Everyone says how low they’re running they’re pressures . From cx to mtb just wondered why ? I like a little absorb tin of small rocks etc but the rolling resistance must be terrible when your dropping below 20’s .

I think Schwalbe? Continental? did a PDF that demonstrated why lower pressure was better off road, I run about 20/25 front and back, and it gives a good trade off between grip, comfort, dragginess.

by the tyre deforming over small bumps rather than bouncing over them rolling resistance is reduced off road. On road is a different matter because it’s a smooth surface having a smaller surface area and less deformation of the tyre decreases rolling resistance

There was a report somewhere that said at higher pressures, the tyre deformation was radial around the circumference of the tyre, whereas at lower pressures the tyre deformed over the width. The width one took less energy at a given pressure*. Think it was a roadie mag that said this, hence why road tyres are going wider. Same must apply to mtb? I much prefer lower pressures now. My mate happily runs his so low they look virtually flat.

Certainly now when I do put soon air in, they feel weird and bouncy at higher pressures – here I’m talking over 25psi. They may well have been talking width of tyre. Whatever, please now shoot me down in flames.

*Physics, not my forte. Could have got this all wrong, but it sounds good.

Its a balance of keeping the pressure high enough to reduce resistance by rolling to low enough to reduce resistance stalling (hitting obstacles). Most people run lower pressures to gain maximum speed offroad, but of course its possible to get the perfect balance to ensure your ride is at its optimum speed for a whole or part of a ride.

Obviously if youve set your pressures at the perfect balance for a certain trail then you’ll want to make sure you wait at the top to get the right sized gap.

It’s also why people go tubeless – low pressures with an inner tube means a greater risk of pinch flats.

If you’re pointing downhill then rolling resistance probably isn’t what holds you back but better traction and less vibration will make you go faster.

If you’re pointing uphill on rough ground then more grip and comfort will also help up to a point.

Obviously if youve set your pressures at the perfect balance for a certain trail then you’ll want to make sure you wait at the top to get the right sized gap.

😈

Presumably you need lower pressure for 650b as the trails come alive?

High enough to stave of punctures, otherwise as low as you can go. Grip comfort, rolling resistance on roads probably takes a hit but you can get a road or cx gnarmac bike for the black stuff.

Presumably you need lower pressure for 650b as the trails come alive?

I don’t have the shit middle size but I seem to be able to run slightly lower pressures on my 29er without an increase in punctures.

(too many variables for a proper comparison, but so far gut feeling says bigger wheels can use lower pressures)

25/27 for me on a HT

Don’t like it myself, all that squirming, 30-35 for me. on biggish vol tyres (rubber queens) anything less and the tyre feels like its giong to come off the rim. Probably just need to get used to it and I’m probably missing out on grip etc. but I’m a bit shit so it doen’t really matter and usually need to stick in a few miles on tarmac before the trails anyway.

The main reason is grip through better conformance to micro terrain. A softer tyre squidges over small bumps instead of striking them and then having to ride up and over them, which also improves rolling resistance.

I’ve seen my tyre pressures progressively drop over the years. When I started mtbing a mere 16 years ago, I used 40-45 psi, with a bit more experience and wider tyres it dropped to 35psi, but that seemed to be the limit with 2.1 inch, 26er tyres with tubes.

I saw a significant drop to about 24-26 psi once I went tubeless with 2.2 26ers and mid width rims like Stan’s Flows, and I’m now down to 18-20 psi on 2.3 29ers with even wider rims.

I’ve learned that tyre squirming is a function of sidewall stability. You can improve this with wider rims and thicker sidewalls. I’d argue that having both is good practice as part of the package to both resist the squirm and provide adequate pinch flat resistance when running low pressure.

This explains it all in simple terms.

The reversion of the move to wide rims in the ’90s held back an earlier move to lower pressures IMO. A wider rim helps with the feel with low pressures.

Sidewall stiffness is definitely important – IME really lightweight tyres are more of an issue at least even on really wide rims. I run ~20psi in 2.25 mid weight tyre (Smorgasbord) on a 42mm 29er rim – but with about twice the load on it most of you have. That’s with a tube. Tried that sort of pressure with a 26″ 2.4 Nobby Nic, but it didn’t feel nice and needed a lot more pressure which spoiled the ride (a 26″ 2.4 Fat Albert which has a more substantial carcass also works well though).

I don’t like to feel the tyre squirming around so keep mine at about 30psi. After my first run at BPW on Sunday I check them as they felt like they were rolling around a bit in corners and turned out they were both about 20psi. Once back up to 30 they just felt nicer. Glad I did as Rim Dinger was the next run and wouldn’t have wanted anything softer for that.

What I mean is that it’s probably more about what feels right for you which gives you the performance boost rather than a set pressure. And I run tubeless to stop flats rather than to get low pressures or weight loss (my ghetto tubeless set is probably about as heavy as running tubes but it’s never flatted or burped off the rim.

Tom KP.

Hate the feel of really low pressure, feels like the tyre will roll off the rim. Mine are about 30 which feels good.

my ghetto tubeless set is probably about as heavy as running tubes

This has been my rule of thumb with tubeless for years.

If you’re feeling bored and want to read up on tyre geek info then have a look at various write-ups here