Aero Vs standard road bike

on-yer-bike

thecycleclinic wrote:

The change in drag coefficent of an aero frame compared to a round tubes frame is around the same a going from a 36H mavic OP rimmed wheelset to a set of 50mm toroidal rimmed wheels perhaps a bit more.

That much?! I would have thought the wheels would have had a greater effect.

bernithebiker

Ai_1 wrote:

thecycleclinic wrote:

.....In fact the time saving is greater at lower power out puts than larger ones (maths works in funny ways)........

I would dispute this.
Aerodynamic improvements are of vastly more value at higher speeds. While a very slow rider may benefit under certain circumstances, the proportional contribution of improved aerodynamics to the equation will be minimal as compared with a faster moving bike and rider.

There is a calculation that shows that over a fixed distance, the slower rider will save more time as he is out there longer. Weird, but that's just the way it is.

(eg. fast rider does the 20km in 30 mins (down from 31), slow rider does it in 38 (down from 40))

I would say that an aero frame is last on the list, after;

1. Position
2. Aero helmet
3. Aero clothing
4. Aero wheels.

Look at the pro-peloton riding Specialized. Very few Venges. Lots of Tarmacs.....

ai_1

Do you know where this worked example can be found?

bernithebiker

Ai_1 wrote:

Do you know where this worked example can be found?

A link was posted earlier in this thread;

http://www.cervelo.com/en/engineering/t ... iders.html

But that's not the only one, there are several other examples.

neeb

Yes, of course the slower rider will save more time in absolute terms but he will save less time in relative terms (i.e. his percentage time savings due to being aero will be less than for the faster rider).

It's a bit of a silly comparison as no-one wants to save time by going slower to lose more time, and even against other slower riders the time saved by being aero will be less significant than it will be for a faster rider against other fast riders.

ai_1

Thanks for the link, didn't realise it was one of the Cervelo ones from earlier.
Okay I don't dispute that argument. However I think, as neeb mentioned, it's the wrong way to look at it. As I said in my earlier post
"Aerodynamic improvements are of vastly more value at higher speeds. While a very slow rider may benefit under certain circumstances, the proportional contribution of improved aerodynamics to the equation will be minimal as compared with a faster moving bike and rider"
Those "certain circumstances" incidentally would include strong headwinds which may severely increase airspeed and thus the effective distance covered increases the slower your pace making any aerodynamic savings more important.

monkimark

If you're doing 15mph with a 25mph tail wind, does the aero bike make you slower?

bernithebiker

Ai_1 wrote:

Thanks for the link, didn't realise it was one of the Cervelo ones from earlier.
Okay I don't dispute that argument. However I think, as neeb mentioned, it's the wrong way to look at it. As I said in my earlier post
"Aerodynamic improvements are of vastly more value at higher speeds. While a very slow rider may benefit under certain circumstances, the proportional contribution of improved aerodynamics to the equation will be minimal as compared with a faster moving bike and rider"
Those "certain circumstances" incidentally would include strong headwinds which may severely increase airspeed and thus the effective distance covered increases the slower your pace making any aerodynamic savings more important.

We can quantify 'vastly' by saying that aero drag increases with the square of your speed (or velocity, if we're being nitpicky), and that the power required to increase velocity is to the third power (i.e. cubed).

i.e. clearly, the faster you go, the more aerodynamics comes into play; I guess everyone kind of knows that, but the function to the power of 2 shows by just how much.

A good tip for a hilly TT is to put in more power up the hills (lower speed) and less on the flats (higher speed). This way, it can be possible to post a faster time averaging say 280W, than averaging 300W.

ai_1

monkimark wrote:

If you're doing 15mph with a 25mph tail wind, does the aero bike make you slower?

Potentially!

ai_1

bernithebiker wrote:

Ai_1 wrote:

Thanks for the link, didn't realise it was one of the Cervelo ones from earlier.
Okay I don't dispute that argument. However I think, as neeb mentioned, it's the wrong way to look at it. As I said in my earlier post
"Aerodynamic improvements are of vastly more value at higher speeds. While a very slow rider may benefit under certain circumstances, the proportional contribution of improved aerodynamics to the equation will be minimal as compared with a faster moving bike and rider"
Those "certain circumstances" incidentally would include strong headwinds which may severely increase airspeed and thus the effective distance covered increases the slower your pace making any aerodynamic savings more important.

