- I used to think that higher tire pressure
is always faster, in fact, when I started cycling,
the only limits for me on my tire pressure
were how much weight I could bring to bear on a track pump,
and the safe limit printed on the tire wall.
Then I crashed a few times on corners in the wet,
and I realized that wasn't so smart.
But this video isn't about the risk of slipping out
or, indeed, puncture resistance, no.
We are gonna do a little experiment to see
what tire pressure is fastest on rough ground.
(bouncy music)
Lab tests do actually show that for a smooth surface,
higher tire pressure does result
in lower rolling resistance than low pressure.
But the emphasis there is on smooth surface.
Not many roads are actually totally smooth,
and on a rough road, every little bump
causes vertical movement of the bike
and the rider, which is a waste of energy.
Now consider a slightly softer tire.
That allows for some damping of the road roughness
and reduces the vertical movement of rider and bike,
makes you more efficient, brilliant.
But if we take that too far, for example,
if we go right down to one bar,
well, then you have a really soft,
sloppy tire that is definitely not quick.
So clearly there is an optimum somewhere,
and we want to try and find it.
(energetic music)
So here we have one rough, 400 meter section of road,
one bike, one power, one position,
seven different tire pressures.
I'm gonna start here and finish here,
and see which one is quickest.
(energetic music)
Now the scientific measurement of road roughness
is known as the IRI or International Roughness Index.
I wanted to quantify the road roughness for our test,
but apparently you need lasers mounted
underneath a car, and they told me
that the GCN budget wouldn't stretch to that.
So I'm just gonna quantify is as pretty darn rough.
(energetic beat music)
This is run one at 8.2 bar.
(energetic beat music)
(active music)
So one important lesson from this experiment
is that if you want to test your tire pressure
on a rough section of road, don't choose a piece of road
that gets repaired halfway through your experiment.
(active music)
Now to the results.
Well, I did see a slight decrease in time taken
for the test run with decreasing tire pressure,
i.e. lower tire pressure was actually a little bit faster.
However, there was quite a lot of scatter in the results.
And like I say, that if I did this experiment again,
I would choose a section of road,
maybe a bit longer and with a few less potholes
because I'm not sure about the consistency of my testing.
However, what I can say, is that the lower tire pressure
felt so much more comfortable on the rough road,
and I really felt like I was pedaling far more smoothly
because I wasn't bouncing up and down as much.
I'd like to emphasize that tire pressure
does depend on your body weight and the road conditions.
For example, I weigh 50 kilos, and my bike is pretty light,
so I don't need a huge amount of pressure
to resist snakebite punctures.
If you're heavier, you will need
a little more air in your tires.
(upbeat music)
Now I have to say that experiment was not
the finest I've ever designed.
And quite apart from the fact that we didn't
do enough test runs at each tire pressure
for the results to be reliable,
I'd say that the course I chose was both
too short to keep a consistent power,
and the time measured too short
to draw any reliable conclusions from.
But, I think the point still stands,
with tires, harder does not always mean faster.
So let's take a look at why.
Well, faster means lower rolling resistance.
What is rolling resistance?
(upbeat music)
Let us look at what contributes to the loss
of energy in a rotating wheel.
Try to draw a circle.
That's okay actually, for a circle.
Anyway, the main contributions to loss
of energy are: aerodynamic drag,
the weight of the tire,
flexing in the tire, and the road roughness.
Now the design and construction of the tire
have a major effect on three of these,
so the aerodynamic drag mostly depends
on the diameter of the wheel and the width
of the rim and the tire itself.
Now weight obviously varies a lot
between different kinds of tire,
and the thickness and stiffness of the tire wall
are what affects how much it flexes.
So there are big differences in rolling resistance
between brands and models of tires.
But we're not comparing different tires here,
all we want to look at is a standard tire, x,
and changing the air pressure.
So that means we can ignore the aerodynamic drag
because frankly, the difference in diameter
of the tire at different pressures is negligible.
