HowTo : tirepressure or rimpressure ?!

The rim is the max !

not the tyre !

After the stop at the aerodromo near Portimao, Portugal I passed a petrolstation where I added about 0.5 bar to each tyre to get them back at 3.5 bar. Not bad after some two months and taking into account the lower temperatures.

But most people don't know that not the tyre but the rim determines the maximum pressure the tyre should have.

The official absolute maximum pressure of my (heavy duty 135kgs each) rims is 3 bar (with a small tyre !). My Schwalbe tires however are 50mms (2 inch) wide and have a pressure-range of 2.5 bar to 5.0 bar*.

The reason the rims determine the maximum tire pressure is that the walls of tires are forcing the walls of the rim to spread (1). But if the tirepressure is (too) low the tire will fold (sack) too far (2) which finally will cause the tire's sidewalls to start cracking (3). Also the tire will ofcourse be flexing more sideways and so will then the whole bicycle, something I don't like at all.

As you can see at (calculate from) the label my rims (so with fitted 50mm tyres) would hold a maximum pressure of some 3.15 bars to 2.4 bars. I compromise at a 3.5 bar which brings me precise steering. Knowing I (my wheels) have only few bumps to handle (because of my low average speed 😁). And knowing about that 50kgs of luggage ...

* There do not exist rims that can survive these kind of pressures so the question is why high-pressure tires are being produced.

You might (will) end up with a rim like this one.

How strong is airpressure ?!

- Let's assume1 you have 55mm wide tires (as I have) and inflate them to 3.5 bar.
- Let's also assume2 that there innerwall (innertube) is some 75% wide, so maybe 42mms.
- Let's assume3 that 80% of the innertube is touching the tyre. The remaining 20% will hit the rim but will not spread it due to the (non-) angle to it.
- And let's assume4 you ride 29-inch wheels (as I do).
- The outercircumference (length) of my tires is 227cms, so let's assume5 there middlediameter is 220cms.

An innertube of 42mms has a circumference of π x 42mms = 13.2cms. So 80% means that 10.5cms across is pushing to the tire with 3.5bar. So 1 "slice" of 1cm exerts a force of 37kgs to the tyre.

Since the medium tubelength is 220cms that means that 2310cms2 are pushing to the tire. Which means that a whopping total of 8124kgs of force is trying to have the tire and the rim explode ...

Btw : the remaining 20% does not try to spread the rim since it is not at an angle to it.

But I expect my OnTheRoad 4.0 supertouringbike 🖱️ to live forever !

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