Tyre pressure variation when riding Mythbusters experiment

A guy at my LBS told me that as you ride along the pressure in your tyre will increase by 10 to 15PSI from its ‘resting’ pressure. This is the reason he gave me for not inflating my tubeless Wheel TA 36" tyre above 38PSI (resting), when I know that a coker tyre (with a tube) can go up to 50PSI. Is this a cycling myth or can anyone with a pressure guage confirm this? Its like Mythbusters for unicyclists!

Here’s the challenge: go you measure the resting pressure of your tyre, preferably with a pressure guage that is accurate to 1 or 2 PSI. Then go and ride continuously for half an hour or an hour jump off and take the pressure again. Report your findings here so that we all may find out something cool about tyres!

Sadly I don’t have a pressure gauge, so I can’t check this for you. However, I’d just like to clarify what it is that he thinks will increase the pressure. As I see it, there are two options; fully laden weight on the unicycle or heat build-up in the tyres/tubes.

When I pump up the tyres on my uni, it’s always lying on the floor, so there is no external influences on the tube other than the tyre stopping it from expanding beyond a certain point. When riding, of course, there’s the weight of the uni itself, as well as my weight all bearing down on the tyre, so that is bound to increase pressure. Things like riding off a kerb, over a stick, etc will naturally give a momentary increase to pressure. I suspect that this combination that will give the extra pressure.

A rolling tyre will build up heat as it continually flexes over the surface it travels over. This, in turn, will heat up the air in the tyre, thus giving it a higher pressure relative to the colder air outside. Car tyres can certainly get quite warm after travelling at speed for even a short amount of time. I’m sure that bike tyres probably do too. The kind of speed that a unicycle travels (regardless of wheel size, it’s more about the rpm) is unlikely to have any significant effect on the temperature.

Please note that these are just my gut-instinct reactions here. It’s been many years since I studied physics, so I could be miles out, but I think it’s the first option. As to the 15psi being correct? Well, that’ll need some experiments, but how you’ll attach and read a gauge on a wheel as it rides down a kerb will be an interesting challenge!


i dont think the weight on the tyre would affect the pressure much really. because when you put weight on it the tyre bulges out to te sides and its got fairly much the same volume as it did before so that wouldnt make much of a difference.

i can imagine the heating of the tyre having affect on the pressure, but negligable affect because we travel so slow really.

I think it also depends if your using the unicycle for trials or for street. If your using it 4 trials. Then you might lose pressure if bunny hops are your prefered choice of tricks. :stuck_out_tongue:
If you prefer street if you corner too tightly causing your uni to add more weight therefore losing pressure as i have found out the hard way. :smiley:

Take care

unless you’re putting the tire into the trunk of a hot car, i think he’s crazy. tubes generally lose a few psi per day to to rubber not being 100% air tight.
also weight has no bearing on pressure, there’s still the same amount of air in the same space, it’s just shaped differently.

Sounds mythy to me. Maybe he’s talking about temperature change, such as what you’d get between an air-conditioned pumping-up environment and a hot summer day’s pavement plus rim heat from braking.

We don’t use brakes (mostly). Temperature change can surely have an effect, though I don’t know if we ride fast enough, except on 29" or larger wheels, to make much difference other than ambient temperature.

How did this guy ever get a job at a bike shop??? :thinking:

Basic gas laws:
You have quantity of gas (mass), volume, and temperature as variables.

Increase any one of those three and leave the others unchanged and pressure will change.

More gas = more pressure
More volume = less pressure
More heat = more pressure

Increase the temperature, and the pressure will increase. (Charles’ Law - see http://en.wikipedia.org/wiki/Charles’_law ) I spent ages studying that stuff when I was a diver.

Riding the uni will do two things:

  • Deform the tyre and tube and this flexing will cause some heat.
  • Compress the air in the tyre. This will increase pressure and also generate some heat which further increases pressure until the heat is dissipated.

Both will contribute to a small increase in pressure.

A tyre is flexible, but not stretchy. If you squash part of it, it doesn’t expand somewhere else to compensate. A squashed cylinder has less volume than an unsquashed one. However, for very slight squashing at the bottom of a large tyre, this effect will be minimal - a tiny fraction of 1%.

As for the change arising from heat, this will be proportionate to the original pressure. A unicycle tyre runs at anything from 10 psi or so up to 30 or 40 psi. (My Bacon Slicer runs at 130 psi, but that is unusual.)

A tyre running at 130 psi (like on a road racing bike) may well incease in pressure by nearly 10% = 10 psi or so. However, if a uni tyre increases in pressure by 10% or so, that will usually only be about 2 - 3 psi, if that.

