Last Friday I got an invited to hang out with some folks from Cervelo for a night at the local tri shop (Bonzai) going through a fairly detailed technical presentation, primarily focused on aerodynamics of bikes. I happily accepted the invite – despite it the day and time. You have to really enjoy this stuff in order to sit through a long detailed PowerPoint presentation on aerodynamics on a Friday evening. But…that’s exactly what I did.
The presentation started off with a bit of an introduction into how Cervelo makes bikes, and some of their general design philosophies. They also went into a bit of history on the company. For some reason I thought it was a much older company than they really are. In the world of bikes, they’re pretty new to the scene actually – I was still in high school when their first bike was born.
They also spent a bit of time talking about the professional Cervelo Test Team group, and how they transformed from sponsoring a team with the Cervelo brand in the past, to having their own cycling team (Cervelo Test Team), where the focus was on getting really good feedback to Cervelo – not just being ‘another ride’. In other words, they enlisted riders with backgrounds in engineering, to be able to get detailed evaluations, and move away from feedback like “Umm, it rode like a bike” – which is what they got before with the old sponsorship model.
(Side note: I didn’t realize you could actually go spend a bit of time riding with the team. They offer that sort of thing tied in with larger races (read: The Tour). One of the guys from the tri shop got to do it last summer, which is pretty damn cool. How do I get that trip? And would I get to meet Bob Roll while at the “Tour Day France”? I’m willing to slap two Cervelo stickers over the Chipotle bike shorts I have. Though, the bike ‘Hot’ portion over the posterior of the ass would have to remain free and clear. Just cause.)
From there the presentation dovetailed into a number of areas around materials – walking through the different types of composites used, and how they flexed depending on the load time and load direction.
This was pretty informative, as they talked through why certain portions of the frame are made with materials that are more stiff, whereas other portions use less stiff composites. Further, why some bikes use entirely different materials than other bikes. For example – a bike designed to ride the Paris-Roubaix race (which includes off-road sections) would be focused on different materials than a time trial bike.
However, the most interesting part of the session was that focused on aerodynamic properties, and the impact on drag because of the shape of an object.
Drag is a relatively simple formula using a few different parameters, and even though I nearly failed AP Calculus, I can at least complete the function below…most of the time. Just not before 9AM (and it’s totally not my fault Calculus was at 7:20AM, WTF?).
Starting with the most easiest – the square in the diagram below – you can see that it has 40 times more drag than the airfoil (bottom shape). In other words, if you put a simple brick wall up in front of your bike, it would be 40 times more sucky than the airfoil.
As you work your way down through the different shapes below, you can see which shapes lend themselves to more aerodynamic postures. As you do that, you probably start to notice certain shapes are utilized in fields where aerodynamics matter – such as flight and cycling. For example, that bottom shape – the airfoil, is the same shape as an aircraft wing.
So now that we’ve looked at pretty pictures of circles and squares – where does it matter? Well, it manifests itself in some places you might not think of. For example, your aerobottle. Many new aerobottles themselves are in an overall aero shape (like an airfoil turned sideways), such as this relatively generic one below.
But while the bottle design itself tapers like an aerofoil, one relatively important part does not: The straw.
Now, last fall I had read a painfully long thread on Slowtwitch about this, based on research from MIT. But deciphering all of the good from the bad in there is sorta tough. Mostly because there’s a lot of good stuff. But, the Cervelo presentation was able to really put a simple visual on the whole straw thing.
Because your typical straw is probably about a foot long, you need to take that and increase it’s size by a factor of 24 (per the chart above). So let’s go with 12” long by .33” wide (these straws are not small). So to get the base area that’s (12*.3= 4sq inches). Now we take that and multiple it by 24 (24*4). Thus, 96sq inches.
Now, how big is 96sq inches you say? Well, it would be a touch bit bigger than an 8×11” sheet of paper. So, now imagine instead of the aerobottle straw, you just put up a piece of flat steal in front of you the size of a sheet of printer paper. And then you went out and rode 112 miles…into the wind. Kinda sucky, ehh?
Of course, the problem here is overlooking the obvious. The obvious being that while simply not having an aerobottle may work well for your average Tour de France rider, or someone making pit stops along the way – it doesn’t work well for your average Ironman triathlete. It’s been well proven through many painful bonks that if you don’t have hydration directly in front of you, you’re less likely to drink it. And cutting the straw length only saves you so much before it just gets fairly annoying.
So what are some options that are aerodynamically friendly then? Oddly enough, just simply laying a water bottle on the aerobars. This has a profile that much more closely resembles an aerofoil than that of a brick wall.
