Aerodynamic and Metabolic Reality

It’s hard work pedaling/pushing a loaded bike up a mountain, but the mountain pays you back with an equal descent, right? Often the same day?

But air is weighty – about two pounds per cubic yard (1.2 kilograms per cubic meter) at the altitudes and temperatures where most of us live. We seldom notice it at walking speeds, but we push a lot of weight out of the way as we displace air, by whatever mode of travel.

Aerodynamic drag accounts for as much as 90 percent of the energy expended by cyclists – perhaps more given the Big’s typical wide load. Worse, it increases with the square of relative air speed. All considered, a 10-mph (16-km/h) headwind increases the work of pedaling the Big by half, and my foolishly-chosen routes during my 48-state initial journey presented predominant headwinds.

That 50-percent increase equated to about 1500 calories of extra work per 60 miles pedaled, my longest repeatable daily goal on the Big.

Worse, descents down mountains are much faster than the climbs, so that square-of-relative-air-speed factor impacts there, too. There is far less aerodynamic drag when climbing a mountain at less than 5 to 7 miles per hour than when descending at 30-plus. Hence, mountains don’t give back all those extra calories spent on ascents.

My plan was to complete all 48 states between May 23, 2016 and August 21, 2017 – 12,500 miles in 16 months. Easy-peasy. 781 miles per month.

But reality again intervened. Factoring detours for food, water, campgrounds, lodging, etc., I pedaled, on average, 1.6 miles for every mile of progress along my routes.

12,500 miles became 20,000. 1250 miles per month. Not as easy. I’d have to cut miles and eat a lot more food than I’d planned.

But, reality had yet another lesson to teach: Consuming ever-more calories does not work. No matter how much more I ate each day – and I was voracious! – I continued to lose weight. And stamina.

And I had no idea why.

Until, I noticed a study reported in Science Advances in June of 2019, which found “… evidence for an alimentary energy supply limit in humans of ~2.5× BMR [basal metabolic rate]; greater expenditure requires drawing down the body’s energy stores.”

So, metabolic reality = ~2.5xBMR.

Do more, and you will consume yourself, as well as your daily bread. No matter how much you eat. Sounds simple … in hindsight.

As a astronomy writer, I should have anticipated that the fundamentals of physics would so govern that aspect of human physiology, as well as all else in the universe.

Considering age, weight, height and gender, my basal metabolic rate is roughly 1330 calories per day. Times 2.5 yields a theoretical limit of 3325 calories.

Subtract my BMR, and my net daily bike-travel energy budget is 1995 calories for everything from pitching my tent to chewing food to pedaling the bike. I estimate that those more-mundane daily movements cost me at least 100 calories, leaving but 1895 to fuel the loaded Big.

At sea level, sans headwinds and climbs, I burn an average of 550 calories every ten miles I pedal the Big. So, my daily-bread range now rounds to 35 miles per day, 245 per week, 1000 per month. Generous, but limited enough to explain why pushing too long to average 300-plus miles per week – week after week – was to consume more of myself than was safe.

Something had to give, and what gave first was my routes. Cutting miles meant cutting corners – literal state corners – and I cut more than a few, reducing those 20,000 miles to a tad under 15,000.

And still, that wasn’t enough. What gave next was my body. At 125 pounds – 20 below my usual 145 – I had exhausted all stamina reserves and shut down, until I could regain at least 10 pounds. Rinse and repeat, and 16 months became 21.

After I’d completed the last state, I watched an interview I had given during the week before the August 2017 eclipse, when I was pushing hard to get to Nashville in time for the event, and I did not recognize my emaciated self.

All because alimentary energy-supply limit = ~2.5xBMR.

I’m often asked, “What would you change, if you had it to do again?” But, I am doing it. No, not 48 states. Instead, a nice, very-long bikeabout, without the artificial goal of checking off geopolitical boundaries, pausing as long as I must, whenever and wherever compelled, to enjoy the people and sights of this huge country.

Making up for corners cut.

(Graphic compliments of Science Advances.)

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