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# Motorsport Wheel Alignment: Cold vs Hot Pressure

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## Cold vs Hot Pressure

### 02.46

 00:00 - As we've already touched on briefly, the tyre pressures are constantly changing during a track session, based on the temperature of the air in the tyre. 00:08 This is simply physics at work and as the air in the tyre heats up, the pressure increases. 00:13 This increase in heat is due to the temperature being transferred into the air from the carcass of the tyre as well as being radiated out from the brake system into the wheel itself. 00:24 What we need to understand is that it's common for the air pressure to increase in the range of perhaps six to eight psi, as the tyres come up to temperature. 00:33 We need to therefore account for this when we're setting the tyre pressures at the start of a day's racing because the cold pressures will need to be much lower than our desired hot running pressure. 00:43 To make matters a little more complex, the amount of temperature gain will depend on the car you're driving as well as the ambient conditions such as the track temperature. 00:51 This means that we can't just set our tyre pressures front and rear to be perhaps six psi lower than our hot target pressure and call the job done. 01:00 Instead it requires careful monitoring so that you can draw a correlation between cold pressures under certain conditions and be relatively confident where they'll end up at operating temperature. 01:12 A good example of why this is the case would be a heavy front engined car where there's relatively little load placed on the rear tyres. 01:19 We could expect this sort of car will put more energy and heat into the front tyres than the rear and hence the pressure in the front tyres will rise more than the pressure in the rear under race conditions. 01:30 We need to factor this in when setting the cold pressures. 01:33 And in this particular case, we might have needed to set the cold pressures at 24 psi in the front and perhaps 26.5 psi in the rear to get to our ideal target hot operating pressure. 01:46 Since we're going to be operating with cold tyres and low tyre pressures for our first lap or two, we also need to consider how this will affect the way the car handles. 01:55 Naturally with cold tyres, we're not likely to have the same level of grip that we would expect when they're at normal operating temperature. 02:02 This is made worse of course because with low tyre pressure, we're also running more on the outside edges of the tyre and the middle section won't be contributing as much to the overall grip as it will when the pressure comes up. 02:15 The last aspect we need to consider is that with low tyre pressures, the tyre will tend to move around on the rim a little more and this can contribute to the car feeling a little less precise in response to steering input. 02:25 When we combine all of these conditions and couple them with a grid of drivers all determined to make it through turn one in first place, this is why the first lap or two of any race is one of the easiest places to come unstuck as it can be difficult, particularly for a novice driver, to properly judge the available grip.

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