Aero frames vs. non-aero frames - difference in cdA?

neeb

I know this is a minefield and that everything depends on how components interact together and with the rider, etc, and also that this single factor (aero frame vs. non aero) is a very small one compared to riding position, clothing, etc, but...

Very approximately, on average, how much cdA difference is there likely to be between a modern aero frame (such as the Venge, Noah, Foil, Madone etc) and a non-aero frame (such as the Tarmac, Helium, Addict, Emonda etc)? I'm always seeing aero figures for bike stuff quoted as "aero watts" at a certain speed, but what I want is actual cdA figures.

i.e., if a rider has a total cdA of (say) 0.35 on a non-aero frame, what is it likely to be on an aero one? I'm just looking for an extremely approximate figure for the difference here, +/- 100% even...

Reason being I want to compare for myself frame weight with frame aero-ness in terms of how they might affect speed at a range of gradients, using the tools on the Analytic Cycling website.

frisbee

You can convert from watts to cDA, they are going to have assumed the best case i.e. a completely flat course, so the maths is relatively simple.

bernithebiker

Good question, and no point trying to convert the watts that the adverts tell you to get a CdA. They will each pick their best case scenario and then add 10% BS factor, so you really can't compare.

As far as the A (area) part of the CdA goes, I'd say pretty minimal. There's only so much you can shave off the frontal area of a frame.

It's mostly about the Cd, which is obviously better for aero shapes like ellipse, foil, truncated wedge, etc. than round tubes. But how much is anyone's guess, and as you say, the interactions between different tube shapes is very complicated.

Personally, I put the frame pretty near the bottom of the list when it comes to easy aero gains, especially when you factor in the $$$.

My wild guess would be about a 0.01 or 0.02 improvement, but then do you give up something else in return? (weight, stiffness, ride comfort, etc).

neeb

frisbee wrote:

You can convert from watts to cDA, they are going to have assumed the best case i.e. a completely flat course, so the maths is relatively simple.

Maybe simple for you, but I had to think about it a bit.. ;-)

So, if Power = Force x velocity I can work out cdA for a certain "aero wattage" and speed if I can express cdA as some sort of speed-independent coefficient of Force.. Or is that what it is already? Does Power ("aero wattage") simply equal cdA x velocity? If the figure I'm using for total bike+rider cdA is 0.35, are the units right?

neeb

bernithebiker wrote:

Good question, and no point trying to convert the watts that the adverts tell you to get a CdA. They will each pick their best case scenario and then add 10% BS factor, so you really can't compare.

As far as the A (area) part of the CdA goes, I'd say pretty minimal. There's only so much you can shave off the frontal area of a frame.

It's mostly about the Cd, which is obviously better for aero shapes like ellipse, foil, truncated wedge, etc. than round tubes. But how much is anyone's guess, and as you say, the interactions between different tube shapes is very complicated.

Personally, I put the frame pretty near the bottom of the list when it comes to easy aero gains, especially when you factor in the $$$.

My wild guess would be about a 0.01 or 0.02 improvement, but then do you give up something else in return? (weight, stiffness, ride comfort, etc).

Thanks! I just posted that reply to Frisbee above at the same time..

neeb

It's worth understanding the maths if for no other reason than to be able to call out the manufacturers on the BS..

Just to expand on why I got interested in this - it seems that increasingly these days if you are after a high-end frame you have a choice between stiffness/weight (often marketed as the climber's frame), and a heavier aero frame from the same manufacturer. How to choose? The vague consensus these days is that "aero counts for more than weight" (e.g. the stuff in this week's Cycling Weekly), but I'm not necessarily convinced that the tiny reduction in total drag resulting from an aero frame alone outweighs the weight reduction you get from the climber's frame if you are focused more on hill climbing. For any given manufacturer's climbing and aero frames, there must be a certain gradient at which the tiny weight savings from the climbers frame trump the tiny drag reduction benefits of the aero frame. I wonder what this is. It would be interesting to have this gradient figure for every manufacturer's climber's and aero frame options..

