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Aero rims – Hype or Truth?(11 posts)

Aero rims – Hype or Truth?wongsifu_mk
Oct 23, 2002 3:18 PM
Seriously, what are the benefits of aero rims? I'm shopping for a set of race(crit) wheels that also perform well with Centuries.
How much of a difference do they really make in regards to overall speed, acceleration, efficiency and ride quality? If they're lighter than or are near the weight of 'standard' rims, does the aerodynamic design really account for a marked performance improvement? I've heard arguments that spokes affect aerodynamics more and that they are the main source of drag. I've also heard that the momentum of a tall aero rims has its benefits as well.

SO confused...

Thanks ahead of time.
How much hype?Kerry
Oct 23, 2002 3:50 PM
The rims are half of the equation. For reference, the best aero wheels will give you about 0.5 mph (at 25 mph) in zero wind conditions. That benefit comes from the rim profile AND the spoke count. Going from 36 to 24 spokes (box section rim) is worth 0.2-0.3 mph alone, so you can figure out the contribution of the rim. In cross winds, you may lose much of this if the rim section is large and you have to fight for control. In most racing situations, aero wheels are of little value since you are in a pack. In a century, you're not likely going 25 mph, so your speed gain is significantly less. The best aero wheels might gain you 4 minutes in a 20 mph century - huge if you're racing but pretty meaningless if you're training or riding for fun.

Aero wheels have an effect on acceleration in proportion to their weight. If you're on the lightest Zipps, they will most likely accelerate faster than what you have now. Heavier rims accelerate (and decelerate) slower, though not by much. Ride quality is totally dependent on the particular build. Bottom line, aero wheels are faster in nearly all circumstances, but light weight is probably the best answer for racing, and the speed increases from aero wheels are mostly beneficial for time trials.
THANKS for the detailed & valuable info.wongsifu_mk
Oct 23, 2002 9:00 PM
Exactly what I was looking for.
Please cite your sources, as I have...sprockets2
Oct 24, 2002 12:51 PM
talked at length with some engineering-type guys who set up a series of trials to try to work this out and they found that there were too many confounding factors, some of which you mentioned. They were extremely frustrated by the results and decided that they needed way more sophisticated apparatus and much more controlled conditions. From the sound of it, they had done a very good job of setting up their runs and it seems like the effects of aero-rims in a strictly aerodynamic sense are very small. I would guess that focusing more on spoke drag would be a wise target for tech weenie/race types.

I have decided to only think about aero-rims for strength reasons in my own purchasing. Even then some of the aero-rims are so heavy that they affect the handling and subjective pleasure of riding.
www.bicyclesports.com; technical section (nm)TJeanloz
Oct 24, 2002 2:17 PM
Pete PensyresKerry
Oct 24, 2002 4:26 PM
Did an extensive series of tests for Bicycling magazine (August 1997), back when they actually used to test products. He did a large number of early morning (no wind) coasting tests on a long, straight, steady grade hill. He did enough tests to get reproducible results. Interstingly, several of the wheels he tested (or VERY close replicas) are still on the market. The most successful ones, of course. These tests were on complete wheels, so they didn't address the rim effects vs. spoke effects, but many other reports point out that low spoke count box section rim wheels (like the 24 spoke Campy "climb-dynamic" wheels) are good for that 0.2-0.3 mph. This is where the conclusion of "rims count for half, spokes count for half."
re: Aero rims – Hype or Truth?dasho
Oct 23, 2002 5:21 PM
This message was posted over on Cycling Forum. Hakan obviously put A LOT of time into this post.
Date: October 15, 2002 03:14 PM
Author: Håkan (Hakan.Carlson@home.se)
Subject: Looks or performance?

(Most of this have been published here a couple of weeks ago, but the search function don't seem to find it)

The most important performance factors of a wheel is aerodynamics, weight and last and least inertia. Aerodynamics is by far the most important factor, at least as long as the speed is higher then 20-25km/h. That the parameters above actually are listed in order of importance can be read about here: http://www.bike.com/template.asp?date=8%2F1%2F2001&lsectionnumber=6

Quote:
"So, what do all these numbers mean? It means that when evaluating wheel performance, wheel aerodynamics are the most important, distantly followed by wheel mass. Wheel inertia effects in all cases are so small that they are arguably insignificant."
Or you can find out by yourself by testing different (REALISTIC!) values for the different parameters here: http://www.analyticcycling.com

The German magazine Tour(http://www.tour-magazin.de) have tested several different wheels in issue 7/2002.

