More powerful, but is it better?
The engineer in me will never be able to stop thinking up new things to try that might help me go faster on a bike. This time of year that sort of "I wonder if..." thinking goes into high gear because I'm spending more time on the trainer.
Some guys I know would rather walk into a burning house than ride indoors. If for nothing else, I like it for the opportunity to experiment.
An experiment I tried recently seems to have really paid off. All the aero testing we've been doing this year has resulted in a radically changed TT position. I've been aware that this newly evolved position has affected power production, so I've been thinking of ways I might get some of that lost wattage back.
The last couple of tweaks seemed to do it - nearly a 10% increase in power.
Running some numbers suggests it may be faster too. If I assume the CdA for this position is no worse than the CdA I started with, I'll still be faster in this new position. Of course only testing will decide whether this new CdA is as good as assumed.
Unfortunately I'm pretty sure I'll have to wait for spring to find out. The mornings this time of year are way too cold for shaved legs and skinsuits.
Ever take air pressure measurements?
Mark,
Since you are a engineer and we both went to the same school, but many years apart I have a question.
I wonder if you have ever thought about using a total air pressure gauge to monitor your actual wind speed? Then you could also back our some other aero data and then compare that to your logged ground speed and power? The more I think about the more I wonder why you don't see more wind tunnel to actual road conditions correlation testing like you do in motorsports? Seems like the bikes are so advanced these days and so light the bike engineers could put a total pressure tap into the aero bars or head tube and log the TT to measure wind effects in a race with minimal weight penalties. I've already applied for a patent on that idea by so don't get any ideas!
James
Aye, but are pressure gauges?
Aye, they bikes are very advanced but are pressure gauges so far advanced?
My engineering skills aren't what they used to be so by all means, check my math here. The way I figure it, a bicycle rider doing about a 1 hour pace for a 40 km TT in full aero mode is pushing against a wind-force somewhere in the neighborhood of 35-40 N. A good aero position will present a frontal area a little under 0.5 m^2.
Putting those numbers together, I get a pressure somewhere near, but slightly less than, 100 N/m^2. (You can tell I'm an engineer and not a scientist because I have no problem whatsoever rounding numbers all over the place.)
OK, last step. Convert N/m^2 to psi because when you do a search for pressure gauges all the spec sheets are in psi. I get about 0.015 psi.
I don't know whether any pressure gauge available today can measure pressures that low. Worse, that pressure is nearer the maximum pressure that we'd need to measure than to the minimum.
Measure in inches of water.
Measure in inches of water. 1psi - 22.5 inH20
Example:
http://www.omega.com/pptst/PX160.html
Also can convert to Pascals if you want to stay in S.I. units. I did that once when looking at a bunch of aero data and it made trends easier to see.
How do you get 40N/0.5M^2 = 100 N/m^2? Round away my friend.
I had a prof who once started every statics problems by drawing free body diagram and stating sum of Forces and sum of Moments equaled zero because the object was not moving. He said if we were scientists we would accept that solution, but the engineers wanted to know the forces of every internal member.
I rounded way up
I rounded up liberally thinking I would show that even if the pressure estimates were inflated (Get it? Inflated. Thanks, I'll be here all week.) by excessive rounding up the results would not be detectable with an ordinary pressure gauge.
Looking at the pressure gauges you recommend, do you think any but a differential gauge would do the job? The 0-6.9 kPa range is far too broad don't you think?
And if we're to use a differential gauge, where do you take your reference pressure?
> And if we're to use a
> And if we're to use a differential gauge, where do you
> take your reference pressure?
No problem. -> Use a "Prandtl probe".
There are sensors available for model air planes that measure speed by a Prandtl probe and a precise differential pressure sensor.
Ref Pressure
You could put an air tap (or the whole sensor) in a water bottle that has the cap on but valve open. That would probably be stable enough to use. Typically automotive or aerospace application look for a steady state pressure that won't change much and are okay if it is slightly above or below the actual ATM conditions. Of course you could get into full blown geek mode and calc. the true altitude adjusted pressure based on each days temp, baro pressure, etc. and then correlate your logged ref. pressure to that.