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Old 04-09-08, 06:29 AM   #1
gregw
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Wind resistance vs rolling resistance

Seeing the old trailer vs panniers thread come up AGAIN, I thought I would post this link to a calculator that can give you some real numbers for your riding set-up. This calculator can help you make decisions about equipment, packing methods and riding styles. Tell us what you find??

http://www.analyticcycling.com/ForcesPower_Page.html

The final number given is watts of energy needed to maintain the speed you set under different variables. Those variables being;

Frontal area, (trailer vs panniers comparisons)

altitude (air density)

weight of rider

rolling resistance (trailer vs panniers comparisons, if resistance for 2 wheels is .004, then 3 should be .006, half again as much

slope (hills vs flat ground)

cadence

and more that for our purposes can just be left on the default settings.


So plug-in your assumptions and see what you get.

Last edited by gregw; 04-09-08 at 07:35 AM.
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Old 04-09-08, 06:43 AM   #2
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First thing I noticed with my example was that the power needed to maintain 15 mph was 131 watts and 12mph was 65watts. So if I slow down 20%, I save 50% of my energy. Pretty good trade.
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Old 04-09-08, 06:51 AM   #3
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I would think that a trailer has more frontal area and obviously more rolling resistance.
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Old 04-09-08, 07:34 AM   #4
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I would think that a trailer has more frontal area and obviously more rolling resistance.
I was just trying to measure my trailer's frontal area, but it's not that easy. I take the frontal dimensions (that part is easy) but then I need to subtract the frontal area of the bike with me on it. The trailer is falling into the slipstream that I have already made. When I subtract this out, my frontal area only increases slightly, maybe 32 square inches (.02 meters squared).

Someone with panniers, put both front and back on you bike and give us an estimate. From bottom of front bag to top of rear, vertically x width of bag x 2 bags. Stuff on top of racks won't count because the bike and rider cancel this out.
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Old 04-09-08, 08:00 AM   #5
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Seems like the best configuration would be to use a handlebar bag, a seat bag and keep all other stuff on top of the front and rear racks. No panniers and no trailer. This would give you virtually no additional frontal area and no increased rolling resistance.

Would this move your center of gravity up too high?? Would handling suffer?? How much could you pack this way??
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Old 04-09-08, 08:38 AM   #6
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Interesting stuff, but just measuring frontal area is probably an oversimplification. For example the drag of a streamlined body and a flat disk are quite different. They probably make reasonable assumptions about the shape of the bike and rider, but probably not about the bike, rider, and trailer. They may not make good assumptions about the bike, rider, and pannier combination either.

My best guess is that a bike with a trailer has less drag than one with panniers, but it is just a guess.

Another issue is if you group ride, the trailer is hard to draft effectively. I suspect that the trailer drafts the bike it is attached to pretty effectively though.
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Old 04-09-08, 08:45 AM   #7
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If you are riding in any type of a cross wind the trailer will be dragging air.
And just because the trailer is behind the bike doesn't mean you can subtract surface area. The trailer also sits lower than the bike.
You would need a wind tunnel to measure this, its just not that simple.
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Old 04-09-08, 09:13 AM   #8
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If you are riding in any type of a cross wind the trailer will be dragging air.
And just because the trailer is behind the bike doesn't mean you can subtract surface area. The trailer also sits lower than the bike.
You would need a wind tunnel to measure this, its just not that simple.
Your right it is no where near that simple, but it's interesting to do comparisons. For my rig, what happens if I can loose 10lbs of gear or fat, or slow down 1/2mph??

Loose 10lbs = 2.3watts difference

Slow down 1/2mph = 8watts difference

So taking my time saves more energy than dieting.

Are these numbers empirically correct, no, but the comparisons are informative.
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Old 04-09-08, 11:57 AM   #9
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The key to minimizing wind resistance is to minimize boundary layer separation. This is why aero helmets are tapered. While this bit of software is useful it is designed for a rider on a bike by the looks of things so extending it to a loaded bike or a bike + trailer may be difficult. More about what model assumptions are used would be needed. At a guess I would say this is an empirical model and thus that point of source from the data my drastically change how applicable this model is. It seems like it may a useful tool for comparing different setups but don't take too much notice of it because you don't know how the model came about / what data / assumptions are used. Fluid flow is very complicated and frequently counter intuitive or at the least gives surprising results.
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Old 04-09-08, 01:14 PM   #10
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Originally Posted by gregw View Post
First thing I noticed with my example was that the power needed to maintain 15 mph was 131 watts and 12mph was 65watts. So if I slow down 20%, I save 50% of my energy. Pretty good trade.

Not exactly, as you would have to pedal 25% longer at the 12mph to reach the same distance as when travelling at 15 mph, so you would have to expend about 81 watts which is still a nice savings.
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Old 04-09-08, 08:09 PM   #11
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Yeah, it is correct, if you cut speed on the HWY from 70 mph to 55, as was done during the last oil crisis, you reduce consumption to 62%. Assuming gearing optimized etc... Same thing as reducing from 15 mph to 11.8. This is also true of wind force generally, double you speed and wind force goes up 4 times. At 25MPH it is estimated 80% of energy goes into cutting through the wind, on flat ground.

I would echo all the warnings abouts simple analysis of load drafting, or even rolling resistance since most trailer wheels aren't as efficient as primary wheels. Also, for peak efficiency, I would argue: zipper fairing, aero bikes, longer wheelbase, hard shell paniers, and two wheels only, with the rear having a mylar disc or a carbon spoke patern. If you want to get the load in line, you don't need a trailer to do it.
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Old 10-31-13, 01:02 PM   #12
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Speed is in km/h!
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Old 10-31-13, 06:05 PM   #13
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I sent this thread to some buds. One, Harry M., replied with this:

Don’t know about bents but trailers versus panniers I have some strong opinions from coming across Kansas. There were three pulling BOB’s and four of us with four panniers each. On the toughest wind day all the BOB’s made it in and none of us with panniers did. Those with pannier were blown all over and off the road. Didn’t see any bents so don’t know how they would have fared. The wind was on about at our 10-11 at up to 50mph gusts.
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Old 10-31-13, 06:53 PM   #14
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Calculating effective area isn't easy. When in doubt, try roll down tests (where you can solve for both aerodynamic drag and rolling resistance).
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Old 10-31-13, 07:56 PM   #15
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My suggestion, as the owner of a PowerTap power meter, is not to put too much faith in any of these online calculators...
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Old 11-01-13, 06:24 AM   #16
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My suggestion, as the owner of a PowerTap power meter, is not to put too much faith in any of these online calculators...
The calculators are not too bad for giving you and idea of what's going on. Knowing what your rolling resistance is and (particularly) knowing your CdA is the hard part (roll down tests, or tests with well calibrated power meters can help answer those questions).

Power meters can be tricky to calibrate well too. But they can be a very useful (if not expensive) training tool in most cases.
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