Watts per KG and gravity question!

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by TomUK

Hi all,

I was having a discussion at work about watts per KG and climbing and was hoping someone could shed some light on this question.. bare with me

Lets just say for instance

Tony Martin can produce 6w/kg for 1 hour and Chris Froome can also produce 6w/kg for 1 hour why do these big TT specialists who can produce huge power for a hour at a time struggle on the big long steep climbs even when there are no attacks just high tempo? So if they are producing very similar w/kg why do they not climb at the same rate?

Is there a sliding scale of how gravity effects Weight X Power X Gradient?

Hopefully this makes sense, thanks in advance for the reply's

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by Calnago

Gravity is evil. It scares me. I don't want to talk about it :).
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by hna

You are assuming that they are producing the same W/kg. They don't. Froome climbs at more than 6 W/kg.

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by TomUK

If Froome can produce more w/kg then Martin why does Martin smash him in a TT? A W/KG is a W/KG on the flat, up or down. There is a 3rd variable and hopefully someone can explain it :)

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by hna

Air resistance. W/kg is only in effect on climbs, not on flat roads. The rider with the best W/cda (aerodynamics) will be the fastest. Because of the small differences between rider aerodynamics we can roughly say that the rider producing the most amount of power will be fastest on the flats. And Froome isn't exactly slow in the TT either.

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by 8bitMartin


Power is king on the flats, low weight is key in the climbs.

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by djconnel

If two riders have the same W/kg for an hour, they should be very close on their ability to climb for an hour. So I'd be surprised if Froome and Martin are the same W/kg for an hour: Froome has more wind resistance per unit mass, I'd suspect, and has higher W/kg.

But even if they did, there's also specificity. My W/kg drops off dramatically when I time trial because I don't practice time trialing. The position is different and the pedal dynamics are different. Martin specializes in time trialing, so can produce close to maximal power in the time trial position. If Froome is less practiced at time trialing, more at climbing, he will gain more power going to a climb than Martin. Martin may even lose power going to a climb.

But my guess is Froome's W/kg time trialing is higher from the start.

Time trialing isn't about W/kg, but W/CdA, so if you for example gain 2% of mass but 1% of power that might be a win for time trialing, but not for climbing.

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by CarpetFibre

Weight doesn't really matter on the flat. It's all about raw power (and of course aerodynamics). The steeper the gradient, the more that the power-to-weight-ratio takes effect. Really, W/kg on the flat is not terribly relevant. The graph above posted by 8bitMartin shows this.

Djconnel also hit the nail on the head with regards to specificity. Climbers may well be more tuned to cranking out the power whist sitting up on the tops, whereas the time-trial specialists can be more efficient when in that position.

Smaller people generally aren't as good at time-trialling, as they simply can't produce the numbers that slightly larger people can. I think the exceptions to this rule are small people who can firstly get a very good position on the bike, but can actually get the power out down there. That's where specificity comes in. You can't expect a really small person to be cranking out 450W on a climb or a time trial, but if they can hold 350W in an efficient TT tuck then they won't be a time trial specialist but they can at least hold their own.

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by mrfish

Simply put in terms of first order effects, power: frontal area counts on a flat TT, power:weight on a mountain climb.

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by Rick

The interesting thing to note from the chart (if accurate) is that gravity is already over 50% of the restraining force at only 3% grade. :unbelievable:

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by Tinea Pedis

Even more interesting, drive train is 3% of resistance. I'll take a well-maintained drivetrain for an extra 1%!

But agreed, at 3% I did not expect gravity to be 58% of resistance.

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by Tapeworm

It should be noted that the above model is based on a constant power output (of 300w). If speed were to increase (because of relative power output) and speed increased enough - aerodynamic drag would start to rise again as a more significant resistance.

But you'd have to be motoring 8)

Conversely, the slower you go (because of relatively less power output) the less aerodynamic drag plays a part.
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by nathanong87

i should really eat less.

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by HillRPete

I'm going for the "train more" (better) option.

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by dominikk

Both of you are right!
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