Tyre Pressures

[Spydaman]

I read somewhere that you can establish what your tyre pressures should be by measuring them cold, going for a drive then measuring them hot. The difference should be x psi. Can someone tell me what x should be and whether to increase or decrease the cold pressure depending on x.

[Spydaman]

It's for a kit car and the manual doesn't specify tyre pressures. It will depend on weight, tyre diameter, width, profile and type of driving. I've set them to the pressures for the donor car but as it weighs less and the tyres are wider it won't be correct. I've also seen it said that you draw a chalk line across the width of the tyre roll it backwards and forwrds then see where the chalk has been rubbed off. If it's rubbed off the centre reduce pressure and increase if it's rubbed off the outer edges.

[Spydaman]

Why would the fact it's a kitcar have any relevance? It's the method of using hot v cold pressures to establish the optimum I'm asking about.
Unfortunately the car is from the mid 80's and the manufacturer is no longer trading so I can't ask them. I could ask Vredestein but doubt the have any info.
I've checked the increase in pressure from hot to cold with the pressures I'm using now and the fronts increase by 1psi and the backs by 1.5psi Does this sound about right?
With the chalk method I assume the load distrbution over the contact patch is uniform when the pressure is correct and rubs the chalk off the centre if over-inflated and and off the outer edges if under inflated.

[Spydaman]

That's the one! Yes it's for the Bonito. The Beetle weighs 870kg whereas the Bonito is about 700 so 20% less. The Beetle tyre pressures are 20F/28R so will try 16F/22R and see how it feels.