Monitor your power output without a powertap!




If you're truly a scrappy triathlete, then you don't just ruthlessly claw your way to your goals, but you do it on equipment that is far inferior to that of your peers. Take for example our bikes - I ride a once $2400, aluminum 2005 felt S22. Gered rides a a 2008 Quintana Roo Seduza. Both bikes are completely stock (with the exception of our saddles - mine died of overuse, and Gered's put his future family in jeopardy). Nice bikes, really... $2000+ is a lot to spend on a bike. But compared to the bikes of the people with similar finish times as ours, our bikes might as well have pink streamers and training wheels. These guys are usually in their mid 30s with enough money to buy whatever kind of equipment they want. Their $2 - $4000 frames (not bikes, just the frames) are often garnished with a nice set of Zipp or HED wheels that price around an additional $2500 for the pair. Can you guess which bikes are ours and which are our competition's?

Setting aside the difference in efficiency that their bikes can produce, they also usually have a very fancy hub on the back called a Cyclops Powertap. The powertap is connected the bikes computer and provides the cyclist with the usual info: speed, cadence, HR, etc., but it also provides the riders wattage output - a very handy piece of information - or so I'm told. Gered has been doing some rides on a computrainer lately... these are trainers that provide the same wattage information as a powertap. He whined and complained in his usual Gered way about how much of a better gauge it is for workouts than HR, and started talking about yet another toy he wanted to buy. Determined to save him $1500 for a wheel hub that provides this information (that's right the powertap hub costs about $1500), I promised a blog entry that would provide an alternative way to monitor his power output (wattage) in workouts at home.

To do this you'll need one solid ride on a computrainer with some 5 to 10 minute anearobic intervals, a speedometer on your bike that has a rear wheel sensor, and a rousing math lesson from yours truly.

Let's start with the math lesson:
To begin, you need to understand the concept of direct variation. It's a very simple term that I teach my algebra 1 students in the first quarter of the year. Basically, it's like this:
"Say Gered goes to his froofy farmer's market and buys some beets. It's $1.50 for 3 beets. So you know that if he buys 2 beets, it'll cost him a dollar." So if he buys 4 beets it will cost him???... Yup $2.00. You see what you did was you figured out how much it was per beet... in this case $.50. Having figured that out, you could determine how much it would cost for Gered to buy 207 beets. Just multiply:
'
Doing calculation with wattage and a computrainer is very similar. For beets $0.50 was a "constant coefficient" that was multiplied by the number of beets to get the total cost. Unlike riding outside, riding on a trainer has a relatively constant rate of resistance against your legs (especially on a magnetic trainer - the resistance of a fluid trainer varies with how fast the rotor turns) - there's no wind, no hills, not even your change in weight affects the resistance of a trainer. Therefore, there is a constant coefficient for the resistance of your trainer. That means that your power output varies directly with the rate at which your wheel spins. (i.e. your speed) Here's how it works mathematically:
So say for example, Gered goes on a computrainer ride (you can usually find these through a bike shop for $20 a pop) and rides anaerobic intervals at an average heart rate of 175, and puts out an average of 290 watts. Then at home on his magnetic trainer, he finds when he does some similar intervals at an average heart rate of 175 that the speed on his personal trainer reads an average of 18.9 mph. 
At this point I'm sure Gered would want you to know that when he rides at an anaerobic effort on real roads, his pace is more like 24-26 mph. The reason for this discrepency is that the resistance his trainer puts on his rear wheel is greater than the resistance that Gered faces while riding on the road. The nice thing however, is that the resistance on his trainer is constant, and therefore calculable, and then useable. Watch:
Since Gered knows that the same heart rate on two rides should yield at least similar power outputs, he can assume that at home, when he was at 175, and his speed was 18.9 mph, his wattage should have again been right around 290. If he plugs these values into the formula, it will help him to find the resistance coefficient of his trainer.
This number - 0.065 is Gered's trainer's resistance coefficient. He can now use this to base his workouts on power output instead of heart rate like so:
Now that Gered has his resistance coefficient he can use it to find the speeds on his trainer that equate to whatever wattages he wants. If he wants to put out 180 watts on the rest intervals between his 290 watt intervals, he can calculate it.
So Gered has to ride 11.7 mph between intervals if he wants to put out 180 watts.
 And there you have it - a nice long-winded lesson in wattage calculation. In case my way of explaining this stuff really didn't click with you, I've created a spreadsheet you can use to calculate this stuff more quickly - it does all the work for you. So actually, you didn't need to read all this, you just had to get to the bottom to get the spreadsheet. MWA HA HA :-) But hey, now you (hopeful) better understand the relationship between power output and speed, you don't need a power tap, and you're a couple steps closer to being a truly scrappy triathlete.

3 comments:

  1. Did anyone find this helpful/understandable?

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  2. yes. i understand that gered is a shopaholic, and needs his good frugal friend brett to outsmart him and trick him into not shopping by scaring him with math. thank you brett.

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  3. This isn't very helpful for long term training. It's a great tool to calculate an average, but is that valuable?

    It doesn't give you training stress, chronic load, power over time, peak power over time, or intensity factor. Watts are the measurement metric, but the resultant number is a tool used to arrive at more important numbers.

    But, ride like you want. Important thing is spinning the wheels.

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