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Power Cranks
While I was in Mallorca this Easter, our
group leaders were using SRM cranks (so
called power cranks), which accurately
measure wattage. They also had their laptops
with them so were able download and display
the information gained.
The point that they were able to
demonstrate time and again was that in time
trials, if you start really hard, you’ll be
putting out a lot less power at the end. If
you then calculate your average power output
over the ride, which you can do with the SRM
system, and then, when you’ve recovered,
ride the same time trial again, but this
time, keeping your power output throughout
at the average for the previous ride, you’ll
go significantly faster and at a lower
average heart rate.
There is however a problem – the ‘budget’
version of SRM cranks cost £1400, though the
distributors in the UK (www.scientific-coaching.com)
make the point that while lots of riders are
willing to spend £1400 on a new bike or a
new frame, the potential performance gains
from buying SRMs and putting them on your
existing bike are much greater.
This raises the question of how you
maintain steady power output throughout a
time trial without an SRM power read out on
your bars. The best advice would probably be
not start too hard, not to go flat out up a
half mile hill when you know very well that
it will take the edge off the pace you can
ride the three miles of flat road that
follow. But it’s not all about not going too
hard on the harder bits, it’s also about
going as hard on the downhill bits or when
the wind is behind you. A heart rate monitor
might help but don’t aim to keep it the same
throughout the race, or you’ll go
progressively more slowly – heart rate tends
to drift gently upwards for a given power
output over a period of time.
There is another watt measuring system
which is less expensive, called Power Tap,
which involves a fat rear hub, a bit like an
old Sturmey Archer 3 speed. It apparently
does the job but the fashion conscious time
triallist probably wouldn’t want one on his
race bike. There is also a much cheaper and
much less accurate system produced by Polar,
not displeasing on the eye, which is
probably better than nothing – ask Bill
Barrie – he’s used one.
Dave Birch
May 2006 |
