Tyre pressure changes

[BigChange]

Checked and set my tyres at the recommended 36 psi well before the heat wave started.  I was very surprised to find, after having been stationery for 3 hours today  (12/8) , that they had risen to 39.5 psi.  Ride did seem a little hard.  Is this normal or do I have a dodgy (digital) guage?

[matt8]

Air expands as it heats, so it's probably correct.

I too was wondering if I should deflate my tyres a bit due to heat.

Have no idea how much pressure can change based on temperature.

[royoftherovers]

Air expands when heated and contracts when cooled.

Science/Physics. lesson over.

[malcolmw]

I have typically found that pressure rises about 0.2 to 0.3 bar from "cold" to "hot" (just after running). If we say this normally represents a temperature rise from 15degC to 35degC then you get some idea of the shift in pressure with temperature.

[BigChange]

Makes me wonder at the need to adhere to "recommended" pressures if such variations are possible.  Recommended is  just a starting point.   We have adaptive this and that so why not adaptive tyre pressures also 😉  

[royoftherovers]

9 minutes ago, BigChange said:

Makes me wonder at the need to adhere to "recommended" pressures if such variations are possible.  Recommended is  just a starting point.   We have adaptive this and that so why not adaptive tyre pressures also 😉  

A sensible idea Peter, but if we have too much "Auto", then it is likely that drivers will not check what they should check on  a regular basis.

Sadly too many have today abdicated their responsibility to "Auto."

[BigChange]

Little bit of rain today (for me) with cooler conditions and bingo - pressure back down to 36.5psi.  So all's well !

[NemesisUK]

The recommended tyre pressures are set when the tyres are at ambient temperature (cold), the tyres have not been used for at least 3hrs.

Once one starts driving the tyre temperature will increase due to friction and flexing. This rise in temperature also increases the tyre pressure, which as it increases, reduces the amount of tyre flex and in effect stops the rise in tyre temperature and subsequent increase in pressure. (which incidentally is why under-inflated tyres are dangerous, as the low pressure allows prolonged flexing and a dangerous increase in pressure) 

The recommended 'cold' pressures are calculated to give the designed running or 'hot' pressure. Ambient temperature changes will obviously change this starting point and tyres should be adjusted from winter to summer and back again. Generally in the UK we don't experience huge variations on temperature within the seasons (there are exceptions...) so I tend to allow small variations in 'cold' pressures as the running pressure always evens out the same.

Obviously one must always check wear patterns and adjust pressures accordingly. 

[royoftherovers]

17 minutes ago, NemesisUK said:

The recommended tyre pressures are set when the tyres are at ambient temperature (cold), the tyres have not been used for at least 3hrs.

Once one starts driving the tyre temperature will increase due to friction and flexing. This rise in temperature also increases the tyre pressure, which as it increases, reduces the amount of tyre flex and in effect stops the rise in tyre temperature and subsequent increase in pressure. (which incidentally is why under-inflated tyres are dangerous, as the low pressure allows prolonged flexing and a dangerous increase in pressure) 

The recommended 'cold' pressures are calculated to give the designed running or 'hot' pressure. Ambient temperature changes will obviously change this starting point and tyres should be adjusted from winter to summer and back again. Generally in the UK we don't experience huge variations on temperature within the seasons (there are exceptions...) so I tend to allow small variations in 'cold' pressures as the running pressure always evens out the same.

Obviously one must always check wear patterns and adjust pressures accordingly. 

A++ Peter.

[NemesisUK]

deleted

Edited August 14, 2020 by NemesisUK
quoted wrong post

[NemesisUK]

37 minutes ago, NemesisUK said:

(which incidentally is why under-inflated tyres are dangerous, as the low pressure allows prolonged flexing and a dangerous increase in pressure) 

Sorry I should have put temperature, leading to increased wear and if severe enough, possible tread separation and a blow-out.