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Why riders like you need to get more aero and wheel weight doesn't matter

Wind tunnel evidence that it really is aerodynamics that makes the difference

I went to a wind tunnel and learnt a few things. The first thing I learned is that a wind tunnel isn't necessarily a tunnel.

I had an image in my mind of a fully enclosed and pristine tube like you see in the car ads. Octagonal, probably. Maybe with smoke trails, and definitely with a big fan at one end. The reality was a bit different. 

Swiss Side Wind Tunnel 25.jpg

The GST wind tunnel at the Airbus Defence and Space facility on the outskirts of Friedrichshafen scores on the fan front. But basically it's just a room. There's a table in it, with a bunch of tools on, and a stepladder propped up in the corner. There's some feather flags filling some space, and a projector hanging from the ceiling. Pristine it ain't.

Swiss Side Wind Tunnel 22.jpg

It's an old facility; it used to be the Dornier wind tunnel when Dornier were making planes, and there's still a goodly number of switches and filament bulbs in the control room. Now it mostly splits its time between bike parts and scale models of drones. But it's a good tunnel, says Jean-Paul Ballard of Swiss Side wheels, who's invited me to spend an hour or two looking at the process. He's got a bit of experience with aerodynamics, having worked for 14 years in Formula 1 as the lead engineer on a number of teams. Between them, the team behind Swiss Side have over 50 years of F1 experience, which obviously includes a whole stack of wind tunnel time. 

"It doesn't look like much but it's incredibly consistent", he says of the GST tunnel. "One of the problems with an enclosed tunnel is the blockage effects. This is important when testing with a rider as results can be significantly influenced. Here the flow isn't constrained". And, for the record, the fan sucks. "We laugh when we see pictures of wind tunnels in adverts with the bike pointing the wrong way". It's obvious once you think about it: if you want a clean flow of air you don't want a massive fan chopping it up a few feet from your test rig.

So the fan pulls the air through, and the intake at the other end is as smooth and uniform as possible to create the best conditions for consistent results. And the extra space in the room means the natural flow expansion and turbulence created has somewhere to go without affecting the readings on the rig. And all that makes for lovely, repeatable numbers. Stepladder or no.

We spent a bit of time running a standard series of tests. There's plenty to take out of the few tests that we did (and some we didn't), so let's pull out some of the most interesting bits of information we garnered. Starting with this...

1. The slower you are, the more important aero is.

Well, up to a point.

That's right: all you choppers out there – and I count myself firmly among your number – need aero most of all. That's madness, right?

Wrong. This seemingly counter-intuitive bit of information is explained by a single graph from the basic tests we ran during our hour at the tunnel. Here it is:

Swiss Side Wind Tunnel 29.jpg
Drag v yaw angle (click image to embiggen)

So what's this showing? It's showing the drag of a range wheels at various angles of yaw. The blue line at the bottom is an Argon 18 time trial bike; ignore that slippery beast for now and concentrate on the four lines at the top. That's four different wheels aboard the same frameset, a Cervelo R5 (Not a P5 like the legend says). They're all Swiss Side wheels: The box section Heidi (light blue) and three depths of their toroidal Hadron rim. The 80mm Hadron 800+ is in red, The 62.5mm Hadron 625 in orange and the 48.5mm Hadron 485 in green. Got that?

What's yaw?

Yaw is the angle of the wind relative to the bike. So if you're cycling directly into a head wind the yaw angle is zero, and if you're at a standstill with a direct crosswind, it's 90°. When you're moving, the yaw is a combination of the wind speed, the wind direction and your speed.

The first take-home point here is to look at the drag from the bike at a yaw of zero. Whatever the wheel fitted, the drag of the bike as it faces directly into the wind is more or less the same. We (well, I, anyway) mostly have a notion that those big, chunky rims make a nice shape that slices through the air straight on. In reality, that's not where the gains come from.

Once the yaw angle starts to creep up then you start to see a marked difference in the aero-optimised wheels compared to the box section. The Heidi wheel keeps increasing in drag over about 6° while the Hadron wheels begin to drop back down again until at 20° of yaw their lines are well underneath the standard rim.

