Training With Power

I think that we have already had a thread or two on here discussing this, but with Brent asking about Power Taps and Karen expressing her confusion about the whole thing I thought it might be good to put out some basic information regarding training and more importantly RACING with power data.

First of all what is "power"?

Power is defined as either work (expressed in joules)/time (seconds) or as more applicable to cycling Power=Torque x Angular Velocity. Both are measured in watts or wattage (interchangeable terms).

How is Power measured?

Power is derived from measuring the torque on strain gagues somewhere on the drivetrain of the bike.

Each product measures torque at a different spot in the drive train:

SRM-The SRM is considered the "gold standard" when it comes to power measuring devices. Torque is measured through the strain gauges located in the crank arms, which logically is the most accurate place. SRM's are not cheap with the base model running around ~$3000-4000.

Power Tap-Power Tap by Saris is probably the most popular of the power measuring devices. The power tap hub is laced into a wheel (either one you already have or you can buy the whole thing) and has strain gagues in the rear hub that measure torque at that point. As a general rule there is ~3-5% difference between an SRM and PT reading but that is negligible. The great thing about a Power Tap is it's portability between bikes (if you ride a road and tri bike) and ease of use. The drawback is you either have to train on your race wheel or race on your training wheel (or buy a race wheel with a PT).

Quarq Cinco-Quarq is the new kid on the block when it comes to power measuring devices. Quarq replaces the spyder on the crank so it can be retro fitted to just about any existing crankset. The drawbacks are that it is also quite expensive (I don't know a firm $$ amount) and from what I have heard the data is hard to interpret as well.

Ergomo-This device replaces the bottom bracket on your bike and measures torque there. The caveat is that it only measures on the drive side so you are only really getting 50% of the data produced. The company that produces these went bankrupt a few years ago but I have heard that someone else has picked them up and will offer US distributorship in the future. New these cost about $1500 but that was 5-6 years ago.

Ibike-I was a beta tester for this device a few years back and for what it does it isn't a bad option for the budget conscious. The ibike itself doesn't measure torque or power directly, but rather through measureing a series of opposing forces calculates the resistance and then derives the power needed to over come those forces (Newton's third law of physics-for every action there is an equal and opposite reaction). Putting it up against my Power Tap the numbers were fairly close until you get to the higer wattage outputs (>400 watts). However the margin of error was consistant so it can be used as an accurate training tool. The drawbacks to this device are the fact that it uses an air port in the front (similar to the pitot static system in an airplane) so it has to have "clean air" to work. If you are riding a tri bike it has to be positioned below the aerobars and out of the slipstream of the hands in order to be accurate. For a road bike you can mount it on the stem or handlebars with no issues.  New these run ~$700.

Garmin-Garmin just purchased a company (the name escapes me at the moment) that has a power measureing device that goes in the spindle of the pedal. I have heard that they are planning to come to market with this sometime in 2011 so stay tuned.

Years ago Heart Rate monitors were big and expensive and pretty much only pro's and elite athletes used them. Today you can buy one at Walmart. Soon power will be the same way as new products and new companies enter the market place and force the price down and quality up.

That is the basics of power and power measureing devices, if you have any further questions you can ask here and I will answer or you can PM me.