All:

I bought a 2013 Rockhopper Comp to get back into biking after a couple decades. I've been struggling with making it fit for a few months, and finally decided that since I'm doing 90% of my riding on the road, I wanted to get some higher pressure tires so it would roll faster.

LBS said Infinity Armadillos would work fine at 100 psi. Not that I don't trust them, but I also contacted the official Specialized support staff, who told me these rims would absolutely not support 100 psi.

So instead of pouring more money into my MTB to try and make it a 90% road bike, I decided to buy something else. I landed on a Salsa Vaya, as it seemed great for the kind of riding I want to do, without prohibiting me from hitting a gravel road, if I want.

This has DT Swiss 470 rims, which are rated only to 58 psi, despite having tires which are rated for 90 psi.

Salsa says 58 is the right number, and not to exceed that.

My question is, what happens to a rim if that pressure is exceeded? And finally, are there any recommendations for a decent rim that will fit up to 35-38mm tires, and has a higher max psi? Or am I chasing my tail here?

And in case it makes a difference, I am quite heavy, at 280. I just want something that will roll well, so I can cover longer distances. Right now, I can only do about 25 miles on my MTB until I'm done.

Thanks!

If you have an Android phone, get the Berto tire pressure app...

For 35mm tires, your weight roughly should be 81 rear and 66 front, the rims should handle that fine.

With wider and higher volume tires, you don't need to run 100psi in them, that's only for the skinny roadie tires.

I'm 260 and with 40mm tires I run 65 rear and 55 front and it is totally fine, I could really run less if I wanted to. The pressure drops with larger and higher volume tires.

Your 25 miles issue is just fitness, keep pedaling. I've been riding since the last week in July and can only get out once a week, 25-30 miles is my ceiling right now...only because I have limited time in the saddle.

High pressures in larger tires exert significant force on the tire bead and rim. At too high a pressure you can blow the tire right off the rim or have the rim fail on you.

And as kingsqueak posted, you don't need as high a pressure in larger tires in the first place.

It's not just the absolute pressure. The tire size matters. A 2" tire at 100 psi exerts a lot more pressure than a 1" tire at 100 psi. I don't know the exact relationship, but I can tell you that once upon a time I spread a rim by inflating a 2" tire to 90 psi. The stupid part was, there was no need to go that high - at 65 psi the tire was plenty hard.

Replace your knobbies with some slicks of a comparable size (Schwalbe Big Apples come to mind) and you'll find it easier to pedal on the street and hardly notice the difference on gravel.

Actually no. The PSI means the pressure on a surface. 100 psi with a 700x28 tire will exert the same pressure on the rim as 100 psi on a 700x35 tire on that rim: 100 pounds per square inch. Now when the tire or rim blows there's going to be a lot more energy stored in the larger tire.

Hah. I'm running 2.35 Big Apples right now at 55, which is max. Been reading some of the Berto stuff, and I'd like to try his pressures, but he's still at 84 psi on the rear tire for my weight in a 37mm. Should I be afraid to ride at that with rims that say 58?

Since your tires aren't 37mm, then you shouldn't be trying to use those numbers. Berto's earlier article has some suggestions for MTB tires like yours at the end: www.bccclub.org/documents/Tireinflation.pdf

Sorry I wasn't clear. The tires I have on the Vaya now are 38mm Clements. On my Rockhopper I have the Big Apples.

Thanks!

The Berto Android app says for the 38mm to run 72/rear and 60/front

If the Big Apples are 2.35/60mm it would be 52/rear and 43/front....and due to that specific tire, it could be a good bit less actually.

Just some more numbers to compare.

Larger tire = higher force on rim. Look up hoop stress or cylinder stress: https://en.wikipedia.org/wiki/Cylinder_stress

Force is proportional to the width of the tire. Twice the width, twice the tension in the cords and twice the force spreading the rim flanges.

Oh, gotcha, that could be poor reading comprehension on my part, too.

Going back to the last page of the 2006 Berto article, it looks like you might get away with using ~58 psi in the rear (that chart is for minimums), and may be able to adjust the front downward for a little more comfort. Even if a rider does have a 35/65 or 40/60 weight distribution on level ground, I prefer to use 45/55 for more balance.

It is true that a larger-width tire places significantly more stress on a rim than a narrower tire inflated to the same pressure.
The fatter tire's cross-section is greater, and the exit angle that the casing pulls on the rim edge becomes more-perpendicular to the plane of the wheel as well.

Many downhill MTB rims have a decal or instructions warning about the matter.
On the Mavic EX 325 Disk rims I have, the website doesn't say anything about max tire pressures,,,, but the decal on the rims says that the max pressure for a 2.2" wide tire is 55 PSI and the maximum pressure for a 3" wide tire is only 33 PSI.

Most other rims don't bother to say anything about this, because they just assume that a fatter tire will always be inflated to a much lower pressure than a narrower tire.

Actually, you're both right - to a point.

100 PSI means 100 pounds per square inch. That's 100 pounds of force for every square inch of surface area. A fatter tire has a lot more square inches of sidewall. Consequently the fatter tire will have more force pushing outward against the two rim flanges.