Friday, April 25, 2008

Aero testing helmets - comments

whodesigns:

Hi Fishboy,

Interesting thread, I have done similar stuff in the past and also spent time in wind tunnels. There are a lot of uncontrolled variables in outdoor testing, hence the increased use of wind tunnels.

I have a few questions:

How do you verify the calibration of the Ergomo, in my experience they can be a bit random?

How are you sampling the data?

What is your calculated total error for each run?
Aero testing is very individual. It has been my experience that a helmet may be fast on one person but slower on another.


Fishboy:

Yep Ergomos can be a bit random, and in fact there are studies (at the Saris web site) that confirm this; there is also no verifiable external test you can perform like on an SRM to ensure accuracy.
The BB unit in question is new and was properly installed to spec. It performs accurately in high climb tests predicting speed vs. weight on certain grades.
Every test I've done allows offset calibration to take place after stabilisation of temperature - all the above tests were conducted with the same offset value. So yes, it is possible that the measurement instrument is not as accurate as it could be, but short of heading off to the tunnel, it should be capable of producing some results. In fact this was one reason for trying these tests - could you actually see a difference between certain types of equipment/position without having to resort to a tunnel.

The data is sampled by looking for sections of constant speed and velocity, where the initial and final velocities are the same. Because it is on a velodrome, the data chosen must be more than 400 metres in length to ensure that at least one full lap of the 'drome is completed. This ensures that if there is some slight wind all angles into and away from the wind are covered (all tests so far conducted with no wind). Average power and speed are recorded and plotted.

By total error do you mean TE = Bias + Z*SD? And what value would you recommend for Z?

And yes, aero [i]can [/i]be highly variable. What works for one person might not work for another. I've stated that a few times through this discussion. But in general terms it is still applicable. The fact that the test data shows an improvement of 0.5 to 1.0 kmh at 200 watts by using an aero helmet is an important finding that people without access to their own tunnel and R&D staff would be interested to know. I've achieved that same result 4 or 5 times now over the course of the last year with testing, and results backed up with racing, so I'm pretty comfortable that this is on the money. And if the choice is aero wheels at $2500-3500 or a helmet for $200 that achieves the same benefit, again, I think people would be interested to know.

There is so much crap that goes on with aero testing manufacturers claiming this much time with this and this much time with that. I'm convinced that most manufacturers don't actually have that much idea what is actually going on (some notable exceptions), but the reality is that saying something is 'tunnel tested' is good for biz. For every tiny bit of data I've collected from every wind tunnel test that I've been able to find, the test result has often depended on who was running the test. As Cervelo says: "We're happy to finish second in tunnel tests paid for by someone else."

whodesigns:

This is so true, you know what, the really interesting data never gets published, the best stuff I have seen in the tunnel will never get published.

Don't even get me started on CFD!!!!

By total error I mean the sum of all the errors, often this is bigger than the difference you are trying to measure so it becomes uncertain if you are really seeing a trend or not. Repeat trials can help, as can use of a control.

As I said it gets really hard to control the variables especially outdoors, so apart from the Ergomo, there is frontal area, rolling resistance, ambient conditions etc etc.

Congrats on the time and effort you have spent on this.

2 comments:

  1. Hey just wonderd if youve done any more Aero helmet testing, Im currently using a Bell Meteor 2 with vents in the front.
    Do you think I would see a big improvement by using the Limar Chrono.?

    Thanks

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  2. Depends a bit ... Bell meteor is pretty good ... A lot depends on how the helmet fits onto your shoulders/back - if the Bell is sitting snug onto your back it might be hard for the Limar to trump it. By the way, Torpedo7 have a special on Limar at the moment, less than $100. At that price you can pretty much afford to buy one and see!

    ReplyDelete