Measured weighs for Ksyrium SSC SL:
Front 730g Rear 884g (excl Q/R)

Standard wheel with OpenPro-rim and 32-spokes:
Front 676g Rear 781g (excl Q/R)

The standard wheels are built with Sapim (http://www.sapim.be) aero spokes and hubs from the German manufacturer Tune:
http://www.tune.de

Tour also tested the aerodynamics of the wheels in Mavics windtunnel. The wheels have almost the same aerodynamics:
Ksyrium SL: 251g, OpenPro: 258g ie less then 3% difference.

The test also shows that the inertia is almost the same for the wheels:
Ksyrium SL: 121Joule(J), OpenPro: 116J, a difference of 4%.

But what about the 'old' Ksyriums?
What about the aerodynamics, weight and inertia of them?

Tour have in earlier issues teste them too, unfortunately not in the windtunnel. Aerodynamics for a bicycle wheel is largely set by the rims surface area or rims with a higher profile are faster. Read more here:
http://www.bicyclesports.com/technical/aerodynamics/watts_drag.html

The rim profile of the Ksyriums are not very good, compare with the CXP33-rim:
http://www.mavic.com/servlet/srt/mavic/road-prod_fiche?produitsid=46&lg=uk

The spokes of the Ksyriums are very different but they are so fat(1,7 x 5,2 mm) that if they make a difference they make the aerodynamics worse. Thin aero spokes like the Sapim CX Ray are 0,95 x 2,3 mm. Wheels built with CXP33-rims and thin aerospokes will in practice have the same aerodynamics as the 'old' Ksyriums.

But what about weight and inertia for the wheels? Mavic is selling the Ksyriums as lightweight wheels.

In issue 4/2000 Tour tested the 'old' Ksyriums and in issue 7/99 they tested wheels built with CXP33-rims.
Here are some of the data:
Weight(f/r) Inertia
Ksyrium 730/934 123J
CXP33 678/845 118J

The CXP33-wheels were this time too built with Sapim CX-Ray spokes (28/32) and Tune hubs.

Ksyrium-wheels both the 'old' version and the new SL don't seem to have any different performance then wheels built with OpenPro or CXP33-rims.

But there are two ways that the Ksyriums are different in:
Looks
Price

If the looks make them worth the price is up to the buyer to decide, but the buyer also clearly shows off that he pays extra for looks not performance.

Homebuilt alternatives

If you like the black look of the KSyriums you can try the following alternative:

Front
Hub: Nav AmericanClassic, black 68g  515SEK
Rim: OpenPro, black 425g 399SEK
Spokes: 32 Sapim CX-Ray, black 145g 736SEK
Nipples: Sapim Polyax, black 11g (incl)

649g 1805SEK

Rear
Hub: Tune MAG 200, black 209g 2477SEK
Rim: CXP33, black
Good post. Thanks (nm)Chen2
Oct 24, 2002 2:00 PM
Question for Chendasho
Oct 24, 2002 2:13 PM
Chen,

You mentioned a while back that a 9 cog Shimano DA could accept a gear as high as 32.Do you now how to do this? I have a 12-27 for the mountains but I'm getting old and the mountains in NC can get pretty steep so I would like to go with at least a 30 or possibly 32. I don't want to change to a triple just yet. I realize I could put a Shimano Mt. Bike rear der and cassette on but I wonder if that is the only way. Thanks in advance!
Partial answer.Chen2
Oct 25, 2002 2:34 PM
I don't think I said you can put a 32 on a D-A. I'm not sure what the maximum cog size is for D-A, as you know, Shimano says 27. The rear short cage 9-speed D-A derailleur is compatible with other 9-speed Shimano groups within spec limits. Many folks have reported using larger than spec cogs with Shimano rear derailleurs, as high as 30t. My personal experience is up to 28t on an Ultegra derailleur. If you want to try 30, go for it. Anything bigger will require a mountain bike rear derailleur. You can buy custom cassettes that go as high as 30t or you can assemble your own using various Shimano parts and parts from www.sheldonbrown.com. Just be careful to keep the spacing correct and make sure your chain is long enough to handle the big to big combination with no chain slack when in the small to small combination. Take a look at the cassette diagrams and charts at the Shimano.com European web site. When building a cassette, keep the ratios from cog to cog as close as possible and still get the job done. I built a 14-28 for my wife to help her get up these Oklahoma hills.
~Al
Thanks!dasho
Oct 26, 2002 3:18 AM
Thanks Chen! I contacted Sheldon Browm and found I can get a 30 and that should be plenty.