8.28.2013

A Trip to the A2 Wind Tunnel


Why go to the wind tunnel? 
After spending a couple days roaming around the mountains in Asheville and surrounding areas, it was now time for business...the real reason for the trip was a visit to the A2 Wind Tunnel.  So why go to a wind tunnel?  Well, science tells us that a large amount of a riders power output will go to overcome the resistance of the air, so getting in a position that minimizes drag is pretty important.  It's also important not to sacrifice power too much when getting in this optimum position.  It's a trade-off: more watts make you go faster, and less drag makes you go faster too.  If your position is super-aero but severely limits power then it may not be the best position.  If your position is very powerful but is not aero at all, it might not be the best position.  The key is finding the best of best both worlds for your optimum position.   The wind tunnel helps you do that.

How do you know if your position sucks?
Well, it's pretty tough to know without the use of a power meter.  Using power allows you to (in a controlled environment) compare average power (watts) vs. average speed (mph) to get an idea on "how much speed you are getting for your power output".  The more "aero" you are, the faster you will go off of a given wattage.

One beautiful thing about statistics is that as sample size increases you can begin to see trends.  I've been riding with power for 4 years now and since I race a lot I've got a rather large collection of power vs. speed data points in races.  Now, I'll be the first to say that you have to be careful when comparing these measures because they can be affected by things like wind and elevation, but as you increase the amount of data points you can begin to see trends and outliers.  With all this in mind, I've created the plot below from all of my races over the past 3 years.  Most of these races were on a Cervelo P2C with a Louis Garneau Superleggera helmet, and a HED Jet disc and Jet 9 with Vittoria Open Corsa EVO tires. 


Firstly, you can see two groups of data points with each group following a somewhat "up and to the right" trendline.  Obviously, this is what you would expect since it takes more power to get more speed.  What I've called "hilly" courses in the chart are races with an elevation gain rate of around 50 ft/mile or greater.  Five of these eight points are actually races on the same course.  These points are all shifted over to the left, and seemingly follow their own curve.  Over to the right you can see another group of points that consist of what I call "flatter" races.

Secondly, we can look at this chart and begin to see how terrible my time trail position is.  Lets key in on a bench mark that may cyclists and triathletes have: how many watts does it take you to go under an hour for 40k, or 24.86 mph?  Well for me, the chart says it will probably take me around 270-280 watts.  Keep in mind that I'm 5'11" tall and weigh 148 lbs.  Now how do you know if this is good or not?  Well, anyone that knows anything knows that this sucks.  But if you didn't you can do a couple things; you can see how people your size compare to you (I've got plenty of friends racing with power), you can ask coach (which I did), and you can get on forums (like Slowtwitch) and read about the vast multitudes of forum users and hear how many watts it takes them to go a certain speed.  As sample size increases you can begin to chunk some outliers and hone in on what a "good, bad, and average" power vs. speed relationship looks like.

I say all this to say that my decision to take a trip to the wind tunnel is the accumulation of knowledge and experiences over the last 4 years of training with power.  The more I trained and the more I raced, the more I realized that I was "wasting watts" somewhere.  And the faster I got, I realized that it was costing me races.  Like if I could just save 20 watts on the bike (which I thought was reasonable based on the above chart) then all of the sudden I've won 3 more races so far in 2013.

My Results
James had the session right in front of mine so I got up there early with Heath and Brian to watch his tunnel session and figure out how things worked.  You can read about his results here, but lets just say that his position was already pretty awesome and the tunnel pretty much confirmed that.  I got a lot of value out of watching his test and seeing how the guys at A2 made their decisions and ran through all the runs.  I also realized that having Health and Brian there was a very good thing, since they were providing insight and offering up suggestions based on their knowledge of James and how he rides his bike...and they would do the same for me.  Alright, enough of the pre-game, now it's time for my results.

The Baseline
The first thing you do in the tunnel is establish your baseline, as this is what all other runs will be compared to in order to determine if any position or equipment changes are "better or worse".  A2 uses a spreadsheet format that is pretty easy to understand.  The easiest thing to do is to look at "aero watts" from the spreadsheet, or the wattage to overcome aerodynamic drag.  Of course this varies with speed so for all cases we had 25 mph plugged in, as this is close to what I would average in a race.  Another note is that all of the runs were done at 0 and 10 degrees yaw.  Basically, yaw describes the idea that airflow doesn't always hit a rider head on (0 degrees) but sometimes at some other angle as determined by the wind.

