Mmm. The 15% is a bit fiddly for my taste and, I think, very likely to be influenced by ambient and operational parameters, particularly air and road, especially tarmac, temperature, wind (direction for cooling and effect on temperatures), load absolute and distribution over time-- just the consumption of water, a very heavy element indeed, can be important, etc.
I don't want to sound like a hypocrite before people who may have read the section of my automobile design book where I talk about tyres and their care and feeding, but it seems to me that while the absolute forces on a car tyre are greater, once the thing is up to heat it leads at least a more constant life than a bicycle tyre.
It follows that the beneficial pressure regime for bicycle tyres is the determination of a minimum inflation that will not cause fishbites in your tubes (or wreck the tyre itself, but with modern construction methods that's a much more distant prospect), and another minimum for the bike with its heaviest likely load. After that the correct inflation for any load can be guesstimated without doing any great harm.
For ten years I operated Big Apples 60mm wide, admittedly on rims internally 24mm wide (it makes a difference to the control the rim exerts over the tyre, and in turn to how the tyres heat up), at the 2 bar/29psi (very easy to remember on a 29er) that Chalo Colina, who has much experience of these tyres, advised. So towards the end of the month, when I would check the tyre pressures, they were often down to 1.6bar. I'm a pretty hard rider, especially on the downhills, and at speed I don't swerve off line for anything, I just ride through it. In those ten years I had two flats, both towards the end after my weight had increased by 7kg, one by fishbites, one possibly by a nail-puncture when I rode across an industrial estate. BTW, these punctures were in the ultralight "racing" Schwalbe tubes, typer 19A, not the hefty standard type 19. One caused what could have been a serious accident but fortunutely ended in scrapes and bruises and some shellshocked motorists. When I took these tyres off, with some of the originally anyway minuscule tread still remaining (say half-worn), at over 8000, there were absolutely no signs of abuse on those tyres, even though for all their life under an abusive rider they had been inflated at pressures that offhand, before I had this experience, I would have condemned out of hand as stupidly low.
It may just be that for twenty years I've ridden on Schwalbe and top-end Bontrager workalike tyres, but it seems to me there's a huge safety margin in at least the Schwalbe inflation recommendations. Whatever the reason, I'd already been riding on very low pressures for many years when the word came down from SJS not to overinflate, so that I was instantly onside.
