle B would have less mechanical advantage and has to work harder to move the bike at every speed.

    Say that an engine has peak torque at 4500 rpm. That's where it feels most powerful. At 26:1 ratio, Bike A feels this power at 13 mph. At 13 mph, Bike B spins at 2250 rpm, or idle speed. However, it would not have the low end power to reach 13 mph, much like a car being in 4th gear. You would have to pedal faster than 13mph before Bike B could weakly push itself forward.

    At 9000 rpm, Bike A is screaming at 25 mph, while Bike B is at peak torque at 4500 rpm. At 9,000 rpm, Bike B will be screaming at 50 mph.

    So the larger diameter the roller, the more the engine has to struggle at low speed, but the bike would be faster, but not quicker to 50 mph. The smaller spindle will peak at a much lower speed, and will feel perkier until it begs for mercy at 25 mph.

    hope this helps.

    5-7
     
  3. Happy Valley

    Happy Valley Active Member

    5-7

    Thanks so much.....again.

    I couldn't have hoped for a better or more thorough explantion. I think I got it.
     
    Last edited: Jul 27, 2008
  4. Egor

    Egor Guest

    One interesting point is that a friction drive is not affected by the size of the wheel. Essentially the friction drive is moving at the speed of the size of the roller as if it were being used directly to the ground. In other words if you had it behind the bike and the roller were touching the ground pushing the bike the diameter would dictate the speed of the bike, the larger the roller the more ground it would cover in each rotation. The way to change the ratio of wheel is to move into more the center, to a smaller diameter and take advantage of the outside being larger making a longer distance in each revolution. This is a little long but sometimes it is a little hard to see that this is real time motion the tire moves at the ground directly in proportion to the circumference of the roller. Have fun, Dave
     
  5. Happy Valley

    Happy Valley Active Member

    Thanks Egor, good analogy, I'm finally understanding, I think, what I want to know.

    0.7/8" diameter = 2.748" circumference
    1. 00" diameter = 3.141" circumference
    1 1/8" diameter = 3.534" circumference
    1 1/4" diameter = 3.926" circumference
    1 3/8" diameter = 4.319" circumference
    1 1/2" diameter = 4.712" circumference

    1/8" increments in roller diameter equals a touch over 3/8" increments in circumfernce.

    The 5/8" difference in diameter, for example, between the 7/8" and the 1 1/2" rollers is almost a 2" spread in circumference! Multiply that times rpm.
     
    Last edited: Jul 28, 2008
  6. Mountainman

    Mountainman Active Member

    Dave Station makes it all pretty simple when someone places an order. He asked me which roller I wanted -- I didn't have a clue at the time. He asked my weight (195 lbs). How many hills in our area - we have soom hills here in San Diego. He asked me if I could peddle up the hills without walking the bike - yes - but may be very slow on some.. He sent the 1" roller - has worked perfect for the layman. Happy Riding from - Mountainman
     
  7. Egor

    Egor Guest

    So to make it easy with a 4" circumference roller at ten thousand RPM's you are traveling at 40,000 inches a minute. A mile has 63,360 inches in it so you are traveling about 3/4 of a mile a minute, so I think that equates to about 40 MPH. Have fun, Dave
     
  8. seabillco

    seabillco Member

    Hi, all
    Just to chime in with a real world experience I just had on this topic...
    I had a 1.3" OD BMX foot peg as a drive roller on my 20" wheel on the Rans recumbent and it was pretty fast with the 25cc Ryobi 4 stroke engine at about 25 mph but it dogged on the hills quite a bit.
    So, I switched to a 1" OD BMX foot peg drive roller this morning and found that my top speed isn't much less but the hill climbing and take-off ability is much better.
    Also, the smaller spindle results in a lower speed before the engine bogs down and dies which is very nice for making tighter turns at slower speeds without killing the engine (it's a friction drive with no centrifugal clutch so I kill it by putting the brakes on).
    In case anyone is looking for the BMX foot pegs, I found a nice pair online that have a 3/8" bore which works on most of my Ryobi engines and one end of the peg is threaded with 24T and the other with 26T. I needed 24T. Very Clever. Here's the web site:

    bicyclebuys.com

    The SKU is 0255552 and the Product Name is: "Pyramid Pegs 26/24T Black. The cost was $20 with shipping.


    Steve G.
    Grants Pass, Oregon
     
    Last edited: Jul 30, 2008
  9. jonboy2five

    jonboy2five Member

    not a "ratio"

    you already got it i know, but just remember that there is no ratio to worry about when using a drive roller (spindle) friction drive. a good number to remember when calculating speeds is how to go from inches per minute to miles per hour. the formula : in/min / 1056 = miles/hour...happy motoring!
     
  10. seabillco

    seabillco Member

    7-30-08
    Hi, again
    Just a final note in case anyone is still reading this thread...
    I took a 16 mile trip today with a 1" drive roller instead of the old 1.3" roller on my friction drive, home made, MB.
    The smaller roller made a BIG difference in hill climbing. I was able, for the first time, to go into town and back, up a big hill, with no pedaling.
    Also, I got 256 mpg which is the most I've ever gotten.
    FWIW, I suspect the smaller roller kept the 25cc Ryobi 4 stroke engine in its optimum power band better.
    It's hard to believe you can get this kind of performance from a home built bike and that, at least around here, no one even seems aware of MB...

    Steve G.
    Grants Pass, Oregon
     
  11. Happy Valley

    Happy Valley Active Member

    Kudos, Steve! Glad your pleased with the mods you made.

    And thanks for the update, I'm still reading and interested as I'm waiting for a Friday delivery on a friction kit with a 1" roller. :lol:
     
  12. seabillco

    seabillco Member

    Hi, HV
    Thanks!
    Oooohhh... What kit did you get???
    I'm very curious to hear the details since I've been thinking about a friction kit, also.
    Any info will be voraciously consumed (at least by me!)

    Thanks again!
    Steve G
    Grants Pass, Oregon
     
    Last edited: Jul 30, 2008
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