>>this means checking your pressures and putting air in.>>
...and not leaving them in direct sunshine...:-))
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What\'s for you won\'t pass you by
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What does the car's owners manual say? If it doesn't mention it, don't do it. If it does mention it, do what the manual says.
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L\'escargot.
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You could always do what I have to. When it snows, my car is SO USELESS it's a deathtrap, frankly - so I have to use another vehicle entirely. I'm in the very fortunate position that I can do that.
And they're all the same, apparently.
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Wouldn't mess with the manufacturers recommendations - potentially dangerous
One way of helping to ensure good traction in wet weather is to make sure that you have some decent tread depth, personally don't think the legal minimum depth is enough should be increased
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There appears to be some confusion here, the das equation is:
PV=kRT
for practical purposes take k and R as constants.
For a given mass of gas if ambient temperature(T) decreases either pressure(P) or volume(V) must decrease in proportion. In a tyre volume is constrained to be nearly constant so pressure will reduce.
In reality if the car is in motion the heat generated by the tyre warms the air raising the pressure anyway and a lower ambient temperature will have little effect on this.
Temperatures in this equation are Kelvin not Celcius and measured from absolute zero of -273C (OC=273K). In other word a drop of ambient temperature from 10c to 0C when you measure your tyre pressure causes a drop of 10/283 or 3% (1 psi if you run your tyres at 33psi) which is not really significant.
If you measure your pressures when the tyres are at 50C (after long run) then the error from 10C would be 40/283 or 14% which is why you measure temperatures when cold.
Yes I know standard temperature is normally taken to be 20c and I have no idea how hot tyres on road cars can get in normal use.
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I think I will leave mine as they are.
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Agreed, I can't believe I'm looking at a general squabble about tyre pressures! :-)
Blue
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Decreasing tyre pressures in cold weather, I'm sure your insurance company would be glad of yet another payout get out, in the event of an accident.
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Good points, well made IMO, and quoting equations is always good ;)
The primary means of heat input onto the tyre is via hysteresis in the rubber. So sidewall deflection and the flattening of the contact patch are responsible.
If you consider one revolution of the wheel, there is a constant energy input via this deflection mechanism. I'm not sure if there is a heat input mechanism that is proportional to the velocity of deformation, via a viscosity like term - it's rubber, it's complicated, so I wouldn't be surprised!
If during the revolution, there is time for all of this heat energy to be dissipated via conduction to the road and to the wheel, convection to the passing air/water/slush, and radiation to the suroundings, then the tyre temperature will not rise above ambient.
When the tyre is stone cold, and the car is going beyond a snail's walking speed, this complete loss of energy usually doesn't happen. The rate of heat energy dissipation increases at higher temperatures (each mechanism of heat loss follows different power laws, but they all work better with a higher temperature differential).
So, the tyre reaches an equilibrium temperature where the energy input per rev is balanced by the energy output per rev.
If you speed up, you put more energy in per unit time, and the tyre heats up. Speed ratings on tyres can be viewed as heat ratings.
If you reduce the pressure, you get more deflection, hence more energy input per revolution. The tyre runs hotter than one at higher pressure. This can cause a runaway situation, and sudden tyre failure due to overheating caused by low pressure is common.
I once decided to drive an old banger back to the garage with quite a flat tyre. After only half a mile or so, at 20-30 mph, the tyre was so hot, you couldn't touch it! I scrapped the tyre off afterwards, and haven't repeated that particular foolishness since.
Putting aside the concerns about handling, I am of the opinin that, for general motoring, if you are unsure about what the pressures should be, or have a poor gauge, then it is safer to put too much pressure in than too little.
Of course, the best and right thing to do is to follow the guidance given in the vehicle's manual.
Number_Cruncher
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The nub (or hub, ha ha, as we are in Punland) of the Mapmaker/Cheddar dispute seems to be what has the most influence on tyre temperature - ambient air temperature, or friction and flexing of the rubber?
This came into the old argument about checking pressures when hot or cold, and the conclusion that all sorts of factors sent the temperatures up or down, and the safe course was to use a reasonable and consistent average - ie the makers' handbook figures.
If a car has been driven fast on good roads, and suddenly encounters ice or snow, and slows down and gets stuck, its tyres will clearly still be hot. But if it has been ploughing slowly for an hour through freezing conditions, and then bogs down in deep snow, its tyres are surely likely to be pretty cold?
And they will cool further in the half hour you spend shovelling snow aside before making an attempt to break out.
Finally there is that lovely old example of pre-planning a trip - the inclusion of two old mats tied to lengths of cord, which you shove under the back wheels but tie to the bumper. Then when you get going you don't have to stop to retrieve the mats and so risk getting stuck again.
It wouldn't work so neatly with front wheel drive, as they'd get caught up in the back wheels or the exhaust pipe.
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The nub (or hub, ha ha, as we are in Punland) of the Mapmaker/Cheddar dispute seems to be what has the most influence on tyre temperature - ambient air temperature, or friction and flexing of the rubber?
The point is that the manufacturers recommended figures allow for the frictional increase in temperature however they do not allow for the effect of a decrease or increase in ambient temperature. I.e. the pressures are supposed to be set when 'cold' at ambient temperature accordingly if the tyres are set to 30psi at an ambient of 20 deg they will increase in temperature when driven (for this example let's say by 10%) though this is accounted for when the recomended pressures are established, i.e. in this example the ideal pressure is actually 33psi, 30psi + 10%.
However if this car is left overnight and the ambient temperature drops to 0 deg then the 'cold' tyre pressures will drop, even though they will still increase due to frictional effects when driven they will not reach the ideal of 33psi, hence the pressures should be reset to 30psi when 'cold' at this new colder ambient temperature.
To get back on track with the original point, if the driver perceives a benefit in having lower tyre pressures in a cold snap he may simply not want to reset the recomended pressures at the new colder ambient temperature.
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