Techie Tuesday

I was reviewing an old post about what to wear in cooler weather, as we are getting close to that season again. This got me to thinking about an online conversation I had with a fellow member in one of my bike clubs.

She was offering advice on all-weather shells, and I realized I would roast, dehydrate, and collapse in less than an hour in her favorite shell, so started trying to devise some kind of framework within which to discuss winter clothing.

I came up with something that is rather succinct, comprised of only two factors, your BMI, and power output. To get your 'heat' number, multiply your BMI times your power output. As explained below, mine is 28x250, which equals 7,000. A 6' rider weighing 170 lbs making 250 watts has a heat index of 5,775 so they'd need 20% more protection. A 5'3" rider weighing 130lbs would have the same BMI, but making 175 watts her heat index would be 4,025, so she'd need almost double the protection. (74% more).

BMI is a pretty good estimate of "stuff that traps heat" divided by the surface area available to get rid of that heat. The only thing missing then is an estimate for how much heat you are making to be trapped. The calculator at the upper-left hand corner of my blog will calculate this for you if you provide it speed, grade and weight inputs. You can also get watts generated on your rides from the RideWithGPS metrics tab if you upload your ride data to their website.

As a reference, my BMI is 28, and I'm usually making 250-275 watts when not coasting. Yes, that means I am 'overweight' by this metric, but if you read the list of caveats, guys who spend 20 yrs in the gym lifting weights will always have a high BMI because they have more muscle at any given height, and muscle is heavier than fat. For the purposes of my framework here, that muscle retains heat, so the BMI still works quite well.

Now when you have a conversation about winter clothing you can ask someone making recommendations about their BMI (or inputs so you can calc it), how many watts they generate, and have some idea if you are going to freeze, be nice and comfortable, or swelter inside your own clothing until you cook from the inside out. Let me know how accurate this estimator is. Winter clothing is expensive enough it's worth coming up with a good estimator.

Would it surprise you to find that this same metric is useful in estimating your hydration and electrolyte requirements in summer?
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