"The 33"-Road Bike Racing - Aero Benefits of Rear Wheel

I'm thinking of getting a set of carbon tubulars. All this nonsense about drag and dimples and rotational mass has deluded me into thinking I might be faster. Therefore, I ask you to humour me and not tell me crap like I won't be faster - I already know this. Consider this purely an academic discussion.

Does anyone know the added benefit of a rear aero wheel? Given the fact that the wheel is behind the frame and riders legs, is there really any noticeable difference? The reason I ask is because it is well within my budget to get a nice front and keep my Open Pro and Powertap rear. Sure, the thing will be ugly as hell, but I could care less. I don't really feel like getting a set just so it looks complete is worth it if the added benefit is like 10% of just the front wheel. Plus, I could always get the rear later.

i applaud you for being willing to ride a nice front wheel and a terrible unmatching rear!

Since so many people TT with rear discs there is obviously some advantage there.

i applaud you for being willing to ride a nice front wheel and a terrible unmatching rear!

Since so many people TT with rear discs there is obviously some advantage there.



More people would use front disks if they weren't so squirrelly.

Since so many people TT with rear discs there is obviously some advantage there.

I agree, however I could see that it takes a rear disc to get any real advantage, so just putting a 58mm on the back won't do jack.

Discs are more aero off the bike and with certain conditions. According to the HED gurus a deep sectioned wheel is equally as fast on a bike with a seat tube cutout for most conditions. Discs also don't have a huge benefit margin until yaw angles above 15 degrees.

Not sure if they wanted me to tell, so I'll leave the names out, but a couple of big name riders are going to be on H3D and Stinger 90s this year because they tested faster than the disc(on their particular bike and position).

As for percentages, I don't know, but the front wheel is definitely the one to get first. Maybe a disc cover in the back to offset the cost, but get a bunch of aero benefit too?

I should show up at my next crit with a 1080 and a rear disc. I wonder if the officials would even let me warm up...

I should show up at my next crit with a 1080 and a rear disc. I wonder if the officials would even let me warm up...

the 1080 is certainly legal. The rear disc I believe may also be legal as long as it is not an event which would have to abide by UCI rules. Still, you would get laughed at.

as to the OP, I remember reading the difference in drag from a slow 32 spoke box rim to a high end disc can range from sucking up 50 watts (for the slow wheel) and 0-10 watts for the rear disc. That is 40-50 watts of savings. Whether or not that is true remains another thing...I can't seem to find the web reference for that. Going to ANY type of aero will get you your biggest return (box rim to say a 58 mm). After that, there are diminishing returns so the 58 to a 108 mm rim will not give nearly as much help.

And for a $100 you can get a wheel cover cut to fit your rear wheel from Wheelbuilder dot com and get 98% of the aeroness of a disk wheel at a fraction of the cost.

Really, what reason not to?

Does anyone know the added benefit of a rear aero wheel?

They sound really cool when they go by, whoop whooop whoopp......:D

Its like a playing card and a clothes pin for adults.

They sound really cool when they go by, whoop whooop whoopp......:D

Its like a playing card and a clothes pin for adults.

Thats still cool. :p

I've also read that after the aero wheels 2nd best thing you can do is an aero helmet. True?

I ask 'cause my "a" goal this year is the district TT and would like to take advantage of everything i can within the rules.

According to MIT (I think?) the helmet will help more than the wheels, actually. Definitely heard that somewhere.

According to MIT (I think?) the helmet will help more than the wheels, actually. Definitely heard that somewhere.

Who wants to take this one? Beuller?

I'm thinking of getting a set of carbon tubulars. All this nonsense about drag and dimples and rotational mass has deluded me into thinking I might be faster. Therefore, I ask you to humour me and not tell me crap like I won't be faster - I already know this. Consider this purely an academic discussion.

Does anyone know the added benefit of a rear aero wheel? Given the fact that the wheel is behind the frame and riders legs, is there really any noticeable difference? The reason I ask is because it is well within my budget to get a nice front and keep my Open Pro and Powertap rear. Sure, the thing will be ugly as hell, but I could care less. I don't really feel like getting a set just so it looks complete is worth it if the added benefit is like 10% of just the front wheel. Plus, I could always get the rear later.

What you haven't mentioned is the, "What are you going to use it for?" Crits., TT, RR... Going tubular is a likely weight savings of tubular over clinchers. I just got an old set of Zipp tubular 404's. the savings of 1lb over my campy 28 hole 14-15 gage std. spoke -Front wheel alone-
Weight savings of the wheels pays every time you accelerate or climb, The aero saves watts in most every situation except a super tailwind.

