FLO Cycling – Aerodynamic Difference between Sapim CX-Ray and Round Spokes


Years ago when, Chris and I were starting FLO, we made the decision to use Sapim CX-Ray spokes.  We’d heard that they were faster than round spokes or bladed spokes.  The CX-Ray’s have a unique aero shape, which likely explains part of the reason the spokes are used on so many aerodynamic cycling wheels.  



While I’d seen studies in the past on spokes in a wind tunnel, we want to study the difference between CX-Rays and round spokes.  Were CX-Ray spokes really that much faster than a standard round spoke?  

When Chris and I visited the A2 Wind Tunnel in November of 2015, we decided to find out.  

If you are interested in our other studies from the A2 Wind Tunnel, please be sure to check them out.  

Does Tire Pressure Change Aerodynamic Drag?
Does Covering the Valve Cutout on a DISC Wheel Matter?

How Much Drag Does a Clydesdale Build Add to Cycling Wheel?

What We Tested
We tested the following wheels in the wind tunnel. 

1.  Front FLO 30 with 20 radially laced Sapim CX-Ray spokes

2.  Front FLO 30 with 20 radially laced round spokes.  

We used a FLO 30 to maximize the spoke length.  The FLO 30 maximizes spoke length since it is our shallowest rim.  



How We Tested the Wheels
It’s important to define how a test is performed in a wind tunnel.  There are countless variables, and if you are not clear about the test, the results are not very clear.  Here is how we tested the wheels at the A2 Wind Tunnel.

1.  Tare was calculated and removed from all tests.

2.  Each wheel was swept from 0-20 degrees of yaw, in 2.5 degree increments. 

3.  Each measurement was taken twice and averaged.

4.  The same Schwalbe Ultremo ZX tire in a 23mm size was used for each test.

5.  All tires were inflated to 95 psi and calibrated with a digital gauge.

The Results
The results were very surprising to me.  While I thought that there would be a difference between the two, I did not expect the large difference we saw.  



Using the FLO 30 with round spokes as a baseline, the FLO 30 with Sapim CX-Ray spokes will save you 9 seconds over a 40km, and over an Ironman you gain 42 seconds.  If you take both wheels into consideration, the time savings will increase.  

While Sapim CX-Ray spokes are more expensive, we’ve known for a long time that they are arguably the best spokes because of their strength and durability.  If we add the aerodynamic benefit into the equation, going with another spoke does not make much sense.  

I hope you enjoyed this article.  Please feel free to comment and ask questions below. 

Take care,
Jon

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7 Comments

  1. I tried to leave a comment a couple days ago but it doesn't seem to have gone through so here's another attempt.

    First I wanted to say I've really enjoyed reading your recent blog posts about the development of the new carbon wheels and other tests you did with them while in the wind tunnel. Keep up the great work!

    I had two questions about your test:
    1. What was the speed you tested at?
    2. What type of spokes did you use for the round spokes?

    November did a similar test a couple years back and came up with a smaller difference than you did: http://www.novemberbicycles.com/blog/2013/1/22/aerodynamic-drag-of-lasers-vs-cx-rays.html. Assuming you used Sapim Race for the round spokes at 30 mph, I was wondering if you thought the difference was totally due to the difference in rim depth since they used 38mm rims, or do you think it might have to do with differences in your NDRV formula compared to their equivalent? From what they say their yaw weighting sounds similar to yours. Or do you think there is there some other reason for the difference?

  2. Which Sapim round spokes were used? November did this test, I am sure you are aware; their test found that there is 1 aero watt difference between CX-Rays and Lasers (which are the same spoke, just not flattened).

  3. A lot of people have been asking what round spokes we used. We used 14g Sapim Leader spokes.

    Ben,

    The wind speed we used was 30mph.

    I would guess the biggest difference between our test and the November test would be the spokes we used.

    I hope that helps!

    Chris

  4. Chris, I really enjoy these articles, but I have to ask… is there a dress uniform at the A2 tunnel? You are always wearing the same hoodie*!

    *Bunnyhug?

  5. Richard Knight,

    I'm not sure what you mean by Bunnyhug? I'm wearing the same hoodie because most of the pictures were taken on the same day.

    Chris

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