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

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!

About the Author: Michael Flood

100 Comments

  1. when i pumped my tire it felt a bit hard lol when i bought topeak gauge d2 psi checker i saw that it was on 35 psi haha and i thought the tire are hard enough lol , so i pumped it more to 90 psi and its so much better now

  2. I started to ride with my 32c tires on 6 Bar, and now after half year I ride with 4.5 Bar and its more comfortable for me and fast too

  3. Hola, Emma, te has pasado¡¡¡¡ a mi también me usta la física teorica pero ha sido demasiado¡¡¡ Vente a vivir a España y quédate en el GCN España, que aquí nos tomamos las cosas de otra forma¡¡¡ Con tu permiso. GUAPAAAA¡¡¡¡ Saludos

  4. Can you do the same for a typical Time Trial flatish course for the UK on normal tarmac roads? This is where the correct PSI could be huge. Thank you.

  5. You didn't elaborate on the wight of the rider, and I think is the most important factor as to what is your ideal tyre pressure. Say you weigh only the 55 kilos, or 60 kilos, or 70 kilos, etc.

  6. I would like to see a tire experiment done on a downhill sans chain (or don't pedal). That would remove a major variable. In fact I need to find an appropriate hill to test 23.

  7. I've never ridden on skinny tires, but I'm genuinely surprised that a 120psi tire can run normally at 29 psi. That would be like my running my MTB tires at probably 3psi.

  8. I have always wondered if there is a formula to figure out optimal tire pressure. I am ever got a snake bike road riding my MTB but my road bike even at near max pressure has on 700cx28mm tires. So does that mean I am too heavy or the tires I run are junk or something altogether different?

  9. I used to ride at 100psi, gained some weight, started to get lots of snake bites (I call them vampire bites).
    Were generally caused by hitting gravel on paved roads (common in towns)
    Was told to raise pressure to 110 and this solved the problem. I also tend tyo ride off saddle to better distribute weight in areas where there is gravel on the paved road.

    So lowering tyre pressure to save a few seconds would result in losing much more time fixing punctures.

  10. After a long bike ride on bad roads, one more thought:
    When you hit a gravel rock, as you pointed out in your video, there is an vertical movement, as well as a horizontal vector that slows you down.
    What your logic omitted is that on the other side, that same rock will give you a horizontal vector force that pushes you forward as you "drop" fropm the rock. (the vertical motions are lost enerrgy).

  11. I have a fat bike. 30 psi max is mentioned on the tyres. Should I go with 30 psi on smooth tarmac highway? I have seen in few videos that we should fill 10% less air than the max. Please suggest

  12. Higher tire pressure gives the tires a wooden feel. The tire does not react properly when turning, this requires more effort from the rider.

  13. you can pretty easily try and find out that high pressure is not always faster on a less-than-ideal-road. go find yourself a strech of cobbles. go into the cobbles at the same speed, one time with high pressure, one time with lower pressure. dont pedal. you will slow down faster with higher pressure.

  14. Placebo effect comes into play as does the speed that a given surface is taken at. Hence I need 20 psi on even the bumpiest of trails.

  15. I personally look directly while riding, by observing the crushing of the tire. If the tire does not deform, it is too inflated so i have to inflate lower pressure.

  16. My height is almost the same as you Emma. When you get bored of this beautiful black Orbea, you could give me as a present. Thank you from Gran Canaria

  17. I would be pretty terrified to run a 25mm tire at 20 p.s.i. even if I weighed just 50kg. That looks like a recipe for disaster.

  18. As you approach your (tested) optimal PSI for your conditions you can actually get the benefits of higher pressure in terms of rolling resistance while keeping your pedaling smooth by looking at the saddle rails and shell, if you haven't already figured out how your frame and seatpost affect smoothness. Tire pressure should be fine-tuning, not the first thing done to find the right PSI before evaluating all of the other factors.

    Generally, if you travel over modern roads and have selected good wheels, tires and saddle for your conditions, you should be able to experiment with 75 to 90 psi for training and not much higher (if at all) for racing. Note that racing requirements are different because of extreme side and braking forces that we don't normally experience during submax and or solo training.

  19. Hello Emma and hello guys. Interesting test, but I find it impossible, I myself did some tests on routes of 40/50 km with wheels inflated to 7.2 bar ahead and 7.5 behind and then with 6.5 bar, and I recorded times better with the wheels inflated to 7.5 bar .. another thing you can do a test test with 3 bikes with price range from 1500/2000 € weight 8 8.5 kg, make a path of 10 km in the plain 5 k uphill , and 5 km downhill and record the times? I would suggest Focus izalco race 105, Canyon, and Orbea, or Wilier Triestina, ok? is possible, hello guys greetings from Italy ..

  20. Find a bumpy downhill section of road and let gravity do the work for you.  No need to pedal just coast down the hill with you legs tucked into the same position and don't touch the brakes.  Simple and the scatter from run to run will be minimal.  Physics 101.

  21. Emma, I want to see a video from you that breaks down best pressure by body weight. Maybe break it down too by body weight with size of tires 25mm, 28mm etc.

  22. Why does she pedaling for the experiment? It would be much easier to reproduce exactly the same conditions if she just focus on standing still, holding the position and roll downhill.

  23. Awesome video and great effort, thanks.
    However, this is no where near science, it is biased and misleading conclusion.
    Showing numbers does not always lead to best results. And I am not saying the numbers are not correct, but under the experiment conditions, this is valid.
    So please, don't say GCN does the science, while you are trying to generate debate.
    You should mention the power and the wind condition in each lap.
    To do this properly, you should conduct these at constant power (not average) and constant speed in race track. These conditions will eleminate important factors affecting the results.