And, if you look back at this year’s Kona bike gallery, you’ll notice about half of the top 15 men were using the bottle in the aerobars approach (or no bottle upfront). Same with the women.
Now, the problem here is really the attachment piece – meaning, what ensures the bottle doesn’t go flying when you hit that massive pothole. The system in the picture above and below sells for a redonkulous $95 once all the parts are included. Seriously? Just look at it below. We’re talking a couple of straps here and a baseboard. This isn’t rocket science. Yes it sells for rocket fuel like prices.
Now, bike companies are starting to build in some of these things into the frames themselves. For example, the relatively new (and expensive) Cervelo P4 includes a much more aerodynamic water bottle:
The problem with that bottle though is refills while underway, and general logistical issues with having basically one bottle (cleaning, etc…). But, the option is there for the taking. There are also other options out there now that are a similarly streamlined to the frame, like the below. Though again, they’re a bit expensive.
So, finally, what about those bottle cages behind you on your rear rack system? Well, it turns out those actually aren’t that bad at all. In fact, that’s a rather ideal place to put them. That’s because your body has already hosed up the airflow in that particular area, so really, it doesn’t get much worse. The best place to put them is as close as possible to the seat (and thus your ass), and tucked up behind you a bit. You just don’t want them hanging out there like a aerial refueling boom or anything. Here’s a post I wrote a while back on what I use, for those interested.
Now, the cool thing is that Cervelo publishes a fair bit of this data that you can go out and read all about the details (starting here). Also, there’s quite a bit the MIT data available as well via the Slowtwitch link above, even for other areas of the bike as well.
Hope you found this interesting (and just feel lucky I didn’t dump 2.5 hours of engineering slides on ya!), thanks for reading!
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Great post. I have been considering switching my bottle set up to the bottle in the aerobars vs the aerodrink and then the speedfil system down below. My food (perpetuem) would go in the bottle and hydration in the speedfill. I’m definitely going to make the move now. Just have to figure out exactly how to mount the bottle, because I’m not paying $95.
Nice recap of the event. I wanted to go…but it was on a Friday ;)
I think when it comes to aerodynamics and water bottle setup, it really comes down to what setup is going to ensure you will get the hydration you need. It obviously gets more important, the longer the race. Many pros use the single bottle up front setup now, but then again, they aren’t out on the course for as long and they are much more efficient, so they don’t need to take in as much. You also have people like Joanna Zieger who sticks with a trusty Camelback. Though it may not be the most aero (though it is flush against her, so it probably doesn’t cause much drag), for her it works.
Profile has a must cheaper version of the XLab product. It’s only $11. Now of course it’s not made of carbon and you probably get what you paid for, but it’s not a bad alternative.
The Girl and I were talking about this afterwards – My ghetto fab solution is put a little bit of electrical tape down on the aerobars (to prevent scratching), and use a cheap metal bottle cage with super strong zip ties. The metal allows you to bend the cage around a thinner bottle, a la Gatorade. I talked to Andy about it, and his only negative comment was “but the XLab set up LOOKS better.”
Regarding the tour, here you go: link to cervelo.com It’s under the test team travel. All meals, a bike to use while there, test team kit, and bad ass experience for $6900 for the first week of the Tour.
Thanks again for the translation from techo to english!
I haven’t read through the endless ST thread on the straw, so maybe this has been answered.
Sure the straw hits the wind and generates drag with a effective surface area that you can calculate.
However, about 12″ behind the straw is a rider with a much larger surface area that would have hit the wind anyway. She is also irregular and will generate plenty of drag.
Is the total drag of two things (straw, rider) going into the same wind purely additive (total drag = drag[straw] + drag[rider])? Or maybe it is more or less than the sum of their individual drags?
I don’t know – just asking, but it is important that one know the answer to this before writing off the straw.
I’ve been a physicist and worked in automotive racing for five years.. the comparative aerodynamics information in this post is hyper-simplified and generally inaccurate. One cannot simply measure and multiply, take a cross-sectional area, and make broad-based comparisons on drag or wind resistance, especially in a complex environment like a bicycle. It works well enough when considering a simple surface in clean orthogonal (ie head-on) air, but not in this instance. Drag isn’t just a function of cross-sectional area, as well, drag friction continues as the air flows over and around an object; in fact, the trailing edge is very important, a poorly designed trailing edge creates eddys and pressure drops.
The straw being round and not oval is absolutely inconsequential at the distances and speeds any cyclist will ever achieve in the real world.
