Monday, May 4, 2009

Wheel CFD

I received a note from Jürgen, one of the principals of the website:

http://www.wing-light.de/

this morning. He gave me permission to host the pdf document that is distributed on his site that describes the work he and his crew have done with aero wheels:

http://www.biketechreview.com/images/wheel_simulation_wing-light_090430.pdf

I asked a lot of inane "cfd groupie" questions offline, ones that I hope he'll be able to shed some light on.

Anyway, I just thought that the work he was doing was cool and figured I'd point it out to folks.

enjoy!

-k

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Saturday, April 25, 2009

Aero Torque/Watts to Spin

This aero torque topic has been on my list for a long time. I've tried many different ways of gaining insight into the deal over the years:

1) reported texas a&m wind tunnel motor measurements
2) srm power meter driving rear wheels in calm air
3) coast down of wheels in a truing stand with a fan blowing
4) srm power meter driving rear wheels with a fan blowing
5) srm power meter driving rear wheels while I stood in the bed of my pickup and my wife drove around at 30mph

more recently:

6) on a really windy day (30mph gusts) I did the srm power meter thing
7) monitoring the motor (that is used to spin the wheels) power during wheel/tire tests in the san diego wind tunnel
8) using all six force and moment component data from the wind tunnel balance during same test described in #7

I've found several independent references on the internets:

http://www.recumbents.com/WISIL/MartinDocs/Validation%20of%20a%20mathematical%20model%20for%20road%20cycling.pdf

estimated two wheels need additional 3-4 watts at 40kph 6-7 watts at 50kph

http://www.soton.ac.uk/~aijf197/Wheel%20Aerodynamics/Results%20and%20Discussion.htm

estimated one wheel to need additional 5-9 watts at 40kph

http://journals.pepublishing.com/content/g1463815454723l0/

estimated a single wheel requires more than listed above (research funded by a wheel manufacturer)

There also used to be a thesis from the same university as the latter link above that included CFD work here:

http://ir.canterbury.ac.nz/handle/10092/1800

(research funded by a wheel manufacturer)

But they have since removed the document until October of 2010. The first time I followed the thesis link in maybe December of 2008, I was able to download, save, and read the paper and the rotational watts are similar to what was reported above – though, they include wind tunnel measurements (translational/rotational) for the zipp 808 and hed three spoke. The manner in which these researchers gathered their rotational power data is clever (the motor they use to spin the wheel in the tunnel is mounted on a force platform that monitors things), and does not use the data acquired from a six component balance.

Based on my SRM tinkerings, I’ve really not been able to discern more than a few watt difference between wheels – independent of whether or not there is ambient wind present (and the numbers on the plot below include drivetrain losses):



This past time in the tunnel, I made the mental commitment to reduce the wind tunnel force and moment data in such a way that it not only reflected the translational resistive force, but also the aerodynamic torque – or “watts to spin”.

I won’t bore you with math and whatnot related to how I independently approached this topic using force/moment wind tunnel data (I'm not the sharpest tool in the shed, so I reckon it took me quite a bit longer to figure this one out compared to the big dawgs at the wheel company's - and I probably stuffed something up along the way :-0 ), but it seems as if the rotational watts can account for a chunk of the whole "fastest wheel" deal. For an estimate on the magnitude I came up with you might want to check out the insider's forum for discussion/further insight.

I’m not sure what to make of this independent finding. It’s encouraging that one is able to use force and moment data to repeatably measure this aerodynamic torque value and this fact will more than likely prompt me to investigate wheels that are different than the three spoke and the 1080 in the future. I would expect wheels like the HED 90 and 808 and shallower rim/longer spoke wheels to get rotationally bad at a relatively faster rate.

So, maybe the data here:

http://journals.pepublishing.com/content/g1463815454723l0/

Is on target?

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Saturday, April 4, 2009

Put 'em up!




This here's another wind tunnel shoot-out! ;-)

Tested a couple of fast wheels and a slew of fast tires (which just might make more of a difference than the wheel itself).

Standby for a heads up on how you can gain access to the same kind of results that manufacturers generate.

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Sunday, April 13, 2008

Flanders and Catastrophic Wheel Failure

I was catching up on some of the spring classics the other morning, and mozy-d on over to


http://www.cycling.tv/


and watched the highlights of the tour of flanders. Oh, that's a bike race!

Anyway, during the race, I saw something I hadn't seen before - and that was a Saunier Duval rider (announcer said is was Gomez Marchante, but not sure if it was...) going au bloc with his head down and drilling a curb perpendicularly.

The resulting carnage was painful to watch - though, shockingly, it seems as if the rider made it through the ordeal in one piece.

The same couldn't be said for his wheel, nor his fork:



higher res here.

It looks like the fork legs de-bonded from the crown, eh? (Any better pictures/video on the net than the original cycling.tv stuff?) Lucky that no-one impaled themelves on those fork legs - but it's a good thing that the wheel passed the UCI safety tests, huh?

Say, in this case when the fork legs debond from the crown and the rim collapses, exposing sharp serrated edges, would this mean that the wheel fails the requirements of UCI rule 1.3.018:


1.3.018 :
Wheels of the bicycle may vary in diameter between 70 cm maximum
and 55 cm minimum, including the tyre. For the cyclo-cross bicycle the width
of the tyre shall not exceed 35 mm and it may not incorporate any form of
spike or stud.


For massed start road races and cyclo-cross races only wheel
designs granted prior approval by the UCI may be used. Wheels will have
minimum 12 spokes; spokes can be round, flattened or oval, asfar as no
dimension of their sections exceeds 10 mm. In order to be granted approval
wheels must have passed a rupture test as prescribed by the UCI in a
laboratory approved by the UCI. The test results must show that the rupture
characteristics obtained are compatible with those resulting from an
impact sustained during normal use of the wheel. The following criteria must be
fulfilled:


· On impact, no element of the wheel may become detached and be
expelled outwards.
· The rupture must not present any shattered or broken off elements, or any sharp or serrated surfaces that could harm the user, other riders and/or spectators.
· The rupture characteristics must not cause the hub to become separated from the rim in such a way that the wheel becomes detached from the forks.

Without prejudice to the tests imposed by the laws, regulations or customs, standard (traditional) wheels are exempted from the rupture test referred to above. A traditional wheel is deemed to be a wheel with at least 16 metal spokes; the spokes may be round, flat or oval, provided that no dimension of their cross sections exceeds 2.4 mm; the section of the rim must not exceed 2.5 cm on each side.

What is your take? It becomes hard to interpret what the "wheel" is and what the "fork" is in this case - ya know, the fork being in multiple bits and all. :-)

I always kind of wondered what would be the consequences when a bike racer hit an obstacle that reproduced wheel failure modes created by the UCI safety tests. I speculated what would happen when something like this happened when I wrote the UCI and Me article for bike.com years ago.

Now, it looks like we know for sure what happens. When will we see safety tests for forks - it looks like in the Flanders incident, the fork was the weakest link - but really, would the consequences change in this instance? I reckon not - the bike is going to go flying with all of its sharp and dangerous edges, and the rider is going to hit the deck. Not much one can do when drilling a curb perpendicularly at 45-50 kph.

I've seen wheels get destroyed like what I saw on the cycling.tv footage - folded in rim, discontinous, not ridable. I saw destruction like that during my days at brand-S in the bike biz many moons ago - I reckon we catastrophically destroyed hundreds of wheels on a test fixture we nicknamed the "bonk" fixture... cuz we'd "bonk" rigidly constrained wheels with this massive pendulum.

ahh, those were the days! I get a kick out of breaking things like that. Is that normal? ;-)

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