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# Bike Wheel Phsyics - some WOW info

Road Cycling “It is by riding a bicycle that you learn the contours of a country best, since you have to sweat up the hills and coast down them. Thus you remember them as they actually are, while in a motor car only a high hill impresses you, and you have no such accurate remembrance of country you have driven through as you gain by riding a bicycle.” -- Ernest Hemingway

# Bike Wheel Phsyics - some WOW info

08-02-11, 10:12 AM
#1
cyclezen
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Bike Wheel Phsyics - some WOW info

some interweb searching discovered this...
did a search to see if this had been presented here before... nadda

bike wheel physics - the down and dirty, 'bling'-proof numbers

A big 'good going BRO' to John Swanson for tacklin this undefined 'performance' profile of the modern Bike Wheel.
John - you a BF member? if so, kudos to you!

the intro - http://www.bikephysics.com/intro.htm

the study paper - http://www.bikephysics.com/paper.htm

The numbers - http://www.bikephysics.com/rails/wheel/list

man, this blows away a whole lotta 'Bling' factor when it comes down to what you're getting for your money...

would be good to see numbers for all the 'Fab' wheels currently feted...

sortta timely excerpt re the recent threads on tires/tubes:
"...I have one other public service announcement. I just showed that the moment of inertia of a front wheel is about 0.04 kgm2. Now let's consider a tire of average radius 0.335 meters. From Weight Weenies the average weight of a Michelin Megamium 2 is 265 grams. The average weight of a Veloflex Corsa is 160 grams. The difference between stock tubes (107 grams) and light ones (60 grams) is about 50 grams.
So... the difference between “heavy” tires and tubes and “light” tires and tubes is about 150 grams. This works out to a difference in moment of inertia of 0.017 kgm2. Compare that to the wheel we just analyzed. Could you imagine the sales coup if someone could claim their wheels had 40% less MoI than their competitors?! Riders would line up to give testimonials about how much faster they accelerate and how much energy they save in a long race. That doesn't happen for tires though. Makes you think, doesn't it?
This just goes to show that nice tires and tubes are a great and inexpensive way to get a significant performance boost. The benefits are even greater if you decide to switch to tubulars since you'll save another 50-100 grams. Also, higher quality tires generally have a better ride quality and lower rolling resistance. There really is no downside and no reason to ride crappy tires."
08-02-11, 10:15 AM
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Runner 1

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Cool find. The effect of tires on moment of inertia has been known for awhile, but for some reason everyone's concerned about saving 3 grams in the hub.
08-02-11, 10:28 AM
#3
Psimet2001
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Originally Posted by Runner 1
Cool find. The effect of tires on moment of inertia has been known for awhile, but for some reason everyone's concerned about saving 3 grams in the hub.
The bane of my existence.
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08-02-11, 10:28 AM
#4
asgelle
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He should have begun his reasearch with a review of prior work. He would have found Kraig Willett did something similar only not considering the wheel in isolation, but using the equation of motion for the total bike rider system. http://biketechreview.com/reviews/wh...el-performance

Take away message. Wheel inertia is a trivial component to the power demands of the entire system. (50% change in wheel inertia changes power required by 0.02%.)
08-02-11, 10:30 AM
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Thanks for sharing! The paper was very informative.
08-02-11, 10:41 AM
#6
Homebrew01
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Too bad it's so old with so few wheels tested and no real aerodynamic testing .....
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08-02-11, 10:46 AM
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cyclezen
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Originally Posted by asgelle
He should have begun his reasearch with a review of prior work. He would have found Kraig Willett did something similar only not considering the wheel in isolation, but using the equation of motion for the total bike rider system. http://biketechreview.com/reviews/wh...el-performance