If I knew the genuine cdA reduction of the aero frame over the climber's frame in each case, I could work it out exactly on the Analytic Cycling site..

bernithebiker

Cd is a coefficient and therefore unit-less. And area is m2.

So the units for CdA are m2.

Velocity is m/s.

Drag force is related to air density x CdA x v2

or kg/m3 x m2 x m/s x m/s

which = kgm/s2 which is the unit for Newtons.

Power required is related to density x CdA x v3

or kg/m3 x m2 x m/s x m/s x m/s

which = kgm2/s3 which is watts.

bernithebiker

neeb wrote:

It's worth understanding the maths if for no other reason than to be able to call out the manufacturers on the BS..

Just to expand on why I got interested in this - it seems that increasingly these days if you are after a high-end frame you have a choice between stiffness/weight (often marketed as the climber's frame), and a heavier aero frame from the same manufacturer. How to choose? The vague consensus these days is that "aero counts for more than weight" (e.g. the stuff in this week's Cycling Weekly), but I'm not necessarily convinced that the tiny reduction in total drag resulting from an aero frame alone outweighs the weight reduction you get from the climber's frame if you are focused more on hill climbing. For any given manufacturer's climbing and aero frames, there must be a certain gradient at which the tiny weight savings from the climbers frame trump the tiny drag reduction benefits of the aero frame. I wonder what this is. It would be interesting to have this gradient figure for every manufacturer's climber's and aero frame options..

If I knew the genuine cdA reduction of the aero frame over the climber's frame in each case, I could work it out exactly on the Analytic Cycling site..

I would say it's not just about weight versus aero, but also handling, feel, geometry, stiffness, comfort, etc.

Both weight and aero will be trumped by a better handling bike on a twisty course (especially in the Alps say).

Also, wind tunnel figures are a long way from real world where wind is constantly, subtly changing direction and speed and the rider's body position has all sorts of effects on the frame (especially as it is never in the same place all the time).

Also, what happens when you rock the bike from side to side (as most of us do)? The aerodynamics change significantly here, and yet noone tests for this (as far as I know).

And even if you could get all manufacturers to agree to a 'CdA standard' is this going to be at 0 yaw, or 10' or 15'? Some frames are excellent at 0' but crap at 15'.

Personally I want to enjoy riding my bike as much as possible which is why I'm happy to give up a tiny bit of top speed for the best handling/geometry bike possible, which is why I like my Tarmac so much. I've optimized other (to my mind more important) aero factors like position, clothing, aero bar, wheels, etc.

pblakeney

I can't remember the guys name, but he is an aerospace engineer involved in military drones and has a sideline in carbon wheels. He calls bullshit on aero frame design tests as they are irrelevant to the real world.

neeb

bernithebiker wrote:

I would say it's not just about weight versus aero, but also handling, feel, geometry, stiffness, comfort, etc.

Both weight and aero will be trumped by a better handling bike on a twisty course (especially in the Alps say).

Oh, I completely agree. But I'd assess those things by riding the bike and/or reading reviews / looking at the geometry, and these days it seems that it's possible to make great handling, stiff & comfortable, great feeling bikes in both the lightweight and aero categories. Some bikes will just be better than others, but the trade-off that's currently being marketed by most of the big brands in terms of their race bike options is aero vs. weight, so I just thought it would be interesting to look at that dichotomy from a speed perspective

neeb

bernithebiker wrote:

Cd is a coefficient and therefore unit-less. And area is m2.

So the units for CdA are m2.

Velocity is m/s.

Drag force is related to air density x CdA x v2

or kg/m3 x m2 x m/s x m/s

which = kgm/s2 which is the unit for Newtons.

Power required is related to density x CdA x v3

or kg/m3 x m2 x m/s x m/s x m/s

which = kgm2/s3 which is watts.

