For all you physics majors: I believe Power= force x velocity and force= mass x acceloration....if so can I calculate power by doing the following coast test:?
Ride at say at a constant 20mph then coast for 5 seconds and check my new speed
(ie my deceloration) and calculate my watts for that 20mph. The test would be done on a flat road. I realize deceloration may not be linear and it would represent my watt
output for that riding position and assumes a flat road my entire ride.
Thanks

Way too simplified to account for anything in real world. Even when you ride 20mph (say on rollers), your power output at any given time is likely to deviate by more than 10 W from average.

Also, you are making a huge assumption that resistance remains the same within a five second time period. If you have a PM, the first thing you'll realize is how untrue that is. Furthermore, what you are proposing has already been accomplished by iBike, which measures everything but power. In a close-to-perfect setting (read, velodrome), it should be pretty accurate, but this is not true on the open road.

PM are nice but not necessary for success.

The true poor man's power meter is a heart rate monitor.

Agree with mcjimbo as above.

What you need to know for training purposes is your power output in real time. Your average power output for a given length of time may be interesting but it is useless.

If you have a powermeter than tells you your instanenous power output in real time, you can determine your training zones based on power. Then you plan a training program based on these zones. For example, today workout will be three 10 minute intervals at zone 4 or maybe 280 watts (ie. you watch your powermeter and put out 280 watts for the 10 min interval), etc, etc, etc..

actually, perceived exertion is cheaper , but you are right on with the HRM.

OP, it is true that you'll likely overdo an interval at its beginning or climb at a power way above your threshold power on even a 3% hill if you use only an HRM, but these are the things you learn to deal with and adapt to after a while. Case in point: say you are doing a zone 2 ride (for which efforts should stay below threshold), and the top of your endurance zone is 145. You don't gun it so that it ramps to 145 immediately because you are likely putting out power in your VO2 max zone when you do it. Instead, your goal is to hit the bottom at 141 and slowly let your HR creep up. The effect of this would be very similar to metering your efforts so your power output is in high tempo.

When you do enough intervals with a HRM you learn how the ramping of your HR should be so that you won't be overdoing anything. As for the short, 1minute intervals, well, those should be all out anyway that if you are looking at your PM, you are doing something wrong.

I'd argue for the opposite as it's hard to nail it to 280W+/- 10W even under perfect conditions. It's the normalized power of the duration of the interval that matters. After all, by standard definition, you average power of a 60min long time trial is FTP, the power you use to determine all your training zones.

I have found it helpful to change my Powertap setting to give me more smoothing of the data (10 or even 30 sec smoothing) so it is easier to try to hold a certain power level for intervals.

My main point though was that a formula to give a one time power output (as the OP described) is meaningless and doesn't serve any training purpose.

Thanks for the reply. The ibike coast test is what trigged my post, but I thought they did it to determine the rider's aerodynamic drag.
Perhaps the coast could be done over 30 seconds to 'average out the variations, but isnt the variation with a pm due to changes in cadence and theres no
pedaling during the coast. I definately agree a pm is not necessary, but would help me to give a similar effort whether tailwind or headwind.
I live in a flat area where we use highway overpasses for hills!

An also good version of the poor man's power meter but more quantitative is getting your rear-wheel speed on a Kurt Kinetic trainer and matching the speed to the power curve chart. Not exact, but pretty dang good AND reproducible.

Since it takes your heart rate a while to catch up with your exertion, heart rate is a better indicator for longer intervals. Much less useful for short intervals.

When I was in college, we tried to calculate drag coefficients for a car using the coast-down method. The main problem is that the measurements aren't very accurate. For example, on my speedometer (a cheapy), displayed speed lags a second or so behind (it allows several wheel rotations before recognizing a change) and then also, speed may change in fairly large increments- so it may show 20.1 and 20.4 without showing 20.2 and 20.3 in between. The result is your measured deceleration is not very accurate.

With the car test, this was done in west Texas, which is noted for wind, and we did the test in both directions in hopes of averaging out the wind factor, but that throws another big unknown into it. Also, you don't normally have a direct headwind or tailwind, and the effects of a quartering wind are not normally figured in drag calculations, so it just makes the whole approach that much more approximate.

The method also assumes you know what you and your bike weigh.

It would likely be about as useful to use one of the online calculators.

good luck.


or just buy the damn power meter.