Altitude vs Power

[cwwees]

I know this is a "duh!" moment, but...

I have been trying to make 100 minutes to Mt Bigelow Rd for about 3 years now. I'm missing it by about 2 minutes (2%). A friend with a PowerTap convinced me that a real-time power meter reading could help me go faster - aiming for constant power across the ride, and I've been happy with the iBike for this. The climb is about 6000 ft (from 2600 to 8200 ft elevation), and I've always struggled to hold constant power.

In my recent Bicycle Tour of Colorado trip, I had the opportunity to ride grades at 10,000 and 11,000 ft elevation. I discovered that I can't produce as much power at high elevation. Imagine that!

My question is whether there is a rule-of-thumb for de-rating power capacity with elevation? If I can produce 300 W at 2600 ft, what should I expect the equivalent percentage-of-max effort to be at 8000 ft?

Charlie

[alienator]

Maybe there's a rule of thumb out there, but the reality is that there's no quasi-accurate way to say. How you perform at altitude depends on your level of acclimitization and a host of other factors that are specific to you.

[rruff]

If I can produce 300 W at 2600 ft, what should I expect the equivalent percentage-of-max effort to be at 8000 ft?

This will give you a good ballpark... F11 is your altitude in meters:
= (0.178*(F11/1000)^3 - 1.43*(F11/1000)^2 - 4.07*(F11/1000) + 100)/100

At 2600 I get a factor of .960, and at 8000 it is .841... so 300W at 2600ft = 263W at 8000ft.

[cwwees]

Thanks very much, rruf. My subjective experience on my local hill is pretty close to that value. My subjective experience in Colorado is probably similar, though I haven't gone back to my records to compare. If it's within 10%, it's better than my ability to control my power output, anyway.

Where did this polynomial come from? I realize it's a curve-fit, but I wonder at the source data.

Charlie

[rruff]

I don't remember exactly where I got it, but I think it was a study of elite runners... a curve fit of the data.