What on earth is going on here... reads like a Monty Python movie

I agree, bust out the graph paper. Few things are more confusing than someone describing a freebody diagram.

There are two possibilities.
1. "They" were not aware of it.
2. "They" ignored it deliberately.

In both possibilities, wheel manufacturers sell deep rear wheels. If from the beginning of windtunneltests the rotational drag would have been measured, no one would have bought a deep rear wheel and everyone would ride around on discs.

I followed 4 years ago the standard tests (without rotational drag) and bought an 808 rear wheel. I got aware of rotational drag and bought a disc now. So the 808 was basically a waist of (a lot of) money.

I think the fact discs are forbidden in Kona is financially also very fafourable for deep rear wheel manufacturers.

...proving my point, even if you got her name wrong.

Also, what are Flo wheels? Are those like copies of Hed?

---

no sponsors | no races | nothing to see here

http://www.flocycling.com

---

Make Inside Out Sports your next online tri shop! http://www.insideoutsports.com/

I think you missed my point. Are you going to show up to the next West River Drive TT with double Super 9s? No, that would be ridiculous, and it would be an enormous amount of money to spend on wheels for our use cases. Everything can be taken to the extreme. Are world class, record setting, sponsored athletes going to be after every last fraction of a watt? Of course. They are also going to be in the velodrome/wind tunnel testing dozens of different fabric patterns, then again while wet, etc. Does that mean we should? Well, not necessarily...we simply don't all have the resources.

I found a couple really great roads for aero testing near me so if someone would loan me a super 9 disc and a set of Garmin Vector pedals I'll tell you home much the difference is. I'll be shocked if it's a very big difference without a strong crosswind... or rolling hills where the extra inertia might actually help.

---

TrainingBible Coaching
http://www.trainingbible.com

Sigh.. I promised myself not to get into time thieves like this one...

Anyway, I submit free body diagram #2 and #3 combined:

This is an instantaneous snapshot of the wheel rolling down the horizontal road in still air, with the instant center (IC) right under the hub.
The forces from the fork , Fw from weight and Fp from 'propulsion' acting through the hub axis.
Reaction from the road, Fy as the integral of the pressure and the unknown Fx from horizontal road interaction.
And last, the aero drag vector Fd...

Torque equilibrium equation is 'separated' into a rolling resistance part and an aero drag part.

Can we agree on this one, and do the tunnel analysis later?

Edit: I left out the X and Y force equilibrium equations, but they are 'obvious'. And I made the Fx relevant again, it was named 'irrelevant' when I edited my post too many times and lost track of the wording..

But I am going to run the most aerodynamic, lab-, and race-proven, wheelset available to me (and also save that same wheelset for race days only).

I feel like your OP borders on a trolling question. We already know the answers to which is faster—disc vs deep wheel—independent of variables such as financial wherewithal, or comfort in crosswinds.

I think better questions can be asked, like, "Why don't Jorgensen, True, Stanford, Gomez, Mola, Murray, the Brownlees, etc. all run 404/808 (or sponsors' equivalent wheel) in what amounts to criterium racing?" or, "When will WTC permit discs to be used in Hawaii?"

---

no sponsors | no races | nothing to see here

So I've had numerous emails now telling me that there was a real life FBD posted to ST.. This is awesome.

From my experience there is only one answer: yes, discs are faster and likely 2-10 watts over even the best deep wheels and here's why:

Drag advantage: 1-6 watts. I've tested a LOT of bikes and bikes with riders and never seen this to not be true.. I'm sure there is some case our there were it isn't, but I've never seen it. Even when the wheels are similar in wheel only drag, the disc always brings something additional to the table and CFD tells us that it's the ability of the disc to just clean up the turbulence off the back of the bike. Again with CFD, we see that the fastest wheels can equal a disc in wheel only testing because they are acting as a very efficient front and rear airfoils in clean air. However, in the turbulence at the back of the bike the efficiency of the rim shapes is reduced, so the deep wheel still has to cut and clean up the air at the rear of the bike while the disc doesn't. Firecrest, Trek D3, ENVE Smart..all exceptional, but still not a disc in tunnel testing. CFD also shows how the disc really does a great job of stabilizing the flow field behind the entire system which is an effect that you can't account for in wheel only testing, so we theorized for years that this flow stabilization is part of the disc magic.. we used to call is the 1+1=3 phenomenon. CFD would later show us that

Wattage to spin: advantage disc wheel ~1-3 watts.
Wattage to spin in bikes with large cutouts is even higher. Think of this from a mass flow perspective..in bikes with large surface area seat tubes that are tight to the rear wheel, you are dramatically increasing the amount of air flowing over the wheel (think small crosswinds here), for the disc, this just leads to more flow acceleration, but with a deep wheel, it leads to more flow into the spinning spokes so wattage to spin goes up. You can quantify this with CFD where the models are built with the spinning spokes as their own meshed region, by putting the wheel in different frame designs you see the flow rate in that region can change pretty significantly depending on frame design. Again in CFD we also see that the disc doesn't just not add additional turbulence in this region, but it actually serves to clean it up so again, advantage disc.

