April 15, 2016
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 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.