Do some bikes roll faster?

[Crackle]

No. The weight ( or properly, the mass) doesn't affect the acceleration. Ignoring friction, they will fall at the same rate.

C'mon, c'mon. Are you saying then that a heavier rider does not go faster than a lighter one down hill because he's heavier?

The heavier rider has the greater potential energy so surely he converts it to Kinetic energy quicker, especially when overcoming wind and bearing resistance?

 

[Dave5N]

C'mon, c'mon. Are you saying then that a heavier rider does not go faster than a lighter one down hill because he's heavier?

I am. Newtonian mechanics agrees with me.

The heavier rider has the greater potential energy so surely he converts it to Kinetic energy quicker, especially when overcoming wind and bearing resistance?

He has greater momentum (mass times velocity). He (or she) does not go faster because he is heavier.

 

[Crackle]

C'mon Dave, I'm on the verge of being proved foolish, I'll hold my hand up and wear a cap for a day but this is like pulling teeth. So their greater momentum will enable them to travel further but not faster.

Why does he/she go faster, given it's empirically observable that they do, as per original post (or is this a myth, the descends like a stone bit)? Are we to discount mass and point purely to air resistance and mechanical friction: I feel there's something missing but I'm resisting a Google as yet.

 

[Chris James]

Going downhill a heavy and light rider will accelerate the same (due to gravity). However, the heavier rider will have more momentum and so will be slowed less by air resistance etc (assuming both riders present the same frontal area)

 

[Dave5N]

C'mon Dave, I'm on the verge of being proved foolish, I'll hold my hand up and wear a cap for a day but this is like pulling teeth. So their greater momentum will enable them to travel further but not faster.

Why does he/she go faster, given it's empirically observable that they do, as per original post (or is this a myth, the descends like a stone bit)? Are we to discount mass and point purely to air resistance and mechanical friction: I feel there's something missing but I'm resisting a Google as yet.

That is exactly what I am saying. Both riders will descend at equal acceleration, although air resistance (not significant at low speeds but very significant as speed increases) and rolling resistance will exert a different opposing force on each person.

 

[Dave5N]

To get back to the original point. Undoubtedly some bikes will roll faster than others.

 

[Crackle]

Going downhill a heavy and light rider will accelerate the same (due to gravity). However, the heavier rider will have more momentum and so will be slowed less by air resistance etc (assuming both riders present the same frontal area)

And there we have it. Put much better than my clumsy PE/KE attempt.

So I don't need to change what I originally wrote, which incidentally was not an attempt to overturn Galileo or Newtons laws.

 

[Dave5N]

Sorry Crackle, but I'm not convinced that's true either. The air resistance is related to the speed of the rider and his (or her) frontal area, as well as a few other things, but not (I don't think - it is a very long time since I looked at a physics book) his (or her) mass.

It may well be true that a heavier rider is wider, though!

 

[fossyant]

Oh god.. this goes back to a huge debate that was on one of the Tour forums, years and years ago..it just went on and on....

The argument was... what makes the fastest descender ....... roll on all these arguments about weight / aerodynamics etc...... all OK in a lab...

Answer...it's the one that does the 'brain in back pocket' and goes for it - takes the risks.

As to the OP's question, some bikes will roll better, but loads of other factors as well like weight, position (i.e. more aero) and stuff...far too complex and get's away from the simple fact you can ride a bike quicker than your mates.....

 

[Crackle]

If two riders being different weights, set off downhill in a vacuum on equal machines, both would reach the bottom at the same time.

Introduce wind resistance and the heavier rider would have the advantage as he has greater mass and hence greater momentum. Take that to an extreme though and make the heavy rider as broad as a sail and the wind resistance will be so great he'll lose momentum rapidly and the lighter rider will come home first.

Empirically speaking, if I set off downhill with my fat mate (who says he's big boned 'casue he's in denial) he will plummet to the bottom faster than me. If however I tuck and he doesn't, I get there first: Try it.

I found this too: We're not the first to have this debate.

 

[fossyant]

And if you pedal, and if there are hair pins...... all bollocks...well the rider with the biggest 'theoretical' ones will win !

 

[Cycling Naturalist]

Funny, but more amusing is the pompous way certain people spout nonsense as if it is established fact, when a cursory glance at a GCSE physics book would put them straight.

I never got past the bit where you have colour in the balls dropped from the tower.

 

[Crackle]

And if you pedal, and if there are hair pins...... all bollocks...well the rider with the biggest 'theoretical' ones will win !

If you pedal then you're introducing another force and not just rolling. Bollocks? no in the sense that lots of professional riders and teams have invested time and effort into improving speed by altering position, for many reasons not just aerodynamics. Bollocks? yes in the sense of 'does it make any difference to me or Joe Soap, then it is, 'cause it doesn't, I'm not that good. The original premise was rolling downhill though, not pedalling downhill around bends.

 

[Dave5N]

If two riders being different weights, set off downhill in a vacuum on equal machines, both would reach the bottom at the same time.

Introduce wind resistance and the heavier rider would have the advantage as he has greater mass and hence greater momentum. Take that to an extreme though and make the heavy rider as broad as a sail and the wind resistance will be so great he'll lose momentum rapidly and the lighter rider will come home first.

Empirically speaking, if I set off downhill with my fat mate (who says he's big boned 'casue he's in denial) he will plummet to the bottom faster than me. If however I tuck and he doesn't, I get there first: Try it.

I found this too: We're not the first to have this debate.

I don't think that's true. Wind resistance or drag is proportional to speed and frontal area (and shape and the viscosity of the air no doubt).

I don't think it is proportional to mass.

 

[Dave5N]

I never got past the bit where you have colour in the balls dropped from the tower. :?:

Did you go over the lines?