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Tyre pressure question

  • 27-11-2022 6:50pm
    Registered Users Posts: 45

    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!


  • Moderators, Category Moderators, Arts Moderators, Sports Moderators Posts: 47,870 CMod ✭✭✭✭magicbastarder

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

  • Registered Users Posts: 532 ✭✭✭Mr. Cats

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

  • Moderators, Sports Moderators Posts: 19,283 Mod ✭✭✭✭Weepsie

  • Registered Users Posts: 532 ✭✭✭Mr. Cats

    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.

  • Posts: 0 [Deleted User]

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

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  • Registered Users Posts: 21,596 ✭✭✭✭Squidgy Black

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

    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

  • Registered Users Posts: 532 ✭✭✭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.

  • Moderators, Category Moderators, Arts Moderators, Sports Moderators Posts: 47,870 CMod ✭✭✭✭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.

  • Registered Users Posts: 1,639 ✭✭✭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?! :-)

  • Moderators, Category Moderators, Arts Moderators, Sports Moderators Posts: 47,870 CMod ✭✭✭✭magicbastarder

    those tyres are faster, on billiard table smooth roads.

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  • Registered Users Posts: 7,769 ✭✭✭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

  • Registered Users Posts: 1,483 ✭✭✭hesker

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

  • Moderators, Sports Moderators Posts: 24,262 Mod ✭✭✭✭CramCycle

    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.

  • Registered Users Posts: 7,769 ✭✭✭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.

  • Moderators, Category Moderators, Arts Moderators, Sports Moderators Posts: 47,870 CMod ✭✭✭✭magicbastarder

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

  • Registered Users Posts: 7,769 ✭✭✭cletus

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

  • Registered Users Posts: 167 ✭✭Al Wright

    Here an article explaining how to optimize tyre pressures vs loading

  • Registered Users Posts: 1,483 ✭✭✭hesker

    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