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  1. #11
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    lmfao its just a tire just slap it on and go! some people just over think to much and complicate things

  2. #12
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    Quote Originally Posted by landuse View Post
    I am trying to figure out if size does matter or not. On this forum there is a sticky which says that it doesn't matter, and other people are saying that it does.

    I need a bit of clarification here.
    How many of us need to tell you it doesn't matter before you finally stop looking for "clarification?" This isn't a matter of opinion but obviously you're treating it as such. I think you just need to go test it out yourself.
    pearls ain't free

  3. #13
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    lol thats cavimike dont listen to him hes like the biggest negative in mbc he will cut down ppl like crazy here and is jus plain miserable ask anyone hes our oscar the grouch of mbc lol we jus close the lid and let him mumble to himself anyways dude jus put a tire on who cares if it has resistance does it really matter? a thick tire a thin tire who cares jus go ride have fun!

  4. #14
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    Tyre size certainly does influence friction, hence the reason why high end road racing bikes use very narrow tyres, to minimise wind resistance.
    The larger the sectional profile, the greater the wind resistance. At the high end of professional competition, any advantage that can be taken, will be taken.

    Technically a larger diameter tyre will also reduce rolling resistance as axle to bearing speed is reduced.

    All these frictional and heat losses add up, but it needs to be put in context of the goal that is trying to be achieved.
    Last edited by Fabian; 04-27-2012 at 01:51 PM.

  5. #15
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    With a friction drive, roller size alone determines the potental speed. A 1" roller will move the bike forward 3.14" per revolution. A 1.5" roller will move the bike forward 4.71" per engine revolution. Be it a small diameter tire or a large diameter, they both can only move forward by the circumference of the roller on the friction drive.

  6. #16
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    imagine a car wheel on a bike yes weight and friction matter!

  7. #17
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    Quote Originally Posted by DeathProof View Post
    imagine a car wheel on a bike yes weight and friction matter!
    Yes they do.

    But, in the context of a BICYCLE, with a small engine mounted, the weight of the bike will fall in a relatively small range, and the tire size will also fall into a relatively small range, and within these ranges, the only truly appreciable determining factor in calculating the maximum theoretical speed of the bike/motor, is the roller diameter and engine speed.

    True, you can obtain small improvements in the attainable bike speed, (which cannot exceed the maximum theoretical speed,) by fiddling with tire width, air pressure, rolling friction, etc.

    What I am getting at is that the roller diameter and engine speed determine the maximum theoretical speed obtainable. Period. Any other factor simply reduces the obtainable speed of the bike.

    From a practical viewpoint, you can fairly easily get to within an MPH or two of the max theoretical speed using standard, relatively inexpensive equipment. (I got a full MPH by replacing the stock tire with a slick, low rolling friction tire, for instance.) Or, you could end up spending thousands to get that final MPH or so. At some point, there is a point of vanishing returns, where you spend more and more to get less and less.
    Lou

    "Lisa, in this house, we obey the laws of thermodynamics!" - Homer Simpson

    "Bicycling is a healthy and manly pursuit with much to recommend it, and, unlike other foolish crazes, it has not died out." -- The Daily Telegraph (1877)

  8. #18
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    Have to agree with you Lou.

    At some point the cost of improving engine performance becomes less costly than reducing rolling resistance or aerodynamic drag, unless you are competing in a control category.
    Last edited by Fabian; 05-01-2012 at 03:56 PM.

  9. #19
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    Back to the OP (landuse) questions. The second reference from the other site tries to equate a "gear reduction ratio" to friction drives... but, that approach is meaningless, in regard to friction drives. As the other person mentions, changing the tire size cancels out the 'gear ratio' change.

    The other site poster said:
    while the roller's surface-speed IS the same as the vehicle's surface-speed, the fact's irrelevant to the math that's used to calculate ratio and speed.
    For a friction drive system, Surface Speed is ALL that matters, and ratio is meaningless. The surface speed is the speed of a point on the circumference (outer surface) of the drive roller. Assuming that there is no slipping between drive roller and tire, or between tire and road:

    The surface speed of the spinning roller is EXACTLY the same speed as the surface speed of the spinning tire, NO MATTER THE DIAMETER, because the roller and the tire are in constant contact.

    Likewise, the surface speed of the spinning tire is EXACTLY the same as the surface speed of the tire against the asphalt, (or, the road moving past the tire,) because THEY are in contact.

    In a system with no slip, the surface speed of the roller is therefore IDENTICAL to the speed of the bike, no matter the tire diameter, and to calculate the maximum theoretical bike speed, you only need engine RPM and roller diameter.

    Even though the RPM of the wheel is much less than the RPM of the drive roller, (By the ratio of Drive roller diameter divided by tire diameter) the linear speed of a point on the surface of BOTH is the same. It HAS TO BE if there's no slipping. They are in contact.

    Assuming a 1 inch roller and a 26 inch wheel, the roller RPM is 26 times the wheel RPM. Your roller would HAVE to spin 26 times for the bike to travel 81.64 inches (pi * 26 inch)

    If you cut the wheel diameter in half, the roller RPM would now only be 13 times faster than the wheel RPM. But, since each wheel rotation will only take you half as far (40.82 inches,) as with the 26 inch wheel, so your wheel would have to spin twice, and the drive roller would STILL need to spin 26 times (13 * 2 times) to make the bike move the same 81.64 inches as with one rotation of a 26 inch tire.

    Remember, with a chain/belt drive, all the power is directed to the drive AXLE, to make it spin at a given RPM. In that case, you NEED to use the wheel diameter to calculate the final speed.

    But, with a friction drive, all the power is directed against the outer surface of the wheel, bypassing the axle entirely (From a power transfer point of view.) Since the power is already applied AT the tire surface, the tire diameter is irrelevant.

    Now, you COULD go through the RPM calculations at each point, but, it's POINTLESS. Because the RPM of the spinning tire varies in direct relationship to the tire diameter, AND the distance traveled with each rotation of the tire also varies in direct relationship to tire diameter, any change introduced by a tire diameter change is exactly and completely canceled out.

    Last edited by loquin; 05-02-2012 at 12:27 PM.
    Lou

    "Lisa, in this house, we obey the laws of thermodynamics!" - Homer Simpson

    "Bicycling is a healthy and manly pursuit with much to recommend it, and, unlike other foolish crazes, it has not died out." -- The Daily Telegraph (1877)

  10. #20
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    Thanks everyone. I appreciate the help

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