And we're also not looking at changes in weight.
What does that leave?
It leaves tire flex and road roughness.
(upbeat music)
So let's look at tire flex first.
Now as a wheel rolls along,
the section of tire that's in contact
with the road is compressed.
That compression of the tire makes the side wall bulge out.
This bulging of the tire dissipates energy
as waste heat in the material of the tire.
And the lower your tire pressure,
the more the compression of the tire, the more the bulging,
the more the flex, the more the loss of energy.
So, higher pressure means you go faster, right?
(upbeat music)
Well not necessarily because of the road roughness.
Now when your tire hits a bump, even a tiny, tiny bump
like a piece of gravel in the tarmac,
that exerts a resistive force on the wheel.
Let's look at the force vector and break it down.
This is a close-up of our wheel rolling forward.
And we're gonna look at the bit really close
to the road, so the road
and real close-up of our wheel and tire.
Now in reality, we've got some compression
of the tire here, as we know,
and we've got a little bump here.
So the wheel is going this way, rotating this way,
and this little bump here, the force it exerts
on the tire is at a normal angle
to the tire obviously, so 90 degrees to the tire.
Now as you can see, the force that this bump,
let's call it a pebble, exerts on the wheel
is not just vertical, it's actually
at an angle to the vertical here.
And that angle depends on both the size
of your wheel and the size of the pebble,
so the bigger the pebble, the bigger that angle.
And the smaller your wheel, the bigger that angle,
which is why in mountain biking,
people often now use 29 inch wheels
because the obstacles exert less of a backwards force.
So let's break this force down.
So the vertical component of this force vector,
this bit here, the vertical bit,
that's what you feel as uncomfortable jolting on your bike.
The vertical up and down movement,
and, well, it makes you uncomfortable.
And that might well be slower because
when you're uncomfortable, it's harder to pedal.
I mean if you can imagine riding on bare rims
or a solid metal wheel, it'd be really hard
to pedal just because of the jolting.
But it's the horizontal part that we really don't want
because that horizontal force, F,
call it H for horizontal.
So clearly if any given road there is an optimum
tire pressure, something that is between too hard
and too soft, let's call it the Goldilocks tire pressure.
But how do we find out that perfect tire pressure?
Well that depends on tire width,
the total mass of you and your bicycle,
and just how rough the road really is.
And of course when you go out for a ride,
you want to ride on many different surfaces,
so you want to take a balance of the roughest
and the smoothest roads you'll be riding on.
Now there are plenty of tire pressure charts out there,
which I would encourage you to look up.
When you're looking at one of these charts of tire pressure,
remember that your weight on a bike
is not normally evenly distributed.
Most people, and most bike geometries
mean that you have about 60% of your weight
over the back wheel and only about 40% over the front wheel,
and of course it does depend on how,
your position on the bike and the geometry.
But that means you normally need to have a higher
tire pressure in your back tire than in the front tire.
Now one other thing that you clearly
have to take into consideration
is the possibility of snakebite punctures.
So clearly, if your tire is totally compressing
over every bump, and you're likely to get a snakebite
puncture, then your tire pressure is definitely too low.
Hopefully this video is at least a little bit
interesting, and it helps you to see
why harder tires, not always faster.
Perhaps you will even be inspired to design
and run your own experiment on tire pressure
to see which pressure is fastest for you.
But of course, it does depend on the road conditions.
Why not let us know in the comments
how you get on, if you run an experiment.
Give us a thumbs up if you liked it,
and if you would like to see a little bit more
about tire pressure, why not check out
Simon's excellent video about tire pressure
by clicking down here.
Quiet now, Granny's talking.
(giggles)
Right, behave yourself, all right.
I won't tell you twice, all right.
(giggles)
Magnet, so exciting.
That's how rough the road is.
Tire track pumps, always so tall.
That's almost as tall as me!