I see you posted your results in August, but going all of the way back to mid June I didn't see anything else.

---

Chris Thornham
Co-Founder And Previous Owner Of FLO Cycling

I can appreciate your general recommendations to people. I've always liked the wheels you make and the price point that you sell them at, the down-to-earth approach that you guys have, and that's the very reason that I bought from you in order #1 (at least, I think it was the first pre-order) and again this past year.

But your answer doesn't get at my initial question...

If a wind tunnel test can be foiled simply by failing to control tire pressure down to a margin of accuracy that consumers don't have, do these tunnel results actually matter in the real world?

Effectively, I interpret comments about your $2K tire gauge as a sign that there is a 'theoretical' (wind tunnel) difference, that in 'practice' (real-world use), is lost in the margins of error.

Now, I don't mean to completely discount the validity of tunnels or aero testing (I've used the Chung method with a some degree of success), but I think we're at a point that some differences that we're being told about in the tunnel, in the end just get lost in the wash.

---

Chris Thornham
Co-Founder And Previous Owner Of FLO Cycling

ok. I'm going to take a long look at these affordable digital gauges.

This sentence is a real stretch:

"They probably don't have as high resolution instrumentation to be able to get the numbers quickly and accurately. So what we can do in 20 minutes, may take someone a few hours at (for example) A2's wind tunnel. "

Instrumentation is not an issue with getting numbers quickly, it's the control algorithms. Any signal conditioning produced in the last 20 years is going to be at least 16 bit and it's not unusual to have temp compensated .1% instruments except for temperature and humidity. 65,000 steps for 10" H2O is a pretty high resolution instrument. If you are going finer you'd better have some extraordinary calibration equipment or you'll be sending your stuff out every 90 days and then the metrology approach is questionable compared to end to end.

So you have a baro, a DP if using a nozzle, some other type of sensor if not, a DP from inlet to baro, some probes to measure eddys, temp, humidity and a few load cells for instrumentation. Any modern signal conditioners work really well unless you're using crxp from Omega.

We’ve had useful data from the TyreWiz. We still need to check the accuracy but the precision is there. I’m not sure i’d put them on a race bike for aero reasons, but I happily run them on back wheels during testing most of the time. When testing we still try and use a clients pump/gauge as that’s what they’ll have available later. So even if they aren’t actually putting a true 95PSI in they putting the optimum in, even if in reality that’s 87PSI.

In answer to an earlier question, tyre pressure will change by several PSI during a ride from temperature changes, and from air loss. We’re in the process of writing this up in a form that’s useful for riders.

Tyre temperature will influence Crr directly through effects in material properties as well as pressure. Even before we had the measurements we have now, it was always more consistent testing outdoors on cloudy days than in sunlight.

---

Developing aero, fit and other fun stuff at Red is Faster

Don't forget to set your cold pressure 2-3 psi lower, because you know, tires do warm up when you ride them. Don't brake too hard on the course, that will throw your pressure off. I just don't know about where you should rack your bike. Shade and pressure will be much higher on the road. Sun and pressure will be way to high to start. Otoh rolling resistance goes down with increasing tire temperature. Sun it is!

Sounds easy enough :/.

Hi everyone,

it’s a very interesting discussion I followed on different forum, roadies and triathletes’, and even if I have not posted here for decades, I thought I could chime in … I think that the most important point in that discussion is how science deals with reality.

Both Flo Wheels and Hambini have a different point of view about wheels’ aerodynamics and both likely did their homework.

Flo seems to be a reputable wheels company, they did much studies about yaw angles, rims shapes, tires pressures and so on … they established a protocol, tested wheels and ended up with conclusions and products !

Hambini and his collegues, who seem to have experience of aeronautical engineering around high speed military airplanes, are at least as legitimate to work on wheels … but they seem to have another point of view, probably due to that aeronautical background, so a different protocol and new conclusions.

Being a scientific guy, I think that neither are totally right or wrong … they “published” different results about a common topic as scientists do every day, with slightly different protocols, analysis methods etc. ending up with different conclusions about the same “reality”.

Both points of view are interesting and have to be taken with a grain of salt … I can understand that after spending huge $$$ on tri equipment, some may be disappointed, lost in their reasoning but as a matter of fact, different points of view are exactly what I’m looking for in science to widen the spectrum of comprehension.

It seems that in recent years, people have more and more difficulties understanding that science is what it is … hypothesis, studies, measures and validation / invalidation of hypothesis, on and on … there can be no definitive conclusion, in my opinion, on any scientific topic … take sport or healthcare, climate, nutrition, agriculture, whatever you want.

Anyway, as Chris from Flo mentioned, we are talking about marginal gains compared to swapping from box to deep rims, etc. … and marginal gains are very difficult to measure because precision, accuracy and cost become more and more important in that situation.

So thanks to Hambini for his work … and thanks to Flo for their approach. I’d add that I have no dogs in the fight, no Flo wheels either etc.

Regards, E

What wheels do you ride?

Having switched from triathlete to recreational roadie in the past 10 years, I got less sensitive to aero gimmicks but I still ride custom wheels with Corima aero rims and Tune hubs when the weather is nice and shallow rims with bombproof hubs in the rain and winter's harsh conditions.

