Apparent wind and aerodynamics

The wind vector thing and how it affects the wheel aerodynamics still bugs me.

If the wind is coming from straight ahead of you, it’s obviously a headwind. Let’s call straight ahead “Zero Degrees”. Directly behind you is 180 degrees. Zero degree apparent wind only happens in three instances when riding a bike: 1) when there is no wind. 2) when there is a headwind directly in front of you. 3) when there is a tailwind directly behind you…but, you are going faster than the tailwind. Of course, the only time there is no apparent wind is when you have a wind from 180 degrees and you are matching that wind speed exactly.

As the wind comes from somewhere within 90 degrees from Zero, it’s direction originates from a point that allows us to refer to it as a headwind, but with an apparent direction (or vector) skewed somewhat towards Zero depending upon your forward speed and the speed of the wind. When the wind originates from Greater than 90 degrees in front of you, it is often referred to as a tailwind, but, the resultant vector (or apparent direction) of the wind is STILL somewhere in front of you, i.e., you still have wind on your nose, as long as you are going faster than the tailwind.

On a relatively calm day, the wind vector is mostly going to stay fairly close to Zero…if you have sufficient speed…say 20 mph. At 20 mph, it takes a 10 mph 90 degree side wind to give you an apparent wind vector of 22.5 degrees. According to the charts, it isn’t until you get to at least 15 degree yaw angles that the tri-spoked wheels finally have less drag than the deep dished wheel. And according to The Committee, I believe it was, the wind speed at the ground is often significantly less than the windspeed at the height of the bicyclist. So, AT THE WHEEL, the wind vector in the above example might be only 15 degrees…something less than 22.5 anyway. Again, the charts tell me the deep dish wheels have less drag than the trispoke until you reach 15 degree yaw conditions.

Summarizing, it seems to me that the greatest apparent wind vector in our example would be something less than 22.5 degrees (assuming riding 20 mph, with the ground wind speed less than the 10 mph wind speed up on the bicyclist’s nose). Just barely enough apparent wind to produce a yaw angle where the trispoke begins to outperform the deep dish wheel. Hold that thought.

Here’s where it seems to get tricky: as long as your bike speed is at least as fast as the wind speed (in our rider going 20 mph, lets increase the wind speed to 20 mph 90 degree side wind) the greatest apparent wind vector is 45 degrees (remember it would probably be less at the ground level of the wheel). Because as we turn away from the wind, it becomes a rear-quartering wind, i.e., the wind is now coming at 95 degrees, then 110, then 135, etc., BUT the vector moves incrementally back away from 45 degrees toward Zero. IOW, when wind speed equals rider speed, the apparent wind direction doesn’t keep increasing past 45 degrees as the wind becomes rear-quartering.

At 180 degrees, the wind vector in the first example is now 10 mph at Zero degrees (20 mph rider speed minus 10mph tailwind speed). In the second example, there is no apparent wind, because tailwind speed matches the rider’s speed. What is enlightening to me is how the vector changes from Zero when going directly into the wind, to 22.5 degrees as we turn and the wind is coming directly from the side, then incrementally back towards Zero degrees as we continue turning away from the wind until we’ve turned enough to have a 180 degree tail wind, where the vector again reaches Zero.

All this makes me think tri-spokes aren’t nearly as aero as many deep dish wheels in relatively calm conditions under a fast rider, because the tri-spoke advantage doesn’t really show up until you get somewhere over 15 degrees apparent wind vector at the wheel.

It just makes me think the higher wind conditions are where the trispoke design shines, and USUALLY, at least around where I live, we don’t often have strong enough winds to get a 15+ degree apparent wind vector if I’m traveling at 25+mph. That would take at least a 12.5 mph wind from 90 degrees to get the apparent wind vector at the wheel up to the mid-high teens (although the apparent wind vector is at 22.5 at the level of the rider’s nose.)

Furthermore, as the wind leaves the front 90 degree quadrant, your speed should increase due to less wind resistance, which tends to move the apparent wind vector closer to Zero at a given wind speed. That means, on a circular course, with a 12.5 mph wind, and a rider averaging 25 mph, maybe the entire course is ridden with an effective yaw angle at the wheel that favors a deep dish wheel over a trispoke.

I know that side force means a lot regarding handling problems with any wheel, and maybe the deep dish wheels experience a higher side force…but, the part of the deep dish right against the ground may be experiencing much less side force than the part of the wheel at the top…and since you are removing the middle of the trispoke area, maybe the resultant side forces aren’t really all that bad on a deep dish wheel, as long as it isn’t too deep, but, that’s another question.

Any enlightening someone could shine on this model? Something I’m missing or mis-interpreting?

I actually did understand what you are trying to say. It sounds about right.

I can’t validate any of this, but the post itself, if we were playing chess, would be a “!!”
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rumor wrote: when evaluating aero wheel drag numbers, I take the average from 0 to 15 degrees. The tri-spokes work out slightly worse than some others taking this approach but we’re talking <<0.1 lbs drag in most cases.