Mikefule is exactly right. We just finished this study in chemistry last month. I also think that your LBS person was maybe suggesting that whilst upon the unicycle the pressure increases. Although I’m not sure that this is true, have someone take the pressure while you are in the saddle as well.


We’ve addressed this before on earlier threads.

Let’s take a simple example:

A wheel with a 20 inch diameter has a 62 inch circumference.

Let’s assume for simplicity that the cross sectional area of the tyre/tube is exactly 2 square inches.

The volume is then roughly 62 x 2 = 64 cubic inches.

(I say roughly because a tyre is not a cyclinder, it is a torus, but I’m trying to keep the arithmetic easy.)

Assume this tyre is at 20 psi.

You now sit on it and the bottom of the tyre deforms slightly.

Nowhere else on the tyre expands. It can’t because the tyre is flexible, but not stretchy. (Unlike a balloon, where if you squeeze on bit, another bit gets bigger.)

But what is squashing? Say 2 inches of the circumference of the tyre may be squashing by about 10% or 20% of its cross section.

Keeping it simple, say exactly 2 inches and exactly 20% loss of cross sectional area.

The loss of available volume is therefore 20% of 2/62

2/62 = 0.03
20% of 0.03 = 0.006

That means that in this simple example, the tyre would be losing 0.6% of its volume.

The amount of air in the tyre would be the same, so the pressure would increase by 100/99.4, making the increase just over 0.6%

There are a few short cuts in the steps I’ve shown to convert to percentages, but the result is right: in the example given, sitting on the uni would increase the tyre pressure by 0.6% which in technical terms is naff all.

On a vaguely related note, do you know any way to calculate contact patch size for various tires?

I switched my Large Marge rim from a 3" Gazzaloddi to a 3.7" Endomorph. I am easily able to calculate the difference in volume, etc, but I have no idea how to mathematically estimate contact patch.

Any ideas or advice on how to do so?

PV = nRT

V might actually increase a bit if the rubber stretches. n will decrease slightly with time as the gas diffuses out of the tube (a neglible effect). T might increase slightly due to friction while you ride, but I doubt very much. So it sounds like a myth to me.

if you left the wheel in your car on a hot day, it could increase T enough to potentially pop it. had that happen before to customers, so in the summer we leave a few (10-15) psi out of most hihger pressure tires if they’re going inside a car.

About that, would a creepy crawler suffer damage if its at off-road (squishes around an inch just riding normally) pressure and is left in car for an hour or two in a summer day? I want to ride it to cross country practice, then leave it in someone’s car, then ride it back home, since practice is only two miles from my house.

absolutely not, it’s a tire and therefore made of rubber which melts at a temperature of 110 degrees celcius which is 10 degrees warmer than the boiling point of water.

Only track-type unicycles (skinny tires) are run at pressures similar to racing bikes. I don’t think it’s a concern for other unicycle types, especially those with fat tires such as MUni, 36", Trials, etc.

Are most tires really made of rubber? Or are they various polymers of rubber-like substances? In any case, if it’s hot enough to melt your tire, it’s definitely not a good time to be out riding… :slight_smile:

We’ve addressed this before on earlier threads.

Let’s take a simple example:

A wheel with a 20 inch diameter has a 62 inch circumference.

Let’s assume for simplicity that the cross sectional area of the tyre/tube is exactly 2 square inches.

The volume is then roughly 62 x 2 = 64 cubic inches.

(I say roughly because a tyre is not a cyclinder, it is a torus, but I’m trying to keep the arithmetic easy.)

Assume this tyre is at 20 psi.

You now sit on it and the bottom of the tyre deforms slightly.


I might have ignored this mathematical blunder, had the next sentence not mentioned keeping the arithmetic easy! But I then had to share my quiet grin with you.

However I do like the word “cyclinder”. As a description of the torus shape of a cycle tyre it is excellent.


Oops. Typed it all twice because my computer locked up the first time I tried to post it. I hang my head in shame. :o :o :o

Of course, if I’d had to place £100 on who might have pointed out such an error…

Oh dear, am I being typecast? Second comment from you along those lines this year Mike. :wink:

…and I thought I was being rather restrained, intentionally ignoring other nit picking opportunities.

Have a good New Year Mike, and please continue to keep us entertained during it.


Yeah sounds like a myth. Thanks for your posts Mike and Jim_Rob. If there is a measurable increase in pressure, it would be really small, nothing like 10 to 15PSI. Myth busted!