Tyre pressure question

DrumcDub1

Hi folks,

Hoping you can give me a steer with a couple of Qs on tyre pressure on my road bike.

I'm roughly 100kg, is there any rule of thumb re optimal PSI here (700x32)? Max PSI on my stock tyres is only 70 PSI which seems low given my weight!

Thanks in advance!

magicbastarder

i'd say you'd be fine with 70; it's almost bang on what this calculator suggests too:

https://cycling-sport.com/en/resources/tire-pressure-calculator

Mr. Cats

Is 70 not really low for max pressure? Are they tubeless by any chance?

Weepsie

https://www.boards.ie/discussion/comment/119928876#Comment_119928876

Wouldn't think so for a 32c

Mr. Cats

https://www.boards.ie/discussion/comment/119929077#Comment_119929077

Had a look and for GP5000s it’s max 102 for a 32c but for the TR version the max is 72.

You might be right for other tyres of course.

https://www.boards.ie/discussion/comment/119929518#Comment_119929518

It's basic physics. Bigger tyre is bigger volume of air so less pressure needed.

Squidgy Black

https://www.boards.ie/discussion/2058273593/tyre-pressure-question

I’m a tad heavier than yourself and running 70 front and rear on GP5000 32c clinchers.

https://www.boards.ie/discussion/comment/119928876#Comment_119928876

A fair few rims actually have a max pressure recommendation of 70 for 32c even if you’re running clinchers, Hunt for example say you can’t go over 70 for 32c

Mr. Cats

Yes, no doubt, particularly if they’re hookless. I’d still be a bit surprised if the max on a clincher tyre casing was stated as 70 though (just based on my limited experience).

Agree on the ‘optimal’ pressure in any case which looks to be around 70 based on MB’s calculator.

magicbastarder

worth noting that that calculator seems to suggest a little lower than others - it's suggesting 80psi for me on 25c tyres, which is actually what i run - but i usually see about 90 suggestedfor my weight/tyre combo.

2011abc

As an oldie I’m still trying to come to grips with how anything could be faster than 17/18mm TT tubs at 170psi let alone super wide tyres . It seems so counterintuitive but I guess that ship has sailed and taken a lot more myths with it . Is steel even real?! :-)

magicbastarder

those tyres are faster, on billiard table smooth roads.

cletus

I wonder is the 90psi recommendation a hangover from the tyre pressures 2011abc is talking about? Like, people are slowly accepting lower pressures, but not too low

hesker

https://www.boards.ie/discussion/comment/119929644#Comment_119929644

I’m no physicist but pretty sure that’s not it😀

CramCycle

https://www.boards.ie/discussion/comment/119930831#Comment_119930831

Not in terms of speed but in terms of how much air to hold you from running the rims off the ground they are right. There are other factors but in equivalent tyres (which you can't make as even the same brand/model will have variations between sizes), this would be it, the bigger the tyre the less pressure you need as its the same amount of air. The amount of air is the pressure by the volume, the bigger the volume, the lower the pressure and vice versa.

cletus

It's all about the hoop stress😁

In basic terms hoop stress means for any given pressure in a cylinder ( for cycling think "tire") the wider the diameter, the higher the overall force that is applied to the cylinder walls. Without getting too mathys, lets say tire A has double the internal diameter of tire B. They are both at 100 psi. As an example we are going to say that if you took a one inch length of tire A its total internal area is 2 square inches. The tire casing has 200lbs being applied to it (100psi x 2 square inches), while tire B only have 100 psi being applied to it, being 1/2 the area (100psi x 1 square inch). This means tire A would feel much, much harder than tire B. Tire A would also being applying around twice the force to the wheel rim it is attached to, trying to pull it apart.

magicbastarder

just for comparison, the nordstream gas pipeline runs at up to 220 bar. not 220 psi - 220 bar.

cletus

https://www.boards.ie/discussion/comment/119931864#Comment_119931864

If they ran it tubeless, they could probably run lower pressure...

Al Wright

Here an article explaining how to optimize tyre pressures vs loading

TireDrop.pdf

hesker

https://www.boards.ie/discussion/comment/119931522#Comment_119931522

You’re saying the same thing essentially but I don’t think it’s the volume that dictates it.

What matters is that you need an internal pressure to balance the pressure the tire feels from the outside due to contact with the road. That external pressure comes from the weight of rider and bike divided by the contact area with the road. Bigger tyres are wider and so have a larger contact area so the pressure is reduced. P=F/A