I can go faster

[Matt2matt2002]

Have I discovered something new?
On my usual 17 miles whiz around the lanes of South West Scotlandshire today, I was on a long straight stretch with a slight downward gradient.
Don't ask why but for some reason I took both legs off the pedals, raised both legs outwards away from the frame, swung them backwards and then forwards. I immediately felt the bike "rush" forwards!
(I didn't feel the bike slow when my legs went backwards)
Any physicists out there who can put me right on what happened?
If I had kept doing the swing thing would I have kept going forever? 

[Danneaux]

If I had kept doing the swing thing would I have kept going forever?

<Psst!  Matt!>  This is confidential, but the answer is...
.
.
.
.
...yes!

But don't tell anyone, or they'll never sell another bicycle with drivetrain, economies will collapse, and the world as we know it will end.

Have I discovered something new?

Ehm, not "new", per se...the knowledge has been around for ages, but suppressed, and for very good reason.

All the best,

Dan. (...who's scared to death of what happens when this becomes common knowledge...)

[Danneaux]

Oh, drat!  The word has already gotten out, Matt. Perhaps this explains the current state of the world economy.

At any rate, you may find the information at the following link related to "dynamic weight transfer" and the concept of "dynamic wedge" to be of interest in light of your own experience:

http://www.ozebiz.com.au/racetech/theory/wttrans.html

Best,

Dan. (...exploring my "off-the-grid" options as I write this...)

[Matt2matt2002]

Thanks for the link.
Very interesting but (I'm) stupid. 
I couldn't spot anything about sticking your legs out the side of racing cars.

Is it a perceived effect? Did I spend energy swinging my legs backwards and then perceive the forward motion?
Perhaps because I was having a free ride down hill at the time, and this combined with the energy I had spent, I felt the forward motion amplified?

I suspect you have the answer to this (for me) conundrum.
I usually abide by the KISS principle, so,please enlighten me and keep it simples. 

[Danneaux]

...Very interesting but (I'm) stupid...

No, Matt, you're no stupider than me, or we're both stuck at the same level of stupitude.

I spent the last hour trying to model this behavior and finally gave it up as too complex and with too many variables to do accurately. All my old physics formulae apply to conservation of angular momentum and such, and this is a linear problem having to do with the conservation of energy and forward momentum. In the interests of time and getting something (anything!) else done today, I've got to admit temporary defeat (though my mind will keep working at it).

What I think is happening is this (and, yes, I got intrigued enough to get the bike out and try it out in the street in front of my house. By this time, the neighbors think I'm nuts for being so interested in bikes anyway, so there's little to lose; they probably figure it's just a continuation of my downward spiral...):

= The bike and your collective mass have forward momentum.

= When you swing your legs back, that counters the forward momentum to a degree, but to move your legs back, you're also propelling your body forward.

= When you swing your legs forward, the forward transfer of their mass (what, maybe 20kg or so?) is additive to the greater mass of yourself and the bike (a seeming gain), but swinging your legs forward propels your upper body back, so that would cancel out.

I don't yet have my bike computer hooked-up to the Nomad, but I would imagine any speed gained by swinging your legs forward would be cancelled by swinging them back. It would be neat to see if there is a momentary gain/loss in speed, but I can't imagine any net-total gain in forward momentum resulting from swinging your legs.

Sadly, I think it is a perceived effect. I think you called it exactly when you opined...

Perhaps because I was having a free ride down hill at the time, and this combined with the energy I had spent, I felt the forward motion amplified?

Perhaps our friend and colleague JimK can save us with a timely and cogent analysis. I'm guessing if anyone can, it is he.

All the best,

Dan. (...proving the neighbors right one Bicycle Venture at a time...)

[JimK]

This is a nice puzzle! It will probably take some experimenting to figure out exactly what's going on!

Here is one fun game I thought about when I was reading Wilson's Bicycling Science: suppose you are on a road that goes up and down every couple feet or so. I think you can propel yourself forwards indefinitely by moving your body around on the bike. Get low on the bike at the top of a hill and then on each down slope push yourself up high. Then on the up slopes you take the weight off the bike by getting low again. There's probably a way to time some back-and-forth swinging motion that will also work, pushing and pulling, working with the varying shape of the road.

It's fascinating to watch skateboarders. I think you can get some turning into the action and make it work. Instead of the road going up and down, you can turn to make your path wiggle back and forth, and then by shifting your weight along with the curves... these BMX bikes have really low seats, the point of which I think is essentially to create more room for getting low, a wider range of rider heights.

I remember back in college, in freshman physics, I was playing with how to pump on a swing. Here was my scheme: stand up on the swing; each time you get to the bottom of the swing's movement, stand up tall. At the top of the swing's movement, both forward and backward, squat down. Wow! You can get going scary high that way, in a hurry! The more usual swinging style is what you were doing on your bike. I confess I have never really figured that out.

The whole business of moving one's body around on a bike and how that affects the bike's motion - it's really fascinating!

[Matt2matt2002]

Thanks folks.
I must confess to having discovered this quite a while ago but only thought of bringing it to this forum today having been continually impressed at the quality of technical debate and friendly camaraderie.
All I can add at this point is to recommend that anyone who tries this out to be unsighted from friends and neighbours.

