Aero frames vs. non-aero frames - difference in cdA?
Comments
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That's a very interesting (and in retrospect obvious) explanation that I'd either not heard before or had forgotten.hambini wrote: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 when the wheels are at the top, any aerofoil-type cross-section is actually orientated at 90 degrees to the horizontal.. Is it actually this that (partially?) explains the supposedly reduced drag of more u-shaped (rather than v-shaped) modern rims? The conventional explanation (generally accompanied by a diagram showing airflow over the front rather than the top of the wheel) is that it discourages air-flow separation when the net air flow isn't coming from straight ahead (yaw etc).
And of course it's also a neat explanation of the advantage of deeper rims, assuming these rims are u-shaped in section. If you imagine a 60mm u-shaped rim with air flowing over the top part of the wheels, there is effectively a continuous surface from the front to the back of the wheel in the top 60cm of the wheel's diameter.
This is probably very basic cycling aerodynamics, but I'd never thought of it this way before. I blame the marketing BS and the less than scientific focus in most of the cycling press!0 -
neeb wrote:
That's a very interesting (and in retrospect obvious) explanation that I'd either not heard before or had forgotten.hambini wrote: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 when the wheels are at the top, any aerofoil-type cross-section is actually orientated at 90 degrees to the horizontal.. Is it actually this that (partially?) explains the supposedly reduced drag of more u-shaped (rather than v-shaped) modern rims? The conventional explanation (generally accompanied by a diagram showing airflow over the front rather than the top of the wheel) is that it discourages air-flow separation when the net air flow isn't coming from straight ahead (yaw etc).
And of course it's also a neat explanation of the advantage of deeper rims, assuming these rims are u-shaped in section. If you imagine a 60mm u-shaped rim with air flowing over the top part of the wheels, there is effectively a continuous surface from the front to the back of the wheel in the top 60cm of the wheel's diameter.
This is probably very basic cycling aerodynamics, but I'd never thought of it this way before. I blame the marketing BS and the less than scientific focus in most of the cycling press!
The shape of the aerofoil has little effect in comparison to how deep the dish is. Aerodynamically speaking the "deepness" is effectively the cord length of the aerofoil. The longer the cord length is in comparison to the thickness, the faster the shape is as it is much more resistant to detachment at a wider variety of Reynolds numbers (This is a parameter that defines turbulence, laminar flow and all that good stuff. For simplicity, golf balls are dimpled because of Reynolds number!)
If you watch this and skip through to the CFD bit, you'll see the difference in aerofoil sections and the effect of depth
https://youtu.be/BUih_emc54M
Hope that helps
Hambini0 -
Really interesting video, thanks!hambini wrote:The shape of the aerofoil has little effect in comparison to how deep the dish is. Aerodynamically speaking the "deepness" is effectively the cord length of the aerofoil. The longer the cord length is in comparison to the thickness, the faster the shape is as it is much more resistant to detachment at a wider variety of Reynolds numbers (This is a parameter that defines turbulence, laminar flow and all that good stuff. For simplicity, golf balls are dimpled because of Reynolds number!)
If you watch this and skip through to the CFD bit, you'll see the difference in aerofoil sections and the effect of depth
https://youtu.be/BUih_emc54M
Hope that helps
Hambini
- You showed the effect that rim depth has convincingly, and how it's much more important than rim profile / aerofoil shape. But in your CAD demonstrations were you always assuming airflow from the front (no yaw)? Would the aerofoil shape not still become more important at greater yaw angles, as the manufacturers claim? Or is this just such a minor factor compared to rim depth (even at high yaw angles) that it effectively doesn't matter?
- One thing you showed in your CAD was that how with a standard aerofoil you get vortices at the back of the wheel associated with the aerofoil shape being reversed compared with the front. Might not a u-shaped profile mitigate that, as the shape when reversed would be much more similar to the non-reversed shape? Or perhaps this is true, but the advantages of the non-reversed shape up front trumps this?
- You mentioned the advantages of having the tyre narrower than the rim - how significant is this relatively speaking, compared to rim depth? Would this be the main reason to get a modern wider/more-u-shaped rim, the fact that it makes it possible to fit a tyre that's narrower than the rim? (given that no one these days uses 21mm tyres).
- Great to know that the dimples are marketing BS.. That's one factor I can now discount completely when weight up pros and cons of new wheel options.. ;-)0 -
If aero frames were useless TT bikes would all have round tubes and quoting power saved is a meaningless stat. Quoting CdA valves is not but the difference would be so small that many would think there is no difference at all however small differences in CdA are signicant in a time trial. Converting to time saved is more useful for people and as pointed out above time saved for slower riders is greater. Aero frames work they make a difference but by themsleves the difference is small but they are supposed to be use din combination with aero wheels obeying the 105% rule, well fitted clothing, aero helmet and an optimised position. Make all the changes and you will go a fair bit quicker. Do one and you playing at the margins.