Yep, that’s the conclusion I come to. It’s just the opposite of what I commonly hear…don’t run tri-spokes in high winds. Well, the fact seems to be that they don’t do as well as deep dish until you get to higher winds. If you don’t have handling problems, run the trispoke in high winds, run the deep dish otherwise.

BTW, I’m talking about only the front wheel here. The back wheel is too complicated from an aerodynamic standpoint for me. It just seems the rear disc is the bomb unless it is ridiculously windy…then, step back to the trispoke or deep dish on the rear.

Thanks to all for your input…just trying to figure it out.

I generally agree…although I think you type too much.

I would guess that 90% of my riding in the summertime here in Floriduh is in the 0-15 degree category, because the winds are usually 5-10mph maximum. But the spring and fall here are notorious for high and consistent winds. In the spring these seem to always be almost directly from the east, and usually 15-20mph whenever I go out to ride…grrrrrrr. In these times I probably exceed the 30 degree point pretty consistently.

As an example, the last two years in a row the Great Floridian Triathlon 1/2 and Full distances have been hammered by a consistent high wind from the east. Because of the race layout this means that 3/4 of the course is run with almost a direct crosswind. Going up some of the hills at 12-16mph with a 20mph 90 degree crosswind is around the equivalent of a 24mph wind from a 55 degree direction. It’s pretty hard to keep up 20mph with that much wind on flatland, much less while going uphill. The worst parts were of course going from dead still in the trees to the sudden 20mph blast from the side.

The reason that people don’t recommend the trispoke in heavy crosswinds is because of the substantially higher sideforce. In these sorts of cases riding a trispoke would be pretty tough…it would probably have less drag on your speed, but keeping it stable on the road would be a bit difficult.

Titan, it seems to me you can divide wheel drag into two subsets: 1) movement of air over the wheel shape i.e, tire and rims 2) and wind drag of the spokes. Doesn’t most of the spoke wind drag comes from the top half of the wheel. If you are traveling at 20 mph with a 20 mph side wind, the top portion of the tire is traveling at 40mph relative to the ground. While the average wind angle on the wheel may be 22 ½ degrees, the majority of the spoke drag is occurring at less than 22 ½ degrees. On the other hand, the leading edge of the wheel shields the spokes from headwind, so who knows?

The problem I see with this model is that it’s too simple. There’s no accounting for the spoke velocity relative to it’s radial distance from the hub. Also, air coming off the rim would contain vorticity that would impart extra z-component (x - direction of travel, y- vertical place) apparent forces. Then, of course there’s also vortices off other spokes to account for in a real world model. One must also not forget the possible effects of the fork, where boundary layer separation at low relative wind angles might increase vorticity in the spoke region.

The model you offer is good for modeling the relative wind angles on the rider, but probably not the wheels. Plus, the drag data out on the web for front wheels is probably highly questionable just on the measure of repeatability and comparison of testing procedures.

Titan is pretty much spot on with the explanation of wind vector and it’s real world effects. We use data from the National Meteorlogical Society which quantify an average wind velocity anywhere in the country at any time is roughly 8mph. To tell us that for an average triathlete, this will result in a wind angle of 0-20 deg (assuming 21-22mph average speed). The key here is that the faster the rider, the lower the wind angle and the more the course moves around, the lower the wind angle. To acheive high angle winds, you need the cross-wind to be at 90deg, if the road curves or turns by 10 degrees or so, the apparent wind angle begins to come down quite quickly, so to quantify something for high angle aerodynamics you need a pretty specific course and wind condition, such as a perfectly straight coastal road with steady 20mph cross-wind, but the reality is that generaly the road is going to curv and twist a bit, which will begin to bring down the wind angles actually experienced.

For a rule of thumb, we say that roughly 75%-80% of conditions in the real world will be between 0 and 20 degrees of apparent wind angle with something like 15-20% of those between 0-10, leaving the spread from 10-20 accounting for more than 50% of conditions. The other 20-25% accounts for conditions between 20 and 90 (with likelihood of occurence diminishing with increased angle such that 90 is theoretically impossible if you are actually moving forward). This is why we have really been working to optimize rim shapes between 10 and 20 degrees, since that is where you do most of your riding. Here’s a paper on it: http://www.zipp.com/tech/documents/ANoteonRimWidth_002.pdf

As for the thought that deep rims have higher side force than 3 or 4 spoke composite rims, I have to disagree. All of the data we have for our own 808 and the Hed Deep show that both have lower side force than any 3 or 4 spoke composite wheel. By our data, the Deep has about 15% less side force than a Hed 3 and the 808 has 35% less side force (slightly less deep, high sidewall curvature, and dimples allowing for lower leeward pressure drag allow for this), while both deep section wheels are more aero in the 0-20 range. The other aspects that is important is wheel stability, with deep section wheels having more consistent handling with varying wind angle due to linear side force characteristics, whereas some composite spoked wheels have non-linear side force such that small changes in wind angle can have disproportionately large increases in side force. This effect and wheel torque are something that we have studied for a while and really lost a lot of sleep trying to optimize in the 808 rim shape. You can see an actual side force graph from the wind tunnel at the end of that paper, and see how much less force any of these wheels have than a disc, but relative to each other, there are some big differences as well.

