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I'd like to understand more about how the design of a bike affects its speed (assume a normal, two-wheeled bike with gears).

  • Can I assume that weight has little effect, on level ground, except when starting and stopping? If not, why not: what is the effect and how big is it?
  • Are the two biggest factors wind resistance, and tire rolling resistance?

Can you point me towards data for the above:

  • Rolling resistances for various types/sizes of tire at different speeds
  • Wind resistances for different rider positions at different speeds

Specifically how much difference is there between e.g. a 700x23 mm tire, and a 700x32 mm tire at 90 psi?

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Another thing that seems relevant although I don't know how you could quantify it is how easy it is for the rider to push power into the pedals. On my cruser I am in a very upright position and it is hard to push power into them (partly because I cannot use my uppper body efficiently). On my road bike I can transfer power my more efficiently. –  sixtyfootersdude Jun 1 '11 at 11:52
    
The rolling resistance will probably be lower on the larger tire (if at same psi), but the wind resistance will be higher on that larger tire, and at higher speeds that wind resistance is a bigger facter. There's no general formula, it depends on more construction details than just the size. –  freiheit Jun 1 '11 at 16:38
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If you really want to get into the details of this, I strongly recommend picking up a copy of Bicycling Science (mitpress.mit.edu/catalog/item/default.asp?ttype=2&tid=10063). –  lantius Jun 1 '11 at 19:56

4 Answers 4

up vote 2 down vote accepted

I like to use an excellent power calculator to answer questions like these. Play with the numbers, and you can see the exact effects on wind resistance of different rider positions or changing the type of tire.

In short, at race speeds, wind resistance requires by far the most power to overcome. It ends up being anywhere from 85%–90% of your overall power expenditure. For example, with that calculator, it takes a fully-crouched 150lb rider somewhere around 650W to maintain a 35mph pace. Remove air resistance entirely and it only requires an astonishing 50W. At the same power output (650W), with no air resistance, the same rider could reach a ridiculous speed of 475mph.

Sheldon also covers the topic, providing an excellent cost–benefit breakdown for various aerodynamic improvements. And Wikipedia makes the claim that "lowering a bike's weight by 1 lb… will have the same effect over a 40 km time trial on flat ground as removing a protrusion into the air the size of a pencil".

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In context, the last sentence is meant to imply that reducing weight is less significant than improving aerodynamics. –  ChrisW Jun 2 '11 at 2:10

In answer to another question moz posted this link to a IHPVA research paper, which includes (on pages 15 though 17) results of testing the rolling resistance of different tires, at different speeds and pressures.

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As others have mentioned, air resistance is a big factor. If you look at the formula for calculating drag, you'll see that the drag is a function of the square of the speed. This means that the drag a 5 mph would be 25 times the drag at 1mph and the drag at 10 mph would be 100 times the drag at 1 mph. For 30 mph you have 900 times the drag of 1 mph, even though you're only going 30 times as fast.

Also of interest is that colder air actually causes more resistance as it is less fluid than hot air. So as temperatures get lower, your air resistance increases. Because of this, wind power generators get more electricity when in colder climates, assuming the wind speed is the same.

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I going to guess that cold air is more or less just as fluid (viscous) as warm air, but that cold air causes more resistance because it's denser. –  ChrisW Jun 10 '11 at 2:36

Note that though impractical for most uses, the speed figures for streamliner and other highly-aerodynamic designs are far greater than anything obtainable from a "normal" bike. The faster you go, the more air you're pushing.

The ultimate bike speeds are obtainable through "motor pacing", wherein the cyclist rides in the aerodynamic "bubble" behind a fast vehicle like a truck or a locomotive. These bikes are so high-geared as to be unrideable at lower speeds, and they are decidedly not light...
Does illustrate the importance of air resistance.

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Yes, but motor paced bike gain from wind resistance, as the airflow is designed to push them along. So they're not even really relevant to the question of "what if there was no air resistance". –  Мסž Jun 1 '11 at 22:08

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