Why? Because the air flow stays attached to the rim, rather than breaking off and causing turbulence that drags against the bike. It's at these higher yaw angles that the gains are most apparent. That's where you're making your time up.

More speed, less yaw

Swiss Side Wind tunnel 10.jpg

So, imagine a rider travelling at the speed of light. The wind will always be straight on, a yaw angle of zero: however hard it's blowing, the wind speed and direction will have no discernible effect. As you slow up, the fact that you're moving more slowly means that a greater range of yaw angles are possible for any given wind speed. And if you slow right down until you stop, anything goes: the yaw angle is whatever the wind direction is, relative to the way you're facing.

Followed all that? The practical upshot is that the faster you go, the lower the range of relative wind angles you'll experience. Professional riders in normal conditions won't see anything over about 10°, whereas us sportive bashers and lower-category chuffers will see much higher yaw for the same wind, because we can't go as fast through it. And it's those higher yaw angles that see the biggest gains, up until about the 20° mark when the airflow detaches from the rim and you lose the aero advantage.

Swiss Side Wind tunnel 5.jpg

"Faster riders generate more drag", Jean-Paul adds, "because drag is proportional to the square of velocity. But faster riders are also on the course for less time, and experience a narrower range of yaw angles. Through our simulations, we see that slower riders actually save more absolute time. They're out on the road for longer and can therefore benefit from the bigger aero gains for longer."

So go out and buy some new aero wheels, ordinary rider. Tell them I said it was okay. Make sure you fit the right tyres, though. Because...

2. Fit the wrong tyres and your wheels won't be aero

This was perhaps the most surprising bit of information from the whole session. It's not something that we tested on the day, because unfortunately there wasn't time to swap masses of tyres out and in, but Jean-Paul from Swiss Side told me they'd been testing plenty, and the results are interesting. More than interesting. It's all about what kind of airflow is passing over the wheel.

You'll often see trips on aerodynamic profiles; small raised or rough sections. Some bike wheels even have them. These trips cause turbulence, and the turbulent layer of air helps the flow to stay attached to the surface. The tread pattern of a tyre can act as a trip and help the wheel to remain aerodynamically efficient at higher yaw angles. It can do that. It doesn't always do that.

Swiss Side Wind Tunnel 28.jpg

"The best tyre we've found is the Continental GP4000S", Jean-Paul told me, and that's what our test bikes were shod with. Is the tread pattern designed for that job? Probably not, he conceded, it's more than likely just a coincidence. But there are other tyre manufacturers that are definitely designing their sidewall profiles with aerodynamics in mind, whether they're talking about it or not.

And how much difference does it make?

"If you fit a GP4000S to one of our Hadron wheels the flow will stay attached to 18 or 20° of yaw," Jean-Paul said. "With another tyre, one with a completely smooth profile, the figure drops to 8-10°".

One look at our yaw graph above shows that you're losing almost all of the aero advantage of a deep section if that's the case. That's a major issue, and one that most people won't have considered at all in the whole aero equation.

So how will you know whether your tyre is a good one, or a bad one? Well, you won't. Except if you're running GP4000S tyres, in which case you're in luck. Or if you're running tyres that don't have any tread profile at all, which is probably bad. The Swiss Side guys have been running tests on a wide range of tyres: there's boxes of them knocking about at the wind tunnel. I'm not sure they have plans to release that information to the general public, at least for now.

Swiss Side Wind Tunnel 27.jpg

So, you've got your deep section wheels and they look pretty pro, and your tyres with the (hopefully) effective trip profile. But those big old rims are heavy: maybe a couple of hundred grams over a climber's wheelset. Isn't that going to negate all that precious aero advantage?

Well, no. because...

3. Weight is a secondary issue almost all the time

What's more important, light weight or aero? That's simple, according to Jean-Paul. Aero trumps light almost every time.

Swiss Side's aero range has - up until now - used a non-structural carbon aero profile with an alloy rim and braking surface. It's simple enough to get the right profile with a non-structural fairing, and the alloy braking surface means more consistent performance from your brakes. The alloy bead section is easy to extrude, too, which means the wheels are cheaper. The penalty in terms of weight is about 100g over going to a full carbon construction, and you're adding that on to the extant weight penalty of just having a bigger rim made of more stuff. What kind of time penalty is that going to add up to?