This is all giving me a huge headache…I think I’ll go for a bike ride!! : )

Even running your own business is easier then trying to figure out which wheel is fastest!!

Wow DUDE!! You are thinking this one to death!! BUT…

One thing that I think is missing is the drag factor of the spokes in the windstream between the forks. As each spoke passes between the forks, it disturbs the airflow, creating turbulence and drag. This is the primary feature of disc and tri-spoke wheels, that only zero (disk) or three such occurences are to be found in each revlution of the wheel. According to John Cobb, the drag reduction is worth approximately 40 watts, and the HED 3 was something like 20-25. But then I’m old and forgetful, so those numbers could be completely wrong.

That said, I was advised that the Nimble Crosswind would theoretically be about 8-10 secons faster than my HED Cx for 40K and the HED 3 would save an addition 3-5 seconds. So in any case, the difference wasn’t worth the investment. Certainly a 45-60mm rim or a tri-spoke would be worth the investment if you’re currently running a 28-32 spoke box rim or something equally cumbersome, but the investment makes less and less sense if you have a good wheel to begin with.

So if you’re debating scrapping a 404 for a HED 3, save your money and spend the time working on your engine.

triguy41+1 wrote: I generally agree…although I think you type too much

You are correct. But, what I lack in conciseness, I make up for in minutiae

USDA racer wrote: The model you offer is good for modeling the relative wind angles on the rider, but probably not the wheels.

I see what you mean. Yep, most of that stuff is probably overly simple to describe what is happening at the wheel, although it’s probably OK to describe the effects on the rider.

JDub, Josh, Paul, Cuz…thanks for your input.

Isn’t this forum good?

Hey Josh

why do they always pick 30mph as a test speed? most of us can not hold that kind of speed for very long.

what happens to the number at 22 to 26 mph?

Dan…

Does anyone have data for different wheels?
There is some at
http://weightweenies.starbike.com/phpBB2/posting.php?mode=reply&t=6167
but it’s only got a few wheels.

triguy42,

In fact, a disc and a trispoke front is pretty easy to ride and control in high crosswinds, where as twin trispokes are much more difficult to control in those same high crosswinds.

From my experience, the stronger the wind, the more reason to ride the disc/trispoke combination until maybe the crosswind exceeds 40 mph.

This combination is more stable and more aero in these conditions than most realize.

bikedude,

John Cobb has demonstrated in the wind tunnel that even as the speed decreases, the aero benefits generally tend to remain the same for both the slower as well as the faster rider. What one must remember is that while the slower rider may have an incrementally smaller aero benefit for any one period of time because of his lower speed, he will be on the course for a longer time than the faster rider and so will continue to accumulate these incrementally smaller aero benefits for a longer period of time and the end result will be nearly equal to the faster rider’s aero benefit.

There is one caveat, however. That is that the slower rider must be able to ride at least 20 mph sustained. At 25 mph, there is very little difference in time saved because of aero equipment between the faster rider and the slower rider.

Ben, in my personal experience, in blustery winds, the ZIPP 808 and a disc rear has noticeably better handling than the H3 front/ disc rear. Don’t get me wrong, the H3 is a nearly bombproof wheel, and proven. And about half the price of the ZIPP. But, from my interpretation of the data on Hed’s website regarding the H3 and their Deep wheels, it sure seems like the Deep wheels are going to be faster in most real-life situations…and, if the rider has a Deep wheel that has less steering force in winds, so much greater the arguement to go that route. Anyway, I’ve never been in a wind strong enough that I wouldn’t run a disc rear with a “normal” front.

I also agree that a front wheel with a bit more surface area seems to settle out the handling when there’s a disc in the rear. They seem to work best together. Once, I ran the H3 in the front with a “normal” spoked wheel in the back due to cassette problems…Yikes…not recommended in the wind!

Titan,

I agree.

I just have not noticed that “big” a difference between the H3 and a deep wheel (high profile rim) in strong crosswinds. I’m wondering if the high profile rim I used to compare was too high profile (nearly equal in surface area to the H3) or not enough surface area to demonstrate a significant difference.

Amazing!!!
I wonder if the Wright brothers would have had time to build an airplane if they had spent their energy dealing with disc and aerospoked wheels in their bike shop in Dayton? Hmmm, more aero data discussed in this thread then applied in the first thirty years of heavier then air, powered, aviation.
Jay