Aerodynamics and Cycling
 

I'm still a bit of a physics geek and I decided to see if I can solve from Cd*A in my riding and maybe for Cr too. Starting with drag I have a simple route around here I did low on the drop bars and normal. Over several trials I get a 4-5% reduction in time which matches up close enough to what one reads.

So then I started reading about Graeme Obree and his riding position and custom made bike built from washing machine bearings and spare parts. It's a really interesting story. Then I got side tracked into recumbents and velobikes. I didn't see any regular bikes with a aerodynamic windshield.

Just in terms of efficiency on roads how hilly would a route have to be to make an upright more efficient than a recumbent given efficient models of roughly the same cost. 500 ft of climbs per 10 miles? 1000 ft of climbs per 10 miles? Assuming normal grades 5-10%.

I'm just fascinated with the design solutions for different issues and how gravity and the air offer quite different challenges that are not easy to combat effectively with a single design.

This is a complicated question you are asking, as there are lots of variables.

Not all diamond framed upright bikes (DFs), and more importantly, people's positions on those bikes, are equal. And not all positions are tolerable for equal lengths of time.

Not all recumbents are equally aerodynamic.

And it takes some time to re-learn to ride on a recumbent to produce as much power as you are capable of.

Some recumbents, while they post good CdAs, might have slightly poorer Crr, if those designs use non-700C or 650c wheels.

Over long time frames comfort starts to play a role in performance.

And keep in mind the difference in effectiveness (i.e. average speed over a given course) vs. efficiency (average calorie burned per mile over a given course). The machine that is most effective may not be the most efficient.

I rode DFs all my life until 2008 when I more or less switched cold turkey onto recumbents. I rode them almost exclusively till two years ago. The past two years I have been mostly on my DFs. But I do mix it up a bit still so I can give you some ballpark anecdotes.

Unless the climbing gets to be over 80 feet per mile or so, I am faster on my recumbents (still, despite me not riding them as much in recent years).

On the climbs themselves, I am anywhere from 5% to 15% slower on the bent. The shorter and steeper the hill, the more difference there is.

The issue for some riders on recumbents and climbing hills has to do with the machine they ride. Some bent designs are terrible climbers. The issue can also be power output. A lot (but not all) riders find their FTP to be between 5 and 20% lower in the recumbent position.

My bike is a Metabike and has a CdA of about .24 m^2, which can be bested by other recumbents, and some riders with good positions on TT bikes. This bike has dual 700c wheels too.

I can hold that CdA all day long though (and I do on 200K and 300K rides).

As far as efficiency goes, there is no doubt in my mind that the bents win. With a lower CdA and postural energy benefits (no energy to sit on the bike), the bent is just simply easier to ride a given distance at a given speed. Unless the climbing is fairly severe, that is.

The bent is about 5 to 6 lbs heavier, but with a total vehicle weight of about 205 lbs, that isn't that much of a penalty.

Yeah, just a bit.;) I, too find this interesting, but I'm not well versed in physics.

I'm honestly not sure why we don't see more small fairings on bicycles, in the strategic places where it matters, and they can serve a double function. Fenders, for example, would seem like a place to start, since they cover the part of the wheel that's moving double speed. A windshield and bark busters can make a pretty significant difference on a motorcycle and should work fine on a bicycle, too.

Jan Heine's blog and magazine have shown aero benefits from front bags and fenders.

Because they are UCI illegal.

And they look stupid.


Anyone who wants to cut wind resistance should have their mommy, daddy, S.O. or whoever drive them somewhere so they can ride back with a tailwind. Or take a bus.

There is also that. But if they were race legal, pros would use them and if pros used them...

True. Just look at all the high socks you see these days.

Small fairings on road bikes (see zzipper fairings for example) are not that effective aerodynamically for two reasons, both related to the mounting position on the handlebars.

First, it's pretty far forward of the body you're trying to screen which leaves a good gap in between most of your body and the fairing. Gaps can "work" with aerodynamics but they have to be close enough to fill in the space before turbulence starts - otherwise it's almost as bad as having an extra object stuck out in the wind. Consequently the fairing has to be pretty large to get your body in the cone behind it, and the other half of that Cd x A equation sinks you. Too much A.

