Friday, June 7, 2013

Training By Numbers 1: Powertap G3 and Garmin Edge 500

This is to be part of a series where I expound upon the merits of training by power measurement, and where I investigate and review a few options.

By now I think it should be obvious to most sport cyclists, but just in case: if you aren't training with a power meter by now, get one. It's the difference between shooting in the dark and having the lights on. Here's how it works.

If you want to improve as a cyclist, you need to train to be able to produce more power. Outside of race situations, you don't have to care about how fast you're going, all you need to concern yourself with is that your power is increasing through your training, for which you test yourself every few weeks. You can train for power over intervals of any duration in order to improve for the specific demands of your chosen challenge. If you're going to race up hills you also need to be very concerned with your power-to-weight ratio.

What you have in a power meter is completely specific, quantifiable information that tells you exactly where you are in relation to your goals, to each other, and also - somewhat harder to swallow - to the pros. It takes a specific amount of power to move a rider of X weight over course Y within a selected time. When I'm on my bike I care about only 3 numbers: my power output in watts, my heart rate, and my cadence. If I'm producing more power for the same heart rate, I'm improving. I can try out different positions, cadences, wheels, sport drinks, haircuts and whatever else I think that might affect my ability to produce power and transfer it to forward motion, thereby refining all of these things.

In training, I can sustain a set power for as long as possible and work on increasing the duration, or I can work on increasing the power output for a particular interval. I can collect data and see what my average power output is for a particular climb or route. No coach will work with you unless you have a powermeter. It is the number 1 training tool after your bike.

I got myself my first powermeter - a Powertap SL+ - about 2 years back. If you're not too familiar with the terrain, the Powertap system bases it's measurement of power on that which is delivered to the rear hub.  It's not the top of the powermeter food chain, but it's the most affordable and regarded by most as accurate and reliable enough - used by many pro teams so it definitely works.

I initially had a few problems with the SL+ and after a couple of pit stops, Cyclops (who make the Powertap) replaced the faulty unit with the brand new G3 model. Already an encouraging recommendation for the product.

Your power meter needs a head unit which displays and stores the data. Most producers of power meters also produce a head unit, but thanks to an agreement on a single interconnectivity protocol called ANT+, any head unit will work with any power meter. Very useful since I'm very attached to my Garmin Edge 500

The Edge 500 is arguably the greatest little gadget a cyclist can have. You no longer need magnets on your spokes and odd little gadgets taped to your forks - it measures speed, distance, position, altitude, gradient (rather obviously) by GPS. It also does heart rate (via a transmitter strap), cadence (using the powertap or a sensor), and pretty much any info a cyclist might need. If you need maps, you go for something higher in the Edge range such as the 810.

Coupled with a power meter it will give you actual power, or averages over a series of durations. It gives you average lap power for your current lap which is a great and vital function for keeping intervals at the right intensity, or maintaining a set average power output on a hill climb for instance. Plus you upload it all to the Garmin Connect website and analyse everything you've done to your heart's content.

So back to the meter itself.

My choice was to get a 32-spoke version built onto a standard training wheel. It's mainly a training tool after all, and I trust the strength and durability of the standard wheel rather than a low-spoke-count lightweight that might have some flex. I might review that decision were I to start again. Once you start training with an indicator of your power output, it's hard to ride without it. I've ended up using it in races - that after all is the place where you really test yourself. In retrospect, a better choice might have been to get a 20-spoker and build it onto a durable but light racing rim. Moot point now.

Being the first power meter that I've used, I'd have to say that it was the expected metamorphosis in my training life. I have rarely ridden without it since. There is an issue of reliability there, as the readings can fluctuate with temperature changes, and I do seem to find myself "calibrating" - resetting the unit to zero - a couple of times during rides where I'm keen to get acurate figures. The batteries don't seem to last very long, and there's no indicator other than extremely dodgy readings to tell you that the batteries are going. The battery is easily replaceable though (easier than the old models).

The main drawback with using the hub-based power meter, is that you never get to use your rear wheels. I suppose it's saved me some money over the couple of years, since there's little point in looking at new wheelsets. However, the adavntage is that you can use the same power meter on all your road bikes - this is a big advantage to me as I live in 2 cities, keep bikes in both, and am constantly travelling between each with only a wheel to carry. Horses for courses.


  1. Greg, intriguing your comments on crank power meters, in my line of work we sometimes use these on gas turbines as it is the only only to quantify the shaft output power with good accuracy. The consensus on the use of such devices is mixed. They are expensive and prone to failures in harsh industrial conditions. I've heard it said they belong in the test bed or lab, not in the field. Anyway, to be correct they are referred to as dynamic torque meters and there are two types: strain gauge and twist measurement. I believe the bicycle ones are the strain gauge type which are affected by temperature and humidity to some extent which requires frequent recalibration as you have alluded to in your experience. The twist measuring type have fixed sensors around the perimeter of the meter set on opposing "teeth". This is actually measuring the deflection of the opposing rows of teeth in real time giving an accurate measure of the twist which can then be calculated back to torque and power. These units can handle any amount of power and are suitable for high speed turbo-machinery up to 100s of megawatt output.

    Regarding the use of power meters for bicycle applications, I suppose I have mixed feelings about it since of course only sports with mechanical drive around bearings, i.e. cycling, rowing (wheel-chair racing?) come to mind, are suitable for power output measurement. For everything else all we can measure is heart rate and velocity. And going back in cycling that was good enough until SRM came along. But food for thought, maybe I'll go for an DuraAce/SRM setup if I win the lottery. regards, jim

    1. Thanks for that insight Jim. Very interesting stuff! As far as I know all the bike powermeters work on strain measurement. Does it pose serious challenges to produce a twist torque measurement for this type of use? Sounds like it might be the cue for a little cottage industry :)

      As far as the use of power measurement in cycling goes, I'd say it was to our great advantage that we use a piece of equipment that can so easily be adapted to give us an actual real-time measurement of output. I honestly feel I'd be going back to the dark ages if I stopped using mine. It's not dependency on gadgets (as I do ride without it often), but when I need to train hard it gives me accuracy, and when I need to test myself it gives me the truth.

      SRM may be the industry top standard, but there are already many in the market that are very reliable and at a price-point that make them much more affordable (around US$1k). Winning races is a lottery. Training with a power meter makes it less so - as long as you put in the required effort of course.