[JimK]

Get low on the bike at the top of a hill and then on each down slope push yourself up high. Then on the up slopes you take the weight off the bike by getting low again.

This stuff is subtle and I think I got this wrong. A nice general principle: in a frictionless system, if you can figure out how to pump energy in somehow, it just has to turn into motion.

With rolling hills or a twisting path, the force pressing you down varies. If you lift yourself up in the high force region and then let yourself back down in the low force region, then you are doing net positive work around the cycle, and that has to turn into the motion of the bike.

On rolling hills, the high force region is at the bottom of the valley where the road is concave upward. The low force region is at the top of the hill, where the road is concave downward. So to pump the bike forward, push yourself up at the bottoms of the valleys and let yourself back down at the tops of the hills. Just stay high on the up slopes and just stay low on the down slopes.

On a curvy path, the high force region is on the curves and the low force region is on the straight parts or the inflection points where the curve changes directions. Even on an elliptical path you should be able to pump: the high force region is the tightly curved part and the low force region is the less curved part. Lower yourself at the inflection points and stand up high where the tightest part of each curve is. Of course this is for a vehicle where the weight is high up dynamically balanced over a narrow support.

I realize this isn't quite the puzzle originally presented in this thread, but that's how I usually attack these things: I start by looking at similar or related problems that I can get some purchase on. Sometimes it will fertilize a little insight!

[JimK]

Another fun way to get a bike going without pedaling is to use the brakes! If fling your body forward and backward on the bike, and apply the brakes at the back to push off forwards but let off the brakes when catching yourself at the front and returning backwards... that ought to be another way to move the bike forward, this time going straight and level. If the road alternated between loose stuff with high rolling resistance and smooth stuff with low rolling resistance, that could work instead of using brakes.

No wonder my girlfriend thinks I am nuts, all this crazy stuff running around in my head. Yeah, I might save the testing for a quieter stretch of road!
 

[JimK]

I should say, there is a fundamental law of physics, the conservation of momentum. If some complex system is sailing along with no external forces being applied to it, then the overall speed of the system can't change. If the system falls apart then the individual bits can go any speed as long as the other bits compensate through some sort of recoil. But if the system hangs together however loosely, its speed can't change.

So the fun here is to get around that "no external forces" clause. Mostly the ground just pushes up so that can't affect the forward movement. But hills, curves, rolling resistance... those aren't simply "up", so they create opportunities.

[Danneaux]

This sure is fun, isn't it, Jim? I'm really grateful to Matt for bringing it up on the Forum. I've enjoyed thinking about the problem all day since!

No wonder my girlfriend thinks I am nuts, all this crazy stuff running around in my head. Yeah, I might save the testing for a quieter stretch of road!

Join the crowd, Jim; you're in good company! It is a shame we don't all live in the same place. What a fun time we'd all have. Given enough of us, we'd comprise the "New Normal"! Meantime, we can give them all something to talk about. 

All the best,

Dan. (...whose neighbors' curtains hardly wiggle anymore unless there's something really exotic, like a bike-trailer crash with sparks at night during high-speed handling trials in front of their houses)

[Matt2matt2002]

So....
I swing my legs out, bob up and down on hills and lean over every so often?

And today I went even faster with Metallica on the ipod!

Anyone else go faster with rock n roll on the 'phones?
 

[Danneaux]

Anyone else go faster with rock n roll on the 'phones?

I'm guessing jags will answer "YES!", but I can't say either way, as I ride without music; I'm skeert I might not hear an overtaking car in time, and it's kinda nice to hear the birds and bike and such.

In a similar vein, I'm still working on suitable helmet-strap-mounted ear wind-blockers. Research continues, but I'm not quite "there" yet with a final version. Frankly, the early versions look a bit like Mickey Mouse ears. This, along with my efforts to replicate and validate your research in a different setting as I...

...swing my legs out, bob up and down on hills and lean over every so often?

...should be all the neighbors need for a competency hearing.

Scientific progress is rarely easy...you have my support.

All the best,

Dan.

[rualexander]

In a similar vein, I'm still working on suitable helmet-strap-mounted ear wind-blockers. Research continues, but I'm not quite "there" yet with a final version. Frankly, the early versions look a bit like Mickey Mouse ears. This, along with my efforts to replicate and validate your research in a different setting as I......should be all the neighbors need for a competency hearing.

Have you seen these Dan? http://www.slipstreamz.com/content.asp?subID=8

[Danneaux]

Oh! Rual!

Nice find!  I had seen this firm's products about a year ago, but at that time, they looked a bit different. As I recall, they had just the little clip to warp the helmet strap (the "Spoiler" model), or a sort of ear-bud kind of thing...but with this latest iteration ("The Slip") -- real promise!

Made in Durban, S.A. ...Ah, that rings a bell. At one time, I knew the daughter of the mayor there.

I'll investigate further, as this is the direction I have been going, but all ready-to-roll.

Nicely found, and much appreciated!

All the best,

Dan.