achoo

Yes, there has to be power to overcome the drag of spinning.

But where does it come from?

The force applied backwards to the contact patch, which spins the wheel.

Right?

OK, but the wheel isn't accelerating backwards, so there must be a force applied to hold the wheel in place, even though it's spinning about its axle.

So where's the forward force to hold the wheel in place come from?

FBinNY 

I think I see where you;re going wrong. The wind tunnel sled measures the external horizontal drag, but the bike is driven from within by torque delivered to the cassette.

If you make a free body drawing of the rear wheel at a fixed moment in time, you can then slot in the various forces.

1- the net horizontal wind drag which can be said to originate at the axle.
2- the torsional wind drag from the spinning wheel
3- the chain tension pulling on the rear sprocket.

Since this in an instant in time, and the wheel is fixed to the ground by tire friction, we can consider that the fulcrum of a lever, running contact patch through axle. So two element are levering backward, and the chain is pulling forward. Measure off the lever arms, and it all comes to equilibrium.

BTW- you can run the same data but use the axle as the fulcrum, and it'll all add up the same way, if you consider the ground to be pushing the bike forward.

All the above assumes no internal friction, but than can be added as originating within the chain just to keep life simple.

That's the mechanics, but I don't know the airflow and air drag effects of a spinning vs. a static wheel moving through air, I'll leave that for folks who design aircraft.

BTW- another way to look at this is to look at the center of wind forces. We assumed it's at the axle, but this isn't true. Tbe bottom of the wheel is stationary, and the top rotates at twice the bike's (and bike generated wind's) speed. Since wind drag isn't linear the reduction in drag below the axle is less than the increase above. So the center of wind drag isn't through the axle, but at some point above that, that changes the lever arm distances, but gets into the aerodynamic math that I wanted to avoid.

achoo

Yeah, I was ignoring friction and rolling resistance, too. I think it's safe to assume spoke shape has little to no impact on rolling resistance and friction losses on a bike.

You're assuming the wheels are being driven by an internal power source (internal to the bike-rider system being measured) via the chain. But that's not the case in the test in question, where there is no rider and no power source internal to the system being measured:


Note that the crankset isn't moving at all. The wheels are being driven via a force applied through the tire's contact patch. Exactly as I described in my earlier posts.

Here's video of a bike in the same wind tunnel, but with a rider pedaling:

#t=17
That bike has an SRM. I'm assuming they were recording the power needed to spin the wheels in that case, and adding it to the power needed to propel the bike through whatever wind they were generating. And somehow figuring out the power lost in spinning the rollers under the wheels, if that's what's happening. Maybe the wheels are floating just above the surface in that case?

You don't need to really know that if all you're doing to trying to determine the drag of an existing object - blow some wind over it, measure the forces on it. That's all you really need to do. Think of it as a sealed box with unknown contents. To find out what it weighs, you can open it, carefully measure each object inside to figure out the volume and then use the data you collect, go look up the materials, do lots of math, and finally calculate how much the box weighs. Or you can just put the bugger on a scale and weigh it - without even knowing what's in it.

The fluid dynamics only comes into play of you want to figure out what's inside that aerodynamic box. We don't care about that. We just want to know what the damn thing weighs.

Again, that's not going to matter as long as the power is externally applied. If the power is internally-generated, as if by a rider, then you do need to add the power used to spin the wheels. Knowing the internal power input and given the relative airspeed, it's somewhat straightforward to determine what the drag coefficient should be - but you can't measure it exactly. And that should tell you why no-rider data can be a lot more valuable - with a limited number of wind tunnel runs, how do you account for variations in pedal RPM, power, and drag due to the rider being inconsistent in power output, cadence, and position? All that's going to affect the aerodynamics, while you're trying to measure the drag coefficient. And while position variations are affecting the system aerodynamics, power and cadence variations are affecting the power necessary to spin the wheels in the wind, which also affects the calculated overall drag coefficient.

Everything plays together in one big mashup when you put a rider on the bike in a wind tunnel.

But if the power to spin the wheels is applied as an outside linear force, all you need to do is make sure you measure that force somewhere. With no rider the force applied doesn't vary, the power applied doesn't vary, the RPM doesn't vary, and the position doesn't vary.

And if you apply the spinning power by pushing to the rear of the contact patch, the power to overcome extra drag generated will be accounted for at the proper speed by the force sensed at the supports holding the bike in place.

