I've been working on this for a while now, and I believe I have come up with the definitive answer to that thorny problem that taxes crossers everywhere - what pressure to run your tires at?
Pressure P = √ y(m/s) x δm ± NA/¥
y = wind speed in m/s
δ = electrical conductivity of mud in S m-1
NA represents Avogadros constant (6.022±0.00000030)x 1023 mol -1
¥ = the amount of Belgian beer drunk in the previous week
Unfortunately this relationship does not hold true for those running clinchers, but I am working on a different formula for those not yet fully surrendered to the dark art of tubs.
Actually it is amazing to see (or rather feel) the difference in pressures in evidence at a local cross race. From the 'I run'em at 50psi all year round' group to the obsessives striving to geek out even further with a reduction from 28psi to 27psi (just me then), there are lots of contrasting approaches to the same problem - get through the terrain as fast as possible. Without puncturing.
Euro PROs like Nys, Wellens and co, reputedly start recce laps at 27/28psi and reduce as they learn the course and conditions, going down to as low as 24psi in some cases. A hand held pressure gauge and pump allows variations of 1psi. Younger, but awesomely talented riders like Lars Boom and Niels Albert have gone on record to say that they do not have the experience yet to run that kind of pressure, but presumably they are working on it. Check out the flat front tires of the riders in particular Vantornout (on the far right) going up the climb on the Koppennberg Cross, a course with a lot of muddy off camber and sweeping descents.
Either way, it is a case of how low can you go on many courses - bottom out the rim at least once a lap and you are in the right ball park pressure zone.
This brings us to the notorious Otterspool Park (cardiac arrest time for Wellens and co) and some of the other slightly rougher NW courses........
Those relationships simply don't hold true when avoiding broken glass, rocks, tree roots, discarded road cones, stolen cars and other debris including stricken riders. Clinchers begin to sound pretty tempting all over again - at 60psi.