Great, thanks.

stueys

I don't think things are that binary anymore. Take the dogma or the R5, both aren't pure aero frames but both apparently stack up pretty well without making any real sacrifices. The out and out aero frames are still there but most manufacturers are ensuring that the 'climbers' frame are aero based, my take is that you aren't giving much up now.

The Rookie

The aero benefits between bikes (leaving the rider out the equation) by a massive margin come from the wheel and tyre setup, the next most important is the fork, as noted the frame itself is relatively uninportnat, even more so if you dont have an aero efficient front wheel and fork putting cleanish air onto the frame (more of which is behind the wheel and fork than above).

Noting the above, I've seen tests on a Cervelo R5 with aero wheels and tyre that put it ahead (bike only, no rider) than more than 50% of the aero bikes they tested (about 10 from memory).

darkhairedlord

The only tangible benefit from an Aero frame is the Aesthetic.

fenix

+1 with DHL.

I've got an aero frame. It looks pretty.

Just look at how much resistance the rider is compared with the bike. The bike is minimal resistance. You'd need to look at your position more than anything.

burnthesheep

I own an aero frame also. I'm an engineer for my work so I simply love the tech and the look.

I went in this order: helmet, kit, frame. The bike as on sale really nicely so figured why not. Non Ebay wheels are still $700 minimum, and that's Yoeleos. You can get an entry level aero bike, complete, for $1500 USD.

Let people buy what they want. The old "aero for pros only" talk gets old after a while. According to some elitists we should all be on endurance framed bikes with Tiagra.

svetty

burnthesheep wrote:

.....Let people buy what they want. The old "aero for pros only" talk gets old after a while. According to some elitists we should all be on endurance framed bikes with Tiagra.

Doing audaxes with full mudguards and with beards, sandals and Carradice bags

pblakeney

Svetty wrote:

burnthesheep wrote:

.....Let people buy what they want. The old "aero for pros only" talk gets old after a while. According to some elitists we should all be on endurance framed bikes with Tiagra.

Doing audaxes with full mudguards and with beards, sandals and Carradice bags

Good advice that...

burnthesheep

Svetty wrote:

burnthesheep wrote:

.....Let people buy what they want. The old "aero for pros only" talk gets old after a while. According to some elitists we should all be on endurance framed bikes with Tiagra.

Doing audaxes with full mudguards and with beards, sandals and Carradice bags

Someone gets it.

I still "buy into" some of the aero wizardry in the real world for even someone as plebeian as myself. I've trained now for about a year, riding and targeted intervals and exercises. Nothing crazy, just to get better.

But I've gotten a few top 10's and top 25's on local flat segments that have about 1000 or 1500 riders on them. And I've seen a few Ironman TT bar riders on the one route for sure. These are short 3 minute to 15 minute flat segments around town.

There is no way, even with a small breeze helping, I'm that quick on my own already. I wear an Evade, typically a "team fit" kit, and have the aero bike (stock 30mm Giant wheels). Given the training with power on the stationary, I know what I can do for 60 minutes, 20 minutes, 5 minutes, 1 minute or 30 seconds.

And the math from that doesn't compute against those flat segments other than some kind of aero wattage advantage at those speeds. Wind be damned, these are days with a forecast wind of less than 10 mph and everyone else also gets that chance on the right day.

What I'm saying is: the wattage I put in from training seems less for the speed I get out.

fenix

Not everyone guns it for a strava segment - if you're doing a focused attempt on a segment then chances are you'll be a lot higher up than a faster bloke out for a longer ride ?

burnthesheep

I'm usually in the 15 to 62 mile range for my rides.

My places on hill segments is more in line with where I would expect them to be. Maybe #250 out of 1500. Not fast, not slow.

I do spend a lot of time in the drops in the flats. I have some leftover upper body from doing the gym before starting to ride. I don't see many rec riders spending a ton of time in the drops.

