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Under 10 psi.

How soft depends on how wide your rims are. There's no way to find what's right for your conditions other than experimenting.

"Normal trails" for me is about 8 psi.
 

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No, that's not phonetic
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I have a digital pressure gauge that reads accurately down to 4psi, but not lower. Few gauges folks have laying around are going to tell them what's going on accurately below 10psi anyway, so numbers are usually guesswork. Add to that the fact that if you pump you tires up in a warm garage and then take them out into a cold environment, the pressure in the tires will drop. It also depends a lot on the rider's weight (Barny can run a few psi less than me) and how wide your rim is (100s will take lower pressures than 65s).

Easy rules of thumb: on soft surfaces go as low as you can where you don't run a risk of pinch flatting or deforming the sidewall of the tire to the point where you see creases or v-shapes in the sidewall. At that point you are doing damage to the tire. On hard surfaces run them as firm as you can stand.
 

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Pressure gauges for trials motorbikes are good. Just get a Presta adaptor.
 

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15 in the summer on road and light off road. 10 in winter in snow and stuff. I can also feel my tires get significantly lower when I go outside and it is sub zero temperatures!
 

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Dr Gadget is IN
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210 lbs, riding trails/sandy arroyos. Larrys on 100mm rims.

9psi front / 10psi rear.

EDIT: I picked these pressures based on no obvious sidewall deformation (on a hard floor).
 

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I am 190 lbs and I don't have a gauge but for the riding I do lately in the snow and mud, when I hook up my Trek floor pump it says "0 PSI".

Basically, I ride low enough that I just barely don't pinch the tube on a big bump.

But yeah it completely depends on terrain, texture of the surface you're riding on, etc etc
 

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It aint gonna ride itself
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According to my low-pressure pencil gauge, 6 front and 8 rear is a good all-around pressure for me (170lbs, 70mm Uma 2 rims.) 12/14 on pavement, 10/12 on hardpack, 4/6 on the really soft stuff...
 

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9 rear and 7 front by my digital gauge for firm snow with 80mm Shebas, down to 6 r and 5 f for soft snow with some base, and as low as 3-4 psi for really soft conditions, though the tire is just barely hanging on the rim at those pressures.
 

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about 1/4" of thumb squish (using a manly thumb)...when the trail gets soft I let air out until i hear the tube and tire start to separate...airing back-up usually takes 100-150 pump-strokes-dependent upon the pump I use...
 

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M8 M12 M15 deez nuts
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damnitman said:
about 1/4" of thumb squish (using a manly thumb)...when the trail gets soft I let air out until i hear the tube and tire start to separate...airing back-up usually takes 100-150 pump-strokes-dependent upon the pump I use...
I've often had fantasies about strapping a fullsized floor pump onto my back for the sake of sanity. If only there were a good way to stow one on a "standard" (including fatbikes) bike. I do carry a Topeak JoeBlow floor pump with me when I ride my Surly Big Dummy cargobike though, it has room to spare… and then some. It's shod with 26"x2.5" Maxxis Hookworms (on 65mm wide Surly Large Marges), so it's a great pump to have handy.
 

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bikeboatbrewski
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I carry a Slime Low Pressure (ATV) dial gauge that I ordered fro Cabelas. It's darn accurate from 2psi to 20... 1st tick is at 1. I usually take the time to adjust pressure for different surfaces. The thing to remember is that such a high volume tire is affected quite a bit by small adjustments of pressure. Especially at the lower end of the scale. I've ran 'em as low as 5psi Rr. and 3ish Ft. with the seat scooted back a bit in really deep, soft sand... and over 20 for longer paved road sections.
 
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