Thrusting for the line

[thom]

In order to attempt to confuse things.... look at rowing!
On the slide forward, the boat effectively shoots forward a bit. Timing this properly at the end of the race is an obvious progression as some races are won by cm rather than metres.

Yes, this is a good analogy because on the slide forward the oars are not in the water causing an acceleration, so the lurch of the boat is purely a function of preserving the momentum of the boat whilst the bodies are shifting.

 

[Dan B]

[QUOTE 2009950, member: 45"]
But as I see it, throwing yourself forward on a bike isn't the same as you're not really pushing against anything. And, given that for every action there is an equal and opposite reaction, you push forward against nothing and so shouldn't move forwards.[/quote]
You don't have to be pushing against anything. Ever sat on a swing as a child and started it swinging by kicking your legs? Same principle

 

[Flying_Monkey]

I agree that a good thrust at the end can make you come first.

It took you a while to get there... but in the end you pulled it off.

 

[Pottsy]

[QUOTE 2010222, member: 45"]OK, but in the rowing example the body is going the opposite way to the direction of travel. Everything being equal means that this pushes the boat forwards a bit. On the line, doesn't Cav lurch forwards?[/quote]
No, he throws the bike forwards by thrusting and extending arms and legs. Effectively his body goes, relatively to the bike, backwards slightly (but it's actually also going forwards still, just briefly at a lesser pace than the bike).

As said before, it's all about timing. You have to do it at the perfect distance from the line to just throw the bike a foot or two further forward for a very brief period.

 

[gavintc]

[QUOTE 2010222, member: 45"]OK, but in the rowing example the body is going the opposite way to the direction of travel. Everything being equal means that this pushes the boat forwards a bit. On the line, doesn't Cav lurch forwards?[/quote]

I do not think this is correct. When the oars (blades) strike the water, the boat surges upwards and forward. On the return to the next stroke, the boat dips into the water and the forward movement is simply residual. I therefore do not believe that there is any additional speed when the rowers are returning to the rear position.

 

[Slaav]

I do not think this is correct. When the oars (blades) strike the water, the boat surges upwards and forward. On the return to the next stroke, the boat dips into the water and the forward movement is simply residual. I therefore do not believe that there is any additional speed when the rowers are returning to the rear position.

Please dont take this wrongly but have you rowed?

The 'Power' mainly comes from the core and larger muscles, the arms give more of a finish! (Isn't it always thus )

On the leg drive and power cycle, the momentum is put into the 'boat' and immediately after a clean pick up of the blade, the main mass in the 'boat' slides forward on the rails to prepare for the next power phase. Dont know the answer but I imagine the absolute fastest the boat actually accelerates and travels is probably immediately after the power phase as the main mass in the boat starts sliding backwards - obviously propelling the boat forwards! Simples....

So, coming back to the bike - the power through the pedals is negligible at the 'thrust' or finish but the momentum causes the bike to shoot or propel forwards as the mass is pushed backwards!

Again - simples

 

[thom]

[QUOTE 2010222, member: 45"]OK, but in the rowing example the body is going the opposite way to the direction of travel. Everything being equal means that this pushes the boat forwards a bit. On the line, doesn't Cav lurch forwards?[/quote]
As already pointed out, he's actually moving backwards on the bike.
Only thing to maybe add is his sprinting position starts out low and far forward over the bars, more so than other sprinters so it's possible he can get a more pronounced effect because he can move is body back further.

 

[Slaav]

As already pointed out, he's actually moving backwards on the bike.
Only thing to maybe add is his sprinting position starts out low and far forward over the bars, more so than other sprinters so it's possible he can get a more pronounced effect because he can move is body back further.

This is a very good point IMHO - if you start in a near 'perfect' sprinters position, then you have so much more momentum to thrust through the bike. If you are sat upright and at the back of the saddle, forget it!

After all, if you were standing with someone else right in front of you and leaning far forward towards said person without falling or tipping, then pushed into them as hard as you could without then falling on your 4rse backwards, the 'thrust' would be soo much more than if you were already leaning back and had no weight to throw into the other chap!

Interesting that it is not pronounced at all in rowing... many races are lost right on the line and timing the equivalent 'thrust' may win races? Or have I just missed it....???

As everything at elite level, I guess that small 0.05 seconds may make the difference between winning and losing

 

[gavintc]

Please dont take this wrongly but have you rowed?

The 'Power' mainly comes from the core and larger muscles, the arms give more of a finish! (Isn't it always thus )

On the leg drive and power cycle, the momentum is put into the 'boat' and immediately after a clean pick up of the blade, the main mass in the 'boat' slides forward on the rails to prepare for the next power phase. Dont know the answer but I imagine the absolute fastest the boat actually accelerates and travels is probably immediately after the power phase as the main mass in the boat starts sliding backwards - obviously propelling the boat forwards! Simples....

So, coming back to the bike - the power through the pedals is negligible at the 'thrust' or finish but the momentum causes the bike to shoot or propel forwards as the mass is pushed backwards!

Again - simples

I have, but I still do not buy that the boat is fastest after the finish and that there is a 'surge' on the return. My understanding is that the power of the stroke is when the blades are in the water - hence why you see a boat lift at this stage. The return allows a boat to run, but the movement of weight backward merely allows the run to continue, does not impart additional power.

 

[Slaav]

I have, but I still do not buy that the boat is fastest after the finish and that there is a 'surge' on the return. My understanding is that the power of the stroke is when the blades are in the water - hence why you see a boat lift at this stage. The return allows a boat to run, but the movement of weight backward merely allows the run to continue, does not impart additional power.

You may well be right

I may ask my old rowing colleagues and coach! I admit my thought is counter intuitive when you put it like that but I imagine it is far more complicated than we both think...

In fact, on a bike, it is nice and simple

 

[iLB]

This years tour, stage 13- mad throwing skills.

 

[Pottsy]

This years tour, stage 13- mad throwing skills.

That shows it perfectly. The body goes (relatively) backward, to briefly throw the bike forward. They were probably about even but the track lunge won it for him.

 

[Pottsy]

Also, excellent, they look like they have bent spokes

 

[thom]

That shows it perfectly. The body goes (relatively) backward, to briefly throw the bike forward. They were probably about even but the track lunge won it for him.

It is a good photo - I wouldn't be surprised if Greipel was behind to start out : the physics is about preserving momentum centered on the centre of mass of the body & bike as a unit:

Let's say the bike weighs 7 kg and Greipel weighs 77 kg, this means the amount the front wheel will move forward will be 11 times more than Greipel's body moves back. So assume Greipel's start position was with his head over the handlebars like Sagan, then you look at the distance Greipel's head moved and you see that ( ok 11/12 'ths of it) is visibly more than the distance between the front of the wheels.

There's a bit more to it than that because the head movement doesn't really represent how much he moved the center of mass of his body, but well, I'm just waffling now...

 

[Pottsy]

Sometimes I agree with a waffle