Aerodynamics

Discuss light weight issues concerning road bikes & parts.
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elviento
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by elviento

A lot of manufacturers are making aero bikes, but they all seem to use very different aero shapes... Any thoughts are which are better, and why?

Here are some examples...
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CasperG
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by CasperG

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This should explain a lot. :wink:

by Weenie


gumgardner
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by gumgardner

Nice post :thumbup:

NGMN
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by NGMN

Trek has a section on airfoil drag around page 22-24 here:
http://www.slowtwitch.com/Downloads/TK1 ... _lores.pdf

It's important to remember that manufacturers choose tubes shapes based on many things like:
1. yaw angle they feel is most important; i.e. Cervelo uses "sharper" airfoils in belief that lower yaw angles are more representative of real world conditions; Trek and Specialized tune their bikes more to ~10-12 degrees of yaw
2. location on bike
3. stiffness/weight/aerodynamic tradeoffs

Casper's chart only tells us a small part of the story, like is that only at 0 degrees of yaw, or and average? I would wager it's only 0 degrees of yaw.

Anyway, if you are trying to design a bike, hire someone to do some CFD and look at companies who know their stuff.

If you are shopping and want to be educated, I would say for time trial bikes there are some good parameters to help guide a purchaser:
1. down tube and seat tube under 30mm wide and over 75mm deep for the majority of their length
2. headtube under 40mm for the bulk of it's length and at least 3:1 depth with blending to downtube and top tube
3. non round, "aero" seatstays and fork

Martin1977
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by Martin1977

Unfortunately, aerodynamicsis not just about tubes shape, but about overall geometry of the bike and components. I can remmember that I've read aerodymamics study of tubes shape and frame geometry from John Cobb and result was, that simply by looking at any bike, you are not able to say, if aerodynamics will be good or bad. You have to perform measurements. There are too many various aspects, that can influence aerodynamics of the frame.
One example was additional bottle on down tube. Surprisingly, if you put one bottle on downtube on standard geometry road bike, aerodymamics is better than without it. Aerodymamics of downtube itself gets worse by adding that water bottle, but airflow around seat tube gets smoother, so overall aerodynamics is better.

User Name
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by User Name

In this aero test done by German Tour Magazine, a drag reduction of 17w at 45km/h was achieved when the rider switched from a 'standard' road bike with aero bars and 2 tri-spokes, to a Cervelo TT bike with aero bars (obviously) and the same tri-spoke wheels. I dunno if you guys reckon that's a lot for a frame -- I guess it is

Required output in watts to sustain 45kph

Stevens San Remo road bike with hands on hoods: 465 Watts
Same bike, hands down on the drops: 406 watts
Same bike, but with Easton Aeroforce aero bars: 369 Watts
Same bike, triathlon position (5.5 cm lower bar, saddle forwards): 360 Watts
Same as above, with 2 carbon Tri-spoke wheels: 345 Watts
Cervelo TT bike + 2 Tri-spoke wheels: 328 Watts
Cervelo TT bike + Tri spoke front + disk rear wheel : 320w
Same as above, but with Giro aero helmet: 317w
Same as above, with speed suit: 307w

It's in German:
http://www.dk-content.de/tour/pdf-archi ... l_0107.pdf
Last edited by User Name on Sat Jul 30, 2011 5:16 pm, edited 2 times in total.

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DMF
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by DMF

Martin1977 wrote:Unfortunately, aerodynamicsis not just about tubes shape, but about overall geometry of the bike and components. I can remmember that I've read aerodymamics study of tubes shape and frame geometry from John Cobb and result was, that simply by looking at any bike, you are not able to say, if aerodynamics will be good or bad. You have to perform measurements. There are too many various aspects, that can influence aerodynamics of the frame.
One example was additional bottle on down tube. Surprisingly, if you put one bottle on downtube on standard geometry road bike, aerodymamics is better than without it. Aerodymamics of downtube itself gets worse by adding that water bottle, but airflow around seat tube gets smoother, so overall aerodynamics is better.



So which is actually better, in overall, bottle on downtube or seattube (standard geometry road bike)? And also, does anyone have any numbers on drag (watt or grams) on either or both, compared to no bottle?

Also, aero bottle on standard geometry road bike - is it actually better than a normal bottle and by what degree (glad if anyone would care to put a number on that)?

NGMN
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by NGMN

Whether bottle on downtube or seattube is better often depends on the rider and the bike. No one can tell you. A general rule of thumb for a standard round bottle is an additional 50-75 grams of drag.

