(soothing music)
- So you want to go faster on your bike
and you're looking for aero gains.
For example, a time trial handlebar
like this Time Zone bar from ControlTech,
or maybe new wheels,
or an aero helmet,
or a new position?
But how do you know if it's actually faster
rather than just feeling faster?
Well here at GCN we like to talk about science.
We even sometimes do proper experiments
and oh how we love to quote peer reviewed
research from scientific journals.
But the problem is that
the results we get in experiments
might not be relevant to you.
It's normally very case specific,
especially for aerodynamics.
If you want to test what's really
faster for you then you'd need
to go to a wind tunnel,
which is quite expensive.
Or you could do a really good field test.
So here is GCN's guide to
how to do a really good field test
to test for aero improvements.
First of all, let's talk about input variables
and response variables.
That's kind of a fancy way of saying
that you need to change something
and measure something.
So the thing you're changing
is what is going to hopefully
bring you an aerodynamic improvement
and the thing you're measuring
is probably going to be
whether you go faster or not.
So in our example what I'm going to change
is the angle on these ControlTech
aero bars from zero degrees,
where they are at the moment, to five degrees.
And that might sound like a tiny tiny change
but it actually is the kind of thing
that we would've measured in a wind tunnel
the year before in Olympic Games
with British Cycling to see
if it brought any benefits.
In fact, we would've measured
even smaller changes than that.
Now the test I'm going to use
is a time trial effort on
this one kilometer stretch of straight road
and the response variable is going to be
whether I go any faster.
The key things you want to pay attention to
when designing an experiment are
validity,
repeatability,
reliability,
and measurement.
Or should that be measureability?
Internal experimental validity
means quite simply making sure
that you are measuring what you want to measure
and not something else instead.
It requires you to make sure your experiment
is consistent so you just have to think
quite carefully about other factors
that might influence the output
apart from the different input variables,
like in my case the bar angle.
So right now I'm thinking about
all the possible reasons that I might
go faster on my time trial stretch of road
that aren't due to being more aerodynamic.
For example, if one time I put down
50 watts more than another time
then I'll probably go faster but it won't be
due to changing the angle of the bars.
So basically what you need to do
is think about keeping all the variables constant
apart from the one variable that you're changing.
And this is also known as local control.
Local control means trying to remove
all other influences on your experiment
beyond the one variable
you are deliberately changing.
For example, the variables you don't
want to change are,
wind conditions and temperature,
so do all your tests on the same day
if possible and in calm conditions.
The power you exert,
traffic,
fatigue,
tire pressure,
body position,
the total weight of your bike,
the total weight of you and your clothing,
and so on.
So to put it another way,
you only want to change one variable
at that time and that does include weight.
So if for example I was to go
and have lunch mid experiment,
well that would change my weight
and that would influence the results.
So lunch stops,
no coffee stops and for that matter
no pee stops either mid experiment
and if the variable you are changing
on your bike changes the weight
of the bike, for example,
if you go from Lo-Pro wheels
to deep section wheels
then you need to compensate for
that weight difference.
For example, what you can
is have a bead on for both setups
but with more water for the lower
weight setup of your bike.
So for my experiment today
I've chosen a route with no traffic lights,
no corners and not too much traffic either.
There is a bit of wind
but it's not very gusty
and it's fairly consistent.
Of course I have a power meter
so I can make sure that my average
power of each time trial test run is the same.
Now if you don't have a power meter it is
still possible to do an aero test like this.
What you need is a very gradual decent
where you can just freewheel down
and measure the difference in time.
But it is better with a power meter
because the movement of your legs
actually affects your aerodynamic resistance.
Another key thing is I'm not going to
ride this test full gas.
It's not just because I'm lazy
it's also because I've got to
do it at least six times
and talk more about repeatability later.
Although I do have to do it fairly fast
to make the test externally valid.
External validity is another important concept.
It means the extent to which the results
of your experiment can be assumed
to be true for other cases.
For example, different people,
different courses or different times.
