I recall reading an article in the past which explained why aero gear is worth it for riders who avg, say 30 km/h over a 40 km TT, versus a rider who avg 40 km/h.
Does anyone have any good links to study’s that explain this? Or the ability to explain it yourself using examples?
…Or am I wrong and the advantage is actually much greater for the 40km/h rider?
You save more time with aero gear the slower you go up to a certain point.
You save more watts with aero gear the faster you go.
evidence?
This is exactly what I needed, thanks!
Basically the longer you’re out there on the course the more total seconds you save with the same aerodynamics. But the fast dude still goes faster and even though their total seconds saved isn’t as great the overall impact of the aero gear still helps their times tremendously.
As Jack suggested play around with the calculators over at: http://www.analyticcycling.com/ForcesSpeed_Page.html but the key is to total up the time it takes to ride a fixed length course with a higher and lower CdA. The slow dude still goes slower but saves more total seconds or minutes.
For instance set everything else equal(sea level, 74kg rider, 0 slope, .004 Crr) and use the calculators to show how much time a rider averaging 300 watts takes to cover a 40K course with a CdA of .300 vs. .250 and then how much time a rider averaging 150 watts takes with the same .300 vs. .250 CdA.
300 watt rider:
.300 CdA ~58.9 minutes
.250 CdA ~55.6 minutes
Time savings ~ 3.3 minutes
150 watt rider:
.300 CdA ~76 minutes
.250 CdA ~71.8 minutes
Time savings ~ 4.2 minutes
The fast rider always goes faster but the slower rider saves more minutes with the same drop in CdA
-Dave
I recall reading an article in the past which explained why aero gear is worth it for riders who avg, say 30 km/h over a 40 km TT, versus a rider who avg 40 km/h.
Does anyone have any good links to study’s that explain this? Or the ability to explain it yourself using examples?
…Or am I wrong and the advantage is actually much greater for the 40km/h rider?
were you thinking of this?
http://www.cervelo.com/en/engineering/thinking-and-processes/slow-vs-fast-riders.html
nice article from Cervelo
So, with the extensive physics that my botany undergrad taught me -
Aren’t there two types of air flow? Laminar (some drag) and turbulent (heavy drag)? Turbulent occurs more frequently at high speeds, right?
So, do these assumptions of the time saved at 30 kph adjust for the smaller turbulent drag at lower speed? Is that represented by the exponential curve for wind resistance? Does aero equipment work to control laminar air flow from becoming turbulent?
Hmm. Now that I’m thinking of it, maybe that’s why one poster said that you save more time ‘to a point’. After that point, the air flow is so smooth that TT equipment doesn’t do anything.
Sorry to spout of a series of inane questions. Your question got the old noodles thinking.
So, with the extensive physics that my botany undergrad taught me -
Aren’t there two types of air flow? Laminar (some drag) and turbulent (heavy drag)? Turbulent occurs more frequently at high speeds, right?
So, do these assumptions of the time saved at 30 kph adjust for the smaller turbulent drag at lower speed? Is that represented by the exponential curve for wind resistance? Does aero equipment work to control laminar air flow from becoming turbulent?
Hmm. Now that I’m thinking of it, maybe that’s why one poster said that you save more time ‘to a point’. After that point, the air flow is so smooth that TT equipment doesn’t do anything.
Sorry to spout of a series of inane questions. Your question got the old noodles thinking.
You’re sort of on the right track.
As cyclists we travel through largely laminar air flow. But our bodies, our bikes, parts of our bikes like the leading edges of wheels, handlebar and tubes and even things like exposed brake housings and cables disturb that laminar flow and create turbulence on their trailing edges. It’s the pressure differential between that laminar flow striking the leading edges and the lower pressure turbulent wake behind the trailing edges along with some viscous skin drag of the air flowing around each object that creates air resistance.
Depending on the dimensions and shape of each of those items the turbulent region stretches out like an eddy or wake behind the trailing edge and eventually the flow ‘reconnects’ and becomes largely laminar again. The cross sectional area or actually three dimensional volume of that turbulent region dictates how low that trailing edge pressure gets and to a large extent how large the overall pressure differential or IOW the large part of the aero drag. Airfoils and other oblong high aspect ratio shapes help minimize the trailing edge turbulence which minimizes the pressure differentials and of course the aero drag.