Oh my gosh! A technical post I UNDERSTOOD!!! I think it was because it did not involve anything that resembled a computer, lol!
Great Information. So with the bottle laying on the aerobars, do you just toss the old bottle and grab a new one from a bottle hand up? I guess the need to amass old toss aways before a big race is necessary.
This is a great discussion. I am an Endurance Nation member, where there has been much talk previously concerning the aero bottle straw.
I think their solution is elegant and easy. Just go to Lowes or Home Depot and replace the plastic straw with a flexible version. Then you can easily tuck it out of the wind until you need it.
Very cool stuff – all of this seems right up your alley. I’d be shocked if you didn’t spend your evening at that clinic.
I definitely see you riding with the Cervelo team in the future.
Hey Steve-
RE: Straw Drag
One of the interesting things was that they actually test WITHOUT the human from the torso up. Meaning, all of the windtunnel testing they do is with a fake human (or real, but generally fake as it’s easier to do for hours on end), as the key thing they’re testing is the bike itself.
As for the straw, it’s the area directly behind it a few inches that causes the air to become turbulent and thus introduce the drag. Meaning, by time it hits the human, the airflow has already normalized again. Interesting, the area directly behind the spacers, where a bento box would go – is also pretty bad. But by placing a Bento box there it can actually in some cases help the situation (in other cases, it’s a wash).
One small thing: in Paris-Roubaix they aren’t going off-road. They just ride on very old streets witch are coverd with a kind of stones. The way they made roads in the Middle ages. Here in this part of Europe you still find a lot of these roads and those in the north of France are in a very bad condition. So it can be nice to have a bike specialy made for it.
Ray, I know this is an old post (I only saw it when looking at your recap post), but I think you might be a bit off on your FA example of the straw. The round shape has drag 24x that of an equivalent FA airfoil, NOT a flat sheet. In fact, if you compare it to a flat sheet, it might actually be better.
The more correct example would be to say that it has the same drag as a proper airfoil, but scaled up so that it has a frontal area the same as a 8.5×11 sheet.
Just a minor nit I just had to pick ;-)
hi,
is the breadcrumb for map route is always horizontal or it can change to vertical? I saw on edge 500 it has vertical breadcrumb. Appreciate your reply.
rgds,
AK
why not just make an aerofoil straw?
DC, while I love reading stuff on your website, in all due respect I’m glad you work on computers, not airplanes. The aerodynamic drag from a 12″ straw is NOT even close to the drag that would be created by a 94-square-inch flat sheet of steel. This can be quickly confirmed empirically by holding a 12″ straw out the window of a car while it is going about 30mph, vs. holding a flat sheet of metal out the window perpendicular to the airflow. The engineering calculations mean nothing if they don’t meet the common-sense test.
What the 24x drag factor means is that the 12″ straw would have the same drag as an ideal airfoil that had a projected area 24x larger, NOT the same as a flat sheet 24x larger. And it is even more important to note that this 24X factor only applies when the airflow over the object is fast enough to reach full turbulence, which will rarely happen at bicycling speeds (for the air temperatures at which we typically would ride).
My upper arms are FAR larger than that straw (and are also approximately 12″ long), so how my jersey fits is FAR more important (Try holding a bike jersey or a flag out the window of a car and see how hard it is to hold on while it flaps in the breeze) . The bike companies are trying to sell us on these things that are supposedly based on rigorous engineering, but are really just sales pitches.
Just sayin’ that we can get faster and more efficient by focusing on many other factors before we start re-engineering straws.
Indeed. It is disappointing that technical information can be so grossly mis-represented, and likely leads to all manner of internet experts claiming aerodynamics theory and testing supports their outlandish claims. Oh well, such is the information age.
Hi,
Wonderful discussion topic makes you think.
But interestingly enough noticing your remark about the oval in the tubes (because they are angled),
looks like the straw is also angled so its actual shape is oval to the direct incoming wind so the relative drug drops towards 3.5 from 24 ..
Still commenting on old posts…
I find two flaws with the arguments against a water-bottle attached to the frame.
1. Your calculation is off, The formula states that your straw is creating 24 times the drag of an perfect airfoil, but it still has about half the drag of a square, meaning it compares to a 2sq inches flat sheet of metal. Which is about the same as having your cycling computer standing straight.
2. The formula is based upon the straw being free-standing. If I had an aerobottle with a long tube, I would have the tube following the frame, perhaps using some strips of tape to keep it in place. Then, only the 3in or something sticking up on the bars would actually lend to air resistance. And those again would be bent and not sticking straight up. Lying on your aerobars, there’s probably next to nothing left of the resistance.