The Rookie

Fenix wrote:

Just look at how much resistance the rider is compared with the bike. The bike is minimal resistance. You'd need to look at your position more than anything.

True, but don't underestimate the wheels, the Cd of a rotating bike wheel is horrendous, ironically it drops if there is a side wind element compared to head on as the rim and tyre become more of an aerofoil (two of, one front, one rear) in effect. A yaw angle of 10 degrees (for bike alone) can reduce aero drag by over 10%.

fenix

I thought we were talking about aero frames though - not wheels - but I take your point.

The Rookie

We were but the comment was 'bike' not 'frame' so I responded to that.

pblakeney

Fenix wrote:

I thought we were talking about aero frames though - not wheels - but I take your point.

No point in discussing aero frames if it is simply marketing BS.

ben@31

Is aero benefit... if there is any... exponential.

Basically at slower speeds theres very little benefit. As the airflow or drag is less. It's only if you can sustain very high mph that you'll see any aero benefit ?

In my opinion aero frames do look nice. But instead I could train harder / wiser and become fitter. Same as "the climbers bike" that somebody mentioned earlier, it's irrelevant if you're installing a heavy groupset + wheels on the frame and you could lose a few lbs of excess body fat.

bernithebiker

ben@31 wrote:

Is aero benefit... if there is any... exponential.

Basically at slower speeds theres very little benefit. As the airflow or drag is less. It's only if you can sustain very high mph that you'll see any aero benefit ?

In my opinion aero frames do look nice. But instead I could train harder / wiser and become fitter. Same as "the climbers bike" that somebody mentioned earlier, it's irrelevant if you're installing a heavy groupset + wheels on the frame and you could lose a few lbs of excess body fat.

Yes aero benefit is there and Yes, power required is a cube function of speed. So to go from sustaining 40 to 45km/h requires vastly more power than to go from 35 to 40km/h.

Which is why most of the marketing gumph is done at 40+ (sometimes 50km/h) to make it sound as good as possible, but these are unrealistic speeds for most riders.

The Rookie

I would say 35-40kph is sensible for most people doing club events to use for comparisons.

darkhairedlord

surely a "climbers" bike should more aero?

burnthesheep

ben@31 wrote:

Is aero benefit... if there is any... exponential.

Basically at slower speeds theres very little benefit. As the airflow or drag is less. It's only if you can sustain very high mph that you'll see any aero benefit ?

It's weird science that polarizes the masses. Thing is, even if the savings is small for a slow rider, a slow rider spends more time going the same distance. So if a set distance race is the benchmark, both a slow rider and fast rider benefit.....in different ways.

Slow riders save very few watts, but save a lot of actual time on the course. A fast rider doesn't save a ton of time on the course since they do the course 2 to 3x quicker, but save lots of watts.

It's kind of both for most Ironman riders. It's time and watts. It's a timed event so saving time is the ultimate goal. But....you want to save watts also at a given speed for energy for the run.

We're already seeing the technology converge. Aero tech and weight tech split as they initially developed. But now they're blending back together.

I forsee in the future that there will once again be one option for both climbing and aero that is both as light as possible, stiff as possible, and aero as possible.

Then, at that time, there will no longer be such a thing as an "aero" bike. Just a cheap recreation bike and a racing bike.

hambini

I thought I would chirp in here.

The rider contributes about 70% of the drag to the bike and rider combination. If you are going to adjust something, then adjust yourself! Just a few degrees of tilting your back down towards the ground will have more effect than the fastest aero frame.

The wheels will have more effect than an Aero frame because the top of the wheels are moving at double the relative velocity you are travelling at.

So the frame is probably about 10% of the total drag. There is a measurable benefit from going from a "round tube" frame to an Aero frame, the two key things to look out for are a scallop around the back wheel and a dropped down tube.

As a few people have mentioned here, the "rocking" from left to right as you get on the gas reaks havoc on the airflow over a bike. Modelling the airflow for this transient type condition is not easy.