tyre pressures

[aedeau]

Originally Posted by cozza

I thought the rear tyres should be higher than the front. Is this not the case?

higher pressure in fonts = less understeer?

current setup is pretty easy to get it sideways, even with traction control on full and in the dry :biggrin:

[ExMX5]

Originally Posted by mattyv

Can't remember, but it was less than $50 for four last time. I think it might have been something like $8 a tyre? I imagine this is going to vary depending on who you go to but it really shouldn't be that expensive.

Most places should know to put red valve caps on too. This denotes that you've had a N2 inflation so mechanics don't go fudging it up on you at service time.

Actually, Nitrogen will escape your tire at the same rate as normal air but is used in racecars and aeroplanes as it is chemically inert (and so will never oxidise the rubber or otherwise damage the tire at high temps).

In your street car the benefits are negligible other than the pressure's usually better regulated.

The red caps are because pure nitrogen is toxic and very dangerous in the enclosed conditions where you'd be working on the car.

All in all the price isn't too bad but you'll see negligible difference in performance...

Originally Posted by Cozza

I thought the rear tyres should be higher than the front. Is this not the case?

That's quite correct - lower pressure increases traction at a cost of stability and tire life. In an understeering car, increasing the rear pressure and decreasing the front pressure will ensure there is more tendency to oversteer.

[smarm]

I thought nitrogen was used because of less pressure differences with variation in tyre temperatures.

[aedeau]

Originally Posted by ExMX5

In an understeering car, increasing the rear pressure and decreasing the front pressure will ensure there is more tendency to oversteer.

Isn't it the other way around?

http://www.tirerack.com/tires/tirete....jsp?techid=58

[ExMX5]

Aedeu, that page does seem to say that - I was pretty confident that lower tire pressures increased grip and higher pressures decreased it (depends on the tire and level of inflation as well). Might need to do a test...

I dislike the way that page refers to "over" and "under" inflation though, as "correct" inflation depends on what you're doing with the car and the load.

Smarm, there is that but it's pretty small around the temps most tires normally see - actually I've just run it through HYSYS, from 21C to 60C @ 320kPa(a) and the increase in pressure is around 13.36% for N2 and 13.37% for Air.

[MikeVictor]

Originally Posted by ExMX5

Actually, Nitrogen will escape your tire at the same rate as normal air but is used in racecars and aeroplanes as it is chemically inert (and so will never oxidise the rubber or otherwise damage the tire at high temps).

In your street car the benefits are negligible other than the pressure's usually better regulated.

The red caps are because pure nitrogen is toxic and very dangerous in the enclosed conditions where you'd be working on the car.

All in all the price isn't too bad but you'll see negligible difference in performance...



That's quite correct - lower pressure increases traction at a cost of stability and tire life. In an understeering car, increasing the rear pressure and decreasing the front pressure will ensure there is more tendency to oversteer.

I disagree on some of your points. I ran a set of Bridgestone RE050A (non-RFT) on my WRX without N2 and one set with.

The set with wore more evenly and felt better to drive on through the hills on the weekend. If you're referring to a peak-hour commuter, then I can understand your comment, but my cars are for fun not a daily drive.

Also N2 molecules are larger than a lot of the 22% non-nitrogen based elements in air, and therefore while there is some leakage the rate is lower than an air mixture.

[harpoon]

I had nitrogen in my last set of tyres and couldn't tell any difference whatsoever - on the track or off. I liked the red caps though!

[takahashi]

You can never have 100% nitrogen. There is still slight variation in pressure with temperature but when you want to reinflate then you are in trouble.

Lower pressure does not necessary give you better traction. It only does when you are on the track when you want to generate more heat to the tyres.

On street and traffic, better to pump it over 38 psi.

[ExMX5]

Originally Posted by mattyv

I disagree on some of your points. I ran a set of Bridgestone RE050A (non-RFT) on my WRX without N2 and one set with.

Also N2 molecules are larger than a lot of the 22% non-nitrogen based elements in air, and therefore while there is some leakage the rate is lower than an air mixture.

Actually O2 has a covalent bond radius about the same as that of N2 but the atoms themselves as O and N are modestly but quite different (around 5-7pm or 8% if you prefer it that way). This will have very little effect on the leakage rate

The leakage rate has far more to do with differential pressure between the inside and outside of the tires than what they are filled with.

While I don't doubt that your WRX did better on the N2 filled tires, I believe this may have had more to do with the accuracy of the fill rather than the contents - your tires have to be very hot and/or very high pressure to benefit from an N2 Fill.

By way of example, a 747's main gear are designed for a 1400kPa fill (vs around 500 max for a car), F1 car tires are *pre*heated to 80C - considering the brakes hit 1200 I assume the tires can hit at least 120C.

I don't know if you can get a car's tires that hot, in all honesty :iono:

[AussieRacer]

taka is right about lower tyre pressures being better on a track, and higher pressures being better on the road. On a track you want the sidewalls to flex to keep the contact patch wider and in better sync with the road. The extra flex also heats the tyre up quicker making the rubber stickier. You don't get that on the road, so a higher pressure gives a more fatter contact patch. Too high and the tyre will start to crown.

Regarding nitrogen....put it this way I only know one state level racer who bothers with it. Again like taka has said it is hard to get 100%, in fact I've heard of percentages a lot lower, even after full purges. The downside is, you must always refill with nitrogen, which is both inconvenient and expensive. Grip wise it has zero benefit, it just provides less fluctuations in pressure as the tyre temp goes up. In F1 where a few degrees of heat, and 0.5 psi makes a difference, it is worth the effort (in fact Ferrari apparently use a mix of gases that include air con gas). But for every other ordinary racer (the majority) a sensible person can pretty easily guess the pressure rise within 0.5 - 1 psi based on ambient temp, sunshine, and track conditions. Especially if you sit your car in the sun before going out on the track :wink: