Tourmalet west side

[Le breton]

Today the stage ends at the Tourmalet.
Bottom of the climb in Luz St-Sauveur at 711m asl.
top, 18.7 km later and 1404 m. higher.

Assume no draft, a 70 kg cyclist, with 8kg equipment.
Take CdA = 0.4 m^2

assume average temperature 25°C. -> av. air density 1.03 g/cm^3.

Neglect the fact that the 1st km is not so steep . 4.5% or so.

Run various climbing speeds on analyticcycling.com

49 min : air resist = 53 watts , grav+rr = 383 W Total 436 watts
ie 6.23 watts/kg
add 2.5% for transmission loss = 6.38 W/kg
add 3% to compare with sea-level effort => 6.57 W/kg.

52 min : air res. 44 W, grav+r.r. = 361 watts total 405 watts
ie
5.78 W/kg
add 2.5% transmission -> 5.9 W/kg
add 3% to compare to sea-level => 6.07 W/kg.
-----

Drafting behind one racer reduces air resistance by about 25%, a saving of 11 watts for 52 min, ie 0.16 W/kg

At 49 min, the saving is 13 watts, ie 0.19W/kg.
---------
So how fast will they climb it?
--------------------
PS : it looks like the temperature might be considerably lower today.

When I write air resistance (which is a force), I actually mean power used to fight air resistance, i.e. FORCE times SPEED. I put 25% of the top of my head, I didn't look at references on what it should be at 20-25 kph.

The problem with looking at SRM data by actual racers is that you don't know if they are well calibrated, so that you have to check that they give at least reasonable values

 

[131313]

depending on the weather conditions, race tactics and the size of the group, I'd say between 48 and 53 minutes.


...which is why I think that looking at times up a climb in isolation (such as putting them as points on a graph) and trying to reach conclusions is downright silly.

The good thing is that Horner's power file (and Sorensen's, to a lesser degree) will give some really good insight into the actual power the top guys are putting out. Horner will have to go up the climb full gas. They're more interested in team GC than Levi's 6th place, so he'll be riding for himself and it will be a full-out effort to the top. It should be pretty easy to figure out the power of the two BFF's based on that info.

 

[Big GMaC]

Going on the other climbs I think I will go for a slightly slower time. I know that so far they have been marking/start stops rather than all out efforts, but still.

56 minutes for the faster climber/MJ Group

 

[David Suro]

To the OP:

Cool post, however I have one correction.

You stated that the power savings of drafting behind one other rider is approximately 25%. That is true if the roads are flat, the riders are of about the same size, and the speed is 40 kph.

Check out this cool article:

http://danpat.net/docs/brekaway.pdf

The benefit of the draft is greater at higher speeds and lesser at slower speeds.

Looking forward to tomorrow's racing...

 

[avanti]

................Drafting behind one racer reduces air resistance by about 25%,
...............

When climbing drafting offers very little aerodynamic savings on steeper slopes. According to one commentator the riders were climbing at 12 mph on the stepper part of Tuesdays stage.
There is a psychological advantage however in "drafting."

What does count is climbing energy required, which is very roughly equal to weight times height (ft-lb). So climbers with the lower weight have an advantage.

 

[131313]

To the OP:

Cool post, however I have one correction.

You stated that the power savings of drafting behind one other rider is approximately 25%. That is true if the roads are flat, the riders are of about the same size, and the speed is 40 kph.

Actually, that's incorrect. Look at his example again. He mentioned that rider wind resistance is decreased 25%, not that the power requirement is decreased 25%.

If you play with the numbers you'll see that the example he used, and the approximate wattage savings, are correct; they take into account gradient.

When climbing drafting offers very little aerodynamic savings on steeper slopes.

Well, it depends on "how steep", and how much is "very little". For the model of the Tourmalet, it's a 11-13 watt reduction, more if there's a headwind. I'd call that significant.

 

[lean mean &green]

Actually, that's incorrect. Look at his example again. He mentioned that rider wind resistance is decreased 25%, not that the power requirement is decreased 25%.

If you play with the numbers you'll see that the example he used, and the approximate wattage savings, are correct; they take into account gradient.

correct.

to clarify, the benefit of sitting behind another rider is fairly constant.

drafting at low speeds becomes less and less beneficial because wind resistance doesn't increase/decrease in a linear fashion. as a road turns upwards the effort required to overcome gradient very soon outpaces the need to overcome wind resistance.

EDIT: drafting while climbing is still beneficial, especially into headwinds.

 

[Kerbdog]

I just want to throw this into the mix also regarding headwind on a climb.Im gonna take for granted that the Tourmalet is going to be wedged with people today. They will be going through a sea of people at some stages. Surely the cover from them also will affect the drafting issue heading uphill?

 

[Animal]

Actually, that's incorrect. Look at his example again. He mentioned that rider wind resistance is decreased 25%, not that the power requirement is decreased 25%.

Don't argue physics with a guy who works at CERN! (If I've got the poster correct!)

 

[Mr.38%]

None of the favourites weighs 70 kg. 62-65 kg should be more appropriate. Bike 6.8 kg, one bottle 0.5 kg, gear/shoes 1.5 kg.

 

[Big GMaC]

None of the favourites weighs 70 kg. 62-65 kg should be more appropriate. Bike 6.8 kg, one bottle 0.5 kg, gear/shoes 1.5 kg.

It is a normalised calculation. You change it so every rider = 70kg + 8kg for bike / shoes etc.

Otherwise watts would not be very comparable as obvious a bigger rider produces more watts to climb at the same speed.

Each rider is normalised to 78kg and the watts adjusted up/down accordingly

Hope that helps

 

[Le breton]

None of the favourites weighs 70 kg. 62-65 kg should be more appropriate. Bike 6.8 kg, one bottle 0.5 kg, gear/shoes 1.5 kg.

Be my guest, run your own simulation on analyticcycling.com and see what you get

Beats me why people, you for example, don't want to do that and see by themselves that the actual weight of the racer ( except extreme cases) hardly makes any difference when the power up a steep mountain pass at a given speed is expressed in Watts/kg.

I guess you are too lazy

 

[Mr.38%]

I would have normalized to 65 kg, that's all I'm saying. Relative power stays relative of course.

 

[Big GMaC]

I would have normalized to 65 kg, that's all I'm saying. Relative power stays relative of course.

It does, it has just always been done to 70kg + 8kg as far as I know. Andy Sckleck does weigh about 68kg (He is 6'1/2), Wiggins over 70, so is Armstrong, Indurain, etc.

Edit: If Andy and Frank are both 1m86 / 6ft1 how come on wiki Frank is listed at 61kg and Andy at 68kg?

 

[Mr.38%]

Arm who? I was talking about *favourites*.

Anyway: If 70+8 is a quasi standard go on and ignore my rude interruption.

 

[Big GMaC]

Arm who? I was talking about *favourites*.

Anyway: If 70+8 is a quasi standard go on and ignore my rude interruption.

I only put Armstrong in as it mean we can refer to past climbs i.e. 1999-2005/2008 as an objective measure.

Not rude at all, you didn't know, now you do and hopefully you will be able to follow better now. Most of the stuff the physics/maths guys get into is beyond my level in that are, just happy to help a bit

 

[Benotti69]

None of the favourites weighs 70 kg.

an outside favourite for the stage maybe Vino 72kg, fat b*astard

 

[Big GMaC]

But actually, no one has a bloody clue what any of them weigh!

Especially 2 1/2 weeks into a stage race

 

[Mr.38%]

...just happy to help a bit

Thank's for the help, very appreciated.

/Mr.38%
8 years on SRM