Following on the tappets hot or cold question, I did a test yesterday comparing tyre pressures hot and cold. I know they are supposed to checked when cold, but cold can be below zero or a warm summer day.
I found a consistent difference of 2 psi as between the pressure after a long run and the same day when they had cooled down. I image the hot temperature would be more consistent than the cold, so why aren't pressures specified hot? And how are you supposed to get to a garage without using your tyres?
Another experiment. Has anyone ever wondered whether the pressure of the spare tyre increases when it is loaded? It does, by between 0.5 and 1 psi.
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Has anyone ever wondered whether the pressure of thespare tyre increases when it is loaded?
No!! :o)
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If you assume that driving the car raises the tyre temperature by a similar amount whatever the conditions, it is reasonable to set a constant pressure 'cold' in the knowledge that it will rise to a (roughly) constant pressure 'hot'.
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The thing to do is drive the car on planks as the co-efficient of expansion of wood is greater than tarmac. Then when you check the pressure, it will be lower than it was before.
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The pressure of a tyre when fitted and loaded on a vehicle surely MUST increase. Take it to the limit where tyre is loaded to the point where it is signficantly flattened by the load. The volume surely must decrease, and hence pressure will increase. Is this not the same as squeezing a balloon which will burst due to increase of pressure
However if the shape and volume of the rest of the tyre expands to compensate, no increase in pressure will occur.
pmh (was peter)
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Just wandering for a mo. I used to work on the motorway, and found that true blow outs were rare. When they did in fact occur, it was usually because of either low tyre pressure, or excessive loading. Either way the effect is the same, the sidewalls, which are relaxed most of the time, find themselves, bulging greatly when that part of the tyre reaches the road surface. Now imagine this flexing from relaxed to bulge, occurring dozens of times a second. It creates enormous heat, which causes the body walls to disintegrate, and then Bang! all of a sudden.
You can easily check your tyres after a run by comparing the temperature of each with your palm. The odd one out normally means trouble!
Cheers Simmo
PS ever noticed why there are so many lorry wheel studs at the end of motorway slip roads?
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You are right Pmh, that is what I was getting at. I was just curious to wonder by how much the pressure in the spare would increase when it became loaded. I know one is always recommended to keep the spare pumped up to pressure just in case, so I mused on how many psi it would actually increase simply by fitting it. Now I know - not a lot!
I asked about checking hot or cold because it occured to me that the flexing of the rubber might be what caused the temperature to rise, more so than the actual air temperature. If that were so then the running temperature might be a more constant factor than the cold one, so I thought, why not quote a hot setting rather than cold?
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Cliff - my point above - as simmo explains - was that the tyres 'work' during driving, causing the sidewalls (especially) to warm up. After a few miles they reach a steady temperature when heat is generated as fast as it is dispersed into the passing atmosphere. Tyres will then be X° warmer than the surroundings on a hot or a cold day; X° will correspond to about a 2 psi rise (for example), but will depend on how fast you are driving (and cornering).
As regards the spare, I set mine several psi above 'normal', because by the time I actually need it, pressure will have dropped a good deal!
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The problem with cold pressures is if the car is parked in the sun then one side may be warmer than the other and can easily add 1 or even 2 psi to the warm side. If you check the pressures when warm after a run then you will get a true reading. Regards Peter
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