Tuesday, August 19, 2014

It's a drag

Last Sunday's bike ride plan called for 30 miles, a modest increase from the previous week's 25 and well within my current comfort zone. That 25 mile ride went exactly as planned, but my time estimate for the 30 mile ride was off by a lot. Plan was for 1:45 but I did it in 2:30. What happened?

The first thing I noticed was that to do 30 miles in 1:45 required an average speed of 17.1 m.p.h. That is a bit faster that my Tinman average speed of 15.2. What made me think I would do a training ride at race pace? These rides are supposed to build endurance, and that means riding predominantly in zones 1 and 2. Not blazing fast. Besides, despite their long duration they are not supposed to leave me drained to where I spend the rest of the day sitting on the sofa watching TV.

When I developed my twenty week plan I used an average bike speed of 16. How did it get to be 17.1? Rounding. Looking back at my spreadsheet, my plan for Sunday was 30 miles in 1:52:30. Going from that data set, which was liner, to the adjusted set which included periodization, I rounded off the times to the nearest quarter hour. Perhaps my rounding rule should have been to always round down, because going faster is not really an option. A little, maybe, but that's it.

That got me to within 30 minutes, but how do I explain those 30 minutes?

The Tinman route included a climb over Diamond Head with cold legs as well as Heart Break Hill, and I was not going all-out in order to protect my run. Sunday's route started in Kahala so no Diamond Head, included Heart Break, and Makapuu with the turn-around at Waimanalo District Park. As for effort, I should have been under my Tinman effort.

Hills take huge chucks of time, which is why they are so useful in creating gaps in races. I could see how my goal pace of 16 m.p.h. was unreasonable when my ride went to Waimanalo, but why was I off by so much? Sure I had to climb up Heart Break Hill and Makapuu, but I got to go down both climbs so shouldn't that be a wash? The speed lost going up come back on the way down. Conservation of energy and all that.

It doesn't work that way.

A bike by itself creates almost no resistance to rolling. Most of what there is comes from chain friction and tire deformation. What keeps a bike from going 100 m.p.h. is drag, the energy required to push the bike and rider through the air. Most of the drag is generated by the rider's body, the other significant source being the wheel spokes.

Drag increases as the square of velocity. The speed loss when climbing is liner. To ride an average speed of 17 m.p.h. with a 5 minute climb at 5 m.p.h. will require a decent speed of 29 m.p.h.. The energy lost to drag will be less during the climb than the preceding flat section, but it will go up sharply on the decent -- there is no way to get back the energy lost on the climb.

It gets worse.

What moves the bike forward is energy provided by the rider. Elite, well financed cyclists can now measure the actual power being expended, by use of a power meter such as made my SRM and Garmin. The rest of us use a heart rate monitor to estimate power output. The harder we peddle, the higher our heart rate. This works surprisingly well.

Even if you do not grasp the significance of a term increasing as the square of another term, all you need to know is that the resistance to speed increases at a much faster rate than the speed increases. But in the case of power, the force required to overcome resistance (in our case, drag), things get ugly fast. Power increases as the cube of speed. The energy you must expend to double your speed will increase a thousandfold. Figuratively, anyway.  It is our power output that we feel when we consider how hard we are working, and our HRM is the tool we most often use to quantify that feeling.

Bear in mind I am not talking about land speed. That would be the speed at which we are traveling along the road, the speed I used to calculate my time and distance estimates. Here we are really talking about airspeed. Land speed and airspeed are the same as long as there is no wind. Sunday, there was wind. A lot more than we had in recent weeks, a lot more than for the Tinman.

By now you should see where this is going. Given an average wind speed of 10 m.p.h. from the east, the ride outbound along Kalanianaole will require far more power than the return. Because this ride was constrained more by power -- perceived effort -- than speed, the result is a much longer ride time than estimated. When two things are related exponentially, equal moves up and down along one term produce wild and crazy changes in the other term. This is why going just a little faster requires so much more energy, why the wind wrecks havoc on bike races, and why a streamlined ride position is so important. 

No comments:

Post a Comment