FBinNY 

All I see is the video, with no captions saying what and where measurements are taken. The classic wind tunnel test mounts the tested object on a sled and measures net wind force, but if that was done, it wouldn't have accounted for the torsional drag. OTOH, knowing the power needed to spin the wheel wouldn't tell you the linear wind drag.

As for the rest, I was only talking about the analysis. Actual measurements would be ideal, but you have to design the experiments correctly to get the right answer.

I was staying aloof from your argument about the issue, and thought I'd step in to make some peace.

Jiggle

Cool. I hadn't seen those.

November Dave

I assume that this refers to our having a prejudged point to prove. If that's the case, I'm sorry that the cycling industry has caused you to be so cynical. It does that to a lot of people. I can, however, assure you that this test, like any others we've ever done (and we do far more than most), had absolutely nothing to do with proving a pre-determined point. A2 testing is as close to a gold standard as exists in aerodynamics testing. The info we gave came straight from their data.

There are ways to separate out power to spin, and lots of arguments on whether it has any validity or not. Without a known, vetted, reference-able protocol for portraying it, it's of questionable benefit anyway. The games that get played with standard aerodynamics testing threaten the credibility of even that. Games even get played with wheel set weight ("claimed weight" - total crock of bs) and they take all of a kitchen scale to debunk, yet still they get played and still the players often/usually get away with it. No such games here.

spdracr39

Yes. Because they look cool in my overall bike scheme I like the bike more which makes me ride faster.

achoo

I'll go back to this, because that's useless for showing the effects of spoke shape. It uses wheels with significant differences in rim design between ALL the wheels. No two wheels in that paper use the same rim. The closest is some specious claim that two of the rims have "similar shapes". I'm unimpressed.

There's no way anyone can claim anything about the impact of spoke shape on drag from that data.

Not only that, the power comparison in that paper is not simply how much energy it takes to spin a wheel - else there wouldn't be a "wind angle" in the plot of multiple wheels with power plotted against wind angle.

That paper is useless in determining the power savings from bladed spokes because every rim is different, and it's not even measuring the power needed to just spin the wheel, as has been claimed - merely spinning a stationary wheel means there's no wind and therefore no wind angle. But that paper's big reveal of data is a graph of power vs wind angle for a bunch of completely different wheels.

And you know what? That paper never once mentions the wind speed used in the test. Sure, it mentions that the wheels were spun at 30 mph. But what was the wind speed? What was the tire pressure? What were the tires used? Was the same tire used on every wheel?

If you want to measure the aerodynamic gains from bladed spokes, you control everything else - keep EVERYTHING ELSE identical - and change JUST THE SPOKES. Rims, tires, hubs. Everything the same. The only difference is the spokes.

And when that's done, the gain from bladed spokes is:

One watt.

Of course, wind tunnels have been around for over a century, and charge hundreds or thousands of dollars an hour. Lots of people have used them for some very expensive things in cycling alone. Nevermind the same aerodynamic folks also work in aerospace and automotive industries where the level of resources greatly exceeds that expended in cycling - but where cycling gets to benefit from all the work done in those other fields.

Armchair internet posters are going to notice something blindingly obvious - "THAT WHEEL IS SPINNING!!!", that an entire industry with over a century of deep and thoroughly reviewed and tested methods has missed?

Uh yeah, sure they will.

The same guys who do aerodynamics for motorcycle and top-fuel drag racing and Formula 1 and NASCAR and Indy car racing suddenly forget that the wheels are spinning when the object of the test is a bicycle?

Carbonfiberboy 

Excuse me? When your results are at such variance with those reported by other testers and wheel builders, I would think you might have had another look at your testing methodology. You admit right here that you don't know if your results have validity, yet you published them? And then accuse me of cynicism and accuse other, unnamed wheel builders and testers of "playing games" with their data?

I don't know why you question your ability to do satisfactory testing. Simply spin a series of wheels at a constant speed, varying only what you want to vary, and recording the necessary power to spin while varying wind speed and yaw. That doesn't seem hard at all to me and answers all questions. Maybe I'm wrong, eh? But we don't know.

Carbonfiberboy 

You may not have read the paper you are reviewing carefully enough. They used identical 303 rims, though from different years, varying only the spoke design. They report the results for the same wheels tested both for drag and for power to spin, and at varying yaw angles. The remarkable thing is the much wider variation reported in the power-to-spin graph vs. the drag graph for the identical wheel series.

