Power/weight question...

[Panter]

Hypothetically, there are two riders, climbing the same, very steep hill at exactly the same pace, with a tailwind of exactly the same speed that they're maintaining, on identical bikes.

Rider A, with bike, weighs exactly 12 Stone

Rider B, with bike, weighs exactly 24 Stone.

If rider A is generating 400 Watts to hold the pace, how much power is rider B generating to keep with him? exactly double?

 

[Oldlegs]

Not quite double.

Some of the 400 watts will be for frictional losses in the bike - more or less the same in both cases.

The bigger rider will get that bit more help from the tailwind

 

[mcshroom]

The bigger rider will get that bit more help from the tailwind

I wish that was true

 

[screenman]

What is the gradient? As this will have a bearing on the an effect on the equation.

 

[Panter]

If the tailwind is travelling at the same speed as the riders, surely it wouldn't assist either?

Does the gradient matter then? is it only double the power required to a certain gradient? If it affects it, then lets assume it's pretty steep, 30% or so.

 

[Steve H]

I don't care how fit he is or how theoretical we are making it...

If a 24 stone bloke attempts a 30% slope, he'll be off and panting!!

 

[Tigerbiten]

I don't care how fit he is or how theoretical we are making it...

If a 24 stone bloke attempts a 30% slope, he'll be off and panting!!

Me + trike + trailer + camping gear weighs is at around 24 stone.
I've cycled up a 30% slope on it.
I admit I've got three wheel and a 12.5" bottom gear
And I was doing sub 2 mph ........
But I did not get off and walk ......

 

[screenman]

I would imagine double would only occur on a vertical plane, as some of your force for both riders would be spent on going forwards. I would have though this would be quite simple for anyone doing maths A level to work out as it should be simple trigonometry. As for me well expelled at 15 seemed to be an end to my education, but one son has a master in maths and the other one is doing it now, so they may have a play with the answer, or knowing kids may not.

 

[Sittingduck]

We need Jimbo here!

 

[ColinJ]

I think the point of the tailwind is to pretty much take air resistance out of the calculation. That done, the only things that the cyclists have to fight against are rolling resistance, transmission losses and lifting the weight of themselves and their bikes up the slope. Ideally, rolling resistance and transmission losses  will be very small compared to the effort of climbing so let's ignore them. I say - yes, (near enough) rider B needs to be producing 800 Watts to keep up with rider A.

(In reality, transmission losses should be roughly the same for the two riders and rolling resistance might be slightly higher for rider B so (s)he would actually need to produce a little over double the power but not much extra - say 801 W vs 400 W.)

 

[chillyuk]

I'm 19 stone and I would burn very little energy going up a 30% slope.

walking, pushing the bike, is easy.

 

[ColinJ]

I'm 19 stone and I would burn very little energy going up a 30% slope.

walking, pushing the bike, is easy.

With respect, that's b******s!

It might be easier, but it isn't easy!

I don't think you have a lot of 30% hills in Essex. In fact there aren't that many even up here in Yorkshire, but I have been up plenty of 25% on foot without a bike and it takes some effort, and I weigh 4 stone less than you!

This one is only 20% or so and the people I've seen pushing bikes up it look pretty knackered!

This little one is about 30% - let's see you walk your bike up that without you getting out of breath!

 

[jimboalee]

Use PowerCalc on the CTC website.

There are two differences to be considered. 1/ Cross section area, and 2/ Coefficient of Drag.

The cross section area of the big guy wouldn't be double the small guy because a human body consists of a collection of ellyptical solids.
The Cd of the big guy might be LESS than the small guy because his Xsection is NOT double, but his mass IS.

The Cd of the 24 stone vehicle is anybody's guess. We would have to recruit a 22 1/2 stone chap to do some Rolldown tests.

If the two climbed the same hill side by side on the same day, their Joules excertion can be calculated. Mass x gravity x metres rise. This is divided by the time it took them, which is J/s, which is Watts.
Watts for forward motion would be negligable, as the 24 stone vehicle might be doing 2 mph.

 

[jimboalee]

I'm 19 stone and I would burn very little energy going up a 30% slope.

walking, pushing the bike, is easy.

You are lifting YOU and a BIKE against gravity. The energy is the same, or very damned close. It FEELS easier because when you walk up a hill, less fatigued muscle bundles are being used.

 

[jimboalee]

Every cyclist knows that an eighteen inch gear is the equivalent of walking.

120 steps per minute ( 60 cadence ) at 3 mph.