How does unloaded spin duration effect on-road performance?

Discussion in 'Cycling Equipment' started by FogVilleLad, Feb 23, 2008.

  1. FogVilleLad

    FogVilleLad New Member

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    The "What is the truth behind bike weight..." thread makes me want to raise this issue.

    I have two sets of 27" wheels, one very old and one fairly new. If I spin their rear wheels by hand, both will spin for approximately a minute. I also have two sets of 700cc wheels, one old and one almost new. If I spin their rear wheels, both will stop at approximately 35 seconds.

    So, how - if at all - does the difference in unloaded spinning duration translate into on-road performance?
     
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  2. Phill P

    Phill P New Member

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    It doesn't in this case as there are a few factors effecting the result.

    What are the builds for the wheels you were talking about?

    The biggest factor is the interia differences between the sets of wheels.

    How much do the rims and tyre weigh on the 27s compared to your 700cs?

    My guess i the 27 weigh a lot more.

    All you are measuring with an unloaded spin test is the deceleration of the wheel due to seal drag mostly, and some bearing drag (plus free wheel on the rear). If the rims and tyres are heavier they have more rotational momentum/rotational energy, so it takes longer for that energy to dissapate away (and the wheel to stop spinning) on the heavier wheels.

    All this proves is that you will slow less when free wheeling on the 27s )depending on rolling resistance), but on the flip side you will accerate slower (for a given output).


    Now if I were wrong and the wheels are of similar weight, then you have crappy bearings, seals, or free wheel in the 700s and you should have them upgraded or serviced!
     
  3. FogVilleLad

    FogVilleLad New Member

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    All four sets were spun without tires being mounted. The 27's are both cross three; one 700 is cross two and the other is cross three (drive), cross two.

    I understand the momentum aspect, but am still surprised by the duration difference.
     
  4. ScienceIsCool

    ScienceIsCool New Member

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    The main components of drag on a rotating wheelare bearing friction, stiction (bearing seal drag), and aerodynamics. At all but the lowest speeds aerodynamics dominate, but as you can see the three parameters add together. This makes the total run down time a poor indicator of what's going on.

    I'm not explaining it properly, but if you'd like I go into this exact topic in lots of detail on my website.

    Short answer is that there's lots of reasons why your wheels could be behaving the way they are. But your observations aren't going to give you enough clues as to why. I hope this was helpful.

    John Swanson
    www.bikephysics.com
     
  5. mikesbytes

    mikesbytes New Member

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    Just book marking this thread. You all know why too.
     
  6. Phill P

    Phill P New Member

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    No....I have no idea why......

    Fogville, what are the specs of the wheels...hubs and rims spoke numbers (although aero is unlikely to be an issue here).

    The near 50% difference is surprising if no tyres.
    Was the spin test done with a chain on the cassette, because the free wheeling would have a large effect as well, but a difference between free wheels alone wouldn't account for the differences.
     
  7. ScienceIsCool

    ScienceIsCool New Member

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    I'm not surprised atthe big differences. Aerodynamics will slow a wheel down very quickly. After that, it's momentum and bearing friction which determine how long it will take. Small chnages can makea big difference in how quickly you go from slow to zero.

    John Swanson
    www.bikephysics.com
     
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