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Piwoslaw 02-24-2009 09:10 AM

Other incarnations of Pulse & Glide
I've been thinking about the Pulse & Glide technique lately, about why it works etc. One of the things I've read is that when P&G'ing between, say, 50 and 70 (speed units) your average speed is 60. Now, I've been repeating that to my family and friends even though I know it's not really true, it's only approximately true. Why? Aerodynamic drag increases with the square of velocity, so the drag between 60 and 70 is greater than between 50 and 60. In other words, above 60 the force which is slowing you down is greater than below 60, so the time it takes to slow from 70 to 60 is shorter than from 60 to 50. This means you're spending more time below 60 than above.

I noticed something similar with the heating in my house: The day temperature is 19.5C while the night temp is 17.5C. When the thermostat switches from day to night I can see that the house cools down to 19.0C pretty fast, like under an hour. After 2-3 hours it's down to 18.5C. It takes 5 hours to get to 18.0C. I think I remember from long long ago that cooling is an exponential function of the temperature difference, but maybe I have it wrong. Anyway, my thermostat seems to use the P&G method. Instead of adjusting my furnace to constantly keep the house at 19.5C, it warms up to 19.75C, then "coasts" down to 19.25C and back again. I'm all exited about this because:
(1) Until a few months ago we didn't have a thermostat. The furnace would keep the water in the system at a constant temperature, meaning the radiators were always warm.
(2) We are saving money and, more important, natural resources. Even though it's been colder than last year and prices have gone up since then, our bill is about 10% less than a year ago. I have yet to check how much less Russian gas we've used.

So these are two incarnations of P&G. Yesterday I read in Bearleener's signature:

You ever notice that birds pulse & glide, too?
I decided to post this thread to discuss other ways of how P&G shows up.

PaleMelanesian 02-24-2009 10:01 AM

Refrigerators use P&G as well.

MazdaMatt 02-24-2009 11:45 AM

So do engineers... drink a coffee... glide for 2 hours... drink a coffee... glide for two hours. :)

skyl4rk 02-24-2009 12:56 PM

Ramp - Soak is a term used for heating liquids, ramp = pour on the heat, soak = let it sit there and cool off slowly.

Peter7307 02-24-2009 09:27 PM

Actually fish use P&G as well as birds.
A flick of the tail and then almost no motion at all for a while then another flick of the tail.

Interesting observations and the more you are aware of them the more you can observe.


dcb 02-24-2009 10:37 PM

flying fish especially :)

and sailplanes

and skateboarders

and me on a bicycle (no fun showing up in a nice shirt all sweaty)

and penguins


Christ 02-25-2009 01:04 AM

Wow... that reminded me of Mario Bros.

So, I've actually noticed since I was young that there aren't too many things in nature that don't take advantage of laziness from time to time... i.e. not expending energy when they don't have to...

Even Polar bears slide on the ice once in awhile.
Penguins understand that it takes less energy to propel themselves directly forward (laying on the ice) than it does to waddle to a destination, so they lay down and paddle their feet.
If you think about it, even basic running is Pulse and Glide, since you're only being propelled by each push off from your foot, while you're gliding until the other foot hits the ground for another propulsion.

Trees also P&G their growth, hence the light and dark rings in a cross-sectional view. So do Humans, hence "growth spurt". Those are getting away from propulsion and more toward generality, though.

More appropriately, most IC engines use pulse and glide as well. Only one cycle of 2 or 4 per piston is actually creating power, then it's relying on the work already created to propel the piston through the rest of each cycle, until it can make power again.

If nearly all things in nature and life use P&G, why are Humans so hard-pressed to accept it in driving?

Piwoslaw 02-25-2009 02:50 AM


Originally Posted by dcb (Post 89460)
and me on a bicycle (no fun showing up in a nice shirt all sweaty)

I was wondering about that. The reason we use P&G while driving is because of how much fuel the engine uses while idling, how energy is lost to friction, etc. Human legs are built and operate differently, so maybe it's not that wise to P&G on your bike. Of course, the only time I spin my pedals nonstop for hours is when cycling in the Tour de France, other than that I coast often to let my legs rest. Sure, human muscles tire quickly when used without rest, but then those muscles are under extra strain when accelerating again. What is going to get you more tired: holding a small weight for a long time, or picking up a big weight, resting, picking it up again, resting, etc.? We don't have built-in ScanGauges to check how much energy we are using and at what rate while steadily cycling and while P&G'ing.
The bottom line of my ranting is that I'm not sure whether P&G on a bike is energy effective or not, but I do it to let my muscles rest.

dcb 02-25-2009 06:35 AM


Originally Posted by Piwoslaw (Post 89514)
...<same old tell you how to use a bike stuff> ...

c'mon dude, don't go there. It's my choice, I'm not training or competing, just transporting myself at a casual pace from one place to another and showing up ready for business. I'm not mr fixed gear single speed, and I did qualify it with "ME", so please don't go tee off on it.

almightybmw 02-25-2009 07:43 AM

Well, I rarely P&G in my vehicles, but I do all the time with the bike to work. It's all downhill, why should I bother pedaling when I've reached a peak eff? (given friction and work)

I do find the T de France comment amusing, as I've never seen in any race form, P&G. There isn't time for FE or resting when going the fastest is the goal. I think drafting makes sense there for work reduction.

BTW, what is the d(lurker)/dx? I've been racking the brain and can't figure it out. Awesome to hear about your thermostat experience. I too have found the function of outside temp to inside temp of my home. Fun finding eff in everything.

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