Well, now when it comes to riding a bike
and specifically the effect of aerodynamics
we know from science that some differences
are person and shape specific,
i.e. sadly what is faster for you
might not be faster for someone else
or on a different course.
That's kind of obvious if you think about it.
So what this means is
that basically your experiment
is unlikely to be externally valid
but that doesn't actually really matter
because you want to find out if you're faster,
not if anyone is faster.
What it does mean though is
that you need to choose a test course
similar to the course that you're targeting
or racing, whether it's rolling,
flat, hilly, windy, calm,
make sure it's as similar as possible.
Choosing the right speed is also
really important for aerodynamic testing.
That's because your aerodynamic
resistance increases as you go faster
so if you do your test too slowly
you might not see any difference
even though you might actually
have improved your aerodynamic drag.
Now we come to repeatability.
For each angle of the aero bars
I'm going to repeat the test
and do it three times.
Why three times?
Well, so that I can see any anomalies.
For each setup the results should be
fairly similar but fluky results
or outliers are always a possibility,
especially when you're testing out on the road.
So if I do it three times,
or even better would be five times
but we're a bit short of time,
then I can see if one result
is really different from others
and that gives me a clue
that something might have gone wrong
with that test and I can repeat it.
It is important to note though
that you should only discount results
that are actually outliers, not just
the results that you don't like the look of.
Reliability might sound like repeatability
but it's actually subtlely different.
Reliability is about how much of an effect
the variable you're changing has
compared to the inevitable effect of
outside influences and of course
how accurately you can measure things.
For example, I'm testing two different bar angles
and I'm doing three test runs for each angle.
Now if there's more a difference
between the times for the same angle of bar,
i.e. the same setup than it is between
the averages for different angles
that means my test is not very reliable,
or to put it another way,
the difference between the two setups
is smaller than I can actually measure
so you probably don't need
to worry about bar angle in that case.
Now it's going to sound blindingly obvious
but you need to be able to measure
your output variable, so in this case
the time taken for each test run.
Now I need to press lap at exactly
the same point at the start
and finish of each lap.
That way I can take the data file home
and analyze it later and see what
the numbers really said
and I don't have to work out averages
while I'm riding along on the road.
While we're talking measurement your choice
of time trial test route is extremely important.
It needs to be similar to your race course, yes,
but not the same length.
It should be shorter because remember
you're going to have to repeat it
three times for each variable.
On the other hand it also shouldn't be
too short because otherwise the slight
differences when you push the lap button
on your bike computer will lead to
quite a significant error compared to
the total length of the time
that you're measuring.
(soothing music)
So here we are back at GCN HQ
and it's time to analyze the results
of my aero bar angle experiment.
So results are in.
It was actually a 1.1 kilometer course
I'd like to point out.
For zero degrees I averaged 212.33 watts
and the average time was 1 minute 47.67 seconds
and for five degree bar angle
I averaged 213 watts and the average
time was 1 minute 48.33 seconds.
And looking at those numbers one might assume
that zero degrees is faster but hold your horses.
I made some real mistakes designing
this experiment, for example,
there's more variation between
the time measured for each setup.
So for example at zero degrees the time measured
varied from 1 minute 43 seconds
to 1 minute 51 seconds and for five degrees
the quickest time was 1 minute 44 seconds
and the slowest time was 1 minute 55 seconds.
So what you can see is there's more
variation in time within the setup,
so for the same setup,
then there is between the two setups.
So if there is a difference it's so small
that we'd have to do a statistically
significant number of tests to work it out
and three is definitely not
statistically significant.
Secondly that large variation within each setup
is probably due to the gusty wind.
So I could really feel the wind
blowing me around when I was out there
and like I said earlier
you want to avoid the influence of wind
on this kind of test.
Well, I hope this video
helps you in your quest for aero improvements.
Give it a thumbs up if so
and share it with your speedy cycling friends.
Don't forget to let me know
down in the comments if you're setting up
your own aero experiment
and if you'd like to check out
another aero geekry video
there's this one about why size
really is important.