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Garfield2010
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We have all heard that it makes a big difference for touring rugby teams between playing at the coast or in Johannesburg.

 

Exactly how much difference will it make for a guy from Johannesburg to do an event in Cape Town?

 

Say you have two exact same route profiles and conditions in Joburg and in Cape Town. What would the same persons time be for both if he did a time trial? Give or take a few? 10 minutes faster on a 100 km's?

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We have all heard that it makes a big difference for touring rugby teams between playing at the coast or in Johannesburg.

 

Exactly how much difference will it make for a guy from Johannesburg to do an event in Cape Town?

 

Say you have two exact same route profiles and conditions in Joburg and in Cape Town. What would the same persons time be for both if he did a time trial? Give or take a few? 10 minutes faster on a 100 km's?

 

Think its between 5 and 10% if I'm not mistaken.

 

Personally I think its over rated and you only really feel it when going the other way round, from the coast up to altitude.

 

The difficulty of the route will make up for the difference you may feel.

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I have read that the increase in power is somewhat offset by an increase in drag due to thicker air, but I haven't run any numbers to confirm this.

I did some playing around with the calculator here (using the default values):

http://www.analyticcycling.com/ForcesSpeed_Page.html

 

I turns out that if you have a sea level functional threshold power (FTP) of 250W, you'd be able to hold 40.4km/h on a flat road at sea level, 41.3km/h at 1500m and 42.1km/h at 3000m.

 

So, you have more power at sea level but the air is thicker and it thus takes more effort to ride through it.

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Cycling is one of the few sports wher altitude can be an advantage. Which is one reason why Eddie Merckx selected Mexico City (6000+ feet) in 1972 for his hour time trial record. The lower wind resistance at that altitude was a bigger advanatge than the more difficult breathing conditions. (Merckx was not altitude acclimatised before the event)

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I did some playing around with the calculator here (using the default values):

http://www.analyticcycling.com/ForcesSpeed_Page.html

 

I turns out that if you have a sea level functional threshold power (FTP) of 250W, you'd be able to hold 40.4km/h on a flat road at sea level, 41.3km/h at 1500m and 42.1km/h at 3000m.

 

So, you have more power at sea level but the air is thicker and it thus takes more effort to ride through it.

 

The air gets a helluva lot thicker when the South Easter is blowing! I would have thought that the impact on your heart and lungs will be the biggest contributing factor and not the density of the air?

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Drag is directly proportional to density, and power is proportional to drag. So in other word 0.8 relative density means there is 80% the density of air at sea level, hence 80% of the drag and 80% of the power to remain at the same speed.

 

I got out my density tables., and it looks something like this:

 

 

feet: relative density

0 :1

1000 :0.9711

2000 :0.9428

3000 :0.9151

4000 :0.8881

5000 :0.8617

6000 :0.8359

7000 :0.8106

8000 :0.7860

9000 :0.7620

10000:0.7385

11000:0.7156

12000:0.6932

13000:0.6713

14000:0.6500

 

 

So if you had to compare JHB at roughly 5000ft (ok its a little higher):

Your threshold power drops by 7%

Your power required to hold a certain speed drops by 13.8%

 

Hence you should have approximately 6.8% "extra" power available at altitude.

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The air gets a helluva lot thicker when the South Easter is blowing! I would have thought that the impact on your heart and lungs will be the biggest contributing factor and not the density of the air?

It turns out it's not. The table I linked to earlier says threshold power (heart + lungs) will decrease by 7% going from sea level to 1500m. For the same increase in altitude, aero drag will decrease by 11%. So, even though you can only do 233W at 1500m it gets you a few km/h more speed.

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Hence you should have approximately 6.8% "extra" power available at altitude.

It seems there's actually a turning point around 3500m ASL where power starts dropping at a faster rate than density. There are actually a fair number of speed and distance records that have been set around this altitude (mostly in Mexico City).

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It seems there's actually a turning point around 3500m ASL where power starts dropping at a faster rate than density. There are actually a fair number of speed and distance records that have been set around this altitude (mostly in Mexico City).

 

Yeah, from my crude calcs using the table in the link I also get a turning point at about 2750m and another peak at 3300m. But I'm making quite a few assumptions. Interesting though.

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