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Tire pressure formula

2746 Views 42 Replies 33 Participants Last post by  GKelley
I just learned of this tire pressure formula online and it puts me within 1psi of my actual favored pressure:

Riding weight in pounds divided by 7 then subtract 1psi for the front and add 2psi for the rear.

How close does this formula match your actual target psi?

Obviously it omits several factors such as riding style, tire casing, rockiness of terrain, use of tire inserts, etc. but that’s for the advanced class.
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This puts me right at my desired pressure for my Nobby Nic TLE 2.35 tires.
Puts me about 8-10 lbs over what I ride. Terravail Coranado 29x2.8 tires
Puts me way over what I ride and I can just feel me deflecting off of things with those numbers. I normally run 22front and 24 rear and I am 180lbs it wants me at 26/28
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Puts me about 8-10 lbs over what I ride. Terravail Coranado 29x2.8 tires
Same here.

I clicked wondering how there could possibly be a "formula" for tire pressure.

I run 19/21 with 2.5 DHF/2.4 Ardent on Hunt Trailwides (30mm inside). No lower. Sometimes higher (+2psi) for hardpack/smooth/flow trails.


Way, way off for me on the front. 10-13psi to be exact (15-18psi my preferred, 27.6psi calculated)

Fairly close on the rear (in the summer). 25-30psi preferred in the summer, 30.6psi calculated. Winter time I'm more 18-24psi.

2.5x29 WTB Verdict light/high grip front, 2.4x29 Michelin Wild Enduro rear... rear, with Tannus tubeless inserts in both at the moment. But the insert in the front is new, and the 18psi I list above was my preferred pressure before the insert. I'm sure the inserts screw with things a bit on the low side. But shouldn't change much on the high side I'd think.
There are too many variables, not the least of which is tire width. But I would say for someone who knew nothing on a typical mountain bike it would tend to get them in the right ballpark...unless they are an extreme outlier on body a kid, for example.
Works out to about 4 PSI over what I use, but with my new XC tires (skinnier) it might be close
Puts me about 8-10 lbs over what I ride.
Same here.
I have no clue what PSI is with the squeeze test. I just know when it's right.
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I’m riding 2.5”/30mm front and 2.3”/25mm rear and if I divide by eight and adjust -1&+2 I get pretty close to my riding pressure. But I have no idea if my pressure is what I should be running: 200#riding weight 23F&27R.
I think there should ideally be another calculation step based on tyre size (considering the sort of pressures I see people using on plus and fat bikes), and perhaps it would be better to use total rider & bike weight...

The method in the first post is 3psi higher than what I actually run, 21/24 as opposed to 18/21.

The other estimate I've seen is 0.28 x rider & bike weight in kg, then subtract 2psi for the front. This gives 19.5/22.5 for me, a bit closer.
The formula would have been pretty close for the 2.0-2.1 tires I ran 20 years ago, but as others have said, about 7-8 psi over what I like in current 2.35s.
8 - 10 lbs over
Puts me 65 psi under the pressure I run on my 25c road tires and nearly 10 psi over what I run on 2.6 tires on 35mm internal width rims. Any formula needs to account for rim width and tire volume.
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160/7= 23

23-1= 22 front

This is very close for me and I will even run these pressures with my smaller tires so I don't rim strike.

If you had wider volume tires then this formula wouldn't be accurate, but it's very close for XC.
Your algorithm needs refined. Don't feel bad. The first version of the Matrix failed, too.
Your algorithm needs refined. Don't feel bad. The first version of the Matrix failed, too.
To be clear, it's not MY algorithm. Like I said in the first post I read it online and wanted to see how well it works with experienced riders like those on this forum. There are definitely a lot of variables that it doesn't take into account.
calculated 18/21
actual pressure I use 15/18.
so, 3psi off.
It needs to specify diameter. If you hold tension constant, psi decreases linearly with diameter.
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