i check my tyre pressures every 4 weeks, and each time i am having to put 4 to 5 lbs of air into each tyre ! is this normal ???

i have a octy vrs with 18's with metal dust valves on

Given that the weather has got progressively colder each month since Sept, then yes I'd say it was normal. The colder air will result in a lower pressure inside the tyre.

No it's not 'normal'

Yes they will drop when the temperature drops, but not "every 4 weeks"

It could be tyres, valves or the wheels that are leaking, although 1psi per week is not a lot.

Given that the weather has got progressively colder each month since Sept, then yes I'd say it was normal. The colder air will result in a lower pressure inside the tyre.

If the temperature drops from 15C to -10C then tyre pressure would drop from 30psi to around 27.5 psi (Boyles? Law - P1/T1=P2/T2) assuming constant volume and NB temperature in deg absolute. That is a fall of around 0.1psi for every 1C temperature fall. So that doesn't explain the loss of 1psi per week over 2 months.

IMO a loss of 1psi per week is excessive - being even across all 4 tyres is unusual - could it be that the alloys have become porous?

If the temperature drops from 15C to -10C then tyre pressure would drop from 30psi to around 27.5 psi (Boyles? Law - P1/T1=P2/T2) assuming constant volume and NB temperature in deg absolute. That is a fall of around 0.1psi for every 1C temperature fall. So that doesn't explain the loss of 1psi per week over 2 months.

IMO a loss of 1psi per week is excessive - being even across all 4 tyres is unusual - could it be that the alloys have become porous?

so it they wany topping up again 4 weeks. should i have it checked out ?

Do some Google searches on "tyre pressure loss". loads of posts on car forums etc, looks like yours is a bit more than you should expect & may be caused by pourous alloys or corrosion on the alloy where the tyre seals to the wheel, both can be easily sorted by a tyre dealer but I would consider just monitoring & doing more research until the tyres need changing & then get it sorted.

The only thread I found re seepage through tyres said 2-5 PSi in 6 months was the norm.

so it they wany topping up again 4 weeks. should i have it checked out ?

I think you may want to check them more than once a month if they are losing air at that rate