Surprisingly its not common knowledge on just how dramatic a difference tyre pressures make to riding and the impact this has on the trail.
High pressures = less chance of damaging your rim but at the expense of lost traction (ie wheelspin)
Low pressures = more traction, less “feel” on the bike and the lower speeds you generally can ride
Low pressures allow the tyre to “bag out” presenting a much larger footprint to the earth, minimising how the tyre sinks into mud or dirt, conforming over rocks and generally providing better traction.
Tim presents a great first hand video here of the difference between high and low tyre pressures and the resulting wheelspin:
As for what PSI to run – this can vary greatly for each type of rider, terrain and weight. Typically between 8-12psi is a usable range, with anything above 18psi considered “high” for trailriding, while “low” would be in the 4-8psi range.
Mousses generally come in “plushie” formats that simulate between 6 and 8psi, while normal mousses are around 10-12psi. Keep in mind that Mousse tubes typically have a lifespan of around 1-2 years and get softer with more use.
Tubes you would rarely get below 10psi without running two rimlocks or risk the tyre spinning on the rim.
Low pressures below 8psi are generally reserved for systems like Tubliss that allow you to run a tyre completely flat (0psi) and yet still be ride able. (We would not suggest riding with no air, it will destroy your knobbie quickly)
The Aussie Hard enduro legend is still hard at work doing his coaching, and has teamed up with the TBRA to put together some informative videos on how to best look after the dwindling trails and areas we can ride.