Who wants to take this one? Beuller?


I will! ZeCanon is perfectly correct. And don't wear gloves.

According to MIT (I think?) the helmet will help more than the wheels, actually. Definitely heard that somewhere.

Here you go: http://www.bicycling.com/article/1,6610,s1-3-12-14995-1,00.html.

"Most people don't realize that a nonaero helmet creates four times the drag of a nonaero wheelset. So you can spend two thousand dollars on a wheelset, or spend two hundred on a helmet and be faster. How you put your race number on matters more than having an aero wheel; today, we glued on our numbers to get them to fit flatter. Then there's water bottle placement: On a round-tubed frame, having a bottle on your seat tube is more aerodynamic than not having one at all, and it's much more aero than putting it on the down tube. And wearing gloves in a time trial will slow you down more than using a nonaero front wheel."

If I might be so bold as to take the pseudo-quote the argument of Asgelle - "the MIT data is at only a zero-degree wind angle"- i.e., straight on... "and XX degrees (generally 15) is the predominate wind angle." My apologies to asgelle if I'm being presumptuous or have misinterpreted.

My argument: The TT's that matter to me have always been dead-wind. And even if they weren't, you'd have to prove to me that the predominate wind angles are in the X-XX degree angle. Even assuming that, you'd still have to prove to me - with person-on-bike data - that helmets are not more of an effect than wheels.

I should show up at my next crit with a 1080 and a rear disc. I wonder if the officials would even let me warm up...

I raced at Ninigret Park RI with a rear disc and a TriSpoke (HED3) up front. P123, I felt I had no chance since I was a 3, the course favors breaks (super windy - beach side airfield, 8 turns), and I can't TT to save my life.

I have to find a picture, I think I even put on my Rakes.

I told my friends/teammates that the 3rd break always wins so I'll go after that one. The first one went, never got a gap, came back. Second one went. Local strongman Amos and another local strongman Leigh. I said "F that 3rd break rule this one is going to work". I bridged up. Worked as hard as I could (i.e. I pulled 1x/lap - they were doing 2 pulls each).

Stayed away for 10 laps or so. Got caught, flurry of counter attacks, and TWO groups went up the road.

I haven't managed to stay in a race down there for more than 4-5 laps since then but the next time I go, I'm waiting for the 3rd break.

Regarding helmets I've considered starting races with a bob tail TT helmet (Lemond 89 Tour helmet) when it's cold. I tape my vents anyway. Giro sold a helmet with a ventless shell (92?) where they simply covered the then-high-number 9 vents with the shell. This was meant as a "road TT" helmet.

In the summer though I never get away and I'm always hot. So vents it is.

cdr

Here you go: http://www.bicycling.com/article/1,6610,s1-3-12-14995-1,00.html.

Actually I was thinking of this, http://forum.slowtwitch.com/gforum.cgi?post=908055;search_string=mit%20aero%20helmet;#908055

and if anyone thinks Zipp is partial, you might want to read what MIT had to say about it. http://forum.slowtwitch.com/gforum.cgi?post=908372;search_string=mit%20aero%20helmet;#908372

I don't have to know much to know that if the guys who did the research disavow these conclusions, there probably isn't a lot of reason to accept them.

Josh is totally right. I'll elaborate on the frames of reference for each claim I made in the Bicycling article very soon, but just so everyone knows most of the quotes were meant to be referenced for general cases (how you'd actually ride the bike, component, etc.). Another thing I'd like to preface everything with is that frontal area and drag are not everything! Disc and aero wheels (at certain yaw angles) can actually contribute forward thrust to the bike due to side forces involved. I'm not trying to say wear an aero helmet and ditch the wheels -- rather I'm trying to mention what should be done if you're trying to get faster on a budget. Cycling and triathlon are expensive sports and the general public should know that they aren't at a loss if they don't have a $10,000 bike!

I'll get into more detail over the next few days, but just know that I didn't make up the things I said in that article and I knew I'd need to back up a lot of the details. Dr. Kim Blair and I are extremely meticulous when we test (and when we release data). I apologize for the lack of context for a lot of the aero notes, but it was Bicycling's choice to not publish the details I included. However, I think the article points to some interesting things that haven't really been written down anywhere.

Thanks for all of your comments. -- Mark Cote, MIT Center for Sports Innovation

Like I always have, asgelle, I still strongly disagree with the use of 'disavow' to describe the above quote. The only objection that 'Joshatzipp' seems to have is that MIT only tested 0 degrees of yaw.

