Anto,
The appropriate tire pressure depends on tire volume, so rim width comes into play as well.
This has been covered extensively on the Forum (
http://thorncyclesforum.co.uk/index.php?topic=3798.0 ), so I'll just give a thumbnail summary: A lot of tires are run at unnecessarily high pressures. The recommended pressures printed on the tire sidewalls may well be in excess of what if required or (in the case of wide tires run at maximum pressures) even hazardous to rim life. A wider tire has more air volume, so can be run at slightly lower pressures for greater comfort without sacrificing rolling resistance. Part of the reason depends on the shape of the contact patch at the pavement for narrow vs. wider tires.
A number of studies have shown that pressures that allow a ~15% drop in rim-to-floor distance when loaded is a good rule of thumb for maximizing comfort without increasing rolling resistance. There are various charts and formulas and even some apps that calculate this for you, making it easy. American cyclist and petroleum engineer Frank Berto did the hard work and research behind these figures, based on surveys of a number of bicycle tire makers.
I used to hammer 125psi/8.6bar into 23-25mm tires in the pre-Internet days of long ago (i.e. late 1970s to early 1980s) when the only high quality tires available to me locally for touring were race-training tires. With tires that narrow carrying touring loads on rough gravel roads, higher pressures were absolutely required to avoid pinch flats and rim damage, and I've got the nerve damage in my hands to prove it!
When high quality touring tires became readily available to me in larger volumes, I was quick to mount them. I use the Android Berto Tire Pressure app, which I've found works well for my needs when the proper frame type, bike loads, tire size, and rider/bike weights are entered. To give you some examples for my various 700C randonneur bikes...
• F50psi/R61psi (3.4/4.2bar) with 700x34 tires actual width on wide rims, unladen except for rider weight
• F58psi/R71psi (4.0/4.9bar) with 700x32 tires actual width on narrow rims, unladen except for rider weight
• F49psi/R60psi (3.4/4.1bar) with 700x34 tires actual width on narrow rims, unladen except for rider weight
Though my weight remains constant, the bikes vary in weight, rim width, and tire cross-section/volume, so the same pressure varies from bike to bike.
Despite what might look like low pressures to a road rider, I find these bikes all roll very well over 300-400km day rides and I have experienced no rim or tire damage or pinch flat even on rough surfaces.
If you would like to send me your weight attired in your usual riding kit, your bike's weight wet (with full water bottles), your tire sizes F/R, and the weight and location of any carried luggage, I can calculate a suggested set of tire pressures for your review*. NOTE: Not all tires are the stated volume/size when inflated, so it is best to caliper them to find the actual size. If you don't have calipers, use an adjustable wrench to measure at the widest point of the tire sidewalls, then transfer that measurement to a ruler.
For example (just plucking figures out of the air...), say you weigh 170lbs, the bike with rear rack, pump, and full bottles weighs 25lbs and has classic road bike design, and you're running a 700x25C tire up front and a 700x28C in the rear, then some starting pressures to try would be F66psi/R87psi (4.6/6.0bar). I used pounds for weight because the app requires data be in that format for entry. If you usually use 90psi F/R, this would mean you could try dropping 3psi in the rear and dropping as much as 24psi in front without a huge increase in rolling resistance according to the Berto's findings. I would suggest you start by reducing only 3psi front and rear and seeing how it feels and rides and would not drop too much air from that 25mm front tire, as there's not a lot of protection for the rim. Nothing like a real-world trial with caution to see how it goes. It might help your back. Still a Very Good Idea to avoid the largest potholes whenever you can.
[EDIT: *Done via PM]
Best,
Dan.
