Tyre pressures

[Delidroid]

Hello There.
I'm sure that everyone of us who take cycling seriously agonise over what tyre pressure should we be using.
Trial and error just adds to the misery. I have an idea but I don't know what or how to handle the data I'm about to explain.

my bike is shod with WTB Thickslick Tyres at the moment. I'm not a great fan of fat skinheads but they're perfect for my experiment.

My idea is simple. Find a nice flat paving slab, wet it and brush off the surface water and ride my bike across it.
Now here's where I need help.
 
1. Do I measure the width of the tyre and the wet patch and calculate the ratio?
2. Do I measure the width of the tyre and paint a water stripe on the slab the same size which will reveal a contact patch along the tyre as well as across.
3. Do I shut up, go back to bed and dream about Kittens or something?

[Sergeant Pluck]

Just go by the manufacturer’s claimed diameter. Most of them are there or thereabouts and close enough for tyre pressure calculating purposes.

There is nothing wrong with just going by feel and experimenting in an up-a-bit, down-a-bit fashion, I am sure the majority of the world’s bike tyres are done this way, but if you want to get into it, this page has a calculator way down under a load of bumpf that is quite useful.

https://silca.cc/pages/app-tire-pressure-calculator?PPC&tw

Gives you an idea. You may find you don’t need such a big front-back difference as you have mentioned elsewhere.

[Delidroid]

Hello Sergeant.
I had a look at the silca website. I'm really surprised how low I should run my tyre pressures.
As you say, I have too much difference. 32 and 33.5 psi is recommended.
Thank you. Del.

[toontra]

Whatever I reckon the lowest I can get away with whilst preventing a pinch flat from hitting a nasty pothole.  I judge this by pinching/pressing with fingers.  It will vary with tyre size and type.

I don't actually use the gauge on the floor pumps - I'm never sure exactly how accurate they are and out on the road you're guessing anyway.  Not very scientific I know but is based on intuition and experience.

[Paul H]

I had a look at the silca website. I'm really surprised how low I should run my tyre pressures.
As you say, I have too much difference. 32 and 33.5 psi is recommended.

That does seem low, though of course it depends on weight. 
I use a different app, with less criteria, and that gives me a 20% higher recommendation than the Silca one.  The higher recommendation  works well for me.

https://play.google.com/store/apps/details?id=pl.jakuborlowski.biketirepressurecalculator&hl=en&gl=US&pli=1

[LittleWheelsandBig]

Excessively low pressures cause me uncertainty with fast corners. Depends on tyre construction and rim width.