Second, the handlebars are always moving around which causes turbulence, breaking up the laminar flow on the surface. That hurts you on the Cd part - you don't get as nice a drag coefficient as you expect. I suspect you can get around it with the right shape, which would be more of an ellipsoid with a long axis in front. But that will have drawbacks that you wouldn't put up with.

I do have a small fairing on my commuting road bike, which hangs down shielding the handlebars, stem and head tube. I built it to carry my commuting load and "might as well make it aero". Ironically it did decrease the drag when I tested it, a tiny amount. Something is going on in that region of the bike, I guess a whole lot of drag. It's on my to-do list to run some more coast-down tests.

Watch your step buddy.

I find the following table from Bicycle Science to be quite interesting in what works the best for cutting down drag

http://i.imgur.com/6Gr4PiW.jpg

True. If monocoque frames were still illegal would they be made at all? Who doesn't like to say monocoque?

As for fairings looking silly, have you seen the outfits cyclists wear? :lol:

There are some wacky numbers in that table.


In just looking at recumbents, virtual elevation type field tests I am aware that have been done are summarized here (with approximate values): BentRider Online Forums - View Single Post - How can elite cyclists learn from aerodynamics?


In the world of velomobiles, I know that a regular sized Quest's CdA is about .08 m^2. A Rotovelo is about 0.15 m^2. A Lighting F-40 is about 0.14 m^2.

I might be able to get you into the wind tunnel at the Georgia Tech Research Institute.

The compressible flow Mach 2 tunnel would probably be overkill but the other two might work.

I can ask my daughter. Starbucks gift cards go a long way with college students. Let me know.


-Tim-

The book is summarized here
It's based on actual verifiable research. I am definitely interested in seeing more published research.

Wow that's tempting, and study aids like Starbucks coffee are worthy donations. I'm in the middle of construction from a new design in fact.

Much of the current work in this field doesn't rise to the point of being published. But that doesn't mean its of poor quality. If you only look to published research you will be scrounging for scraps and the data will either be relative to equipment that is not available anymore or simply is no longer state of the art or doesn't answer the specific questions that you personally have.


If you are serious about exploring these matters, you really ought to investigate field tests using Robert Chung's virtual elevation methods. They are quite robust. They only examine zero yaw, but that's the only major deficiency.


But if you have access to a wind tunnel then nevermind.

There's a fair bit out there even from the last five years. Granted there's more studies of time trial cyclists specifically than a broader more general approach. But by published it doesn't have to be academic, peer reviewed, but there should be a clear methodology and results.

The most aerodynamic riding postures are at the expense of comfort..

Like this guy : http://https://42ndblackwatch1881.fi...09/06/c-1.jpeg




"/,,

True that the most efficient isn't the most comfortable, but that makes engineering and bicycles so fun. You have gravity which is really annoying but not constant, then the air which is almost as annoying and is always there, and then the mechanics and the tires. I mean they've done studies that show how much time shaving helps the super fast :).

It's also interesting to see how people respond to marketing and to what the cyclists on TV do even when it offers little real world benefit.

Let
s face it cycling has everything in the human experience distilled onto two and sometimes three wheels.

If everyone rode around on fat tires and behind big canopies, and that's what you were used to seeing, you'd think an upright road bike looked stupid.

No. They are objectively stupid looking, regardless of how many people use them. That will never change, just like high socks will always looks stupid even if I am the last person in the world wearing low socks.

[QUOTE=Darth Lefty;19147007 Fenders, for example, would seem like a place to start, since they cover the part of the wheel that's moving double speed. [/QUOTE]

Fenders tend to create their own turbulence. For a slick tire, the tire itself going at twice the bike's speed doesn't seem to affect air resistance all that much (in my experience.) What makes a bigger difference is the spokes that are moving much faster than the bike. That's what spoke covers and disk wheels are all about.

Of course, taking the Shelbroco approach to engineering, if the top half of the wheel goes too fast, just invent a wheel that uses two bottom halves! :lol:

Or use smaller wheels with aircraft-like cowlings.

This one seems legit:

http://www.ibikelondon.com/wp-conten...lying-bike.jpg