Swiss Side feed aero and weight data into a model that crunches the numbers for different types of ride profile and length, and then spits out the likely speed and timing penalties based on a reference ride. One of their programs is a 120km rolling ride with 1200m of height gain. Their 'average' rider completes this parcours at exactly 30km/h (211.4W average power), which left me wishing I was average. But enough about my failings. What difference would one hundred grams (from an 8kg bike to an 8.1kg bike, with a 75kg rider) do to the ride time?

Swiss Side Wind Tunnel 31.jpg

Well, it would increase it.

By three seconds.

Adding weight to a rider going that fast, over that terrain, makes precious little difference, really. 100g is 1.25% of the wheel weight; even at four times that, the penalty is just 17 seconds.

Changing that 1.25% weight penalty to an aero penalty - upping the overall drag of the bike by the same percentage – gives a 22-second penalty, and quadrupling the drag penalty pretty much does the same to the time lost: 87 seconds at 5%.

Now these still aren't big numbers: just under a minute and a half in four hours of riding. But the difference is certainly significant: aero gains are worth six times what weight gains are, and a fair conclusion from Swiss Side's stats would be that on rolling terrain it's worth going heavier and more aero.

But surely there's a tipping point? Of course, but it's a long way north of where you might expect. 

This graph shows what a average rider might do on a 20km climb with an average gradient of 4%, knocking out 211W for not much under an hour. That's certainly achievable for many of us.

Swiss Side Wind Tunnel 30.jpg

This is the tipping point, more or less, for this imaginary rider. If your ride has an average gradient of 4% or more then the weight gains will mean bigger time savings than the aero ones. So you're still permitted to take a drill to your levers for the local hillclimb. But unless it is a hillclimb – or a really, REALLY hilly ride – then you're better off optimising your bike to cut through the wind than you are shaving off the grams. If that's a surprise to you, well, it was to me as well. Obviously it doesn't mean that a 10kg beater will be as quick as a 6kg superbike. But any time you're spending stressing over the odd hundred grams here or there is basically wasted.

A big thanks to Swiss Side for their time at the wind tunnel.

Dave is a founding father of road.cc, having previously worked on Cycling Plus and What Mountain Bike magazines back in the day. He also writes about e-bikes for our sister publication ebiketips. He's won three mountain bike bog snorkelling World Championships, and races at the back of the third cats.

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55 comments

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srchar | 6 years ago
1 like

"100g is 1.25% of the wheel weight"

Those are some heavy wheels. 8kg a pair?!

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dottigirl | 6 years ago
0 likes

I got a bit lost - what shape wheels do I need then to be aero please? Being on the slower side, I need some kind of benefit!

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Bigtwin | 6 years ago
5 likes

I'be been into various types of aero for years - especially the mint ones.  I'm still slow.

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Nick T | 6 years ago
4 likes

Company selling aero stuff discovers that slower regular riders will benefit more from aero stuff, and aero stuff is more important than lightweight stuff using science. That's bloody lucky for them isn't it, otherwise I'd worry how they'd sell any of that aero stuff to regular riders if they'd have learnt it was useless

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Dnnnnnn | 6 years ago
2 likes

Aero gains on the bike have to seen in the context of (I believe) 80%+ of drag typically being due to the rider.

So a 10% more aero bike probably only makes 2% overall difference.

You'd be better to work on riding in a more aerodynamic position - but there's no money to be made there so don't expect many articles about it (although, in fairness, I think there was one on road.cc a while back).

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KiwiMike | 6 years ago
0 likes

...so would the Schwalbe G- and S-ones, with their overall dimpled patterns, be more aero?

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part_robot | 6 years ago
0 likes

Just remove the rotors - you'll go super fast!

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IanEdward | 6 years ago
3 likes

Quote:

I got massively fitter when I joined the uni's cycle club a couple years ago. I came across the local sportive when I was out on a ride - a lot of the cyclists were 40-ish and on incredible carbon aero road bikes with deepish wheels.

