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Spokes in a bicycle wheel

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  1. Sep 14, 2015 #1
    • Poster has been reminded to post homework questions in the Homework Help forums
    We were asked a question about why there are spokes in a a bicycle wheel. Our teacher hinted that It was related to moment of inertia. Ok so this kind of a wheel will have a higher moment of inertia than say, a solid disk-like wheel. But how is that useful?
     
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  3. Sep 14, 2015 #2

    andrewkirk

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    No, the solid disk will have a higher moment of inertia, because it will be heavier. But the spoked wheel will have a higher ratio of moment of inertia to mass.

    On a bike it's good for that ratio to be high because mass makes the bike harder to ride up hills (bad) whereas moment of inertia helps keep the bike upright by increasing the angular momentum of the wheels at a given speed (good).

    Mass doesn't matter much when there are no hills, which is why riders use solid rear wheels in time trials when there are no major hills, and on a velodrome they use solid front wheels as well.

    Other reasons to avoid solid wheels are that they are very expensive and they make you vulnerable to being blown over by the wind - especially on the front wheel.
     
  4. Sep 14, 2015 #3

    SteamKing

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    I always thought a bicycle wheel had spokes to keep the axle from dragging on the ground. It's much easier to pedal the bike when the wheels turn. :wink:
     
  5. Sep 14, 2015 #4

    rcgldr

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    Bicycle wheels have spokes because they are lighter. There are also some competition bicycle wheels similar to motorcycle wheels (see attachment), which allows for tubeless tires. The reason for "solid" wheels (they're just covered, not actually solid disks), is reduced aerodynamic drag, but a bit more weight. In most of the other competitive events, bicycle racers are drafting in a pack, until near the end, and then light weight is signficant for the final sprint.
     

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  6. Sep 14, 2015 #5
    Well I suppose I will get a zero if I write that answer. :smile:. She was pretty insistent on the moment of inertia.
     
  7. Sep 14, 2015 #6

    rcgldr

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    OK, moment of inertia. Although the ratio of moment of inertial versus total mass is less for a solid disk than a spoked wheel, the total moment of inertial of a spoked wheel will be less than that one made from solid disk due to the lighter weight. The lower overall moment of inertia allows for greater acceleration, allowing a bicyclist to recover sooner from disturbances in speed.

    Self-stability of a bike is due to steering geometry, like trail other physical configuration that steers the front wheel inwards in response to lean angle that tends to keep a bike vertically oriented (although the direction will change due to disturbances). Gyroscopic precession along the front wheel's steering axis is a reaction to roll torque, and in general, opposes / dampens the corrective steering reaction related to steering geometry.
     
    Last edited: Sep 14, 2015
  8. Sep 15, 2015 #7

    cjl

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    Unfortunately, I have to disagree with your teacher here. Bicycle wheels are spoked because spokes are an inexpensive and simple way to obtain a very high strength to weight ratio. To elaborate:

    A solid wheel or a wheel with a few large spokes (like this) will largely support the weight of the rider through compression of the structure between the wheel axle and the road. The problem with supporting the weight through compression is that only a very small cross section is needed to support the weight, but because of the length from the hub to the rim, a small cross section would be prone to buckling. Therefore, a much larger (and therefore heavier and more expensive) structure is needed just because the weight is supported in compression.

    Spokes elegantly solve this by making the rim a major part of the load bearing structure. Spokes are under tension when they are installed, and are not designed to take any real load at all in compression (you could probably easily bend one by trying to compress it by hand). However, they are very, very good in tension. A spoked wheel is supported by the spokes between the hub and the top of the rim - the hub is basically hanging from the top of the rim, and then the rim and tire transfer that load down to the ground. Because a structure in tension cannot buckle, you are now limited by the tensile strength of the steel spokes itself, which is incredibly high. This allows for a lighter, cheaper wheel. Inertia has little to do with it, other than that the spokes allow for the lowest weight (and lowest inertia) wheel designs. for a given material.
     
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