FLO Cycling – A2 Wind Tunnel Tire Study Part 1

In Part 1 of this series, we are going to look at the effect tires have on aerodynamic drag.  The first time we visited the A2 Wind Tunnel back in April of 2011, we were shocked by how much of a difference tires could make.  We always wondered how much better our results could have been if we had the budget to test a long list of tires.

In November of 2015, four and a half years after our first round of testing, Jon and I went back to the A2 wind tunnel with our new 2016 wheel line.  During our two day visit, we conducted what is rumored to be the largest tire study to ever take place at the A2 Wind Tunnel.  In total, we tested 20 different tires on our new FLO 60 Carbon Clincher to get an accurate look at how tires effect aerodynamics. 

What Will Be Covered in Part 2
Aerodynamic drag is only part of the tire equation.  On top of overcoming the aerodynamic drag created by your tire, you also have to overcome it’s rolling resistance while riding.  A tire that has a low aerodynamic drag, could have a high rolling resistance.  When you combine the two components, the tire as a whole would be a poor performer.  Tires that have the best combination of aerodynamics and rolling resistance are the best tires to use while racing.

I sent Tom Anhalt the majority of the tires we tested in the A2 Wind Tunnel to test on his rollers.  In Part 2 of this series, we will combine both aerodynamic drag and rolling resistance to find the best overall tire.

For now, let’s focus on the results of the aerodynamic testing.  

The Test
Below are the parameters of the aerodynamic drag test performed at the A2 Wind Tunnel.  We used the parameters listed below to eliminate as many variables as possible. 

– Wind tunnel wind velocity was set to 30mph.
– All tires were tested on the same FLO 60 Carbon Clincher.
– Tire pressure was set to 95psi and calibrated with a digital pressure gauge.
– Drag measurements were recorded at 0, 2.5, 5.0, 7.5, 10.0, 12.5, 15.0, 17.5, and 20.0 degrees of yaw.
– At each yaw angle, two measurements were taken and averaged.
– Tare was measured and removed from the results.



The Tires

We tested 20 tires in total.  The tires ranged in size from 22mm to 25mm in width.  We did not test tires less than 22mm because they are too narrow to use on our wheels, and we did not test tires wider than 25mm because they too commonly do not fit on today’s road bikes.  Here is the list of tires we tested.

* Listed tire sizes are based on the manufacturer’s naming conventions.  Please keep in mind that a tire’s width may not measure as stated when on a rim.  For example, a 23mm tire may measure 26mm in width when installed on a rim.  Please keep this in mind when selecting tires for your frame.
The Results
We have created two Interactive Drag Charts.  The first in Grams of Drag and the second in  CdA (m^2).  These interactive charts allow you to turn individual tires on and off.  Placing all 20 tires on the same chart makes things very cluttered, as you can see below.


Time Savings
Aero charts are great, but what does all of this mean.  Exactly how much time does each tire save you on the road, and which tire is the best aerodynamically speaking?  In the table below, we have listed how much time each tire will save you over a 40km race, and an Ironman.  We have used the slowest tire as the baseline with a time savings of 0 seconds.  These time savings are calculated using our Net Drag Reduction Value (NDRV) equation.  Our NDRV equation was developed using the 110,000 real world data points we collected while developing our new wheel line.  Learn more about NDRV.  

I hope you enjoyed this blog article.  Stay tuned for Part 2 of this series where we look at the rolling resistance of each tire, and combine that data with our aero results.  Please leave your questions and comments below.

Take care,

Chris

Join the Conversation

16 Comments

  1. I've read that, although counterintuitive, 25mm tires are actually faster than 23mm. I find it interesting that your chart indicates the opposite.

  2. wacomme,

    I don't have a definitive date at this point. Sorry about that. Tom is currently roller testing all of the tires I sent him. Once we collects his data, we'll have to put everything together.

    Chris

  3. Generally, as far as I'm aware, that's referenced in regards to rolling resistance. Allegedly the contact patch is wider but shorter, giving better cornering traction where lateral contact is king and less resistance in a forward plane.

  4. It's a pity you didn't measure *actual* tire width with vernier calipers once tires at 95psi that would have given a more exacting impression of the tire *actual* size, from my studies for example that GP4000 25mm is more like 29mm on your rim geometry. Although hopefully Tom A. covers this in part II!!

  5. These numbers seem incredible to me. (Admittedly, I know nothing about aerodynamics!) What could explain such massive differences in drag between two tires of the same width and roughly the same material? The time savings you're posting are similar to the type of savings you'd expect to see for aero vs. non-aero helmet, road vs. aero frame, skin suit vs. road jersey, etc. Anyway, glad I've got a set of Continentals on my bike!
    Bill

  6. William,

    From what we know, I think the difference primarily comes from the tread pattern and the shape of the tire. It's amazing how the smallest changes can have the biggest affect in aerodynamic drag.

    Chris

  7. Thanks for publishing; this is great work.

    @Tony — these results don't look at all strange to me. Every rim has a different shape and width. When built into a wheel, each tire fits a bit differently. That fit affects airflow.

    There is no "fastest" tire; tires and wheels work together as a system. If the Flo guys extended the testing to include a bike and a rider, the results might be different.

    A lot of the "25mm is faster" was influenced by Hed and was referring to rolling resistance. In terms of Aero performance, I believe they still recommend an Attack/Force combination, which are 22 and 24 mm, even on the "plus" rims which are 21mm internal (I think the older Ardennes are about 17mm internal, as (I think) are the Flo carbon models).

  8. Here's my only issue with this test. You chose 95psi as the standard for a reason. As previously tested, the Conti GP4000S performed best on a flo 60 at that psi. As you saw in that test, a 5psi change could make quite a difference. Some of the tires selected, particularly ones of different widths may have benefited from more/less tire pressure. As an example, I'd love to see if the GP4000S 25mm, would move up or down the list with a change in tire pressure. I'd be willing to bet 95psi is not the ideal pressure for the 25mm version of the same tire on the same rim.

  9. Mike C,

    You could be right. With wind tunnel testing there are always more variables that you can adjust. That said adjusting the tire pressure for all 20 tires tested would have meant quadrupling our time in the wind tunnel. We can average about 3 tires per hour when testing, and at $595/hr, running 80 tests in total for this blog article alone simply wasn't in the budget.

    The goal of the article was to show that tire selection is very important especially when considering the rolling resistance like we did in Part 2 of this article. We feel that this article is an excellent resource, and if individuals would like to do more testing on a specific tire, then we invite them to do so.

    Take care,

    Chris

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