running ust tyres on flows (which setup tubeless a treat) im running the rear at 32 psi and the front at 30psi (rear ardent & front advantage)

now on my old xm819 ust’s these felt nice and soft at this exact pressure (but not flat if you get what i mean tubeless users), and on the flows they still seem incredibly hard and pumped up…is this due to the change in profile of the tyres due to the rim being wider? just seems really odd, using the same digital gauge so no difference there…i dont mind dropping the psi some more, just wondered if it was a characteristic of the flows

Try double your weight in stone, -1 for the front, +2 at the back, as a starting point. The width of the Flows mean that you’ve got more tyre on the ground and more total force on the carcass, hence the pressure feels higher than on skinnier rims. Fatbike tyres feel way harder than their pressures read for the same reason (like sub 10psi).

also, if you’re going to have this kind of discussion it’s really helpful if you can include the tyre sizes as it can make a difference to recommended pressures and feel etc.

The width of the Flows mean that you’ve got more tyre on the ground

Incorrect, assuming the same pressure and weight of bike+rider. The contact patch will always be the same size. What it will change is the shape of the contact patch – a wider rim will make the contact patch wider side to side and shorter front to back – and the way that the tyre as a whole deforms.

Good explanation here:

http://sheldonbrown.com/tires.html (tyres and width section)

The simplified static behaviour of a tyre is very different to the actual behaviour in use. You’re right about the contact patch area remaining constant for a given pressure and rider weight when at a standstill but as soon as the bike starts moving it gets way more complicated.

The first thing to consider is that the wider the tyre, the less the tyre has to deform from being perfectly circular to establish the contact patch. This deformation causes losses in the carcass (thus increasing rolling resistance) and the stiffness of the carcass causes reduced contact force at the extremities of the contact patch (thus reducing grip/traction).

On that basis alone a narrower tyre requires lower pressure to maintain the same contact patch when moving as a wider tyre. A wider rim effectively widens a tyre, hence my original statement remains true. Oversimplification may help explain concepts but it screws up the details and details matter in the real world.

cheers guys a few more notes then –

same tyres (advantage 2.1 and ardent 2.25), same pressures, same weight rider etc same digital gauge used

not sure of the science behind it, but they are deinitely harder on the flows than on the 819’s, i never really dropped the pressures on the 819s below 30, and to get the same sort of tyre feel i may have to drop further on the flows…

im not complaining, the flows are great and a breeze to run tubeless which is a bonus coming from proper ust, just genuinely intrugied really

Sorting pressures by ‘feel’ (and then checking the numbers) worked for me.
Find a short trail, start with hardish tyres and keep dropping the pressure till you get the fight balance of grip/feel vs rim-bashing burp-hell.

I’ve done this with my Crests, 717s and DTs (all with varying tubeless tyres) and was very surprised to learn than on forestry single-track the ideal pressure with some tyre/rim combos was less than 18psi on the front (am 70kg kitted).

To expand on the pressure vs area vs stiffness thing further, the smaller and more flexible the carcass, the softer it will feel. As you increase the carcass size, hitting a bump causes more of the carcass to be compressed. As more of the carcass is being compressed the reaction force from the internal pressure is greater as force equals pressure times area, thus the reaction you feel is greater so the tyre rides harder. As the carcass stiffness increases, then hitting a bump also causes more of the carcass to be compressed, with the same result.

Personally I’d use both gauges and feel to decide on pressures. Your thumb will tell you the static feel, which takes into account carcass size and stiffness as well as pressure. However the calibration on thumbs can be prone to slipping, usually in the downward direction, resulting in pinch flats / burping / squirming problems.

Run mine at 24 psi at the front and 29 at the back. After starting at 35psi the pressures i run now seem to be perfect for my bike

By total freak this is pretty much bang on what I use…

Try double your weight in stone, -1 for the front, +2 at the back, as a starting point

So for me on 2.2″ and 2.5″ Rubber Queens, and 2.4″ Baron’s I tend to go with 28psi out back and 25 up front.

cheers folk – nice little write up chief, makes logical sense when reading that! thanks

it looks like for my weight of 11stone, perhaps i need to ditch a few more psi then….im quite happy with the pressures climbing road sections, but then they just feel a tad to hard descending rocky stuff…

will just drop a few more then and go from there…i reckon by the sounds of some of the pressures above i can afford to lose about 4 on the back and a a couple on the front without causing to many issues to pinch flats and dings etc