So yeah, laminar vs turbulent flow is a big part of the aero puzzle. But at the speeds we travel the surrounding air is largely in a laminar flow region for all of us, fast or slow and all of us generate trailing edge turbulence with our bodies, bikes and individual parts of the bikes. That doesn’t really change for fast vs. slow riders but in certain situations, particularly crosswind situations there are differences related to the effective angle that the apparent wind (vector sum of our velocity relative to the ground and ambient wind velocity) strikes us or the yaw angle. For the same partial or full crosswind faster riders will experience lower yaw angles or more direct effective headwinds and slower riders will experience greater yaw angles (more of a crosswind). That impacts aerodynamics as components like deep section wheels can be designed to perform better in pure headwinds or better in high yaw angle situations.
So it’s not like fast riders get less relative advantage from an aero position, they get plenty of advantage relative to their speed. It’s just that slower riders get more of an absolute time savings because they’re out there for so much longer. In the example given above both riders saved roughly 5.5% from their slower time with the decreased CdA, it’s just that 5.5% of the larger number (the slower rider’s time at higher CdA) is more total seconds or more absolute time.
-Dave
typo…
Or in relative terms:
300 watt rider: Time savings ~ 3.3 minutes = 5,602%
150 watt rider: Time savings ~ 4.2 minutes = 5,552%
So the improvement is about the same.
…
300 watt rider:
.300 CdA ~58.9 minutes
.250 CdA ~55.6 minutes
Time savings ~ 3.3 minutes
150 watt rider:
.300 CdA ~76 minutes
.250 CdA ~71.8 minutes
Time savings ~ 4.2 minutes
…
Or in relative terms:
300 watt rider: Time savings ~ 3.3 minutes = 5,602%
150 watt rider: Time savings ~ 4.2 minutes = 5,552%
So the improvement is about the same.
…
Exactly, the relative improvement is about the same but in absolute terms slower folks save more time which can be counter intuitive at first glance.
On a bike your Reynolds number is > 10,000 so the flow is basically always turbulent (http://link.springer.com/chapter/10.1007%2F978-2-287-99054-0_40?LI=true). For turbulent flows, power scales with velocity cubed. So, assuming all the opposing forces are aero (which becomes a better assumption the faster you go), a 20% increase in speed with the same drag needs (1.2)^3 - 1 = 72.8% more power.
http://en.wikipedia.org/wiki/Drag_(physics)
So, put it this way. Getting from 20 mph to 24 mph requires either 73% more power on one extreme (a huge increase) or a 25% reduction in drag through better aerodynamics on the other extreme. So if your FTP is currently 200 watts and you’re at 20 mph you would need to get to ~350 watts. That’s pretty hard to do. Or just get a bit more aero and then the power increase is more realistic.
Just having a Re > 10,000 does not mean you have turbulent flow. Look at gliders with 15m wingspans and Re ~150,000 that get full laminar flow. The surface roughness and aero shaping is also a very important consideration.
Also, all flows (not just turbulent) would have P ~ V^3, since P=DV=(.5rV^2*CdA)*V. Granted the CdA will change based on your skin friction.
ok, but bikes are not gliders; speeds are less, geometry is not smooth, the flow field is nonuniform (outside). it’s been a few years since i took transport but by definition laminar and turbulent flow don’t have the same power/velocity relationship (not that it matters for bikes).
“At very low Reynolds numbers, without flow separation, the drag force 
is proportional to 
instead of 
”
Every week someone new arrives and thinks the bike industry didn’t hire anyone who took Aero 101 in school.
ok, but bikes are not gliders; speeds are less, geometry is not smooth, the flow field is nonuniform (outside). it’s been a few years since i took transport but by definition laminar and turbulent flow don’t have the same power/velocity relationship (not that it matters for bikes).
“At very low Reynolds numbers, without flow separation, the drag force is proportional to instead of ”
Thats OK soon the road season will be in full swing and we can have the ST experts telling everyone how Garmin, Sky etc don’t know anything about equipment choice.
But they don’t!.. Do they? ;0)
Here is a good site that has all the math and sample spreadsheet to answer aero questions: http://tri-it-and-you-may-like-it.webs.com/bikeaerodynamics.htm
.
You save more time with aero gear the slower you go up to a certain point.
On a course of fixed distance. Which is most courses, of course, but it’s an important caveat. If it were a race of fixed time (hour record), the slow rider gains no relative bonus.
Every week someone new arrives and thinks the bike industry didn’t hire anyone who took Aero 101 in school.
Based on some past posts from Gerard V., I think he’d agree with those new people, for much of the bike industry.