Like I always have, asgelle, I still strongly disagree with the use of 'disavow' to describe the above quote. The only objection that 'Joshatzipp' seems to have is that MIT only tested 0 degrees of yaw.

If you want to use another word, that's fine, but the point is, as MIT have agreed, they only tested at 0 yaw where helmets perform the best and wheels, the worst. As they've also agreed, 0 yaw is a very rare condition for races and a more realistic value is somewhere between 5 and 15 degrees depending on the assumptions. And finally they've agreed that at more realistic yaw angles, the decrease in drag from aero wheels is greater than the decrease from an aero helmet.

Like I've said, if the group doing the testing doesn't stand behind the Bicycling report, which by the way never speaks to relative value, only absolute performance, I don't see how anyone reading it could.

If you want to use another word, that's fine, but the point is, as MIT have agreed, they only tested at 0 yaw where helmets perform the best and wheels, the worst. As they've also agreed, 0 yaw is a very rare condition for races and a more realistic value is somewhere between 5 and 15 degrees depending on the assumptions. And finally they've agreed that at more realistic yaw angles, the decrease in drag from aero wheels is greater than the decrease from an aero helmet.

Like I've said, if the group doing the testing doesn't stand behind the Bicycling report, which by the way never speaks to relative value, only absolute performance, I don't see how anyone reading it could.

what's your take on the latest roues artisanles wheel testing? apparently they just fit a weighted curve to the degrees of yaw:

http://idata.over-blog.com/0/02/72/10/Tests-Acheteur/base-de-donnees/gauss-560--.gif

do you think this greatly detracts from the study? my guess would be that giving too much weight to the 0 degree measurement would likely overvalue non-aero wheels since the differences between them and aero wheels are less significant at 0 degrees.

http://www.zipp.com/Technology/Aerodynamics/RimShape/tabid/103/Default.aspx

Check out the graph of drag vs. yaw angle. Yes the rear wheel is drafting the frame but only in an absolute headwind.

^To talk about something that would require asterisks, all of Zipp's tests (to my knowledge) are done without a rider. The rear wheel will draft off of the legs as well, which hasn't been tested.

what's your take on the latest roues artisanles wheel testing? apparently they just fit a weighted curve to the degrees of yaw:

I haven't read the article, but in principle, I like the idea of a weighted average yaw angle for general discussions. That would collapse the data over the range of angles to a single number. The problem is how to do the weighting. As you say, the results will depend on the specific implementation so the question becomes does the weighting used reflect reality. It seems this article overweights 0 yaw, but I haven't seen any data to show it does, or to show a single weighting is robust enough to cover a wide range of riders. Until that is available, I prefer to see drag vs. yaw so readers can perform the weighting for themselves.

All of this is just aerodynamics though. I would think that on a flat to rolling course a disk would be faster in a TT just because it would be slightly heavier than a deep aero wheel. I a flat TT you obviously aren't having to climb and you aren't having to deal with the accelerations of a crit, you are just trying to maintain your own momentum so a extremely light wheel could be a disadvantage. The Discovery channel team actually requested the Bontrager Aeolus wheel be made slightly heavier because they believed it to be faster on flat terrain, at least thats what I read.

But Astana went with the Lightweight.

But Astana went with the Lightweight.

But they got banned from the TdF.

Wow get back form some racing to find a lot of good responses.

marc66thomas - I should have mentioned that theses wheels would server all purposes other than TT wheels. They will be crit, RR, and training wheels, as I don't really have the loot to upgrade to SRM or multiple Powertaps. Therefore, I wanted some all rounders. The 58 mm Zipp (williams), AC 58 mm (team sponsor), 50 mm Neuvations, etc, are all at the top of my list based on the fact that they seem to have the most aero benefit while remaining fairly light. I'm also going with tubular, as I'm finally convinced at how much difference good tires can make as well.

cdr - As usual, a great story :)

^To talk about something that would require asterisks, all of Zipp's tests (to my knowledge) are done without a rider. The rear wheel will draft off of the legs as well, which hasn't been tested.

Just to clarify, in a slightly off-topic post...

Many of our tests are conducted on bikes, with riders (most of which don't get publicized quite as heavily as our last test with CSC, if at all). These results are typically not released as there is considerable variation in results due to different frame and fork configurations, rider positions, etc.