I could quantify this and have done in the past with numbers but I have also found it does not add much the debate as many dont seem able to put the numbers into context as numbers without real world context are well just numbers.http://www.thecycleclinic.co.uk -wheel building and other stuff.0 -
To answer your questions.
1. The wheel manufacturers are aiming to delay detachment of the airflow across the rim. By making the rim deeper, they effectively make the thickness to length ratio more favourable. If you crank in some yaw angle then you start to get detachment. The difference between a U profile and a V profile (in the real world) will be small, they will all start to detach at roughly the same angle, therefore the yaw will have little effect. If you are interested, the NACA profile (Mavic) has publicly published yaw (alpha) angle v drag ratio - the relevant chart is cd v alpha
http://airfoiltools.com/airfoil/details ... aca0024-il
Using a shallower section wheel will cause detachment at roughly similar angles (it will be slightly more tolerant) but the overall drag will be less because the detachment occurs over a smaller area (hence less drag).
2. The vortices are caused by the abrupt shape change which is the tyre, so you can't do much about this. The rounded edge is favourable to oncoming air but unfavourable when air leaves the object
3. I would say the changing of the tyres to suit your rims is a more cost effective option than spending your money to try and get your wheels to fit your tyres. If you are going at speed and not in a peloton, IMO it's more efficient to have 23mm tyres.
Hambini0 -
Great, thanks!hambini wrote:To answer your questions.
Hambini
Of course there are other reasons to have a wider rim - potentially greater wheel stiffness and the fact that (for similar drag?) you can run a wider tyre at a lower pressure.
Which brings me to a subject I'd be really grateful to have you opinion on! (fully aware that I'm hijacking my own thread by talking about wheels rather than frames..
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I'm thinking of getting some new wheels built up on hubs I already have (including a Powertap rear).
Current wheels are old-style Reynolds 46 and I'm using 22mm Schwalbe tubulars. These rims are 46mm deep with a V profile. Width at the brake track seems to be a smidgen under 21mm while the actual width of the tyres is a smidgen over 22mm. So the tyres are about 1.5mm wider than the rims. The wheels are very light, doubtless helped by the narrow width.
It'd be pretty difficult to find tyres that would be /narrower/ than these rims, and I'm not sure I'd want to run even 21mm tubs on a regular basis. Also I've heard lots of good things (from non-marketing sources) about the better stiffness and handling of wider rims. And I'm a big fan of the Schwalbe ones, and these only come in 22mm, 24mm and 26mm.
I'm looking at two options (both tubulars again). One is a 45mm deep, wide & toroidal section rim that's 26.4mm wide at the brake track and 27.5mm at the widest point (very similar to a Zipp 303 FC). If I went for this I'd run Schwalbe 24mm tubs (probably around 24.25mm wide in practice).
Other option is a 38mm deep, slighty-less-wide-but-still-wide, u-profile rim (possibly slightly toroidal) that's 24mm wide at the brake track. If I got these I'd use them with the 22mm Schwalbe tubs.
So in both cases the tyre would be a bit narrower than the rim. Presumably the 45mm wheels would be more aero because of the extra 7mm of depth. But: the total weight of the 38mm wheels would be much lighter. I reckon almost 200g lighter once the extra weight of both the rims and the slightly wider tyres is factored in. Now, I know that aero trumps weight in most situations, but I'm a 63kg inveterate weight-weenie, and for reasons that I'd rather we didn't go into here (call it an irrational addiction if you like), it would make me very happy to save that 200g of rotating mass... There's also the advantage of less susceptibility to cross winds of course.
What I'd like to know is roughly how much aero benefit I'd gain from the 45mm rims over the 38mm, so that I can work out (in my own sick and distorted weight-weenie calculus.. ;-)) whether it's worth the extra 200g and slightly greater cross-wind susceptibility... I can average a decent speed on solo rides so I would be reaping some advantage from reducing drag (and before anyone asks, I don't have much body mass to lose, I squeeze into tight-fitting Castelli size S and my position isn't too bad).0 -
Don't take my opinion as gospel. I'm just an idiot who decided to put some CFD on the net.
The difference between 38mm and 45mm is not going to be much but nonetheless there is still a difference. Unless you are going to go up hills where the 200g will make a tad of difference, the 45mm will be faster in every scenario EXCEPT when you are accelerating.0 -
Don't worry I don't take anyone's opinion as gospel, but you clearly know more about drag than I do and have access to a few tools, so your opinion is interesting..hambini wrote:Don't take my opinion as gospel. I'm just an idiot who decided to put some CFD on the net.
