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Re: Pirelli v Avons

Posted: Mon Oct 31, 2016 3:07 pm
by omsracer
Andy the Pirellis I have are:
250 x 575 x 13 Rears
200 x 540 x 13 Fronts

When enquiring with Pirelli they said it was one compound for the F3/F4 tyres they make.

I hope that helps.

Re: Pirelli v Avons

Posted: Mon Oct 31, 2016 3:33 pm
by andrewcliffe

Re: Pirelli v Avons

Posted: Tue Nov 01, 2016 7:11 pm
by AndyY
Thanks Peter and Andrew,

Can I just clarify whether the Club F3 tyres are the same as the BRDC F3 (nee F4) tyres?


Re: Pirelli v Avons

Posted: Tue Nov 01, 2016 7:46 pm
by andrewcliffe
Yes, BRDC F3 and F3 Cup tyres are the same. I understand Pirelli are taking there same compound to Macau this year, replacing Hankook? as tyre supplier.

Re: Pirelli v Avons

Posted: Tue Nov 01, 2016 9:11 pm
by AndyY
Thanks, very useful

Re: Pirelli v Avons

Posted: Tue Nov 08, 2016 5:31 pm
by Wingnut
Can we start the discussion about what to inflate the tyres with now? Nitrogen or air?

Re: Pirelli v Avons

Posted: Wed Nov 09, 2016 11:08 am
by tristancliffe
I'm not 100% sure on this, but by my understanding of the ideal gas laws and the partial pressure equations...

Using ANY dry gas will result in the same pressure change with temperature. There might be small effects due to different specific heat capacities of different gases, but for most gases we'd consider using they are very similar and so can be ignored. Helium and Hydrogen take, for example, around 5 and 14 more joules to heat up the same mass of gas by 1 degree Kelvin, but they are lighter so you'd have less mass in the tyre in the first place, almost cancelling out the 5 and 14.).

Normal air has some water vapour in it (and this is usually exaggerated by compressing it), and it's this bit that causes 'problems' because the pressure rise with temperature is much greater than the oxygen/nitrogen/carbon dioxide in the rest of the mixture. And the amount of water vapour varies greatly depending on the ambient humidity, condition of your compressor, when you last drained the tank etc etc, so it becomes harder to predict the pressure change for a given temperature change.

Simply put, the gas you use DOES NOT MATTER, but water vapour content MIGHT.

Normal air, compressed in an average compressor
Pros - cheap. Does the job for 99.9% of racing vehicles and non-racing vehicles.
Cons - WILL contain water vapour, so you'll get MORE pressure increase with temperature, which means you will have to use lower cold pressures to achieve the target hot pressure. Because you won't know how much water vapour there is you won't know how much it'll increase, so one day you might get 6psi increase, another day 7psi, and another day 5psi, even if EVERYTHING ELSE IS IDENTICAL.

Pros - it'll probably be dry, so it will expand less with temperature, and expand the same every time.
Cons - expensive in comparison. Have to carry bottles around. Need to purge your tyres by filling with nitrogen, sucking all the gas out, filling with nitrogen, sucking it all out and filling with nitrogen again. Every time you change the tyres.

Dry air (perhaps from a bottle, or with an expensive compressed air drier in your system)
Pros - maybe cheaper than nitrogen, and will have the same pressure change with temperature
Cons - almost the same as nitrogen. You still have to purge the tyres a few times each time, and you'll need specialist equipment or bottles.

If you want to "improve your tyre inflation", firstly use a decent pressure gauge. Decent doesn't always mean expensive though! Then consider something like bottled air or nitrogen or whatever is cheap to get hold of with all the hoses and fittings needed (avoid Calorgas or Oxy-Acytelene!). Yet you'll really need to be doing the setup on your car every meeting, and set downs after every session/day, and taking pressure readings (and temperature readings) at least once per session, and having a vaguely consistent driver that can benefit from having the cold pressures 2 psi higher for a given target pressure...

And don't bother with nitrogen in your road tyres. The temperatures generated are so small that pressure changes are also small, and road cars are less critical - rubber bushes, squashy suspension, rubbish tarmac - so it's just not worth the cost. They won't stay inflated for any longer than with air, and the humidity in the tyre won't damage the structure of the tyre in the normal lifetime of a tyre.

I'd love to be corrected if any of the above is inaccurate, and of course it's dipped in my own opinions as well, so make of it what you will.

Addendum - a bottle of nitrogen at several thousand PSI will probably fill, purge and maintain lots of tyres. I once tried to work it out for F3 size tyres, and from memory (ha!) it was around 500 tyre fills, so maybe 166 tyres after some purging/refilling, which is still 41 sets of tyres.

Re: Pirelli v Avons

Posted: Sun Nov 13, 2016 10:00 pm
by schomosport
Just a couple of comments:

Simply put, the gas you use DOES NOT MATTER, but water vapour content MIGHT. Don't take this absolutely literally and do something silly like use the oxygen out of your welding set. Oxygen can react explosively with things like oil and grease.

Secondly, another thing to bear in mind is a several thousand psi gas bottle needs handling with respect since if you should knock the regulator off said bottle may well become an unwelcome projectile. And if you carrying such bottles in a vehicle you will need to carry them properly and put one of those nice green stickers on the back. Something else to attract the attention of Vosa too.