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Tuesday
Jan222013

Aerodynamic Drag of Lasers vs CX-Rays

One of the things we've learned is that offering choices invites questions. One we get all the time is the difference between Sapim Lasers and CX-Rays, which we offer in all our wheels. Or rather, the question really is whether CX-Rays are worth the extra money.

The answer we've always provided is that the spokes are the same weight but CX-Rays are purported to have some aerodynamic advantage. If you're looking for "every last watt of speed," they're the way to go. But we have never seen anything that quantifies the difference between the two in a wheelset. So we decided test Lasers against CX-Rays in the tunnel to give a more informed and specific response than "every last watt of speed."

It turns out, however, that we've been exactly right all along.

We sent two RFSC 38 (38mm) wheels to the wind tunnel, one built with 20 radial laced Sapim Lasers and the other with 20 radial laced Sapim CX-Rays. Here is how the wheels tested against a range of Angles of Attack (AOA):

At all AOAs, the wheel with the CX-Rays was a smidge faster, generating about 11 fewer grams of drag on average at 30mph. If you recall the calculations from yesterday's blog, you'll see that 11 grams of drag at 30mph is - yep - 1 aero watt. You really do save "every last watt of speed" with CX-Rays, and not a watt more.

You remember also from yesterday that aerodynamic impact is diminished at lower speeds. Here is the difference in aero watts between the Laser and CX-Ray wheels at 30mph, 25mph and 20mph. In these calculations, the average drag is calibarated by the frequency of different AOAs at different speeds, which is why at 30mph the difference between the two wheels is 1.8 watts instead of 1.

Most brands assume that if you're spending between $1K and $3K for a carbon wheelset, you're after that every last watt of speed and they make CX-Rays or other bladed spokes standard. The logic starts to break down with shallower alloys though, where the upgrade to CX-Ray spokes may net you a watt, but still leave you a handful or two behind your training buddy on deep carbon, or oblivious if you're training on your own. For alloys in particular, we think it makes a lot of sense to offer the choice so people are not paying for performance they don't need.

 

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Reader Comments (13)

Parts is parts, and watts is watts, and an approx. two watt saving at 30 MPH probably won't net you a PR.
But taken in context, I noted that the diff. in watts absorbed, from the RFSC 38 (w/ CX-Rays) to the 404 FC, is a rather unexciting ~ 5.5 watts. That's the lot,from a 38mm to a 58mm deep rim? So, chasing an extra watt or two saving from an aero-profile spoke doesn't sound that loony. Heck, we must ALL be happily loony...5.5 watts, indeed.

Big thanks, from all us in the loony bin, for testing and publishing this data. When do my 38s ship?

January 22, 2013 | Unregistered CommenterRico

Difference is so very little. Really thanks for this tests! Good job

January 22, 2013 | Unregistered Commenterkavitec

One test must be done with complete bike and two wheels. Here is only front wheel - so on rear with more spokes but in turbulented air will be close to front - around 3watts at 40mph for wheelset?

January 22, 2013 | Unregistered Commenterkavitec

Were the wheels spinning in the wind tunnel? If not the savings may increase slightly as some of the spokes would be traveling at a much higher speed than the bike or stated wind speed or velocity, i.e. top spokes spinning over have bike speed + translational speed from the rotation of the wheel. Although on the opposite side some spokes would be traveling with the air so it might all cancel out.

January 23, 2013 | Unregistered CommenterChris

Rico - I'm not in the business of calling anyone loony. Or fat. We've all seen how badly that can turn out.

Kavitec - It's been credibly shown that the aero impact of the rear wheel is much less than the front, so I wouldn't expect that the impact would be as large as you guess, but it could be tested. Our purpose in testing was just to find out if there was a measurable benefit to CX Rays over Lasers, and if so how big. The conclusion we drew is that if you're looking for every last watt, use CX Rays.

Chris - Wheels are tested while spinning, yes.

January 23, 2013 | Unregistered CommenterDave

An extra $120 for, arguably, a saving of 2 watts? If that isn't diminishing returns, then I don't know what is.

I would infer that the added savings garnered by the CX Rays would diminish further still as you move to a deeper rim.

Regarding the aero impact of a rear wheel, why is it that people still use full discs back there?

January 23, 2013 | Unregistered CommenterJoe Ajello

Joe, a lot of people would see it differently, and that's why we gave people the choice. Choice sometimes equals inefficiency though, as it does in this case - we deal with 4 spokes per wheel (2 types, black and silver each) instead of just one. The Rail is going to be CX Ray only, which will enable some efficiency to that so the incremental cost of that watt will go down.

You could also make the argument that as rims get more slippery, spokes represent a bigger portion of the drag and therefore reducing their drag becomes more important.

Good discs are pretty darn fast. Say a disc saves 5 watts over a reasonably fast wheel when you test them alone, without the bike. Even if that savings is only 20% translated when the disc is acting as a rear wheel, if your job is to go fast in TTs, you use the disc. No question.

January 23, 2013 | Unregistered CommenterDave

You can also make the argument that if there is less spoke length in a deeper rimmed wheel those spokes would be subject less wind.

January 23, 2013 | Unregistered CommenterJoe Ajello

Difference appears to be about 5% improvement with the CX Rays, regardless if the test speed was 20 MPH or 30 MPH. 25 MPH data only shows about a 2.5% improvement. Do you think this anomaly is real world or something that resulted from too many calculations being stretched too thin?
Kudos on breaking the piggy bank and doing your own testing. Until you face that cold, hard data, you're just guessing!
Andy

January 24, 2013 | Unregistered CommenterAndy Hanson

Andy - The raw drag data's pretty clean, and the only calculation is the drag at 30mph being scaled down to 25 and then 20.

January 24, 2013 | Unregistered CommenterDave

Cxray' s are stronger than lasers + slightly more aero.... I always go for the stronger spokes... The aero was just a bonus.

January 26, 2013 | Unregistered CommenterMario

http://sheldonbrown.com/rinard/aero/aerodynamics.htm

October 3, 2013 | Unregistered CommenterJosh

Thanks for taking the time to share your test information.

One critique: Was the power required to spin the wheels during testing measured? From the analysis produced by ZIPP in the past it appears that the energy required to spin your tested wheels may be 0-5 watts of difference.

For reference:
http://www.aeroweenie.com/assets/backup/zipp_data/spokecount.pdf

November 3, 2013 | Unregistered CommenterYancey

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