The difference between 38mm and 45mm is not going to be much but nonetheless there is still a difference. Unless you are going to go up hills where the 200g will make a tad of difference, the 45mm will be faster in every scenario EXCEPT when you are accelerating.
That's the thing though, the accelerating - I'm a zippy kind of guy and just love the responsive feel of a light bike and light wheels (I refuse to believe it's not detectable), and probably the only competitive riding I'll be doing this year is hill climbing. So if I'll get some significant aero advantage from 38mm rims and (proportionately) only a little bit more from 45s I'll take the 200g any day for everyday summer wheels. Then if I want proper aero for flat TTs I'll get some 60mm wheels at a later date.0 -
neeb wrote:
Don't worry I don't take anyone's opinion as gospel, but you clearly know more about drag than I do and have access to a few tools, so your opinion is interesting..hambini wrote:Don't take my opinion as gospel. I'm just an idiot who decided to put some CFD on the net.
The difference between 38mm and 45mm is not going to be much but nonetheless there is still a difference. Unless you are going to go up hills where the 200g will make a tad of difference, the 45mm will be faster in every scenario EXCEPT when you are accelerating.
That's the thing though, the accelerating - I'm a zippy kind of guy and just love the responsive feel of a light bike and light wheels (I refuse to believe it's not detectable), and probably the only competitive riding I'll be doing this year is hill climbing. So if I'll get some significant aero advantage from 38mm rims and (proportionately) only a little bit more from 45s I'll take the 200g any day for everyday summer wheels. Then if I want proper aero for flat TTs I'll get some 60mm wheels at a later date.
Hi Neeb,
I'll leave the detailed aerodynamics to Hambini who knows his stuff, but my opinion, FWIW, is that you got it right in your conclusion.
The aero difference in 38 and 45mm rims is going to be minimal, and rim shape, (and don't forget spoke shape and number!) will play a part too, that could narrow the difference.
The 200g is well worth having, and the unquantifiable 'feel' of the bike is very important and contributes to how much you enjoy riding it.
I love riding a light bike and how it snappily reacts to every input, be it accelerating, cornering, braking, etc. I love aero and going faster too, but I won't compromise on the feel of the bike.
So I'd say go for the wheels that ride best, and then switch to deep, super aero for special aero occasions.
(Oh and don't forget my 'silicone the gap' trick (between tyre and rim!)0 -
Glad I'm not the only person these days who appreciates a light bike and light wheels! (sometimes it seems that way). Clichéd as it is, it's basically stiffness to weight that determines responsiveness of wheels. You have to add material to increase stiffness - deep section wheels are stiff even if they are narrow, but they necessarily get pretty heavy if they are wide as well as deep, and if they are narrow it looks as if much of the aero advantage may be lost if you have to run tyres on them that are wider than the rims. So I'm betting on a modern wide section 38mm rim being as stiff or stiffer than my current narrow section 46mm rims, and possibly even more aero if fitted with tyres that are narrower than the rim (and they'll be about the same weight).bernithebiker wrote:Hi Neeb,
I'll leave the detailed aerodynamics to Hambini who knows his stuff, but my opinion, FWIW, is that you got it right in your conclusion.
The aero difference in 38 and 45mm rims is going to be minimal, and rim shape, (and don't forget spoke shape and number!) will play a part too, that could narrow the difference.
The 200g is well worth having, and the unquantifiable 'feel' of the bike is very important and contributes to how much you enjoy riding it.
I love riding a light bike and how it snappily reacts to every input, be it accelerating, cornering, braking, etc. I love aero and going faster too, but I won't compromise on the feel of the bike.
So I'd say go for the wheels that ride best, and then switch to deep, super aero for special aero occasions.
(Oh and don't forget my 'silicone the gap' trick (between tyre and rim!)0 -
Generally speaking the deeper the rim, the stiffer it will be. For twisting of the rim, it's based on something called polar moment of inertia. For bending of the rim it's called second moment of area.
To describe this if you take a ruler and bend it like you used to twang at school it is quite flexibile but put it on it's side and try bending it across the non-twanging axis and it's very stiff. This is second moment of area in action. A similar analogy applies to your wheels. The deeper wheel will be geometrically stiffer. Assuming the wheels are all the same material the deeper wheel will be stiffer.
Sorry if I have complicated your decision process.0 -
If I've understood you correctly you may have reinforced my current opinion.. ;-)hambini wrote:Generally speaking the deeper the rim, the stiffer it will be. For twisting of the rim, it's based on something called polar moment of inertia. For bending of the rim it's called second moment of area.
