Bike weight..?

[marcusjb]

You beat me by less than a minute.

Those seconds are everything to our more competitive members.

 

Well I had just finished emptying my cup of coffee at Kings Cross. Ning may have been carrying a full cup or something?

I also had semi-skimmed milk in my coffee, must be lighter right?  Marginal gains.

[mattc]

When people spend £1000's to make weight reductions of just a kg from their bike, there has to be some merit to the effect of fluid being carried on-board.

Yes of course - adding a kg of fluid slows you down just as much as adding a kg of steel.

But most of us are realistic about the effect of that kg - and generally don't value it at £1000 (or anywhere near).

To paraphrase Kim; the effect is lost in the noise.

Remember what a prat Chris Froome looked on that mountain cos he refused to carry an extra mars bar.

[marcusjb]

Well your body is constantly perspiring with fluid being evaporated from the skin. So it is not merely a case of 1.5kg being transferred from bike to body as it is not a static measurement.

When people spend £1000's to make weight reductions of just a kg from their bike, there has to be some merit to the effect of fluid being carried on-board.

Sure, but you are still claiming you can feel the difference between a bike with 2 full bidons and one without.

Only one thing for it - blind test. How well do you know your local climbs?

[Chris N]

I'd say that £1 per gram (or £1000/kg) of weight reduction is a pretty good target, if you're going to get all weight weenie about your bikes.  Rotating mass first, ti skewers last.

Diet and exercise are pretty much free.

[Glover Fan]

I need a power meter. I think it would be interesting to see what difference in times there would be at a constant 100w of power up the same hill with and without bidons.

[JamesBradbury]

Mainly because I am not worrying about playing that old favourite Audax game of 'Gas or Solid?'.

Oh! So that's what the pad in my shorts is for?

(Poor stoker   )

[Cudzoziemiec]

You beat me by less than a minute.

Those seconds are everything to our more competitive members.

 

Well I had just finished emptying my cup of coffee at Kings Cross. Ning may have been carrying a full cup or something?

I also had semi-skimmed milk in my coffee, must be lighter right?  Marginal gains.

No! Fat is lighter than water! Therefore full-fat milk is lighter than the same volume of semi-skimmed. Real weight weenies drink their coffee with double cream!

[Glover Fan]

No, just a single espresso...

[Biggsy]

Well your body is constantly perspiring with fluid being evaporated from the skin

and exhaling water vapour.

So it is not merely a case of 1.5kg being transferred from bike to body as it is not a static measurement.

When people spend £1000's to make weight reductions of just a kg from their bike, there has to be some merit to the effect of fluid being carried on-board.

Indeed, and even if all the weight of the water was retained, there's a difference in handling between weight on the bike and weight on the rider.

[Greenbank]

The dead weight of the bike can't help push down on the pedals.

1kg extra added to the rider (in a jersey pocket) will result in a slight speed increase compared to putting the same weight on the bike somewhere. It's just physics.

The trade off is that the extra weight on the rider can become uncomfortable (quickly if there's a lot of it).

[mattc]

I don't believe that's right GB:
Any weight that you use to push down on the pedals also has to be lifted back up ready for the next stroke.  It's just physics.

(This sort of thing can usually be discounted by the well-known TANSTAAFL principle.)

[Greenbank]

It's not true all the time, just pootling along at 10mph there won't be any difference at all.

But if you ever get out of the saddle (or push down hard enough on the pedals that the effective weight on the saddle is nothing) then you'll be getting the benefit of the extra weight. Yes you will work harder (since you have to lift your CoG back up to the top of the pedal stroke, which is now more work because of the extra weight), but the point is that you can put more downward force through the pedals because of the extra mass. More downward force available = faster.

The m in F=ma (for the entire bike) remains the same, but the F (pedal force) will be greater because of the increased mass of the rider.

It's not magically making you faster, it's just a device to allow you to extend beyond an existing limit (of downward pedal force) bounded by your physical mass. Much like a bicycle doesn't provide any energy itself, but it allows me to cover ground much faster and more efficiently than I'm able to even when sprinting.

[zigzag]

a comfy light bike with light wheels is a joy to ride - it's worth the expense for that reason alone . there's one bike that i really liked (test) riding, shame it costs £7.5k..

[mattc]

The only benefit will be to increase the peak-force available (at a cost in overall energy expended).

This is only relevant when you run out of gears and are barely capable of getting past TDC. You didn't specify that situation 

Even in that case, pulling on the bars makes a difference - so the extent to which you have trained your arms and thigh muscles etc will have far more effect than the kg of ballast in your back-pocket.

99% of the time there is much more to how fast you ride than the peak force applied.

Conclusion; it's almost never beneficial, and even where it is, there is a cost.

[Greenbank]

The only benefit will be to increase the peak-force available (at a cost in overall energy expended).