Martin's story makes me think of another John Cobb story that is on point. John took one of the old Easton "aero" downtubes in to the wind tunnel. If you've ever seen this tube, the trailing edge is blunter than many we see today. John tested it faced forwards and backwards. Faced forwards the tube was faster at 0 degrees yaw and 5 degrees by a little bit but by 10 degrees the backwards tube was leading and at 15 degrees the backwards tube was generating negative drag.

Knowing what we know now, I'd say that it'd be a fair guess to say that the "backwards" Easton tube made a pretty good leading edge like Trek's truncated airfoils.

This actually further demonstrates how the above graph tells only one story. 0 degree aero performance is different that performance over a large range of angles.

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DMF
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by DMF

NGMN wrote:Whether bottle on downtube or seattube is better often depends on the rider and the bike. No one can tell you. A general rule of thumb for a standard round bottle is an additional 50-75 grams of drag.


Yes sure, on it's own. But I've read plenty of times online and in magazines that there is actually more drag from the total bike without a bottle, or atleast it is when the bottle is mounted on the seattube... or is there no truth to that? How do you figure that the rider has any serious impact on which of the mounts are better to use? I would suspect that the difference between the mounting points is way bigger than impact of different q-factor, leg size and cadence?

So if a bottle actually reduces total drag, then maybe it wouldn't be too far fetched thinking that an aero bottle could actually be less effective than a concentional bottle on a standard geometry road bike where the aero bottle usually doesn't sit very snug between both the seattube and downtube. Ofcourse that's just speculation on my part - so I was curious if anyone had numbers on this - is it worth the hazzle (and extra weight) going for an aero bottle with a standard frame?

And still, in that case - which mount should be used?

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CharlesM
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by CharlesM

...


:lol:

Dalai
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by Dalai

An old study in regards to various bottle locations on an early 2000's Quintana Roo Ti-phoon bike - http://www.slowtwitch.com/mainheadings/ ... ttles.html

Photo is a 2003 model.

Image

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DMF
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by DMF

Thank you! Interesting and useful article!

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Kayrehn
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by Kayrehn

Martin1977 wrote:One example was additional bottle on down tube. Surprisingly, if you put one bottle on downtube on standard geometry road bike, aerodymamics is better than without it. Aerodymamics of downtube itself gets worse by adding that water bottle, but airflow around seat tube gets smoother, so overall aerodynamics is better.


hm... Thought it was the seat tube? http://www.bicycling.com/bikes-gear/bike-and-gear-features/revenge-nerds

Martin1977
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by Martin1977

Kayrehn wrote:
Martin1977 wrote:One example was additional bottle on down tube. Surprisingly, if you put one bottle on downtube on standard geometry road bike, aerodymamics is better than without it. Aerodymamics of downtube itself gets worse by adding that water bottle, but airflow around seat tube gets smoother, so overall aerodynamics is better.


hm... Thought it was the seat tube? http://www.bicycling.com/bikes-gear/bike-and-gear-features/revenge-nerds


So we have to perform measurement :mrgreen:
I can remmember, that John Cobb porformed wind tunel measurements and result was, that it is better to have bottle on downtube, than even without any bottle at all. I'll try to find that article again.

edit: now I can see, that that QR article is also from John Cobb and according to that, downtube is better placement.

by Weenie


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elviento
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by elviento

I never cease to be amazed by the depth of knowledge and resources on ww... Thanks~

I think this is the BEGINNING of great discussions. Aerodynamics probaby make more difference to riding than weight but can be considered less because weight is so much easier to measure.

So in that sense, the UCI limit may have some positive impact on bike design (I am willing to bet if UCI drops the limit to, say 6kg, then the manufacturers will be back to the super light race).

I want to throw out a few topics for your consideration:

1. Brake design -- will it have to be changed? Maybe hydraulic brakes should be cosisdered? What about water bottle design? something more manageable than the Cervelo version but still provide significant aero improvement?

2. which of the modern aero road bikes are most aero (S5)?

3. The Trek article says Trek has applied for patent over Kamm Tail design on all bikes, but Scott already has the F01. So how does that work? Infringement?

4. Is there any article showing the conversion of weight savings (say 200g) to wattage savings in various riding conditions (flat, climbing, etc.). I wonder how the results stack up against the aero savings chart...

The bike has improved greatly over the past 100+ years with the past 15 years being most dramatic, yet I still see a lot to be improved in the future.

Of course, this will not affect the fact that we could all lose that gut. Or do an ear job like Pantani... maybe not...
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