Power to Weight Ratio

sfichele

This has probably been asked before...

Power to weight ratio is important for hill climbing, but how is this actually defined? Is it

1) (Peak Power) / (rider's weight)

or

2) (Peak Power) / (rider's weight + bike)

Because if its (1), then its hideously wrong to lose weight whilst maintaining this ratio, you'll actually be slower.

It seems like its the second quantity that you want to maximise, since you drag yourself and the bike up the hill.

Pross

???

Most bikes will be at the UCI minimum so there's no opportunity to reduce weight there. If you lose weight off yourself then you will reduce the ratio as no-one in their right mind is going to then increase their bike weight to keep the ratio the same! The only danger is if in losing weight you decrease your power by such a degree that the ratio is less favourable.

Murr X

sfichele wrote:

This has probably been asked before...

Power to weight ratio is important for hill climbing, but how is this actually defined? Is it

1) (Peak Power) / (rider's weight)

or

2) (Peak Power) / (rider's weight + bike)

Because if its (1), then its hideously wrong to lose weight whilst maintaining this ratio, you'll actually be slower.

It seems like its the second quantity that you want to maximise, since you drag yourself and the bike up the hill.

Hi sfichele,

Power to weight is usually talked about in watts produced relative to body weight in kilograms - W/Kg.

Peak power is rarely discussed in hill climbing but much more commonly (and usually much more importantly) it is longer periods of time ie 20 - 60 minutes that you will hear about with climbers. Bike weight is not factored into this figure, so when you hear about a rider being able to produce 5W/Kg that is simply power produced per kilogram of current body weight, it is that simple.

For sustainable power output - 20minutes or above, adding muscle and therefore weight will not* aid in generating more power. That is why the top climbers are about as thin as they can be without sacrificing sustainable power output.


Murr X

*There can be exceptions to this but it is not often the case.

Stuey01

Lets do some sums... I'll make the numbers nice and easy for you.

Rider A has a peak power output of 400W and weighs 100kg

therefore his power to weight ratio is 4W/kg.


Rider A goes on a diet and loses 20kg, he is able to maintain his peak power output of 400W

His new power to weight ratio is 5W/kg. An improvement of 25%.

The whole point of the exercise is lose weight and maintain as much as possible your power output, thereby improving your power to weight ratio.
Losing weight and maintaining your power to weight ratio would mean losing proporionately as much power as you did weight. Why would anyone want to do that?!

Edit to mention that time trialists and trackies are typically less concerned power to weight ratio than simple power output, as the usual course is flat(ter) in profile and therefore weight has less (note, not none at all) influence because you aren't hauling your ass up a big hill.

sfichele

Obviously the whole point of the exercise is to lose weight whilst keeping the same power.

But, the reason I ask is because people like Wiggins have apparently lost weight whilst maintaining their power-to-bodyweight ratio. i.e. he's dropped weight and dropped a bit of power but maintained the ratio. [ I hope I've got this right, articles seem to state that he has maintained the ratio ]

This seems wrong to me (is wrong), because when you factor in the weight of the bike, that would mean he's actually slightly slower when going up hill.

Despite this defining the ratio by just body weight seems daft. Why because a heavier rider with the same ratio will be faster than a lighter rider going up hill because of the weight of the bike.

Pross

I'm pretty sure that what he has been trying to do is drop the weight but not the power and therefore increase the ratio. Whether he has actually done this is another matter but he won't have been aiming to keep the same ratio as that would be counter productive as you say.

Pokerface

sfichele wrote:

Obviously the whole point of the exercise is to lose weight whilst keeping the same power.

But, the reason I ask is because people like Wiggins have apparently lost weight whilst maintaining their power-to-bodyweight ratio. i.e. he's dropped weight and dropped a bit of power but maintained the ratio. [ I hope I've got this right, articles seem to state that he has maintained the ratio ]

This seems wrong to me (is wrong), because when you factor in the weight of the bike, that would mean he's actually slightly slower when going up hill.

Despite this defining the ratio by just body weight seems daft. Why because a heavier rider with the same ratio will be faster than a lighter rider going up hill because of the weight of the bike.

I think what you've read about Wiggins is that he lost weight but maintained his POWER. Not his power to weight ratio.

That would mean his power to weight ratio actually improved.

Pokerface

sfichele wrote:

Despite this defining the ratio by just body weight seems daft. Why because a heavier rider with the same ratio will be faster than a lighter rider going up hill because of the weight of the bike.

The weight of the bike is irrelevant as it is basically a constant. Bikes will weigh the same for all riders so it doesn't come into the equation.

Two riders, regardless of their own body weight, with the same POWER TO WEIGHT ratio will climb at the same speed.

No_Ta_Doctor

Pokerface wrote:

sfichele wrote:

Despite this defining the ratio by just body weight seems daft. Why because a heavier rider with the same ratio will be faster than a lighter rider going up hill because of the weight of the bike.

The weight of the bike is irrelevant as it is basically a constant. Bikes will weigh the same for all riders so it doesn't come into the equation.

Two riders, regardless of their own body weight, with the same POWER TO WEIGHT ratio will climb at the same speed.

Actually, no.

As the OP correctly points out, the heavier rider will be marginally faster. The weight of the bike is constant between the two, but the heavier rider produces more power.

