i run around 36/40 psi on a 2,25 mtb tyre.
just wondering if anyone can give me a pointer to what i would tun in a 4.8 fat tyre ?
body weight is 285 pound.

Start with 10-12. Then you're trying to maximise grip/minimise undamped rebound.
Oh - and tubeless makes a huge difference. Moreso than on a skinnier tyre.

Starting point in psi would be weight in kg divided by ten, add 0.5psi for the back, subtract 0.5psi for the front.

Fat tyre pressures are a bit of a black art - too high and you pogo around off any and every bump; too low and you get the dreaded self-steer. Part of the problem is that there may be only 1-2psi between those two points. For me (85kg) 7.5psi up front was scary with self-steer on the road, 8psi was OK, 8.5psi was perfect. On road there's so much rubber gripping the tarmac that sometimes you really are fighting things.

As Colin notes, tubeless makes a massive difference with fat tyres. There's a big difference between tyres as well, try and get some with a high tpi count, that suppleness helps.

Just to say, if you find yourself with a fair bit of road or hard surface to ride, cranking them up to 20 / 25psi will make them roll remarkably easily.

Playing around with pressures is all part of the fun. I just tend to go by feel these days depending on conditions and terrain. You will soon get to find the sweet spot between bouncing all over the place and thinking WTF is happening to my steering.

The Jumbo Jim's are good all round tyres, but you will definitely find their limit in the mud. I switch to Surly Bud & Lou when winter properly arrives.

Anyway, have fun!

Not to hijack the thread, but I'm assuming to manage such accurate pressures it's common to carry a digital pressure gauge ? Are there any go to units I should look at? I doubt the gauge on my cheapo track pump is accurate enough.

ootini wrote: ↑Tue Oct 05, 2021 4:17 pm Not to hijack the thread, but I'm assuming to manage such accurate pressures it's common to carry a digital pressure gauge ? Are there any go to units I should look at? I doubt the gauge on my cheapo track pump is accurate enough.

Track pump one is very inaccurate. I use a Topeak digital gauge, but don't carry it with me.

ootini wrote: ↑Tue Oct 05, 2021 4:17 pm Not to hijack the thread, but I'm assuming to manage such accurate pressures it's common to carry a digital pressure gauge ? Are there any go to units I should look at? I doubt the gauge on my cheapo track pump is accurate enough.

TBH, I just tend to check the tyres by feel before each ride. If they're " a bit soft", I'll attack them with my normal track pump. If it's very snowy I may let some air out. That's about it.

ootini wrote: ↑Tue Oct 05, 2021 4:17 pm Not to hijack the thread, but I'm assuming to manage such accurate pressures it's common to carry a digital pressure gauge ? Are there any go to units I should look at? I doubt the gauge on my cheapo track pump is accurate enough.

I don't think the Topeak gauge that most people get is accurate enough at low pressures. There is another digital gauge that I forget the name of that does work at sub 1 bar/14psi but the gold standard is an Accu-gauge which is an analogue gauge.

As Colin says, once you get a feel for what's right for you then just do a press/pinch test and remember what it feels like. Big temperature changes also have an effect on the pressure - think ideal gas law: PV/T is a constant and since the volume of the tyre is a constant you are left with P/T so if one drops the other has to drop.

Snowy? I've been down at maybe 2psi in soft snow!



The refrain is: When in doubt, let it out! - if you have soft sand/snow then over-inflated tyres just try and bury themselves in it, dropping the pressure gives you more "float" but it's not for fast riding.

As a lover of all things fat I think folk often overthink the whole tyre pressure issue.

I look at the surface type and conditions and then I whack loosely what I think will be about right (without looking at any numbers). Then once I'm out I'll realise if it's too high or too low and then higher/lower it accordingly (again, no measurements involved).

It usually works out nicely.

Oh, and I'd avoid any fatbike tyres with the word 'Vee' on them* (*other opinions are available!).

One final thing, don't be reluctant to adjust on the go - you'll almost always save time in the long-run by stopping and adjusting the pressure if it's wrong at a given point in the ride.

Run schraeder valves and you can use a trials bike low pressure gauge. Must admit I don't bother much these days. In extreme snow I just pull the valve cores and bung them back in when the tyre is nearly flat.

managed to get some pedals on it today and put 15 psi in the tyres and took it for a spin down the road.
it is not more draggy than my tourer.
i will drop the psi to 10 tomorrow for a proper ride offroad.

It's worth trying slightly lower on the front than the rear, just beware you don't invoke that self-steer mode (though the JJs are pretty good in that department).

just been out for a couple of hours playing at the Leeds Urban bike park.

1st run was a 15 psi. felt ok.
2nd run was at 10 psi. was loads better. smooth like a full susser.

I once checked mine with a trials gauge.
4.8 Buds, tubeless were 7 rear and about 4 front.
As above, get used to tweaking on the fly, a couple of psi makes a fair difference.
Getting it just right is most satisfying. A lot of the bad press for fatbikes comes from folk giving it a spin with too much pressure and pinballing off bumps.
When the pressure is just so, the bouncy effect is lessesned and the bumps get swallowed up with squishy awesomeness
Cruising out on a fresh morning with perfect pressure can turn into a bouncy mess as the sun hits the tyre by lunchtime and expands it. Also, ride down a big enough hill and you might need a few stops to top up as the increasing air pressure softens the tyre.
Riding off the Veleta in Spain, and in the high Atlas were memorable examples of this, but it's noticeable even riding off the top of Macdui down into Glenmore or Braemar.
Welcome to the gang
I love my fatbike, it would unquestionably be the one I'd keep if I had to choose just one to do everything.
(A fatty was my only bike for several years, but I have choices now.)