The fun begins when evaluating the accuracy of our mathematical models developed for comparing wheel-only savings to that in a typical bike. We've refined them to the point where they are typically accurate within about 1-2 percent, and actually on the conservative side. As an example, our calculations were suggesting a savings of 58-60 seconds per hour for Fabian Cancellara with a switch from the Ambrosio wheels typically used in the classics to a pair of 404s. When we got into the tunnel with Fabian last year, it actually worked out to a 63 second savings (so in this case we were actually 5-8.6% conservative). This is based on the bi-modal distribution of effective angles we have arrived at via data collection and examination of NWS data.

Just to clarify, in a slightly off-topic post...

Many of our tests are conducted on bikes, with riders (most of which don't get publicized quite as heavily as our last test with CSC, if at all). These results are typically not released as there is considerable variation in results due to different frame and fork configurations, rider positions, etc.

Yup, rider position/pedaling is difficult to duplicate test-to-test; and errors in rider position would be of the same magnitude or higher than the difference in wheel-to-wheel changes.


This is based on the bi-modal distribution of effective angles we have arrived at via data collection and examination of NWS data.


As they've also agreed, 0 yaw is a very rare condition for races and a more realistic value is somewhere between 5 and 15 degrees depending on the assumptions. And finally they've agreed that at more realistic yaw angles, the decrease in drag from aero wheels is greater than the decrease from an aero helmet.

1. Asgelle - although I believe you, I have not seen MIT agreeing that 5 to 15 degrees is appropriate. Even granting that (I believe 5 degrees is reasonable), I have not seen MIT concurring that at those angles, helmets don't matter as much as wheels.

2. The Michigan state TT championship starts about two miles or so from my house. On the ride there, I pass by a college weather station. The anemometer, located fifty feet in the air and hundreds of feet from treelines, etcetera was dead still, just like it was for the other TT's held on the same course.

Has anyone here ever been out in "20-40 MPH" headwind? If you can't ride at 30 MPH normally, how can you ride against it at all?

One problem is that the measured windspeed has little to do with the actual windspeed; because weather stations do not measure wind three feet off of the ground, near the trees that I ride by on my trip through the local parks.

Moreover, averages are even worse - because much published data is based on average wind power; which, being a cube function of windspeed, overweights higher velocity wind.

Estimating an average TT windspeed based on published data is warrantless and without merit.

Taking data? Sure. I haven't ever seen the data, so I don't know. I *do* know that my TT's are generally held in the later morning/early afternoon of summer days - when the windspeed is almost always below the "average" for the location.

When pulling a 25 MPH average, the wind speed AT THE GROUND needs to be 10 MPH for the average wind angle to be 15 degrees. Using Zipp's 30 MPH testing criteria, the average windspeed needs to be 12 MPH.

What 'weather-station-measured' speed does this work out to be?

Well, this reference (http://www.stadtentwicklung.berlin.de/umwelt/umweltatlas/ed403_06.htm) says that the ratio between near-ground wind speed and measured wind speed is between 0.2 for urban areas, 0.3 for (more or less) vacant lots in urban areas, and 0.8 for farmland type spaces. Should we use 0.5 for our purposes? Riding 50 feet from the treeline, I think it's reasonable. So, for these assumptions, in order to have an 'average' 15 degree headwind, at the 30 MPH mark, the measured windspeed would need to be about 24 MPH.

That's one hell of a wind for mid-summer afternoons. And I live in the midwest.

Of course, that doesn't mean Zipp isn't a great wheel. I have some 404's, and would've bought some 303's this past week if a teammate hadn't wanted/needed them more. But 15 degrees? I've never raced in those conditions.

http://hedcycling.com/technical/yaw-calculator-app.php

And I don't know the specific wind speed, but I was out last year pushing 350-360w steady and going 13 mph. It was the worst day of my life at that point. I swear the wind was 80mph head on.

One problem is that the measured windspeed has little to do with the actual windspeed; because weather stations do not measure wind three feet off of the ground, near the trees that I ride by on my trip through the local parks.

I know of one person who does has used an anemometer placed 3 ft. above the ground to take direct wind measurements during his testing. He believes 0 yaw is not the best choice but somewhere between 5 and 15 degrees is a better angle.

But they got banned from the TdF.

incorrect.

I know of one person who does has used an anemometer placed 3 ft. above the ground to take direct wind measurements during his testing. He believes 0 yaw is not the best choice but somewhere between 5 and 15 degrees is a better angle.