By the way, I essentially ride tubulars ... and I use to build my own wheels.

The aero question came back recently, just because I have a deep section wheels project in mind and was thinking about rim depth and shape ...

Regards, E

@hambini - great work.

stealing from your comments on weightweenies:

Myself and my 3 colleagues came to some broad conclusions

- If you are at mortal speeds, there is not much difference between any deep wheel.
- For general overall riding at 25-35km/h a 35 to 50mm wheel would be recommended.
- If you are going really fast, deeper is better.


I compiled your results as a drag-vs-rim depth chart, which shows that general trend of decreasing drag with depth, though drawing general conclusions (say "go for 65mm+") is possibly limited by the sample size.






But is it possible to generalise any other tips for spotting a fast wheel? (using eyeball aero)
eg is there any general trend with respect to rim width, shape, etc? eg lack of sharp edges/brake track lip, sharp corners at the trailing edge, toroidal, vee, square, rounded, etc.


The last point I think would be interesting to know more about is steering/yaw stability - something eg Swissside claim a lot of work on - the idea that making the bike handle well in fluctuating crosswinds will improve rider confidence, and hence the ability to keep mashing the pedals without backing off to steer, or worse come out of an aero tuck. Any thoughts? Is this why you suggest 35-50mm rims at lower speeds, or is it because the magnitude of the differences is just too small to care about?

@MattyK

Someone asked this question about crosswind stability on weightweenies so I have copied and my response

To emphasise this here are two popular standard aerofoils that are widely studied. NACA 0012 and 0024.

http://airfoiltools.com/airfoil/details?airfoil=n0012-il

http://airfoiltools.com/airfoil/details?airfoil=naca0024-il

If you look at the Cl vs alpha (graph on the top right), this is how the aerofoil generates lift when used on an aircraft, the alpha denotes attack angle and the CL is lift coefficient. If you change the orientation through 90 degrees and apply it to a bike, it is the force generated by wind when the yaw angle is anything other than zero.

The take away is to note that flow separation occurs around 12 degrees irrespective of the shape of the aerofoil. Having a blunt rim will take you up only to around 15 degrees (Reynolds number of ~100k) but it's a heavy price to pay at low yaw angles. Blunt aerofoils *generally* have poorer reattachment.


And for your other questions

Generally, if you experience a crosswind that is constant, the bike will be totally controllable and you simply tilt your body to compensate. If the wind gusts or pulses then this causes instability. For the average individual, they will be able to control a bike with a 50mm rim set at gusting crosswinds of 40km/h. At 80mm, it will be uncomfortable and may result in a meeting with the deck. Hence the guidance to go for 50mm as a general wheel. It is more important on the front wheel than the rear.

Hope that helps

Hambini

---

HELLO HAMBINI FANS!!!

Also don't forget road surface "impedance". Optimal pressure is seemingly highly dependent on surface roughness. So you need to calculate a time-scaled mean impedance for all the road surfaces on a course, and then use that as input into an impedance-matching function solving for pressure. Taking into account losses over the day, and, of course, solar loading.

Even if you had a rectangular box, the break off point would be around 12 degrees.

Optimisation of the shape would only get you round a little further.

---

HELLO HAMBINI FANS!!!

Not as much as you may think...don't forget that the "T" in that equation is in absolute temperature units (i.e. Kelvin, or Rankine) ;-)

Since V, n, and R are constant in this situation, you can simplify it to P/T = Constant. That means that P1/T1 = P2/T2. Rearranging you get P2 = P1 * T2/T1

Let's say the temperature rises from 10C (50F) to 30C (86F), in absolute temperature thats from 283K to 303K. T2/T1 = 303K/283K, or 1.07, meaning that the pressure goes up by only 7% over that HUGE temperature swing.

---

http://bikeblather.blogspot.com/

Understood. I totally misread your previous post.

Reading all this, do you have any intuition on how these results could apply to bike frame choice? Will a frame with very deep tubing like Felt's IA be better suited for dynamic wind conditions or is the P5, which still seems to be the best low yaw choice, still be faster? Obviously this is a bit of a stretch to extrapolate these results and there are a lot of different frame designs, I'm just curious if you have any thoughts.

On a frame I would go for the deeper tubing. It's not like a wheel because it's not rotating. Additionally the rider's legs are disturbing a large proportion of the exposed area so the frame is much more immune to any gusts.

This is just my opinion, I haven't stuck anything in a wind tunnel to prove it.

Hope that helps

Hambini

---

HELLO HAMBINI FANS!!!

I don't think it would be really hard to see a 20C delta.

A Quarc TyreWiz would be able to settle this if anyone has one.

Makes sense, thanks for replying

First you have to decide on what "quite a bit" means...and 20C is a pretty large temp swing...heck, 20F is as well.

---

http://bikeblather.blogspot.com/

Inside a tire? With the sun and braking? I don't know.

DC Rainmaker should come out with a long term review of the Quark TyreWiz, hopefully he has plenty of data to share how tire pressure changes under various riding conditions.

TyreWiz is a handy gadget. :)

I’m going to write up what I’ve found when I get a moment free.

5PSI+ changes are definitely possible. The heatwave has finished here so I need to get out and see what the range is like with cloud cover.

---

Developing aero, fit and other fun stuff at Red is Faster