And here-in lies the case for bought performance advantages, I'm now on the tipping point (I think) between natural youthful fitness and earned fitness. Problem is, I'm now short of time to earn that fitness, a newborn on the way, a wife to devote time to, a well paid job to devote (as little as I can get away with) time to, etc. etc.

Seems entirely fair that I make up for the loss in natural fitness and time to ride by buying expensive shiny parts! Thankfully this article justifies my preferred set up of some RS81 C35s and Conti GP4000s smiley

Avatar
CXR94Di2 | 6 years ago
3 likes

It's all marginal really. What makes a significant difference is the engine on the bike. I've seen young powerful riders destroy aero setups with a standard bike. Power to weight for hills and power on the flats and downhills. The aero advantage comes over long periods of riding.

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Canyon48 replied to CXR94Di2 | 6 years ago
1 like

CXR94Di2 wrote:

It's all marginal really. What makes a significant difference is the engine on the bike. I've seen young powerful riders destroy aero setups with a standard bike. Power to weight for hills and power on the flats and downhills. The aero advantage comes over long periods of riding.

Hit the nail on the head.

I've been riding a few years now, still on my first ever road bike (Dawes Giro 500 - but with Shimano 105 and Campy wheels now).

I got massively fitter when I joined the uni's cycle club a couple years ago. I came across the local sportive when I was out on a ride - a lot of the cyclists were 40-ish and on incredible carbon aero road bikes with deepish wheels.

I had no problems dropping the lot of them on my cheap Alu road bike (and that was 3/4 the way into a 70mile ride)...

This isn't by any means a dig at older guys, fair play to them! Just a comparison of bikes.

 

Interesting what it says about disc brakes though, still don't think it will deter me from going discs on my next bike though!

Avatar
schlepcycling replied to Canyon48 | 6 years ago
4 likes

wellsprop wrote:

CXR94Di2 wrote:

It's all marginal really. What makes a significant difference is the engine on the bike. I've seen young powerful riders destroy aero setups with a standard bike. Power to weight for hills and power on the flats and downhills. The aero advantage comes over long periods of riding.

Hit the nail on the head.

I've been riding a few years now, still on my first ever road bike (Dawes Giro 500 - but with Shimano 105 and Campy wheels now).

I got massively fitter when I joined the uni's cycle club a couple years ago. I came across the local sportive when I was out on a ride - a lot of the cyclists were 40-ish and on incredible carbon aero road bikes with deepish wheels.

I had no problems dropping the lot of them on my cheap Alu road bike (and that was 3/4 the way into a 70mile ride)...

This isn't by any means a dig at older guys, fair play to them! Just a comparison of bikes.

 

Interesting what it says about disc brakes though, still don't think it will deter me from going discs on my next bike though!

How do you know they were trying to keep up with you?, go join in a vets race and see how you get on.

Avatar
alotronic replied to Canyon48 | 6 years ago
2 likes

wellsprop wrote:

CXR94Di2 wrote:

It's all marginal really. What makes a significant difference is the engine on the bike. I've seen young powerful riders destroy aero setups with a standard bike. Power to weight for hills and power on the flats and downhills. The aero advantage comes over long periods of riding.

Hit the nail on the head.

I've been riding a few years now, still on my first ever road bike (Dawes Giro 500 - but with Shimano 105 and Campy wheels now).

I got massively fitter when I joined the uni's cycle club a couple years ago. I came across the local sportive when I was out on a ride - a lot of the cyclists were 40-ish and on incredible carbon aero road bikes with deepish wheels.

I had no problems dropping the lot of them on my cheap Alu road bike (and that was 3/4 the way into a 70mile ride)...

This isn't by any means a dig at older guys, fair play to them! Just a comparison of bikes.

 

Interesting what it says about disc brakes though, still don't think it will deter me from going discs on my next bike though!

 

Not really a comparison of bikes... you would have had to run the test with them on your Dawes and you on the Aero - but I take your point. Over 35 you just can't produce the same power and all these maginal few watts here and then mean nothing in the face of a strong, young rider. Of course strong young riders have to beat each other not us old men - and we gracefully retreat to thinks like Audax and Masters events and (finally) have the money to buy nice bikes and enjoy them. For me a nice bike it not about speed so much as the experience of riding it, so even though you are going to kill me up a hill I am still very much enjoying myself .