Take away message. Wheel inertia is a trivial component to the power demands of the entire system. (50% change in wheel inertia changes power required by 0.02%.)
well ok, I will look at it closer
but already the problem is putting the human variable in the system
yes, it all comes down to human performance, but measuring any 'component' in a system means removing as many variables as possible, the human variable being and having the most variability...
2nd - he discounts 'acceleration' and inertia completely'
in a race, acceleration and decel happen constantly - he discounts wheel inertia as 'placebo',
using the same wheels, anyone having to ride the rivet for a few laps would easily discern the difference between a 300g/110g tire/tube and a 200g/70g tire/tube and a hi-perf tubular...
under a relatively constant velocity aero is certainly king; but when having to brake down to 22 mph and then accel back up to 30, 4x times in a mile, for over 1 hr, I think other factors might also come to play...
08-02-11, 10:55 AM
#8
ahsposo
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Yeah, this was very interesting.

The author not only is pretty good (as far as my feeble understanding allows) at analysis he's pretty funny, too.

What he can't codify (if that's the right word) is how some wheels seem to be "lively" and some aren't. When I was growing up almost all of my wheels were 32 spoked 2X box section tubular rimmed. Maybe it was a subtle difference in spokes and their tension but some wheelsets just seemed to have a little more life in them.
08-02-11, 10:57 AM
#9
asgelle
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Originally Posted by cyclezen
2nd - he discounts 'acceleration' and inertia completely'
in a race, acceleration and decel happen constantly - he discounts wheel inertia as 'placebo',
using the same wheels, anyone having to ride the rivet for a few laps would easily discern the difference between a 300gm/110gm tire/tube and a 200gm/70gm tire/tube and a hi-perf tubular...
I think you mis-state his conclusion. He doesn't begin by assuming acceleration and inertia can be discounted. He looked at the results of the analysis (verified by race data) and based on the results says those factors are insignificant. It's a conclusion not an assumption. As to discerning the difference in equipment, a rider might be able to tell there's a difference between two sets of equipment, but that doesn't mean she can correlate the perceived difference to a change in performance. I can certainly tell the difference between my clinchers and tubulars, but when I considered it closely, what I was noticing was the difference in the sound they made. There's no reason to believe on or the other is faster based on the sound they make.

Accelerating from 22 mph to 30 might sound like a lot, but looking at the data from the P,1,2, crit accelerations are only 0.02-0.03 g. Compare the power required to perform such accelerations to riding at 28-30 mph and you'll see that inertial effects truly are trivial.

But really this is old news. Plenty of other people have confirmed these conclusions through analysis and experiment.
08-02-11, 11:32 AM
#10
cyclezen
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Originally Posted by asgelle
I think you mis-state his conclusion. ... As to discerning the difference in equipment, a rider might be able to tell there's a difference between two sets of equipment, but that doesn't mean she can correlate the perceived difference to a change in performance.

Accelerating from 22 mph to 30 might sound like a lot, but looking at the data from the P,1,2, crit accelerations are only 0.02-0.03 g. Compare the power required to perform such accelerations to riding at 28-30 mph and you'll see that inertial effects truly are trivial.

But really this is old news. Plenty of other people have confirmed these conclusions through analysis and experiment.
I will read it more closely, as I said. However my initial reaction is still 'human' is the 'fly' in the system.
Higher level crits usually run at 28 to 32 and sometimes but rarely fall to 25, and what brings the avg down ARE the corners and the decel/accel.
so the power requirements are usually as extreme as a steady 28, since one is rarely below that, the added burst of power needed for the constant accels, put the overall effort way higher than a steady 28-30.
maybe old news, but I haven't read 'plenty' of other people' yet.
Thanks for this study, I will read it throughly and any ref he gives...
Aero is king... and was even for when we did TTs on our road bikes
But what works in a crit or a shorter circuit race with lots of rollers might not be that easy to asnwer.
Codifying the difference between less aero wheels - Ksyrium elite, Neuvation MX and a WH540 is also a worthy thing.
08-02-11, 12:22 PM
#11
nhluhr
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The author is quite enamored with himself for somebody who extrapolated aerodynamic data from a coast-down test and neglected countless real-world variables.
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