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a rather naive physics question.....(19 posts)

a rather naive physics question.....mealex
Jan 6, 2003 7:56 PM
......ok. two identical humans are racing two identical bikes, except that one human has 650 wheels and the other 700. each is putting out identical wattage numbers for the entire race, and all other factors are equal(ie...aerodynamics and such).....which human will win the race, the one on the 650 wheels, or that one on the 700?

Tie! Why would wheel size make a difference? nmUncle Tim
Jan 6, 2003 8:14 PM
not enough informationDougSloan
Jan 6, 2003 8:17 PM
Is it flat, rolling, rough roads or smooth, lots of corners, drafting, are they large or small riders???

It could make a difference, but we need to know a lot more info.

not enough informationmealex
Jan 6, 2003 8:40 PM
......let's say that it's a 50k time trial, with no drafting. the course is of a varied terrain...... (nm)niteschaos
Jan 6, 2003 8:31 PM
And the winner is !...indurain
Jan 6, 2003 8:54 PM
If all things were egual, the rider with the bigger wheels would win the race. The bigger wheels cover more surface are in the same amount of time with the same amount of energy expended. Of course wheel size makes a difference. Think about it this way. Two four year olds are identical in every way. One is riding a tricycle the other a bicycle. The bicyle has bigger wheels. Who do you think would win?
oh my...yeah right
Jan 6, 2003 10:45 PM
your argument is pathetic.

why don't you get 30 foot diameter wheels, you'll go sooo fast?

...wait...crap...drag...that's what i'm using power to overcome.

drag is proportional to velocity, so ignoring drivetrain friction, which should be largely negligable, on level ground, they would go the same speed with the same power output if they had the same drag.

that's why in tri's and tt some racers use 650c wheels and don't get whooped by the 700c riders. or even in road races, look at the results of a particular female rider, who you might have heard of by the name of Longo, who uses 650c wheels, i think she's won 14 world titles or something unbelievable like that.
650 vs. 700bsdc
Jan 7, 2003 4:36 AM
The difference would be in the gearing of the bicycles. Since both riders would be putting out identical wattages in identical situations they would need to be in slightly different gears to compensate for the different wheel sizes.
If the race is long enough the 650c tires would blow up 1st nmdzrider
Jan 7, 2003 5:13 AM
Engineering considerations:Alexx
Jan 7, 2003 5:29 AM
OK, here's my $.02 worth:
All things being the same, the smaller wheels on the 650 bike will be geared shorter, helping on acceleartion, but likely cutting top speed.
The 650 wheels weigh less.
The 650 wheels, spinning faster, will have more internal friction.
The 650 wheels are more aerodynamic, since they have a smaller frontal area.
As for which will win-it depends on the course, rider, weather conditions, etc., etc.
Few questions in life have easy answers..........
Alexx is on the right track...joekm
Jan 7, 2003 6:43 AM
Also, the 650 wheels will have less rotational intertia permitting more rapid acceleration (all else being equal). However, the energy required to spin the wheels is proportional to inertia times the rotational speed squared. So, spinning the smaller wheels at a given forward speed will require more energy. Remember though, there are also aerodynamic, bio-mechanic, road friction, and other considerations.

Bottom line is, it's not obvious. I imagine that the analytic cycling website probably will take this to greater detail but I would call the original question niave.
Alexx is on the right track...yeah right
Jan 7, 2003 10:04 AM
spinning smaller wheels would only take more energy if both wheels had the same rotational interia.

remember that rotational inertia goes with raius squared, so that while the rotational velocity squared term will be larger, the radius squared term will be smaller.

obviously, with proper gearing, it comes down to aerodynamics, and on a course with vertical gain, weight.
You're right...joekm
Jan 7, 2003 1:02 PM
my mistake, it will actually boil down to rotating mass, which will favor the smaller wheel.
use the equationsbigrider
Jan 7, 2003 5:57 AM

rolling resistance

Tom will have your answer in the wheel comparison section
Just plug and chug( if your an engineer) or think about why
( if you are a physicist)

BTW the larger wheel theory made my dull day bright
another variableDougSloan
Jan 7, 2003 8:28 AM
Bike size. For a larger bike size, I've read that the taller head tube needed for the 650c wheels actually increases total drag. Bottom line: smaller wheels work better for smaller bikes, and larger wheels for larger bikes; the break point is around 54 cm. Note that we are talking about solo time trialing here.

another variableWoof the dog
Jan 8, 2003 2:16 PM
yeah but the headtube is not moving.
sure it isDougSloan
Jan 8, 2003 2:24 PM
It's moving into the wind the same speed the wheels are.

thank you...............mealex
Jan 7, 2003 1:47 PM
I beleive it depends on the gears that they are using-However...benja15
Jan 7, 2003 4:01 PM
If they are using different gears which would provide speed output they would tie

However, if they use the same gears the 650 (not sure on this) would probably go faster because they would have to spin more quickly for the same speed and thus using less power, or wattage