Login required to started new threads
Login required to post replies
I have a question for the engineers/aerodynamics folk among us.
There are a number of websites that claim the aerodynamics don't really come into play on a bicycle until 23 mph or approx 36 kph. Let's be honest, not many AG'ers average that fast in a IM. If this was really so then all the money we tri-geeks have spent on aero gizmos would be a bit of a waste.
Writings by Cobb, Martin, etc. suggest to me that bike aerodynamics comes into play at much lower speeds than this.
What is the thinking on this? Is there actually a theoretical "minimum speed" before which aerodynamics becomes important? Where did this idea of 23 mph come from?
aerodynamics "come into play" at all speeds. It is just that speed improvements are much harder to get at higher speeds so it is easier to improve aerodynamics than the engine.
If one improves aerodynamics at 10 mph one will ride faster. But the previous time was so slow that a smaller percentage increase in speed results in almost the same "number of minutes" saved over a faster (more powerful) person making the same changes for 100 mile course. It is just that this is a smaller percentage of the total time for the slow rider so it becomes less significant. Would you spend $1000 dollars to save 10 minutes off a 10 hour ride or 10 minutes off a 5 hour ride. Or, for the slower person, would that $1000 be better spent hireing a coach or a baby sitter (so they can get more time on the road)?
Frank
--------------
Frank,
An original Ironman and the Inventor of PowerCranks
I'm an engineer who does not specialize in aerodynamics, and I haven't read the articles to which you refer. But, I can tell you that much of engineering comes down to "rules of thumb."
Experience shows us that aerodynamics becomes more important than other factors above certain speeds, and I can remember 13 mph being cited as the point beyond which we fight more wind resistance than rolling resistance. Is this true for everyone, on every ride, on every bike, everywhere? Obviously not. But, it's a rule of thumb that we can use to form generalizations about how bicycles work, and what our biggest obstacles are to riding faster.
I'm wondering if the articles you've read state that the RIDER's aerodynamics (clothing, position, etc.) are more important than the shape of the bike's tubes and/or style of wheels at speeds below 23 mph. And this makes intuitive sense because, as you pointed out, most people don't achieve those speeds. Those who can have probably already addressed things such as clothing and position and now have to focus on the bike itself for improvement.
It's not that the aerodynamics of the bicycle don't matter until until 23 mph; it's probably that there are other obstacles that get in the way first.
The reason why you see the fact that aerodynamics have more importance with faster bikers is becuase aerodynamic drag doe not play a large role in slower moving objects. However as you increase your speed your drag increases at a exponential rate... in other words it increases very quickly at higher speeds. So if you are going 2 mph. and you increase your aerodynamics by 10 % you are making a minimal change becuase the drag is minimal. However in the faster speeds where the drag is very large a 10 % decrease amounts to a very large jump in performance.
This is why you don't hear about runners talking about aerodynamics while triathletes obsess over it.
Having said that... unless you are going 10 mph on your bike it is going to make a difference... is it worth the money it costs... depends on the person.
The figure of 23mph probably comes from empiracal evidence wheer the change in performace was statistically significant for whatever testing protocol they were using...
some of these adages are historical and don't really stand up any more...
My non-scientific rule of thumb is...when I'm going up a hill and drop below 17 mph...I sit straight up, slide back on the seat, and pull against the handlebars. I get better power, better breathing, and change the muscles used to some extent, and I don't feel much wind pushing on me, so I don't think the aerodynamics play much of a role at this speed compared to the increased power output I get from a change in position on the bike.
Of course, if I'm going 17 mph on the flat into a headwind, I try to make like Steve Martin....and get small...
Quid quid latine dictum sit altum videtur
(That which is said in Latin sounds profound)
Well in Athens look for the 100 meter and 200 meter folks in aero suits. They are too hot to go more than 400 meters. But with the top end speed on the sprinters and standing straight up any thing may help.
As far as aero on us slow guys, I also heard the word that around 15-17 mph on a hill air in the lungs is probably more important than a tuck position. Although when you are going up a hill into a 30mph headwind you still need to get small. Aloha G
point taken...
I meant to say... age group runners do not talk about aerodynamic positioning in running.