by Rob Raulings
What is a power meter?
A power meter is a device for measuring how much work a cyclist can produce, normally measured in watts.
Watts are important - in our article on aero drag, watts is basically the power input on one side of the equation for modelling bicyle speed and resistance.
Power meters are not cheap, even the lower end units are more than $1000, however if you are serious about training and racing, they will give you more information on your cycling than perhaps any other possible piece of gear you could own.
A power meter can also SIGNIFICANTLY change the way you train, race and recover.
Commercially available meters:
There are numerous commercial power meters available each with their own merits (and drawbacks):
| Brand | Approx Cost $AUD | Type | Advantages | Disadvantages |
| SRM Science | $6470 | Crankring, 8 strain gauges | -0.5% accuracy -Very accurate -Reliable -Independant to wheelsets -Can swap chainring b/n bikes | Cost |
| SRM Pro | $3460($2995 USD SRP) | Crankring, 4 strain gauges | -2.0% accuracy | Cost |
| SRM Amateur | $2,700 | Crankring, 2 strain gauges | As above | Cost |
| Ergomo Pro | $2500 | Bottom bracket | -Accurate when installed correctly | -Harder to install correctly |
| Powertap | $1500 | Rear wheel hub | -Wireless version
| -Maintenance issues, return to factory (USA) for servicing |
| Polar | $1200 | Chain vibration sensor | -Wireless version | -Not as accurate in some gearing ratios
|
| Quarq | ? | Dec 07: Crankring | -May be cheaper -Wireless -Independant of wheelsets | -Unknown at this stage |
| iBike | $350 | Combination of sensors for air/ground speed, rolling resist, incline and aero drag | -Cheap
| -Less accurate
|
Why use a power meter?
There are numerous reasons, explained in the following sections:
- CTL/ATL/TSS/TSB/IF measurements
- FTP
- specific training
- aero drag testing
- better race pacing
CTL/ATL/TSS/TSB/IF measurements
Dr. Andy Coggan, and American exercise physiologist (and top athlete) is credited with the development of a system for measuring the training load and recovery placed on cyclists over time. The three key components of this method of analysis are: CTL - Chronic training load (your training load over the last 42 days), ATL - acute training load (your training load over the last 7 days), and TSB - Training Stress Balance, which is a measure of how fresh or hammered you are. In addition, each workout completed can be given a Training Stress Score (TSS) which contributes to CTL, ATL and TSB over time. Each training session also has an intensity factor, which typically ranges from 0.7 to 1.5 or so in any given ride.
The system works like this. You ride - hard - you get a big TSS score, but the next day you are feeling pretty hammered. CTL increases a bit (since this is calculated over the last 42 days), ATL increases a lot (since it is calculated over the last 7 days), and TSB takes a nose dive (since you are feeling hammered.) Now imagine you ride a bit easier for the next few days, your CTL will still be increasing (but at a slower rate), ATL will be increasing very slowly or even going down, and TSB will come back from the hole you dug yourself into.
Using graphic software like Excel, or a commercial analysis program like CyclingPeaks, you can track and manage these scores as they respond to your training stimulus.
An example of a CTL/ATL/TSB graph which shows 2 hard 6 week blocks of training with a 2 week break in-between. Even though ATL decreases towards the end of the first 6 weeks CTL is still increasing.
FTP
Using a power meter you can calculate your functional power threshold (FTP). This basically means the maximal power you can produce in watts sustained over a period of 1 hour. Its a great measure of how your current form is stacking up, and once you know your FTP you can use it to perform specific workouts aimed squarely at improving your ability to produce power, and therefore your ability to race faster.
Specific training
Once you have an FTP wattage you can use this for the basis of setting highly specific training sessions, some examples might include:
2 x 20 minute intervals, 5 min rest b/n, 1.05 - 1.10 x FTP wattage. For example, if your FTP is 250 watts, you ride 20 min @ 265 watts, take 5 min active rest, then another 20 min @ 265 watts.
5 x 3 minute intervals, 1:30 rest b/n, 1.1 - 1.2 x FTP wattage.
20 x 30 sec intervals, 3:00 rest b/n, 1.5 - 2.0 x FTP wattage.
If you are lucky enough to have a coach that understands wattage, you can start getting a lot more from your training, and your coach will have much more of an idea about how you are performing.