PS: And yes it's a blast beating other people on crappy bikes, I remember doing that myself  3  http://bikeby.bike/bikes/7-the-divine-comedy/

Happy riding!

 

 

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srchar replied to Canyon48 | 6 years ago
2 likes

wellsprop wrote:

I came across the local sportive when I was out on a ride - a lot of the cyclists were 40-ish and on incredible carbon aero road bikes with deepish wheels.

I had no problems dropping the lot of them on my cheap Alu road bike (and that was 3/4 the way into a 70mile ride)...

It always makes me chuckle when riders say, "Nobody overtook me!" or "I overtook loads of other riders!" as though they're the fastest guy in the west. In reality, there are loads of other quick riders out there, but because the speed differential between you and them is much smaller than between you and the sportivists in your example, you never see them.

Avatar
Canyon48 replied to srchar | 6 years ago
1 like

srchar wrote:

wellsprop wrote:

I came across the local sportive when I was out on a ride - a lot of the cyclists were 40-ish and on incredible carbon aero road bikes with deepish wheels.

I had no problems dropping the lot of them on my cheap Alu road bike (and that was 3/4 the way into a 70mile ride)...

It always makes me chuckle when riders say, "Nobody overtook me!" or "I overtook loads of other riders!" as though they're the fastest guy in the west. In reality, there are loads of other quick riders out there, but because the speed differential between you and them is much smaller than between you and the sportivists in your example, you never see them.

Agreed. To be fair, I come across plenty of people who absolutely rip the life out of me and eventually leave me trailing up hills behind them slowly... A few of the guys in the uni cycle club are bloody good racers, not a patch on them.

And funnily enough, one of the guys I've met out cycling a few times (and he's killed me up every hill I've tried to follow him up) is late 30's early 40's.

 

alotronic wrote:

Not really a comparison of bikes... you would have had to run the test with them on your Dawes and you on the Aero - but I take your point.

 

....For me a nice bike it not about speed so much as the experience of riding it, so even though you are going to kill me up a hill I am still very much enjoying myself . ....

Happy riding!

I wonder if aero bikes/positions make significant differences for many older cyclists. With about 80% of the drag coming from the rider not the bike.

I imagine in a fairly decent test, the aero bike would make a difference, though the difference would decrease with age (due to decreasing flexibility and power). I can only assume that the companies selling the aero stuff won't tell consumers as the target market is probably the older guys with a bit more money. Just a thought, though.

That's how it should be, cycles should be rated in quality, not quantity of miles or speed.

Avatar
earth replied to Canyon48 | 5 years ago
2 likes

Canyon48 wrote:

CXR94Di2 wrote:

It's all marginal really. What makes a significant difference is the engine on the bike. I've seen young powerful riders destroy aero setups with a standard bike. Power to weight for hills and power on the flats and downhills. The aero advantage comes over long periods of riding.

Hit the nail on the head.

I've been riding a few years now, still on my first ever road bike (Dawes Giro 500 - but with Shimano 105 and Campy wheels now).

I got massively fitter when I joined the uni's cycle club a couple years ago. I came across the local sportive when I was out on a ride - a lot of the cyclists were 40-ish and on incredible carbon aero road bikes with deepish wheels.

I had no problems dropping the lot of them on my cheap Alu road bike (and that was 3/4 the way into a 70mile ride)...

This isn't by any means a dig at older guys, fair play to them! Just a comparison of bikes.

 

Interesting what it says about disc brakes though, still don't think it will deter me from going discs on my next bike though!

 

When you are young you have all the fitness without the finances.  As you get older the fitness decreases and the bank balance increases.  So natually, older guys can spend more on the bike to counter the fitness drop off.  The trick is to get the best bike while you are still young.

Avatar
Deeferdonk | 6 years ago
4 likes

I find it weird when people spend excessive money to make their bikes more aero or incrementally lighter. You may easily drop the poor  fat chap on his steel bike, but where's the honour in that when you've purchased your extra watts of power, may as well buy and fit an engine. They should change strava KOMS so that they are based on a function of speed and weight and then you will get an honest picture of who the best cyclists are.

