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Monday
Jul162012

What Do Sail Trimmers Have to Do with Wheels?

Last time, we chewed on some points about aero wheels, but time ran long before I got to dive into what happens between 10 and 15 degrees angle of incidence or apparent wind angle.  If you look at the chart, you'll see that there is a dramatic - nearly vertical - increase in drag for most wheels somewhere between about 10 degrees and 15 degrees.  In the case of the Stinger 7, you have about a 150 gram drag (or approximately 15 watts) difference between 15 and 17.5 degrees of wind angle.  A 2.5 degree windshift is NOTHING, they happen continually.  Also, if your speed is constant but wind speed increases, the apparent wind angle will widen since it is a vector sum (to figure out what your apparent wind angle will be in various situations click this link and substitute bike speed for board speed.  Tacking angle will be the angle between where the wind is coming from and your direction of travel - so if you are headed due east and the wind is coming from due north, your tacking angle is 90*).  As you lay with that page, notice that as wind speed increases relative to bike/board speed, apparent wind angle widens.  Increase board/bike speed and it narrows.  This relates to why I postulated that a HED H3 is an attractive wheel for fast riders in light winds even though it has a narrow range of superiority. 

Back to sail trimmers - what have they got to do with it?  Well, just like the wheels in our chart, a 2.5* difference in sailing is HUGE.  If you take a competitive but unexceptional boat and improve its ability to sail close to the wind by 2.5*, that boat is going to have an awful lot of trouble ever losing a race.  Since sailboats have to tack upwind, the speed they go through the water converts into speed directly into the wind as a function of the cosine of their angle relative to the wind.  If boats A and B are going through the water at 10 knots but boat A is only able to get as close as 38* while boat B is able to get 2.5* closer, boat B will essentially go .3 knots faster into the wind.  After a half an hour, boat B is going to be .15 nautical miles (or 910 feet) ahead of boat A.  Setting aside the issue of the tactical opportunities that this kind of advantage would give you, a 910 foot advantage on a typical 30 minute upwind leg is an ass whooping.  There are also a lot of boats that have "cliffs" just like you see in the wheel chart - a 2.5* difference in your angle to the wind is going to be enormous.  In some catamarans, it's not unrealistic to say that a 2.5* change in your angle to the wind could change your boat speed from 6 knots to 18 knots. 

THE BIG POINT BEING that a 2.5* window of resolution, in wind terms, is not very precise at all.  A wheel that's working just awesomely at 14.9* apparent wind angle might fall off that cliff at 15.1*.  No hyperbole at all there.  The lines are sharp.  A talented sail trimmer could, by very gingerly changing the wheel's apparent wind angle, keep the flow attached for a valuable few portions of a degree, or maybe even a degree, and change relative positions of one wheel to another on this chart quite dramatically. 

I'm not saying that this happens.  Since real world conditions when you are not trimming sails see much more herky-jerky changes in apparent wind angle, the best protocol is to blast air at wheels at wind angles of a somewhat (but as you hopefully now see, not very) precise window of resolution, and graph them from there.  Basically what I'm saying is that some wheels very clearly have windows in which they are psychotically low drag, and the approximate relative places along both the horizontal axis are noteworthy.  A wheel with a wide strip of really low drag numbers is a good aerodynamic choice, but saying that one wheel is better than another because it shows its cliff 1* or 2.5* further out on the horizontal axis is a lot less significant.  Rounding errors and test noise can make the relative differences between two wheels look a lot more different on a chart than they might act in the real world. 

Believe it or not, I have no ulterior motive in this, no desire to see one wheel's apparent relative strengths augmented or diminished.  You get wheels that show really low drag numbers (relative to other options - the H3 never gets absolutely low but it does get relatively low) across a band of 7.5 or 10 degrees, that wheel has got some strengths.  But this does also show that we aren't talking about "I WAS GOING 1 MPH FASTER THAN WITH MY OTHER WHEELS" in any case.  We're talking about differences of, in big cases, 15 watts.  If I could have or not have 15 extra watts, I'd take them.  And after an hour I'd be more than a couple of bike lengths ahead of where I'd be without them.  In a lot more cases we're talking about 4 or 5 watts, which again you'd rather have than not have but the benefit can be very malleable in a dynamic environment.  Those of you who are familiar with how agonizingle obtuse I can be will think that no topic gets raised without a reason, but reason and motive are not mutually inclusive. 

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Reader Comments (7)

Dave, can you teach me how to sail? I have an ongoing bet that I can jump on my father in law's boat, and sail the thing.

July 16, 2012 | Unregistered CommenterMike B

OK, so we’ll need an aero front wheel with a loose leach that twists off to dump power during hard gusts of wind, thereby curing the dreaded “round up into the wind” syndrome common to deep section front wheels. Maybe if we increase downhaul tension on the wheel? Just kidding … a good blog posting 

July 16, 2012 | Unregistered CommenterBill Cullins

Mike - Sure, you bring the boat I'm fresh out (though we have a dearth of good windsurf stuff...)

Bill - I've often wondered if maybe the whole rim depth thing wasn't a dead end and the real road is section profile. If you had on-the-fly-controllable section shape? I gotta get Steve Clark onto this.

One thing my wife got me thinking in her 'I read it bc I love you but God was the boring and painfully nerdy' conversation this afternoon was that this cliff that the wheels show where they lose laminar flow is the better part of one rear shift. I'd love to do a controlled experiment with wind angles right at the cliff point, and control for constant wattage, and see if cadence/gearing has hard lines that correlate to the wind tunnel generated cliff zones. That would take one hell of a cool wind tunnel. And big.

July 16, 2012 | Unregistered CommenterDave Kirkpatrick

That sounds AWESOME! Lets talk after Lost River.

The scientist in me thinks this chart is useless without knowing the error associated with measuring loss in a wind tunnel for these wheels. How accurate is this thing?

July 16, 2012 | Unregistered CommenterMike B

Mike -
I only used data that I knew had the standard 30mph wind and was reported at the same angular increments and measured in grams of drag. As none of the data in the graph is mine, I can't say how any company arrived at their reported points. Is it a mean/mode/median of all runs? Standard deviations? Points that best illustrate what they want to say? Who knows? I think the data is worthwhile in a general "how do different wheel depths/shapes perform" big picture, but useless in a "this will WILL exhibit 76.8 grams of drag at 12.438 apparent wind angle in 30.00 mph wind." There's also the different responses that different wheels will show in different frames, and the assumption of how much transferability there is from tunnel to road. In the absence of a better way to measure things, this is what there is.
Dave

July 17, 2012 | Unregistered CommenterDave

Somewhat reminds me of Eddy B's early '80s book talking about how to avoid wind on TT courses-almost seems if you had a good wind read on say, a long dogleg section and knowledge of how your wheels work at certain angles that you could possibly gain a few meters with an optimal line.

July 17, 2012 | Unregistered CommenterJoe Bond

You'll need to change your perspective, seriously... bike frontal wheels are just a matter of convenience. Using top made gears would be an easy alternative.

July 24, 2012 | Unregistered Commentercastors

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