Similar to what November claims, the drag values reported for differing spoke shapes at zero yaw were extremely close together. I don't argue with that data, only its value.

November Dave

Our results are not at significant variance to that test, which contains one of a few power to spin tests out there. The historical 303 with oval spokes was well within a watt of aero drag than the same-rim 303 with bladed spokes, and needed about 3 or 4w more power to spin than the 303 with bladed spokes.

Any other comparisons in that paper are invalid, because that is the only time two otherwise identical wheels are compared to each other, with only the spokes changed. The butted (round) spokes used in this test are 2.0/1.8/2.0. The Lasers that were our round spokes are 2.0/1.5/2.0. The Italian 46mm rim and the American 46mm rim are nearly identical, but we don't know how nearly, so only suspect comparisons can be drawn from them.

We have complete faith in A2. It is where Zipp does the majority of their testing, as well as many others. The simple question we posed is "what is the aerodynamic difference between two identical wheels, but one has CX rays versus the other one having Lasers." We are profoundly confident that the answer to that question is 1 watt on the wheels that we used.

We did only measure aerodynamic drag, not power to spin. A2 does not offer a measurement of it. Other tunnels may, I don't know. It is not commonly discussed nor included with aero data from wheel builders. Perhaps power to spin is of tremendous value. Guys on SlowTwitch discuss it all the time. Here is a quote from Andrew Coggan, from a comment he posted on our FB page That is the power used to overcome rotational drag. Although typically not measured in wind tunnel tests, we did measure it for our wind tunnel validation study, and others have as well. It amounts to only about 5 W for a pair of conventional wheels and about 3 W for a pair of aero wheels. The difference is therefore small enough that, generally speaking, it is not worth worrying about (although a disc brake rotor might magnify things a bit).

As to the cynicism comment, that wasn't an accusation, that was sympathy. When you said that our data was just another company with a pre-determined point to prove, that sound very cynical to me. In no way was I mad about it, my response was more "I understand that a lot of what the bike industry does tunes people out." How often do wheels weigh more than they are claimed to? How many times can bottom brackets get 30% stiffer? And yes, people play games with aero data all the time. Removing tare, comparing tests that occurred on different days or even in different tunnels - all of this stuff happens.

wphamilton

I'd expect to see the lower range also, but on the other hand I could reasonably imagine several watts from the spokes beating the air like fan blades. In fact, I wouldn't be surprised if that part was more difference than the straight up aero drag difference. So I don't think that the 10 watts is out of the question.

I'm pretty sure that the bladed spokes on my Vuelta Corsa wheels aren't giving me back any 10 watts so I'd have to dig deeper in the wallet for those ellusive micro-gains.

bbattle

At last! A voice of reason and sanity.

cellery

Main noticeable difference for me is you get to listen to your budddy bragging about how much they got the wheels on discount for while they ride at the same effort/speed they always did. They are also noticeably more effective at decapitating small woodland creatures.

OldsCOOL

That is what bladed spokes are for, right?

FBinNY 

You have to hone the leading edges.

Ghazmh

Would polishing the spokes make a difference?

jtaylor996

Well, I either have the best or worst of both worlds. My Synapse came with Aksium wheels with round spokes on the front, and round on the non-drive side of the rear. The drive side rear has bladed spokes (that side is radial). WTH is up with that craziness?

rpenmanparker 

Simple, drive side rear spokes are the most likely to break. Their oddball OEM parts choices are how Mavic makes sure you have to come back to them for replacements. I don't know what their warranty covers, but I wouldn't want to have to buy replacement, bladed spokes from them. Cost and delay would both be horrendous.

TimothyH

The difference is very noticeable.

I went to bladed spokes and guys in the club were like, "Nice wheels!" and "Hey, did you get a new bike?"

Everyone noticed the difference right away.

woodcraft

The squirrel-o-matic, as seen on TV

(well, on hot or not actually)

noodle soup

I've seen the opposite.
NDS spokes fatigue faster due to their lower tension.

thump55

I have bladed spokes, and my girlfriend is hot.

I'm not saying there is a correlation, but I wouldn't take any chance if I were you.

RPK79

QFT

Also, the aero advantage will help you outrun zombies. Like this thread.

BillyD

Exactly. Hold the line, don't change anything!!