I have some cool data on this from the old days when quite a few of our athletes were wondering why Chrissy Wellington was on a P2 and never rode a disc. I think the real reason was that she liked that bike and not discs and at her level is certainly wasn't keeping her from winning, but we found some great stuff, like the wattage to spin the deep wheel was lower in the rear of the P2 than the P3..most likely because that lack of full coverage cutout was 'venting' some flow from the seat tube that otherwise would end up being fed right into the top of the spokes.
Josh

---

http://www.SILCA.cc
Check out my podcast, inside stories from more than 20 years of product and tech innovation from inside the Pro Peloton and Pro Triathlon worlds!
http://www.marginalgainspodcast.cc

Fx would be 0 in a flat road if the wheel is rolling without slipping (RWS)

---

http://cds-0.blogspot.com

you forgot to say...thread over

Yep, Josh killed it. Now we can all get back to work.

---

---

Save: $50 on Speed Hound Recovery Boots | $20 on Air Relax| $100 on Normatec| 15% on Most Absorbable Magnesium

Blogs: Best CHEAP Zwift / Bike Trainer Desk | Theragun G3 vs $140 Bivi Percussive Massager | Normatec Pulse 2.0 vs Normatec Pulse | Speed Hound vs Normatec | Air Relax vs Normatec | Q1 2018 Blood Test Results | | Why HED JET+ Is The BEST value wheelset

In Evelyn's case the front is a 900. Zipp doesn't make front discs anymore, but I picked a used one up pretty cheap.

Ummm... no.

Where would the power "to spin" come from? What's driving it?
All other forces resist the "spin" or act through the hub.

In fact, "Fx" is "the power to spin".
IRL (on the road) and in the tunnel setup.

Maybe the other two equations wheren't so 'obvious'...

Thanks for stopping by.
My high school teacher would be proud seeing me on the barricades with FBD's.

So, follow up question: is the air clean enough once it is close to leaving the trailing edge (the tire), that disc/tire width effects the size/energy of the wake. I.e. does the tire width play a role in "punching the hole in the air"? Or has the hole already been punched by that time?

And a very specific one: Vroomen (or Damon?) said that the Cervelo P4 wasn't designed for/didn't need/should not use a lenticular or 'wide' disc, in fact it was best served with a flat&narrow disc. Do you have any anecdotes about that?

Nicko,
The disc width plays a role only in how cleanly it can transition air onto the tire and then off the back of the system. If the air is all nice and smooth on the wheel and then has to bulge out to get around the tire, things aren't clean.. Ultimately this is what we realized was the true advantage of the Sub9 wheel..it allowed for cleaner trailing edge wake which led to the Super9 which was flat and then had the taper toward the tire in the last 30mm or so. This is also what is happening in some lenticular discs that work well and can work with wide tires.

Damon and Girard are well experienced with both of these, we had lots of tunnel overlap in those days and while we never directly designed frames and wheels together, we did end up testing with each others prototypes and such which really allowed the system to develop together in a very natural way. The P4 was similar to many bikes of that era in that everything was really packed tightly together, so a flat disc allowed for better flow between the wheel and stays. As the wheels become more lenticular in shape, it is advantageous to widen everything to allow those slot gaps to breathe a bit more. I've not done a lot of disc testing in P4's but have seen that generally you want to keep a healthy gap between the wheel surface and seat stay or fork blade, I have seen lens discs that test very, very well on their own, test only average in a frame for this reason.

---

http://www.SILCA.cc
Check out my podcast, inside stories from more than 20 years of product and tech innovation from inside the Pro Peloton and Pro Triathlon worlds!
http://www.marginalgainspodcast.cc

Good stuff! It's also important to keep in mind that aerodynamically, the tire one chooses for a given wheel can effect axial force related power by more than 6 watts and the watts to spin by more than 3 watts.

---

=================
Kraig Willett
http://www.biketechreview.com - check out our reduced report pricing
=================

So the tire has more effect than the wheel for "watts to spin?" How do you know this, and is it known what tire characteristics promote lower watts-to-spin? E.g. is smooth better than treaded or vice versa? I assume you're not talking about Crr, which would be more than 3W.

Power to spin comes from the assymetric pressure distribution in the contact patch that induces a resisting moment with respect to wheel's axle.

---

http://cds-0.blogspot.com

Ummm, no.
Now you are completely mixing up rolling resistance (that scales with vertical load) with the topic at hand, the "non-measured" aerodynamic property called "power to spin" (that scales with air speed).

Completely different entities. Like Crr and CdA. Try again.

And your textbook graphs deals with slip and slip angles in relation to horizontal forces that are 0-100% of the vertical load.
In the case of "power to spin", the force that is required for that is ~0.04% of the vertical load.
There will be no significant slip, and yes, there will be a horizontal force that actually "spins" the wheel.

I've been studying/quantifying bicycle tire aerodynamics since 1999 when I was an engineer with Spinergy, so I have independently acquired specific data for the topic at hand. Tire size & shape are big influences when it comes to this sort of thing. There can also be significant interactions between wheels and tires and I think that's just something to keep in mind when discussing the topic.

---

=================
Kraig Willett
http://www.biketechreview.com - check out our reduced report pricing
=================

OK....

In a Tour test on tt bikes, it was found at the time that a BMC bike was faster with a set of 808's than a disc on the rear 808 combo.
It was a surprise to them and the only bike to be so.
So your answer is as always, 'it depends'.