 

...........I obviously have issues  1

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Filliny | 6 years ago
3 likes

Trust me, on a hills or in mountains if u have wheels from Remerx, builded from cast iron aluminium  - weight have big matter. When u need to spin out heavy bagel and lost on it huge amount of power...

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CXR94Di2 | 7 years ago
7 likes

There was a video article in cycling weekly where they compared 3 sets of wheels on the same bike on an outdoor velodrome at 200W then 300W.   Upshot is that 40mm deep rims give significant speed improvement over standard shallow rims, and 40mm rims are only a little slower than full TT wheels. laws of diminishing returns to cost!?!

Re disc brakes causing 16% extra drag, keep in context its a percentage of a small amount, so hardly a major detriment.  Wearing a slightly baggy top will cause far more drag.

 

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davel replied to CXR94Di2 | 7 years ago
3 likes

CXR94Di2 wrote:

Upshot is that 40mm deep rims give significant speed improvement over standard shallow rims, and 40mm rims are only a little slower than full TT wheels. laws of diminishing returns to cost!?!

Exactly... and 40s-50s also strike a reasonable balance vs weight, in order to be decent all-rounders.

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StraelGuy | 7 years ago
16 likes

What they need to do to reduce drag from disc brakes is to sharpen the edges so they slice through the air more efficiently.

Er, no, wait, hang on a minute...

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tritecommentbot | 7 years ago
5 likes

Pretty damning stuff for disc brakes there. Makes you doubt Cervelo, Canyon and Specialized's claims about discs only adding a watt or so of extra drag.

Generally in favour of disc brakes, but if drag really is that bad then it's quite a step back in terms of aerodynamics.

 

 

Avatar
davel replied to tritecommentbot | 7 years ago
9 likes
unconstituted wrote:

Pretty damning stuff for disc brakes there. Makes you doubt Cervelo, Canyon and Specialized's claims about discs only adding a watt or so of extra drag.

Generally in favour of disc brakes, but if drag really is that bad then it's quite a step back in terms of aerodynamics.

 

 

I dunno... We're still talking tiny amounts: if the article and Python's links are accurate-ish:

Purely talking drag: we're looking at 16% increase of wheel drag, which is 8% of overall drag, so roughly a 1% increase in overall drag.

To keep things nice and easy, let's assume you're putting out 200W: 1% of that effort is 2W, and not all (between 70-90%?) of that effort is eaten by drag.

So even if we're talking stage race pros at 300W+ (who also know drag-defying tricks like hiding in the peloton) discs will still account for say 3W max lost in extra drag, overall. Maybe 2 or 3x that in a sprint? It's small beer, %wise.

Until the next study, anyway  1

Avatar
tritecommentbot replied to davel | 7 years ago
8 likes

davel wrote:
unconstituted wrote:

Pretty damning stuff for disc brakes there. Makes you doubt Cervelo, Canyon and Specialized's claims about discs only adding a watt or so of extra drag.

Generally in favour of disc brakes, but if drag really is that bad then it's quite a step back in terms of aerodynamics.

 

 

I dunno... We're still talking tiny amounts: if the article and Python's links are accurate-ish: Purely talking drag: we're looking at 16% increase of wheel drag, which is 8% of overall drag, so roughly a 1% increase in overall drag. To keep things nice and easy, let's assume you're putting out 200W: 1% of that effort is 2W, and not all (between 70-90%?) of that effort is eaten by drag. So even if we're talking stage race pros at 300W+ (who also know drag-defying tricks like hiding in the peloton) discs will still account for say 3W max lost in extra drag, overall. Maybe 2 or 3x that in a sprint? It's small beer, %wise. Until the next study, anyway  1

 

Alright, back to liking discs again.

 

Bit of a rollercoaster this morning enlightened

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crazy-legs replied to tritecommentbot | 4 years ago
0 likes

tritecommentbot wrote:

Generally in favour of disc brakes, but if drag really is that bad then it's quite a step back in terms of aerodynamics.

Ah but they're more powerful so you can brake later which means you can carry speed for longer so overall they're still faster.   3

Avatar
Freddy56 | 7 years ago
4 likes

Super article. Lots of food for my aging brain

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