parameters are one of the best upgrades
you can make to your bike it's fair to
say that they have revolutionized
training for cycling and so it's
understandable that they are much sought
after but the fact is they are one of
the most complicated and misunderstood
components on your bike so we thought we
would try and explain just how power
meters
work firstly what is power i mean we
talk about it all the time 200 W here
400 W there but what actually is it well
very simply it is a measure of the work
that you're doing now power itself is
absolute but the outcome on the road so
for example the speed that you're riding
at will vary depending on external
factors like the gradient that you're
going up or the headwind that you're
plowing into now to actually find out
the power that you're putting out it
requires a very simple equation which is
force time distance / time simple to say
but much much harder to measure out on
the bike but to get our heads around the
concept right now if you put it in the
following context it might make it much
more simple so if you think about the
force as to how hard you're pushing down
on the pedals and then the frequency
which you pedal so your cadence as being
the time then you should start to grasp
it so for example if you're riding up a
hill at a set speed pedaling at 100 RPM
will require just half the force of
pedaling at 50 RPM but because you exert
that force twice as frequently the power
stays the same
see right well hopefully that's sorted
then how do we actually measure it well
almost all power meters will use strain
gauges to measure force they're
generally just a little sheet of metal
that bends as force is applied to it and
as it bends the resistance of the metal
increases meaning you can actually
detect it simply by an electrical
circuit that should be able to quantify
it and then distance well that is
pre-programmed and it's easiest to
explain in terms of a pedalbased power
meter because the distance is related to
the length of your cranks time well that
on pedal and crankbased power meters
relates to your cadence and then you can
measure that very simply by a magnet or
in the case of certain power meters like
these two an internal accelerometer
means that you can actually detect it
inside the crank in the factory your
power meter will then be calibrated to
make sure the output from the strain
gauges corresponds correctly to the
right torque calibration by the way is
actually a term that's often misused
here because the calibration that you
perform on your head unit isn't actually
calibration is zero offset which is a
little bit like making sure that your
kitchen scales read zero before you put
any food on them so in this case making
sure that your power meter reads zero
when you're not actually pedaling and
it's an essential job but how often you
have to do it does depend on the type of
power meter you have for example this
power to max one on the end here does it
automatically so we've got our
combination of strain gauges and
accelerometers now where do we actually
put them well here's where we as
consumers get to choose so you can put
them at your pedals to measure force and
cadence similarly you can put them on
your cranks or you can put them on the
spider here to measure the force between
the crank and your chain rings you can
even put them on the chain rings
themselves or you can measure the force
coming off your chain at your hub here
like this power tap if you use a
hubbased system or a spiderbased system
then you will only need one power meter
to measure but for your cranks and your
pedals you'll actually need two because
they measure your left and your right
leg output independently like these
rotor twin power ones or the Power Tap
P1 pedals there are though a number of
left-hand only power meters available on
the market and so it's quite a common
decision for us consumers to have to
make do we opt for a slightly cheaper
power meter but one that only measures
on one side and then has to double that
value in order to get our total power
output so assuming literally that your
left leg puts out 50% of your total
power of course the assumption that you
pedal with equal power with your left
and right legs is prone to error
potentially quite serious ones depending
on just how wonky you are but there is
an added complication to that as well
because even riders that may record a
50/50 split on average over the course
of a ride will always pedal harder with
one leg over another for short periods
of time not even the smoothest of
pedlers is a metronome in fact there is
research out there that shows that over
the course of a ride a single-sided
power meter could end up as much as 10%
out and worse than that it wouldn't even
be consistent on a day-to-day basis
because your pedaling style is open to
fluctuations depending on the terrain
for example or actually your fatigue
levels but there is of course a caveat
here and that is that as I've already
mentioned a left side only power meter
tends to be quite significantly cheaper
so it is a great way of getting data on
a relative budget so they are definitely
very good the only thing is that it
depends on exactly how important
accurate data is to you and I guess how
much money you've got finally then what
about software like Straa that estimates
power data based off your speed just how
accurate is that well to a certain
extent it can be relatively accurate but
it does depend on certain conditions
because you see the way that it actually
calculates your power is through an
algorithm it takes into consideration
your weight your velocity and also the
gradient of the road so on a smooth road
with no wind and over a long segment you
may end up getting a figure that's
relatively accurate however if you have
a tail or a headwind that will skew the
results so will riding in a group so
will a poor road surface in fact so will
your choice of bike so the data is fun
to look at then but you may struggle to
read anything meaningful from the
numbers well hopefully that has given
you an insight into just how power
meters work if you're interested if you
weren't interested I strongly suspect
that you won't be here watching the
summary to this video but cudos and a
big thumbs up to all of you who are
anyway that was how power meters work
what about how you actually use them
well if you are new to them then a good
idea would be to click just up there and
watch our getting started with a power
meter video or if you click just down
there we've got our top power meter tips
video which is definitely worth a watch
as well otherwise make sure you
subscribe to GCN to do that just click
on the globe