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Hi Guys,

Long story short, I used my first ever Rspecs yesterday at RD1 super sprints and had a bad time.

Had to fight to just match my worn old AD08 times.

I know a few here have raced on these and am just asking for what your hot pressures were.

I am getting Conflicting info varying from 28psi to 40psi.

I never ran them under 36 but only got about 6 laps on them.

Any help much appreciated !!

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I found for 205/50/15s on 8" rims, they need to be below 33psi hot. Preferably around 31-32. Anything over 34psi and they lost mucho grip, 36psi+ and they were diabolically bad.

YMMV.

F&*K!!!!
Wishing I had of asked before yesterday!!

Thanks dude.

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Agree with Chris.

On an MX-5 I had 205/50/15 on 6.5" rims all last year, and found 30 to 31 hot to be best.
This year I have the same size tyres on 8" rims, and so far the best pressures seem to be the same.

At Wakefield I typically start the day at 24 cold, and then usually need a minor adjustment after the first run.

YMMV on a FWD however.

I start at around 24psi cold and see 30-32psi hot when I get back in. When I first got semi slicks I was told that if the psi is too high when you go out cold you don't get enough movement in the tyre to generate the heat needed.
Running 15x7 205/50 NT01's

Thanks all.
I think the consensus is I was too high.

It felt like no heat was being generated and the tyres looked chalky not sticky/melted like I've seen Semi's look.

I feel stupid as I have lost points towards the year but hey, live and learn!!

I am running 205/50/15 on a 7" rim.

I might get myself to the SOTS next week to experiment properly.


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Dweezle wrote:I might get myself to the SOTS next week to experiment properly.

23/4? I might see you there... If car is ready...

That would be awesome!!

I can bother you in person

They really felt bad, just zero grip, dropping to 36 was a little better but then i suffered fuel surge as i only had 5 litres in the tank and no extra and was the 2nd last run.

Will be nice to have a proper go at them again!!

Get an infra-red temperature probe and the tyres will tell you what they want. I've found a 66-72C spread from inside-outside on the hardest working tyre gives good grip and near even tyre wear. For me on a regular warm day that's a 26psi start and a bleed from 32psi as they grow after each session. Takes about 1 %90 lap to get them to temp and then they crawl to 34psi on-track (I run a TPMS). It's funny I always thought I ran 32psi...but that partial cooldown you get after the start/finish consistently pulls them down 1-2psi. Wish I could run the TPMS into the data logger but glancing at it you do get light temp/pressure spikes on high G corners too. It was a good learning experience having it until I parked the car for a year to convert it to turbo..then the batteries died in the wheel sensors

Thanks so much Hellmun!!

So a Infra Red temp readers work ??
I thought I'd have to buy a expensive probe style one!!

That's good news

Always wanted to run TMPS, then i could easily bleed air quickly.

So me running over 35 and that includes a cool down lap is exactly the reason they felt no better than AD08!

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Supercheap have infrared temp guns at the moment for $20.


As for accuracy, it's probably not as accurate as a $500 professional thing, but I can guarantee it's hundreds of times more accurate than guessing.

I just always thought infrared used reflection for temp readings and the surface of a Tyre would cool to much to be accurate.
But if I am wrong that sounds awesome.
Might have to grab one for $20.
Be useful in quite a few situations.

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Another question,
I have been reading and have got a little confused.

If I do say 3 laps come in, 32psi hot.
Go back out.
Would ambient temperature then come into it as to whether I need to run higher or lower pressure to get the correct temperature into the tyres?

Meaning it is the heat not the pressure that creates the grip ??

Or are you guys using the hot pressures as a indication of tempreture?

Sorry for the bother
Trying to get a better picture


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Ah this is where it gets complicated. It's approximately 10C temperature change per 1 psi pressure change but 32psi isn't some magical number target that tyres work at. It just happens to be where you get a good contact patch on NB/NA mx5's running 15"s. So if you cross ideal temperature range for the tyre compound and have ideal contact patch you'll get great grip. So you actually need the ideal temperature range and ideal pressure not just a set pressure target... 32psi just seems to be a common value many of us find through experience, temperature is then controlled by load applied. Tyre compound is why you'll find people start much lower on semi-slicks than street tyres, it's the different operating temp range targets.

The lower the pressure you start the more aggressive the increase in temperature due to deflection. So to answer your question I need to say that "it depends". If it's a stinking hot day and you constantly push the tyres to the limit it doesn't matter if you maintain 32psi, the temperature will be too high and you'll lose grip even if the contact patch was great. It's a simple physics limit, friction will generate heat and if you can't dispel it somewhere due to ambient temps you have to accept less available time in the tyres sweet spot temperature. If it's really hot you'll notice some people will do 1 hot lap, cool down and then a hot-lap for this reason. Alternatively to curb some temperature build up you can leave the tyre to gain pressure above 32psi, the higher they get the less they deflect on the sidewall but you will get uneven wear. Again in that scenario you compromise contact patch for compound temperature.

This is an example where I've gotten good performance out of tyres going against common advice I've been given. On a cold day people often say keep the pressures up which does give you a good contact patch but compromises getting grip from the compound (you've limited deflection). However if you go out aggressive enough on a cold day, start at 26 or lower you can grow the tyre and get heat as long as you keep right at the limit. My best laptimes are always on cold cold days for this reason and the data loggers shows that I don't get max G drop-off as the session goes on (due to going above the compounds happy place). If however you don't push hard enough and dropped the pressure...you'd have a bad contact patch combined with cool low-pressure tyres and it'd be much worse than just having cold tyres and a good contact patch starting at your hot temps. So you need to setup for your driving ability and experience.

The infra-red pyro won't be as good as those thermocouples you stab into the tyre carcass and will be more suseptible to cooldown laps...but I still found the data useful. I mostly use a long acre racing tyre gauge, infra-red pyro and the TPMS to monitor my tyres. Then later I'll compare to lateral G on the data logger. If you correlate the cold pressures between the good gauge and TPSM pressure you can watch how fast the pressure and wheel temperature drops (orange TPMS I have also has a temp sensor which gives wheel temp). I used to see about 10-20C and 2 PSI drop when skipping the cooldown lap and measuring at the wakefield garages after pulling over immediately. So I used to write down the values with a notation to add the approximate change value and pressure. This is why I alluded that 34psi seems to be my sweet spot...but to get that I always bleed and maintain the tyres at 32psi if that makes sense. I was quite surprised when I had the TPMS working how much change the tyre can have in a single lap. I think though I only got about 6-7 meetings before the batteries died...over 1 year (they're supposed to last 10 years ). So I don't think the batteries appreciated my 700C+ brake temps. Can't recall exactly but there were wheel temps in the 80-90C's on-track and the fronts used to get hotter again sitting in the garage after a stint. The unit new is $190...4 new sensors are $150 so they're kinda expensive to play with for it's short life.

Great post!


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