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Kicking the bike backwards when standing... what's the cause?

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Road Cycling “It is by riding a bicycle that you learn the contours of a country best, since you have to sweat up the hills and coast down them. Thus you remember them as they actually are, while in a motor car only a high hill impresses you, and you have no such accurate remembrance of country you have driven through as you gain by riding a bicycle.” -- Ernest Hemingway

Kicking the bike backwards when standing... what's the cause?

Old 02-10-09, 09:10 AM
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Kicking the bike backwards when standing... what's the cause?

Shifting ones weight forward? starting your stand with the pedal down (no force)?

/discuss
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Old 02-10-09, 09:22 AM
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Physics.
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Old 02-10-09, 09:25 AM
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Originally Posted by jmgorman
Physics.
Gravity too
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Old 02-10-09, 09:31 AM
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If you have someone on your wheel, warn them you are about to stand.
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Old 02-10-09, 09:36 AM
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user error
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Old 02-10-09, 09:37 AM
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Originally Posted by jmgorman
Physics.



Naaaaaahhh!

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Old 02-10-09, 09:45 AM
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I could tell you, but then I'd have to kill you.
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Old 02-10-09, 09:55 AM
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Originally Posted by wfrogge
Shifting ones weight forward? starting your stand with the pedal down (no force)?

/discuss

In the sense of center of mass (COM), if you're COM is moving up a steep hill at 10mph and you stand, you're probably moving 90% of the bike weight (your body) forward faster than 10mph, meaning the bike itself has to either slow down or go backwards (just for a moment). Now, I suppose you could apply sufficient force just as you started standing up that you keep things moving forward.

Even if all the wheel does is slow down, someone close on your wheel could bump it.
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Old 02-10-09, 10:03 AM
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How do you counteract this mysterious and dangerous effect?

/discuss
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Old 02-10-09, 10:03 AM
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If it's gravity why does it happen on the flat too?
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Old 02-10-09, 10:15 AM
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Originally Posted by MONGO!
If it's gravity why does it happen on the flat too?
Because gravity doesn't go away when the road gradient is zero?
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Old 02-10-09, 10:20 AM
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Originally Posted by SpongeDad
In the sense of center of mass (COM), if you're COM is moving up a steep hill at 10mph and you stand, you're probably moving 90% of the bike weight (your body) forward faster than 10mph, meaning the bike itself has to either slow down or go backwards (just for a moment). Now, I suppose you could apply sufficient force just as you started standing up that you keep things moving forward.

Even if all the wheel does is slow down, someone close on your wheel could bump it.
It depends on your frame of reference. As the rider who's standing, it appears that you are moving forward when you stand, because your frame of reference is anchored at the bike.

If you're another rider, your frame of reference is your own bike. When the guy in front stands up, his weight moves forward relative to his bike, but his COM continues at the same speed. Since the bike is lighter and the COM didn't move relatively, the bike moves closer to yours.

The effect would be much smaller if we just rode much heavier bikes
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Old 02-10-09, 10:27 AM
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Originally Posted by SilentShifter
How do you counteract this mysterious and dangerous effect?

/discuss
It's not so mysterious. When some people stand, the first thing they do is pull themselves forward off the saddle with their arms. When you do this, you are pulling the bike back by the handlebars. If the first thing you do is push hard on the pedals and let your legs raise you off the saddle, then the bike doesn't jump back.
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Old 02-10-09, 10:31 AM
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Originally Posted by bdcheung
Because gravity doesn't go away when the road gradient is zero?
So your saying the reason a bike moves "backwards" when a rider stands on flat ground is because gravity pulls it back?


Originally Posted by nafun
It's not so mysterious. When some people stand, the first thing they do is pull themselves forward off the saddle with their arms. When you do this, you are pulling the bike back by the handlebars. If the first thing you do is push hard on the pedals and let your legs raise you off the saddle, then the bike doesn't jump back.
Is the correct answer.
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Old 02-10-09, 10:32 AM
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I didnt say it was mysterious..... just wanted a sanity check.
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Old 02-10-09, 10:45 AM
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Originally Posted by wfrogge
I didnt say it was mysterious..... just wanted a sanity check.
Sorry wfrogge, the "mysterious" part was a reply to post #9.
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Old 02-10-09, 10:47 AM
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Old 02-10-09, 10:47 AM
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Originally Posted by MONGO!
So your saying the reason a bike moves "backwards" when a rider stands on flat ground is because gravity pulls it back?
No, I was answering your question.

In my previous post, I said the reason a bike moves backwards when a rider stands on flat ground is because of the law of reciprocal actions.
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Old 02-10-09, 10:48 AM
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it's basically the reverse of throwing your bike at the line. If there is someone on my wheel, I make an effort to stand smoothly and without throwing the bike backwards. It takes some effort.
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Old 02-10-09, 10:54 AM
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Originally Posted by bdcheung
No, I was answering your question.

In my previous post, I said the reason a bike moves backwards when a rider stands on flat ground is because of the law of reciprocal actions.
I don't see that post anywhere
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Old 02-10-09, 10:56 AM
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Originally Posted by MONGO!
I don't see that post anywhere
https://www.bikeforums.net/showpost.p...62&postcount=7

or do I need to spell everything out?
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Old 02-10-09, 11:08 AM
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Originally Posted by bdcheung
Because gravity doesn't go away when the road gradient is zero?
It has nothing really to do with gravity.
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Old 02-10-09, 11:10 AM
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Originally Posted by bdcheung
No, I was answering your question.

In my previous post, I said the reason a bike moves backwards when a rider stands on flat ground is because of the law of reciprocal actions.
You posted a picture of newton, you didn't specify if you were alluding to his 3rd law of motion or gravity.
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Old 02-10-09, 11:12 AM
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Originally Posted by Phantoj
It has nothing really to do with gravity.
Originally Posted by MONGO!
You posted a picture of newton, you didn't specify if you were alluding to his 3rd law of motion or gravity.
Both of you are correct in your observations.
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Old 02-10-09, 11:54 AM
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Originally Posted by nafun
if the first thing you do is push hard on the pedals and let your legs raise you off the saddle, then the bike doesn't jump back.
+1
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