We can quantify 'vastly' by saying that aero drag increases with the square of your speed (or velocity, if we're being nitpicky), and that the power required to increase velocity is to the third power (i.e. cubed).

i.e. clearly, the faster you go, the more aerodynamics comes into play; I guess everyone kind of knows that, but the function to the power of 2 shows by just how much.

A good tip for a hilly TT is to put in more power up the hills (lower speed) and less on the flats (higher speed). This way, it can be possible to post a faster time averaging say 280W, than averaging 300W.

Agreed on both counts. Similarly if doing a solo circuit with some wind it's worth pushing a little harder on the upwind legs even if that means backing off a smidge going downwind.

t4tomo

Ai_1 wrote:

monkimark wrote:

If you're doing 15mph with a 25mph tail wind, does the aero bike make you slower?

Potentially!

yes if you crash because the wind blows you off when it turns intoa cross wind on a corner.

The canyon looks way nicer than either Rose btw in reponse to the OP

pkripper

understand the maths, get an aero bike.

bernithebiker

pkripper wrote:

understand the maths, get an aero bike.

I've just replaced my Sworks Tarmac with......an Sworks Tarmac. Not the Venge.

Even though I get the whole aero thing, I still think the frame is the smallest part of the equation.

I would rather have the frame engineered for best weight / stiffness / handling, than aero. You can't have your cake and eat it.

I often see aero frames with 2 huge bottles and a Di2 battery hung off the downtube. Not sure what all that does to the complex fluid dynamics calculated by the frame maker......

ai_1

bernithebiker wrote:

pkripper wrote:

understand the maths, get an aero bike.

I've just replaced my Sworks Tarmac with......an Sworks Tarmac. Not the Venge.

Even though I get the whole aero thing, I still think the frame is the smallest part of the equation.

I would rather have the frame engineered for best weight / stiffness / handling, than aero. You can't have your cake and eat it.

I often see aero frames with 2 huge bottles and a Di2 battery hung off the downtube. Not sure what all that does to the complex fluid dynamics calculated by the frame maker......

I guarantee you that none of the bike manufacturers are doing really rigorous fluid dynamics analysis. That's not to say it's all nonsense, it's not, but it's simply impossible to do really thorough fluid dynamics analysis on complex problems like a bike frame+rider+ancilliaries with variable rider positions, wind speeds, wind directions, wheel RPM, etc, etc, etc. Even with vast funding, not available to bike manufacturers, this would be a massive undertaking to fully model. Every experiment or calculation is a massive simplification based on lots of assumptions.
I used to have a CFD professor who continually ranted about how any reasonably competent aerodynamicist could paint a pretty picture just as accurate as 99% of the CFD output used by most companies without ever doing a calculation. Wind tunnel testing is very expensive. Basic CFD is cheap, but only good for producing pretty pictures. Good CFD that produces credible data needs a lot of expertise. The answer your computer spits out from a CFD calculation is entirely dependent on how you defined your grid and selected your mathematical model to provide a representative and convergent solution. Unless you have a very good understanding of what you're doing and why, there's far more likelyhood that your solution is nonsense than a good representation of reality.

Incidentally pkripper I'm an aerodynamiscist by training (though I don't work in the field and I'm quite rusty) and I just bought a Canyon CF SL! I'd like an aero bike for some reasons but decided against one for others.

lawrences

bernithebiker wrote:

pkripper wrote:

understand the maths, get an aero bike.

I've just replaced my Sworks Tarmac with......an Sworks Tarmac. Not the Venge.

Even though I get the whole aero thing, I still think the frame is the smallest part of the equation.

I would rather have the frame engineered for best weight / stiffness / handling, than aero. You can't have your cake and eat it.

I often see aero frames with 2 huge bottles and a Di2 battery hung off the downtube. Not sure what all that does to the complex fluid dynamics calculated by the frame maker......

Most of them design the frame around the bottle, Giant propel for example. I do agree with the battery statement though. Now Shimano do a good internal there is no excuse.

cycleclinic

The problem with the reply to my earlier post is the maths do not back up your statement.

An aero frame reduces the drag coefficent and yes that effects the power required to sustain a speed. Yes the power saving climb with speed cubed but that does not the time savings for a given power output over a fixed distance increase with the power supplied by the cyclist. The reverse is true and can only be shown by doing the maths.