Just to provide a quick but extreme thought experiment - put an ant with the same power-weight ratio on the same bike (it's got very long legs and can actually cycle). The ant simply cant generate enough power to get up the HC climbs.

sfichele

Two riders, regardless of their own body weight, with the same POWER TO WEIGHT ratio will climb at the same speed.

Wrong ! The bike weight does factor.

Take a bike of 7 kg, 5 % gradient, no wind, Lets go for 5W per kg of body weight

Rider A 50 Kg -> 250 W, rider B 100 Kg -> 500 W

http://bikecalculator.com/veloMetric.html

Gives rider A @ 21.87 kph, and rider B @ 24.94 kph

Pross

Pokerface wrote:

sfichele wrote:

Despite this defining the ratio by just body weight seems daft. Why because a heavier rider with the same ratio will be faster than a lighter rider going up hill because of the weight of the bike.

The weight of the bike is irrelevant as it is basically a constant. Bikes will weigh the same for all riders so it doesn't come into the equation.

Two riders, regardless of their own body weight, with the same POWER TO WEIGHT ratio will climb at the same speed.

Not if you go down his route where the ratio stays the same despite a rider losing weight, the bike would then be an additional weight with the rider having less power is I think what the argument is

e.g Rider currently 80kg, bike 7kg, power 400w. Power to (rider) weight ratio = 5, power to overall weight ratio = 4.60

Rider loses 10kg but maintains a power to body weight ratio of 5 so power is now 350w. Power to combined weight would therefore be 350 / 77 = 4.55

A bit of a pointless exercise as no rider in their right mind would look at losing weight but keeping the ratio the same and in any case the respective weight of bike to rider will mean the differences are negligible even in the above example with extreme weightloss.

Verbal

So once all the sums have been done and everyone is in agreement, does anyone have any tips on losing weight but maintaining power output? Nutritional ideas, training etc.?

meanredspider

I may be talking complete bo...rubbish but losing weight and maintaining power-to-weight would benefit other areas such as braking and cornering

jezcc

of course on the flat it's all about power to drag ratio, drag is pretty much a function of physical dimensions and body position. So you need to be strong for your height and flexible to be fast on the flat (Fabian Cancellara)

jezcc

Sprinters meanwhile have to have a very high anaerobic power output. Again power to drag ration, but they aren't trying to minimise drag, just produce a maximum power output. Mark Cavendish is much smaller than Allesandro Petacchi, so is less draggy in a sprint, but his peak anaerobic power output is obviously similar or more.

Pross

Verbal wrote:

So once all the sums have been done and everyone is in agreement, does anyone have any tips on losing weight but maintaining power output? Nutritional ideas, training etc.?

If you are using cycling to lose weight then I would have thought (assuming a novice) that your power will increase as you lose weight as you will be strengthening your cycling muscles. This is getting to a stage where it needs to be moved to th etraining section though I would say. Someone on there will be able to advise specific routines for increasing power.

andyrr

One point the OP is maybe touching on is the fact that the pros will be on the edge often of losing weight such that it also loses muscle but the fne balancing act is to kepe power as high as possible whilst doing so, ie muscle = weight but obviously muscle also = power. When you are running at 3 - 5 % body fat and I wonder if it is possible to keep the lack of weight gain down to purely not putting on body fat as opposed to losing muscle.
I'm sure I've read of it being acceptable for a pro to be slightly katabolic (think that's the word ?) where they will slightly decrease their muscle as it results in a more efficient body.

Gingerflash

My understanding is that the power figure is usually taken at anaerobic threshold, i.e. a power output that can be maintained for quite a while, say 30 minutes on a climb. For the best climbers, this is often 400W, and it's thought that you need power-to-weight of at least 6.2W/kg to have a shot at a grand tour.

Someone like Cancellara might produce say 500W at threshold but weighs a lot more. On the flat, the extra power produces great performace when it's power versus drag. add gravity so that it's power v weight (and a bit of drag) and he's a lot slower.

Peak power is absolute max, usually way over 1000W, and is of most relevance to sprinters.

Pross

andyrr wrote:

One point the OP is maybe touching on is the fact that the pros will be on the edge often of losing weight such that it also loses muscle but the fne balancing act is to kepe power as high as possible whilst doing so, ie muscle = weight but obviously muscle also = power. When you are running at 3 - 5 % body fat and I wonder if it is possible to keep the lack of weight gain down to purely not putting on body fat as opposed to losing muscle.
I'm sure I've read of it being acceptable for a pro to be slightly katabolic (think that's the word ?) where they will slightly decrease their muscle as it results in a more efficient body.

This is the concern I have that Wiggins may have done but I'm sure he has regular body fat scans and power tests that would prevent it. I certainly don't think he will have deliberately sacrificed power to lose weight.

Bronzie

Gingerflash wrote:

it's thought that you need power-to-weight of at least 6.2W/kg to have a shot at a grand tour.

6.2W/kg is often quoted as the limit of what is physiologically possible (without "assistance")

Analysis of this year's TdF climb of the Tourmalet seem to point to lower threshold powers being achieved by Contador and Schleck (albeit towards the end of 3 weeks of hard racing)
http://www.sportsscientists.com/