To describe this if you take a ruler and bend it like you used to twang at school it is quite flexibile but put it on it's side and try bending it across the non-twanging axis and it's very stiff. This is second moment of area in action. A similar analogy applies to your wheels. The deeper wheel will be geometrically stiffer. Assuming the wheels are all the same material the deeper wheel will be stiffer.
Sorry if I have complicated your decision process.
From your explanation I'm guessing that you're defining stiffness in terms of horizontal compression resistance? Just as the ruler doesn't bend easily across the non-twanging axis, a narrow, deep section wheel won't bend easily if pressed from the top or bottom, but will "twang" if bent from side-to-side? While a wider, shallower rim would compress more laterally but twist (twang) less? The latter is exactly what you want from a bicycle wheel - you want it to compress slightly horizontally to absorb bumps in the road but resist sideways bending (becoming pringle-shaped) under high lateral loads when climbing hard out of the saddle (the sort of flexing that causes brake rub).0 -
I can't believe I'm referring to this as the twanging axis but nontheless...
Assuming the wheels are exactly the same width, the stiffness in the twanging axis of either rim will be the same. This is in theory, there are all sorts of effects due to the reduced spoke length, carbon layup etc to consider.
What I am trying to say is I've given you a theoretical approximation, as cyclespeed has said, there is no substitute for "feel"0 -
Unless someone can explain to me why lateral stiffness matters, I think we can probably file it under the same category as 'weighing your valve caps'.0
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less lateral stiffness = brake rub under load.0
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hambini wrote:less lateral stiffness = brake rub under load.
Well yes, but you need pretty 'un-stiff' wheels for that to happen. If that's the only issue then it sounds like another 'valve weight' argument to me.0 -
Imposter wrote:Unless someone can explain to me why lateral stiffness matters, I think we can probably file it under the same category as 'weighing your valve caps'.
You mean you dont do that? jeez lost all respect for you matey.
fwiw when i got my daughter an aeroad last year, her avg speed on her fav 3hr training ride went up between 1 to 1.5 mph for the same power, does it matter? i guess not, her race performances have'nt dramatically improved.
its a hilly ride with 1200m of climbing, the bike is light, responsive and doesnt beat her up, set up is pretty much the same as her old bike, aggressive.
from her pov its clean lines mean its a doddle to clean too 0 -
Depends how closely the brake blocks are set to the rim. But the same slight flex that will cause brakes to rub if the wheels are really flexy will cause them to feel slightly less stiff and responsive when climbing hard out of the saddle. Some of the power you are putting into stomping on the pedal goes into flexing the wheel (between the more or less fixed points of the hub and the ground) rather than into torque. Of course the frame also flexes, and the tyres, and perhaps if the wheels are stiffer some of the energy that would have flexed them flexes those other things instead, but the less that flexes the more power goes into torque.Imposter wrote:hambini wrote:less lateral stiffness = brake rub under load.
Well yes, but you need pretty 'un-stiff' wheels for that to happen. If that's the only issue then it sounds like another 'valve weight' argument to me.0 -
mamba80 wrote:Imposter wrote:Unless someone can explain to me why lateral stiffness matters, I think we can probably file it under the same category as 'weighing your valve caps'.
You mean you dont do that? jeez lost all respect for you matey.
fwiw when i got my daughter an aeroad last year, her avg speed on her fav 3hr training ride went up between 1 to 1.5 mph for the same power, does it matter? i guess not, her race performances have'nt dramatically improved.
its a hilly ride with 1200m of climbing, the bike is light, responsive and doesnt beat her up, set up is pretty much the same as her old bike, aggressive.
from her pov its clean lines mean its a doddle to clean too
Blimey what was she riding before? That's like the improvement slapping tri bars onto a road bike. Impressive0 -
bog standard carbon road bike from hung fu..... CC made up some very light kinlin alloy wheels, its maybe a kilo heavier than the canyon.
personally, i ve never found fitting tri bars to a bog std road bike makes the slightest difference, geo all wrong and disrupts air flow with all those hand swaps for braking and gear changing, maybe on pan flat courses its better.0 -
mamba80 wrote:bog standard carbon road bike from hung fu..... CC made up some very light kinlin alloy wheels, its maybe a kilo heavier than the canyon.
personally, i ve never found fitting tri bars to a bog std road bike makes the slightest difference, geo all wrong and disrupts air flow with all those hand swaps for braking and gear changing, maybe on pan flat courses its better.
I think if you shorten the stem and rake your back over more, you'll probably see the difference but just putting them on in the "road" position might not have much effect as you observed.0
