Aerodynamics question

FSR_XC

Just been reading the article Chris Boardman did for the Triathlon Plus preview edition.

He talks about the importance of aerodynamics for triathlons and TT's, giving guidelines to how much power and time is saved with different aero equipment.

I was wondering what sort of speed you need to be doing for aerodynamics to make a real difference?

dennisn

Not to sound stupid but "the faster the better". It would seem that the general
consensus is that somewhere around 20 MPH is where areo things REALLY start to work.
At least that's the "conventional wisdom" for now(as I understand it).

Dennis Noward

jimycooper

in all the tests in procycling and the like i've read, its been based on 40kph.

so you better be quick to have a decent effect...

Steve Kish

I once read in an article for Shimano Dura Ace AX that at 50kph (31mph), over 80% of your effort goes towards overcoming wind resistance. :shock:

Jeff Jones

Most people don't realise this but the slower you are, the more time you'll save through aero improvements. It's the same percentage (of overall drag) but of a larger number (total time)

robbarker

Jeff Jones wrote:

It's the same percentage (of overall drag)

Aerodynamic resistance increases as a function of speed squared. A fit athlete riding at 28mph will, therefore, experience roughly twice the drag of someone like me going at 20 mph. Hence the faster rider will get a significantly greater percentage improvement in their time by becoming more aerodynamically efficient.

lateralus

it's actually a function of speed cubed.

So double the speed means 8 times the air resistance.

cougie

jimycooper wrote:

in all the tests in procycling and the like i've read, its been based on 40kph.

so you better be quick to have a decent effect...

Isnt that because thats ball park figure for speed for decent amateurs who are probably the main target area.

jimycooper

your probably right, remember the spesh tests posted about about a month ago on bikeradar, i seem to recall that was done at 40kph

Jeff Jones

I knew I shouldn't have posted in such a hurry :-) I'll explain:

Take a rider who has a coefficient of drag x frontal area (CdA) of 0.25. At 200W, that rider will go at 10.34m/s (using the default values on analyticcycling.com) and take 967 seconds to complete 10km. At 300W, that same rider will go at 12m/s, taking 833 seconds to complete 10km.

Now, say the rider reduces their CdA to a super slippery 0.20. At 200W, they will now travel at 11.09m/s, taking 902 seconds for the 10km. At 300W, they will go at 12.88m/s, taking 776 seconds for 10km.

Let's take the fast case (300W). The rider will save 833-776 = 57 seconds by lowering their CdA from 0.25 to 0.20.

In the slow case (200W), the rider will save 967-902 = 65 seconds by lowering their CdA by the same amount.

That's what I meant about the percentage of drag savings being the same. It's a 20% saving in both cases. You save more time by going slower, even though your speed differential will be lower (1.66m/s going from 0.25 to 0.20 CdA at 200W compared to 1.79m/s at 300W).

STEFANOS4784

Oh, i see........ :? :roll: :arrow:

hamstrich

lateralus wrote:

it's actually a function of speed cubed.

So double the speed means 8 times the air resistance.

I think that robbarker's actually right - drag forces are proportional to speed squared, but since power = force x velocity, the power required to maintain that speed will increase with speed cubed.

scherrit

obviously as a fat (masters) sprinter I find aerodynamics pretty frustrating.....

I get to ride at 60km/h (for a little while only!!) AND I punch a big fat hole in the air for other sprinters to follow... not good, so I am embarking on a) a serious weight loss programme and b) a aero improvements programme!

I'll be trying lower bars, narrower bars and possibly an aero helmet this season.
Cheers,
Scherrit.

lateralus

hamstrich wrote:

I think that robbarker's actually right - drag forces are proportional to speed squared, but since power = force x velocity, the power required to maintain that speed will increase with speed cubed.

oops, have checked and you are quite right. :oops: Apologies robbarker!

robbarker

I didn't think my memory was that bad :-)

robbarker

Jeff Jones wrote:

That's what I meant about the percentage of drag savings being the same.

That's not the same thing as saying the same percentage of overall drag :-)

What your data (which I assume is correct) shows is that a significant increase in power only results in a small increase in speed. This becomes more of a problem as speed increases, since the power required to overcome aerodynamic drag increases as a function of speed ^ 3. Our 28 mph cyclist needs roughly three times the power output of our 20 mph cyclist to maintain the speed, which is why I don't tend to ride at 28 mph very much.

Incidentally, I make the percentage time savings 6.72% and 6.84% for the 200w and 300w riders respectively for the same reduction in drag, but only a small difference in speed. As the riders' speed increases, the maths tells us that this gap will widen.

To answer the OPs question, we can see from the data Jeff has posted, there's not a massive difference at 20-25 mph, but it's maybe enough even at 20 mph to be the difference in winning or losing a club TT, all other things being equal.

Being slippery is only part of the story of course, it's no good getting into a flat-backed super-aero position on the bike if you can't then breathe or get any power down!

FSR_XC

Sorry haven't posted again, but following the comments and learning.

likewoah

So what is the typical CdA of a mountain bike and rider?