Cycling efficiency

[chrisrob]

I asked this question in How it Works last week but didn't get an answer so thought I'd post it here:

Like all mechanical devices, my bicycle is less than 100% efficient. Therefore, if I traverse a circular route to end up where I started, I must exert more energy than if I was just walking.
(I know I can freewheel downhill (utilising potential energy) but on a circular route, I'll have just as much uphill to travel.)

Why is it, then that if I cycle my 24 mile circular route in 2 hours, I take less time to recover than if I walk the same route in 8 hours? (OK I'll use fewer pedal strokes when cycling than footfalls when walking but each requires more effort.)

OR have I made a faux pas and not realised that walking is actually far less efficient than cycling? If so, where is the wasted energy? Do my feet get hotter than when cycling? (I'm sure this is really a red herring. But IF walking uses more energy, it presumably must become heat - though I certainly feel hotter cycling!)

[another-view]

So am I right in thinking the question is "why do I recover from doing 2 hours exercise quicker than doing 8 hours"??

[chrisrob]

In part it's to do with recovery perhaps but why does it feel as if I've used less energy cycling when I've actually used more?

[another-view]

You haven't. You've obviously used more in an 8 hour walk than a 2 hour bike ride.

[gen2]

Walking is certainly not 100 percent efficient, but I don't know its exact value.

You are correct that most of the wasted energy is in the form of heat, but you don't feel as hot when walking because the heat is generated over a longer period of time and has longer to dissipate before raising your body temperature.

If you run the same course, you will generate roughly the same amount of waste heat, but in a shorter time and so you will feel much hotter.

[jake-the-peg]

I thought I gave you an answer saying that walking was less efficient and that the heat your body dissapated was spread over a longer period of time which is why you don't notice it..

Perhaps I forgot to press submit - sorry if that was the case.

To understand this you have to look at the numbers

A mile of walking uses about 100 calories; an hour of cycling at 10-12 mph is about 400 Calories (so 10 miles:400 calories = 1 mile 40 calories)
http://www.nutristrategy.com/fitness/cycling.htm
http://walking.about.com/cs/howtoloseweight/a/howcalburn.htm

So cycling is about 2.5 times as efficient as walking

Of course if you cycle faster you will encounter greater wind resisitance an the efficiency will drop

[chrisrob]

Thanks for your answers.
I'll get back on my bike, now.

[ABerrant]

jake-the-peg... you definitely pressed submit!

http://www.theanswerbank.co.uk/How-it-Works/Question818759.html

[Helveticus]

An interesting (?) related issue is just *why* cycling should be so much more efficient than walking. My guess (and it is no more than that) is that when walking one raises and lowers one's centre of gravity (CG) a little bit with each step. The amount by which the CG rises and falls is not much, but over the thousands of steps on a 24 mile walk, the energy required to lift it each time adds up. On a bike, by contrast, the CG remains at an almost identical height with each pedal stroke (assuming one remains in the saddle). Or is there some other, more important factor?

[Rev. Green]

Human joints generate heat through friction of parts sliding on each other. On a bicycle, the ball bearings in the ball races are rolling, not sliding, so the friction is less.

[ll_billym]

It is all here but not as exciting as I thought it would be it has to be said.

http://showcase.unis....et/MECH_frameset.html

[Helveticus]

Interesting point, Rev. Green. However, a bicycle has to be pedalled by a human, whose joints are subject to frictional losses. It may well be that these are lower when cycling than when walking, since the weight of a cyclist is to a large extent borne by the saddle. Frictional losses in the knees and ankles are probably less when cycling than when walking, where the knees and ankles bear a higher load. Could the smooth pedalling motion in cycling be intrinsically more energy efficient than the repeated impacts of walking?