  24. Possibly free wheeling down hill thus eliminating pedaling forces which constantly change over the repeated 'circuits" giving inaccurate results?

  25. Hi, what if your weight is 95kg? How close to max tyre pressure should I have then compared to this video lightweight lady 😉

  26. I like ggeking out on this stuff. The geek factor is especially high in this video. I have recently switched over to a slightly fatter tire on my road bike and I don't think I'm losing speed and it is a lot more comfortable to ride. The guys in my bike shop told me that the rolling resistance is not any higher even though I'm only running 80PSI vs 100 on my old tires. This video confirms it

  27. Might have been more accurate to measure power output at different pressures and keep the time/ speed consistent.

  28. Gnc did a while back a video comparing a road, cyclocross and mountain bike on cobblestones. The mountain bike was the fastest. The ability to soak up bumps clearly helps you go faster, even on an ostensibly slower machine.

  29. They do lab rolling resistance tests, but the equipment I've seen used looks like it has a smooth rolling surface. Does anyone do these test simulating a rough surface? That would settle it pretty quickly

  30. Interesting question, but that 'experiment' lacks controls and consistencies. What pressure is faster will be different for different tires, different rim designs and different road surfaces. You could earn a PHD on such a project. Comfort is something tubular tires provide along with low rolling resistant. Sew-ups don't need stiff side walls for high pressure. Pinch flats don't happen with tubulars. There are still professional racers riding tubulars that they pay for out of their own pocket. Comfort is a major reason for their choice.

  31. What about doing a test with rolling over a 10mm bump and a 20mm bump with a high frame rate camera capturing the bottom of the wheel at the bump – you could probably easily do this with some plywood fixed to a flat surface with some cane fixed to the plywood. It might also give people a really good view of the wheel deflection, and it would allow for a series of strips of cane simulating bumpy roads.

  32. My weight is 110 lbs. (50kg) plus clothes, helmet, shoes, full water bottle and 21 lb. Bike. Tires are 700c X 25 . I use 105 front and 128 rear.
    Works for me.

  33. Quantitative analysis on this may be too difficult. Qualitative analysis based on riders' experiences might be good enough or even the best we can hope for. Speed is essentially what the mass (rider + bike) achieves from the "same" propulsion power of pedaling which we want to assume is constant from test to test. Any up and down movement caused by the roughness of the road will impede the forward momentum and reduce the speed. Air in tires will absorb loss of pedaling output due to this up and down movement but so would the flexible frame and the bones and muscles of the rider's body. All of them will try to mitigate the loss of forward momentum by trying to make as much of the total mass move forward in a gyroscope like way as possible. We riders accomplish this instinctively by momentarily standing on pedals when we go over a bump. Standing allows the body's joints to move freely so that body's mass can preserve the forward momentum. This will help more than adjusting tire pressure in many cases. Since this experience tells us that our, and the bike's, gyroscopic reaction depends on the condition of the road, design of the test like the one in the video is flawed. The result is really dictated by the choice of the road condition.

  34. Mechanical Engineering: Statics 101! A good job GCN and Emma! (but, yeah- randomize your trial runs and add duplicate runs! 🙂 )

  35. Just watched this vid and really enjoyed it. Just thought, if tyre pressure should vary with weight of bike and rider maybe tyre deformation would be a good measurement. Inflate tyre till deformation is 2mm for smooth road, 3 for bumpy, 3.5 for cobbles type of thing.

    Anyway thanks for the great vid.👍👍

  36. The horizontal force component of the pebble doesn't really harm the ride, since it rotates the wheel to a direction it is free to rotate towards and is quickly rotating towards anyway.

  37. Wouldn't rider weight have to be taken into consideration? Sometimes I feel like I have so much force on the front tire when I come out of the saddle to sprint. If I suddenly hit a pothole, I can see that tire bulge out all sick! lol

  38. The backwards force generated by the smallest bumps – wouldn't any frame with built in compliance absorb that?

  39. There's no debate, None…you use the highest recommended pressure printed on the tire…or only slightly lower.
    Geez these things come with instruction.

  40. “Pretty darn rough.” Ha! That was funny! Loved it! Emma, you’re humor is brilliant. 😂🤣😂 And apparently you’re really brilliant, literally as well! 👍

  41. First, I really was listening to what you said–and I got it. Honest! Second, I can't help but notice most of your videos occur in very interesting and beautiful countrysides.
    Would it be inconvenient to, at some point, print where you were shooting the videos? Just asking…. By the way, Emma, you do these just as well as the guys do. You're good at this.

  42. Well, you mentioned a few good points with the end result of "experiment yourself to find the best tyre pressure".

    There are more things to consider, though.
    Stiffnes of the frame and wheels and other components, which can or cannot act as a spring.
    Do we use or body, mainly legs, as a spring, too?
    Tyre pressure heavily depends on temperature. I guess heat from tyre-road contact dissipates pretty well on a bicycle, thus making ambient temperature more important (it usually is the other way around on motorcycle).
    What about grip?
    What about tyre wear?
    What about turning abilities?

  43. Sometimes, I would unintentionally hit potholes would lower pressure be better or high pressure. I’m also concern about damaging my rim. Cheers

  44. These types of experiments are so flawed by design. Human error/effort in different attempts. Also the lines you took onnthe road.

  45. Can we really opt for larger tires? Just pressure is the only thing under our control in response to quality of the road.
    Will gator skin tires help with puncture, at the same low tire pressure compared to other tires?

  46. Wow, I wish all the "GCN does tech" videos were, well, …tech! Some have great comical value but this actually seems legit.

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