Yup, which is why I welcome 'measured' (in this case, data measured specifically for analysis in cycling) data. I'm sure that 0 degrees is not the best single number; and I'm sure a weighted number would do better. Of course at altitude in the western states, the numbers would probably be higher. At low altitudes/in the east, the yaw numbers would probably be lower. For my most important events those numbers have somewhat coincidentally been 0. If I had to guess what the angle would be for this year, I would guess 5 degrees (about a 7 MPH weather-station measured wind). That's still a pretty brisk breeze for mid-summer mid-afternoons.


The difference between 202's and 808's at zero degrees? ~5 watts. At five degrees? ~7 watts.

Just a few thoughts/posts from a few well known sources;

From Coggan

As I mentioned, it's rare that we have absolutely still air through which to pedal, even on an indoor track. The consequence is that there is practically always some crosswind, such that yaw angles of approx. 10-20 deg are far more common than yaw angles <10 deg or >20 deg. Indeed, even in absolutely still air on an indoor track, the yaw angle approaches 10 deg in the turns, due to the fact that the cyclist is constantly changing directions.


And John Cobb

This has been an interesting thread and I wanted to share a couple of my observations on this topic. I think that testing at a single yaw point like 6-7degrees might be a very good idea. Of the "bicycle tunnels", the A&M tunnel is the one that can rotate or yaw very quickly, it can sweep from 0 - 90 degrees in under 1 minute while giving data points. I test there because it is 2 hours from my house so it's convenient and I have become accustomed to doing multiple yaw sweeps for each run. I did see early on that the results that were best at "0" were not always the best for an average yaw run but it also depends on what the rider's goal is as to the importance. As an example, for track racers I often test at every degree from 0-5, for triatheletes/TT racers, I test at 0-2.5-5-7.5-10-12.5-15-20. I would encourage any of you that go to tunnels to test, to do the test with standard wheels and not a disc or very deep rims. The air flow does not break off these race wheels consistantly and getting small data changes is a lot more difficult. I would also encourage you to do several repeats of various runs to verify the changes, all tunnels tend to "creep" and we are dealing with such small drag numbers it is easy to go down the wrong path without double checking the data. I established the standard testing of Drag @30mph, this was done to give high enough force numbers so that they would be repeatable. I would like to get some of you racers opinion as to a good "average" yaw, I will invest the time[and money] to go gather some data at that yaw point and maybe we could establish a new single point yaw for everybody to use. I would still encourage you to test at multiple yaws when possible but a consistant 5-6-7or 8 degree call out might be very helpful. It would be very easy for any tunnel to test at a single yaw point and as more people learn about this type of testing, more speed will be found for everybody.

Just a few thoughts/posts from a few well known sources;

From Coggan
And John Cobb


interesting. it seems as though all of us are in favor of somehow collapsing the data into a more accessible single number, and the distress is over the weight of measurements at different yaw angles. even cobb doesn't seem to know which angles are the most common for typical tts. that does not bode well for a small brain like me.

Yeah, he mentions that he tests at many different angles. Even HED data shows a 90mm rear is very close to a disc up until 15 degrees. They said that some people are more aero until 15 with a 90mm or H3D, while some are still faster with a disc. At 15 degrees the disc has 1/4 the drag of a Stinger 90, but at 0 it's less than 5 grams. I took a chance and went with the Stinger 90 rear because they say a disc seems to matter less when you have a seat tube cutout and the new disc "redesign" was going to leave me without a rear race wheel at one of my big races this year. Plus I can now run a rear 90 in Lynchburg or similar races where it's windy but flat.

Indeed, even in absolutely still air on an indoor track, the yaw angle approaches 10 deg in the turns, due to the fact that the cyclist is constantly changing directions

Bull.

I would still encourage you to test at multiple yaws when possible but a consistant 5-6-7or 8 degree call out might be very helpful.

These numbers might be reasonable. I like the 5 degree. 15 is out to lunch; much more rare in my experience than 0 degrees. Of course, at 5 degrees wheels only make 7 watts of difference, and at 15, they make (guestimating without looking at the chart again) 20. So we only hear about 15 degrees.

They sound really cool when they go by, whoop whooop whoopp......:D

Its like a playing card and a clothes pin for adults.

hehe, I love that sound too :D

Bull.

Hey, he's on the site, lure the good Doc out for discussion.

Hey, he's on the site, lure the good Doc out for discussion.

Who, moi? :D

FWIW, the velodrome yaw data in question were collected by Chet Kyle et al. as part of Project 96, and can be found in Burke's "Cycling Science" (don't recall what page/figure number, though).

Well, I was hoping for Dr. Day or Paulo, but I guess you'll do since you're here and all. :D