What’s The Fastest Tyre Pressure For A Road Bike? | GCN Does Science


– 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!