Correct PSI?
 

Hello!

I have a 2014 Trek X-Caliber 4 29er hardtail with Continental Trail King 2.4 on the front and Mountain King 2.2 on the rear. Though they are a huge step up from the Bontragers it came with, both tires are wider and I'm not sure what the correct PSI i should be running them at is. I'm 5' 11", 240 lbs, and it's a 17.5 frame. Being new to riding mountains instead of flatland, I'd hate to take it out with the new tires and too much air and have a blowout halfway down the trail.

Any help would be much appreciated.

Thanks

Blowouts are more likely due to under-inflation than over inflation. Your tires will have a max inflation pressure printed on the sidewall. Look carefully sometimes it is molded in text and hard to find. Tire that big usually tops out at ~60psi.

One is 58 and one is 60. I was just curious about being a heavier rider, and riding on a rocky surface. I have them set at 40 for rear, 35 for front currently.

58 is a very odd number for a max inflation, I'd want to be sure I wasn't looking at a metric unit or something. As a bigger rider you should want more air in the tires, not less.

Theoretically the correct tire pressure is defined by tire deflection under load. Simply, the lighter the load the less PSI to achieve the desired deflection, the heavier the load the more PSI to achieve the desired deflection.

Tires ALWAYS flatten a little at the bottom and this is desirable. This allows for some contact area as a tire/wheel that doesn't deflect at all will have next to no contact area. Also a tire that doesn't deflect at all is constantly pushing up on every bump and going up rather than forwards takes more energy. This is where flexibility is desirable in a tire. As the tire rotates it is continually deflecting into and then out of the flattened contact patch with the ground. Tires that flex more easily take less energy to rotate and are therefore have less rolling resistance. Now there is a happy medium to be found for best rolling resistance. Too much contact area is a drag but too little contact area also slows you down by forcing the wheel up on every little bump.

Anyway, as a heavier guy I would have more PSI in the tires or the contact patch will be too big and there will be too much stress on the tires. You should be running higher PSI than a featherweight. Try and assess the sidewall flex while you ride.

Anthony

Awesome, thanks for the info. I'll get out and see what I need to do to get set up. After my ride this morning, I'm thinking another 5-8lbs in each tire might do me some good.

The really crude Stan's formula provides a decent starting point (for mountain bike tires).

Rider weight/7 (-1 front/+2 rear).

In your case, ~35-40 psi is a good starting point. I weight 160, and usually run 18/23, so your 35/40 sounds like a good starting point, then fine tune from there.

That's an interesting formula, and easy enough to remember. Thanks for the info!

Formula's such as this can be useful but this formula is not taking into account tire size. Generally larger tires run lower pressures and smaller tires run higher pressures. What size tire is this formula for?

I'd experiment a little.

Anthony

It's from Stan's NoTubes for MTB tires. They don't say exactly, but I'm assuming 2-2.4in MTB tires. Since the OP is at the upper end of common size range (excluding fat/plus bikes), he should be fine.

It's meant to be taken as a starting point. Especially for MTB tires, optimum pressure depends a lot on trail conditions, in addition to size and rider weight.