The bicycle power meter explained

GTC has two power meters for hire but maybe you’re thinking: “I’m not that good on my bike so a power meter isn’t for me.” Or: “Er, hmmm, I don’t really know what a power meter is.” Or: “I have heard good things but what is it all about?”

In fact, a power meter can be very useful for all levels of triathlete. Here we hope to enlighten you, or, at least, take away some of the mystery of power meters.

So, what is it? A power meter is fitted to a bike to measure the power output of a rider.

Where does it go? A power meter can be fitted to different parts of the bike, including the rear wheel hub, the bottom bracket/spindle, chainrings and crank spiders, a crank arm or the pedals. The meters each have various advantages/disadvantages and cost variables.


What can I hire from GTC? The two club power meters take the form of replacement pedals: Garmin Vector 2S pedals.

What’s the point? Measuring power output, or effort, while cycling allows riders to ensure they ride in the most efficient way for the distance, type of race and their fitness level.

But I am still not sure I get it. Power meters measure your effort independent of outside factors, unlike speed and heart rate. Heart-rate, for example, can be affected by illness and how much coffee you have had.

In terms of power, if you think about 15mph uphill into a headwind, this is very different from 15mph on a flat road. Power is always power. By measuring the effort you’re putting into your bike, you can train to specific and repeatable levels day after day, no matter what the terrain or weather.

How is the power calculated?: Power meters generally work by using “strain gauges”. This gets a bit technical, but stick with it… By measuring the torque (force) and combining it with angular velocity, power can be calculated. The power is measured in watts.

But what is power? Actually? If you can recall maths and physics from school days you’ll know that:

power (W) = force x distance / time

Watts are the energy required to a move a mass a certain distance in a known time period.

So, 1W = 1Nm/s.

In other words, to move one Newton one metre in one second requires one watt of energy.

When is comes to the bike, the mass is you plus your bike and the distance is how far you go.

But away from the basic maths, moving a bike is a more complex because there is the resistance to motion, which is never consistent.

So, in this way, power = force x velocity (Are you still with us?!)

So when thinking about a power meter, you apply this equation to a given part of the bike, such as the pedal or crank.

Indeed, the accurate measurement of this force is one of the biggest challenges power meter manufacturers face. So the smallest details become important, such as where the gauges are placed and the quality of the gauges.

But really you don’t need to know any of this because the power meter gives you the data to use.

How do the power meters talk to you? I mean, do they talk to you? Most transmit via ANT+, which connects data and info to most bike computer gadgets, including Garmin.

Some of the most modern power meters also have Bluetooth Smart connectivity, so they can connect straight to a smartphone.

But what about all the other gadgets? Do I really need another one? There are Garmin bike gadgets, heart rate monitors, wrist monitors, VO2 max tests, training plans and dedicated coaches to assist with optimum training. All of these are very useful tools for improving training and performance and a bike power metre is part of an overall package. Plus, you can hire them form the club before buying.

Knowing your power output is a quantitative, repeatable way to assess how hard you are actually working.

Do I ever get to rest? Knowing your power output can also tell you when it’s time to rest.


Does it make any difference? Really? According to GTC’s Craig Ross it does.

He said: “If you have ever finished a bike leg and thought, ‘I’ve not paced that correctly. Or gone through a few months of bike training and wondered, ‘Could I have used the last few months of training more effectively?’

“Then the chances are you would benefit from training and racing with a power meter.”

He has made useful bullet points.

The main benefits of training and racing with power are:
Immediate and real-time feedback on how much effort you are putting in

Removal of variables such as natural heart rate variability/ wind (outdoors)/ fatigue

The ability to tailor your effort to work at the right intensity for your given session.

His key observations are as follows:

Riding any gradient outdoors causes a surprisingly large spike in power output and therefore effort/ fatigue on even undulating courses

You need to establish your target training zones by doing a functional threshold power (FTP) test – this gives an estimate of how much power (in watts) you can sustain for one hour, which is the basis for setting training zones and future session planning.

Completing an FTP test is one of the most enriching experiences of your life.

Your ability to pace time trials or bike leg splits is infinitely easier using a power meter. You can use your knowledge of your own FTP and gauge your effort accordingly, and there shouldn’t be any surprises, even on a hilly route.

He said: “Finally, power meters really are for everyone, and no matter what your ability or current speed. Using one will help you train and race more effectively, and ultimately, go faster.”

Contact Lochlan O’Sullivan to book the Garmin Vector 2S pedals for only £20 per month for club members.