Analysing aero drag
If I told you you could go 1.5km/hr faster on your bike, with exactly your same gear, without doing any training, and without taking anything stronger than a coffee you'd think I was loopy - but believe it or not this actually is possible in a lot of cases, you just have to have a power meter to be able to run the calculations. You can of course resort to spending time in a wind tunnel, but after a couple of hours testing you'll have a sizable chunk out of your wallet that could have gone a long way towards a power meter. Dr Andy Coggan, in reviewing the difference in measurement accuracy between wind tunnels and power meters, believes that wind tunnels are more accurate, but perhaps by only 0.5-1% more than a power meter. The commonly accepted accuracy for power meters is 1-2%, which is normally OK for calculating aero drag.
So how does it work? Simply put, you wait for a windless day (or as near windless as you can get), then head with your bike and power meter to the nearest velodrome or quiet road you use as a test strip. After a warmup, you start riding in your body position that you want to test. By riding a number of runs at a constant speed/watts you can start to build the data you need to calculate aero drag. You need to ride at a range of speeds and watts, from 20 kmh to as fast as you can manage for about 1 minute. Your power meter will be recording all this good info for you. You also need to record the temperature and air pressure from the closest weather station to your test site - you can either get this from the Bureau of meterology, or see if you can find someone with a portable weather station capable of measuring air pressure or wind speed (nerdy, yes, but then so is a power meter it can be argued!)
To do the analysis you need an Excel file created by Alex Simmons (a Sydney cyclist), and available on his blog at http://alex-cycle.blogspot.com
You need to download the data and find sections of data about 1 minute long where the start and end speed are constant, then input the average watts it took to produce that speed. When you have all the data, and have input the air density (calculated from temperature and air pressure), you'll be able to get 2 measurements, CdA (co-efficient of aero drag) and RR (rolling resistance), which are the 2 hardest variables to measure in the equation for cycling power. Sepending on what power meter you have this may be harder or easier. The best method for analysing power files from power meters is Cycling Peaks - which is commercial software, but is not that expensive (compared to the meter anyway).
If your CdA comes out at 0.260 or higher, well the good news is that with some expert positioning advice you will be able to go about 1-1.5 kmh faster if you can drop CdA to about 0.235. The bad news is that with really super slick time trial bikes, aero helmets and aero wheels AND good body positioning, you can get that drag number down to 0.200 and maybe a fraction lower. (In fact it has been estimated that Dave Zabriskies CdA during his world record breaking TT in the tour de France was about 0.195). And the boys (or girls) with those low drag numbers will run rings around you with the same power, and may even be faster than you with LESS power than you produce.
If you can average 40 kmh, with CdA of 0.265, you will be able to average 41.5 kmh with a CdA of 0.235, and 43.5 kmh with a CdA of 0.200, all at the same power, same energy input and same fatigue levels. What other investment in cycling technology can give you that kind of result? (other than a maybe a 600cc Yamaha engine strapped on your bike, which unfortunately isn't legal).
If you're interested in going faster, and would like help to run drag numbers, call in to TCR - they can help advise you on the best choice of power meter for you, can put you in touch with experts in aero calculations!
Better race pacing
Lets face it, triathlon and time trialling is just about you and the clock, 1 on 1, with the time from A to B being your score. You might even have a goal time in your head that you want to achieve. But how do you put that into practice on the road? Well without wattage it is pretty hard, you have to rely on speed and distance/elapsed time to get you through. This might work fine if the course is dead flat, with no wind, but what happens when you start to encounter head or cross winds, or have to ride up hills on the course? Suddenly your actual speed is nothing like your goal speed, and trying to calculate how this is affecting your average speed will surely be difficult (math under low oxygen conditions is a lot tricker than sitting at home!).
This is where wattage can help. If we know that your race distance is 40 km, and that your FTP over 1 hour is 250W, this means you should pretty much be riding at 250W for the entire race distance. Exceeding 250W for more than 20-30 seconds in any part of the course may mean you end up blowing up well out from the finish. Head wind? No problem, just ride at 250 watts, whatever speed you get at 250 watts into the wind will be the fastest way, or very close to it, of getting through the course.
Ironman pacing? No problem. If FTP is 250W, your pacing should be about 0.80-0.85 x FTP, in this case 200-212 watts. Note that environmental conditions, like very high temperatures, may mean you have to further reduce this power goal.
By pacing evenly, you are much more likely to arrive at the finish in the fastest possible time, with the best chance of success (if you have to get off and run).
In summary, there are a lot of excellent reasons why you'd use a power meter, and it is no longer the domain of just elite competitors having access to this technology. If you are serious about training and racing, then you should be serious about considering a power meter - you won't look back!
Further resources:
Consider joining the Google wattage group (discussion list for cyclists with power meters). The site contains a lot of resources for different power meter users, and there is a wealth of information that is searchable.
http://groups.google.com.au/wattage/
For more about aerodynamics, check biketechreview.com.