This is only relevant when you run out of gears and are barely capable of getting past TDC. You didn't specify that situation 

Even in that case, pulling on the bars makes a difference - so the extent to which you have trained your arms and thigh muscles etc will have far more effect than the kg of ballast in your back-pocket.

99% of the time there is much more to how fast you ride than the peak force applied.

Conclusion; it's almost never beneficial, and even where it is, there is a cost.

Good, so even with your interpretation you agree with my statement of:-

1kg extra added to the rider (in a jersey pocket) will result in a slight speed increase compared to putting the same weight on the bike somewhere. It's just physics.

However, I disagree with your view on how often it will be beneficial:-

I'd guess that even on my own commute I'm putting my full body weight on the pedals more than 30% of the time (either by standing on the pedals or just by pushing hard enough that there's no weight on the saddle). It's pretty flat and I never have to get to the point where I'm wrenching the bars, so I'm a long way off 'peak' power input. But for this 30% of the time the extra weight does lead to greater downforce which leads to me being faster.

I never said the extra speed was free.

[Ningishzidda]

Well your body is constantly perspiring with fluid being evaporated from the skin. So it is not merely a case of 1.5kg being transferred from bike to body as it is not a static measurement.

When people spend £1000's to make weight reductions of just a kg from their bike, there has to be some merit to the effect of fluid being carried on-board.

At approx 30 g per minute, which to my mind, is not really a fast enough flow to neck an energy drink and then hope that weight has gone within the next 200 metres before a hideous climb.

[mattc]

GB:
I note you've conflated power with force in your post. This is a big part of why you are wrong.

Your fitness limits your power available over any meaningful period. (Your speed is related directly to power - far more than to peak force.) So anything that saps power at some point in the stroke is going to slow you down overall. There is also the "perpetual motion machine" problem - you can't simply raise the 1kg and expect to get 100% of the energy taken back in some other form.

[ I think that without blackboards-n-stuff we are not going to get past this impasse, but at least I tried  ]

[Cudzoziemiec]

Well, I've just ridden up "my" hill in a higher gear and at a faster speed than normal and with less effort - because for once, my panniers were empty! (Bloody useless stocking at the Coop) Anecdote, of course.

[Ningishzidda]

"Effort".

There's another discussion topic.

[T42]

When the weight's on you then in balancing you move it automatically to the best place when you manoeuvre or accelerate. When it's on the bike you just have to make the best of it. Most of the time it'll work against you.

I noticed this a couple of years back, riding with a chum who had a 7-kilo pack on his back. I had much the same weight on my rack. He could throw the bike about and accelerate as normal, whereas I moved like a tank.

Mind you, he was the one who finished the ride with deep purple welts on his shoulders and his backside blistered to hell.

[Greenbank]

GB:
I note you've conflated power with force in your post. This is a big part of why you are wrong.

Mixing the words power/force doesn't matter. The amount of downforce on the pedals will be more for our unseated (or as good as unseated) rider if the rider is heavier.

And, again, I never said that it doesn't mean you need to work harder with more weight on the rider.

You're the only one talking about its effect on peak force. Maybe that's why you're getting so confused.

[mattc]

You could have the highest pedal-force in the peloton for the first 10 degrees of pedal-stroke - and still come last in every race.

OK, I'll try to outline a scenario to illustrate why the cost of the extra kg will slow you down overall.

Imagine we find you a 40kg backpack to wear. We setup a flat standing-start speed trial. You put all your weight over the pedal (at about 1o'clock position) to get your best start. Sure enough, that extra 40kg helps give you a really good shove off, and all your weight is on the pedal down at 6o'clock.

Now you've got to shift your pack-and-body-weight back up and onto the other pedal as it waits around the 12o'clock position. You might not even be strong enough to get back up there! But assume you're a bad-ass Laarndon Audaxer, so you struggle back up, while the bike freewheels along for a bit, gradually losing speed.

You manage another big push (with your 115kg or whatever). Great - you're back upto speed. You're also  a bit tired. Getting ready for the next push will be even harder. And so on ...

I would even hypothesise that by this stage you are already behind where you would have been without the 40kg. Especially if you'd chosen a gear that didn't need a massive force to get it turning!

I hope this makes it clearer.

[Ningishzidda]

Power requires a movement. Force making something move over a distance in a time.

If your force can’t make something change speed and/or direction of travel, its worthless.

Except when the force stops something falling with gravity, like a cyclist coming to a dead stop up a hill. 

[Greenbank]

I hope this makes it clearer.

Not really, your 40kg is taking it way too extreme. My original statement talked about the odd kg said:-

1kg extra added to the rider (in a jersey pocket) will result in a slight speed increase compared to putting the same weight on the bike somewhere. It's just physics.

The trade off is that the extra weight on the rider can become uncomfortable (quickly if there's a lot of it).

Also remember that the comparison is 1kg on the rider or 1kg on the bike. It's not 1kg on the rider or not.

[mattc]

The physics of 40kg is the same as for 1kg!

Oh well, I think we've reached stalemate. 'Til the next time ...