As seen below, my baseline is 230 and 225 watts at 0 and 10 degrees yaw.  Basically, what this says is that to go 25 mph on my bicycle I would need to generate 230 watts plus any additional wattage required to overcome rolling resistance.  For the sake of this example we'll assume that 40 watts is a good number to assume for rolling resistance, although this depends on what kind of tires you are running.  Adding that 40 to the 230, it should take me around 270 watts to go 25 mph.  Not that the wind tunnel needs any validation (because really the tunnel is validating my previous thoughts), but this matches up pretty good with the power vs. speed chart from my last 3 years of racing.  Here is the data and side/front view from my baseline:




Position and Equipment Changes
After testing the baseline, the guys came out basically saying "yea you've got a lot of room to improve".  I expected this, and it was music to my ears.  It's also kind of obvious from the front view that my helmet is really wide, and I've got this weird lean to the right thing going on.  I'm not sure why...maybe one of my arms is shorter than the other.  One thing to note about the following test sequence is that given the current setup on my Specialized Shiv (stem slammed, Specialized proprietary stem/bars), there is no way for me to drop my front end without changing the stem and base bars.  That being said, the plan was to test a couple of other things first before moving to some new bars that would allow me to get more drop.

Pad Widths, etc
The first thing we did was play with pad widths and extension angles.  We tried moving the pads in 1 cm, 2 cm, and even out 1 cm.  We also tried angling the extensions up just a little bit.  As you can see from the table below, this didn't' change things very much.


pads in 2 cm

pads in 2 cm, extensions rotated up

Bottles
Next, we took off the bottle on my seat tube to quantify its impact, and this resulted in a 6-7 watt savings from the baseline, going from 230/225 down to 224/218.  I repeat, taking the bottle and cage off of my seat tube resulted in a savings of 7 watts!  In other words, I could now go 25 mph off of 7 less watts just by taking 30 seconds to remove a water bottle and cage.  So awesome!  We also verified that at 0 yaw the wind does not see a bottle between the aero bars, and at 10 degrees yaw I was 2 watts better without the bottle.


no more bottle on the seat tube

Helmets
After this, we tested helmets.  It was evident that my current helmet was rather wide when viewed from the front, so I wasn't going to be surprised if I found some savings here.  Given that my head is a weird shape (long from front to back, narrow from ear to ear) I had trouble fitting in most of the helmets that they had there.  The 2 helmets that I tried were the POC Tempor and the Giro Selector.  The POC was Brian's idea because based on watching me on camera he thought that the shape of it fit my contour well.  According to the tunnel guys, Giro usually tests pretty well on most people and the shape of the Selector seemed to fit me well.  As seen below, both helmets tested significantly better than my LG Superleggera, with the Giro getting the slight nod.  It resulted in massive 7-8 watt savings from the previous run!  My aero watts (at 0 and 10 degrees yaw) are now down from 230/225 at the baseline to 216/211...that's a total 14 watts...huge!


Giro Selector front view

Giro Selector side view

POC Tempor front view

POC Tempor side view

Drop
We spent the next 5 minutes or so changing the stem and base bars to an adjustable stem and Vision base bars.  This would allow me to quickly test lower positions.  We tested the front end 1 cm lower and 2 cm lower and while we didn't see any savings at 0 degrees yaw, there was a 4 watt savings at 10 degrees yaw.  I felt like anything more than 2 cm really started pinching my hips so we stopped there.


drop 2 cm front view

drop 2 cm side view

The Final Position
We started playing with extension angle again and went through a run or two with no savings.  At this point, either Heath or Brian had the idea to try me in a full praying mantis position with extensions rotated up as far as they would go...saying that just based on watching me on the computer screen it looked like it "might work out good".  Well...they were right!  This modification resulted in another 5 watt and 2 watt savings at 0 and 10 degrees yaw!  The stats are below, and the final position is #14.





Final Thoughts
In summary, I started out with a baseline of 230 and 225 watts at 0 and 10 degrees yaw and after making a couple of changes (remove seat tube bottle, new helmet, drop front end, rotate extensions) my new position ended up at 211 and 205 watts.  That's a savings of 19 and 20 watts at 25 mph.  Here is the brief summary of the changes.  I've been referring to all savings in terms of watts at 25 mph, but keep in mind that at higher speeds these savings will be even higher.


Anyone that uses power knows how massive this is!  Seeing as how it used to normally take me around 270 watts to go under an hour for 40k, the wind tunnel would suggest that I could now do it at 250 watts.  Or I could go 270 watts, which is my normal Olympic distance power output, and just go a lot faster.  Well, after 2 races with the almost new setup (I've done everything but the 2 cm drop, waiting on bars to come in), I've updated the power vs. speed plot:


You can see the two new green dots that represent the last 2 races.  Even after only two data points, it is clear that the changes have produced a faster setup.  Most notably, the far green dot to the right (273 watts, 26.2 mph) was on the same course that I raced the previous 2 years at 289 watts and 25.7 mph for both years.  I'll do the math for you.  That's 0.5 mph faster off of 16 less watts.  Free speed is awesome!

4 comments:

Mark Florence said...
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Mark Florence said...
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Mark Florence said...

Great article, thanks for sharing. Have you had a chance to ride the new position real world yet and if so have you been able to verify it is faster as per the tunnel testing?

Lynn said...

Hi there, what do you mean by seat post bottle cage? Do you mean the down tube single cage or the double bottle cage that racks off the back of the seat post?