P= (0.5*CDA*rho*Vg^2(Vg+Va)+(Mr+Mb)*g*Crr*Cos(theta)*Vg + (Mr+Mb)*g*sin(theta)*Vg)/0.97

I have removed the term for power required to keep the wheels spinning as it around 1% of total power output. The 0.97 term reflect the 3% power loss from the drivetrain.
Vg is ground speed.
Va is air speed
theta is the angle of any slop you are on can be worked out from gradient.
Crr is rolling resistance coeff
CDA is the important bit here as is aero drag coeff x frontal surface area.
rho is the density of air (1.2kg/m^-3)

Now solving for V is a bit difficult as you have (if your set theta and Va to zero) an equation were P is proportional to (V^3+V) but spreadsheets help here.

If you remove the rolling resistance tern to get a rough approximation of the
CDA is 0.31 on the drops

Therefore V = P/(0.5*0.31*1.2)^-3 ^-3 is the cube root by the way.

So if P =250w V= 11.04 m/s-1
If P = 400W (alex doucett for example) v= 12.9 m/s-1

Over a 10 mile TT on the drop no other aero aids time taken for 250W rider is 1458 secs
Over a 10 mile TT on the drops time taken for the 400W rider is 1247secs

Go on the tops and CDA is 0.41 now time taken on the mythical (with no gradient no wind consistant road surface) TT is 1600secs for the 250W rider and for the 400W rider it is 1368 secs.

So time saving for 250W rider from going to the drops is 142 secs and for the 400W rider it is 121 secs so proving my original statement correct. When thinking about physics problems the the maths has to be run or something measured other urban myth results.

If I run the full model including everything like rolling resistnace, power to keep wheels spinning and drive train loss but with zero wind speed and zero gradient.

250W rider on the tops takes 1673secs
400W rider on the tops takes 1419 secs
250W rider on the drops takes 1547 secs thats 126secs saved
400W rider on the drops takes 1310secs thats 109 secs saved.
O.K the % time saved for the higher power rider is a bit more 8.3% vs 7.5%

So the higher power rider in the full model saves less time as a total going from tops to drops. the same is true for any other aero aids. so lets dispell this myth once and for all that aero aids do not make low power riders faster they do by quite a bit. That said we should only worry about it if we are racing or doing TT's, tri's e.t.c. Aero aids for anything else is just for keeping up sake/vanity and nowt wrong with that I suppose to a point.

It is this sort of model that strava uses to calculate average power outputs for segments but without wind speed.
Position and clothing/helmet will make the biggest contribution followed by frame wheels e.t.c. It all adds ups.

on-yer-bike

:shock:

kettrinboy

Ive done some comparisons over several sections of the same loop on successive days on Sportracks ,wind was the same and ave HR within a beat or two , between my 2014 Venge Expert with Shim DA9000 C50,s and my 2006 Scott CR1 Pro with Kysrium SL,s and found the Venge around 0.5 mph faster, that's using the drops on both bikes ,also the Venge is a bit faster into a block headwind by 0.5-1 mph and on fast downhills ive hit 50 mph on a hill where I could,nt get past 45 on the Scott in 5 attempts , but overall averages seem to be pretty close between the two, for the record I am 62 kgs and have an FTP of 240-250w

Bar Shaker

Your wheels are making much more difference than your frame shape. If you swapped wheels, I suspect your Scott would be just as fast.

Cycle Clinic all of that is accepted but you would get a far better aero advantage from a TT hat than any aero frame. Even just putting cling film over your helmet would probably give more advantage.

Aero frames are notoriously thin on the down tube and BB. This gives poor frame dynamics for power delivery. Given that the frame aero advantages are so small, I would always chose a frame with good FEA to ensure optimum power delivery.

mulletmaster

I have done field testing between a Cervelo s5 and a more traiditonal carbon framed bike and found it was about 12w faster at 22mph. Is that 12w going to impact you, then yes buy it, if not then buy it anyway if you like it don't if not. I don't know why people get so worked up on here apart from a few with agendas in selling more "traditional" products shall we say, it's obvious why they think anything that doesn't come with down tube shifters ia new fangled tosh.

DKay

Bar Shaker wrote:

Aero frames are notoriously thin on the down tube and BB. This gives poor frame dynamics for power delivery. Given that the frame aero advantages are so small, I would always chose a frame with good FEA to ensure optimum power delivery.

I personally wouldn't class a Scott Foil, Giant Propel or Felt AR as lacking sufficient stiffness for good power delivery. These days, aero frames are so good, you can have the best of both worlds.