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Discussion Starter · #1 ·
I've recently purchased mountain bikes with "tubeless ready" rims and there seems to be an issue with max tire pressure ratings.
I've got 4 bikes each with a different wheelset a Shimano, Stans, Roval and Fulcrum.
The Shimano and Fulcrums both state follow the manufacturers tire pressure rating, the Roval and Stans both have max tire pressure ratings.

The Roval is 45 max psi period the Stans has max ratings for different size tires.

I've never owned a wheelset that had max pressure ratings, max tire pressure was always determined by the tire.

My LBS said to ignore the wheelset pressure ratings and use the tire pressure rating. Last week a new 1.50 tire on the Roval carbon wheelset had a failure at the bead and it was at 80 psi, and it was on asphalt with no drops or jumps.

So was the tire failure because the pressure was too high for that rim or was it just a tire manufacturing flaw?

Is my LBS correct in stating that I can run the tires pressure rating or should I follow rim max pressure rating?
 

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mtbrer
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I have seen aluminum rims where the inner wall split, letting the beads spread. The bigger the tire the more force exerted on the rim beads at a given pressure. I would hate to see a carbon rims failure mode, should it fail, due to excessive pressure.
 

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Absolutely follow the rim pressure rating. I learned the hard way. I pumped up a tire to 60psi to get the bead to set on a hookless rim that had a max rating of 35psi. The tire blew off and I was lucky not to permanently damage an eye or ear.


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Discussion Starter · #8 ·
I don't understand why there's a pressure limit, mountain bikes are intended to be used off road, road bike rims are ultra light and could fail if used off road but will hold 120+ PSI. Why is a mountain bike rim limited to 50 psi, they must withstand much more abuse than a road bike rim.

If a rim can't withstand more than 50 psi then how can it be expected to take the abuse a mountain bike rim is designed for.

I'm too old to ride a mountain bike the way they were intended but prefer them over road bikes so I like to use 1.5 tires with about 80 psi as I only ride on asphalt.
 

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Some rims have limits because tires tend to blow off over that. PSI is pounds per square inch, and mountain bike tires have a lot of square inches. Its a lot of force exerted by the tire. A tiny 23mm tire at 100psi isnt actually all that much force. Your 38mm (1.5") tire at 80psi is an insanely high amount of force! You should try those things around 50psi. Maybe less.

Basically, you massively overloaded your tire and rim until failure.
 

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Definitely follow the max limits on the wheel, there are quite a few posts I have seen where people have gotten inner wheel wall cracks by putting new tires on and pumping to 65-80 psi on roval wheels. There is no reason to pump a tire that high.

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Please keep in mind there is a tremendous difference between 120 psi on a 25mm road tire and even 40 psi on a 2.5" mountain bike tire. The hoop stress on the rim is going to be equal to the volume x the pressure. Note that volume is squared so there is a much higher stress on the rim coming from a wide mountain bike tire.
 

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I don't understand why there's a pressure limit, mountain bikes are intended to be used off road, road bike rims are ultra light and could fail if used off road but will hold 120+ PSI. Why is a mountain bike rim limited to 50 psi, they must withstand much more abuse than a road bike rim.

If a rim can't withstand more than 50 psi then how can it be expected to take the abuse a mountain bike rim is designed for.

I'm too old to ride a mountain bike the way they were intended but prefer them over road bikes so I like to use 1.5 tires with about 80 psi as I only ride on asphalt.
Like Porkchop_Power wrote, you are completely missing the part that MTB rims have a much larger surface area which the air is pressurising, so the force on the rim construction is much higher at the same pressure.
 

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I don't understand why there's a pressure limit, mountain bikes are intended to be used off road, road bike rims are ultra light and could fail if used off road but will hold 120+ PSI. Why is a mountain bike rim limited to 50 psi, they must withstand much more abuse than a road bike rim.

If a rim can't withstand more than 50 psi then how can it be expected to take the abuse a mountain bike rim is designed for.

I'm too old to ride a mountain bike the way they were intended but prefer them over road bikes so I like to use 1.5 tires with about 80 psi as I only ride on asphalt.
Road bike rims are not light, partly because they have to withstand the high pressure.
 
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