Which wheels & tyres do I have? And what pressures?

[RobManser]

Apologies for bringing up a much discussed topic, but I've searched the Blatchat archives and I'm still a bit confused.

Last week I bought a 2004 k series R400, and I'm trying to figure out the tyre pressures. Blatchat archives seem to suggest 18psi all round, whereas the popular 7 retailer I bought it from recommended 20psi all round. It has Avon ZZS in 215/55/13 rear, and 185/55/13 front. The manual for the car quotes the R300 and 400 as 195/45/15 CR500s, running at 18psi all round, but it's clearly not got those standard wheels and tyres. The R500 in the handbook is closer to my tyre spec: 175/55/13 front and 205/55/13 rear, CR500s, so that's 10mm narrower than mine, and they recommend 18psi front and 20psi rear. I don't know the spec of my wheels, but are they perhaps R500 wheels (edit - the same size, but they're not the iconic mag split rims) and the more recent ZZS tyre just came in a size 10mm wider, and they fit ok? So what do we reckon? 18? or stagger it 18F/20R like the R500 in the handbook?

Obviously when I do a track day I'll ask Avon and measure hot, then I'll know; but until then, for the road I'd like something to go on.

Many thanks in advance.

Edited July 26 by RobManser

[David JLM]

Recommended (by Avon) is 18-21psi for the road, and 22-27 psi HOT on track. 

 

 

Avon_Tyres_Motorsport_Lotus_Seven_Club_2021_Edition_1_1pdf_210527.pdf

[RobManser]

Thanks David. I see from the PDF in the link that the 18-21psi on the road is cold (it doesn't actually say, so I assume it is), and the 22-27psi on the track is hot, as you say.

So that verifies Blatchat advice on 18 all round (esp for me seeing as it's a superlight and a bit lighter than a roadsport) and the retailer's 20. Both are in spec. It also leaves open the possibility of 18F/20R. I guess I should just experiment?

[Ainsley]

Why would the fronts need less psi than the rears ?

Is it because its not a 50/50 weight distribution ?

Is it to get more grip at the front for sharper turn in ?

Or something else ?

[RobManser]

Good point, cause a wider tyre would run less pressure if all else is the same? I presume it’s to manage temperatures with the high power going to the rears? Just a guess. 

[Nigel B]

For me, 18psi all round (cold) works very well. I have the same tyres, the same sizes as you Rob. I have experimented with different pressures & I’ve tried higher pressures in the rear tyres, but I find that simply makes the car oversteer more readily.

[RobManser]

Great, thanks. I’ll try 18psi. I’ve got a 120 mile drive tomorrow morning, so will give it a go. I tried 20psi last weekend.

[Dan R]

18 psi all round as a good starting point is very common and well tried and tested.  Bear in mind that at relatively low pressures a change of a small amount is a greater percentage of the total pressure

[GulfSeven]

Possibly a dumb question, but why would you want higher pressure on track?

I've always taken a simple approach to track pressures by letting out air with a gauge to keep them at road pressure (18psi) after each session.

Have I been doing it wrong?

Edited July 27 by GulfSeven

[David JLM]

1 hour ago, GulfSeven said:

Possibly a dumb question, but why would you want higher pressure on track?

I've always taken a simple approach to track pressures by letting out air with a gauge to keep them at road pressure (18psi) after each session.

Have I been doing it wrong?

Good question!

Tyres get a materially harder workout on track and the lower the pressure the more movement of the tyre/tread so potential for too much heat,  and less control as the tyre deforms more, and moves on the rim. 

Even brisk road driving isn't a patch on what tyres get on track, and a lower pressure on road gives more compliance. 

Just my thoughts, I didn't even Google - I could be very wrong 😁.

[David JLM]

Tyre pressure changes with heat predominantly due to the moisture in the air within the tyre, which is why we measure pressures when hot - that's the operating temperature/pressure, not when cold. 

Really we want the tyre to be at its optimal working temperature for grip, and have an even temperature across its width - we adjust pressures to get the right level of inflation such that that can happen - even temp implies wide contact patch across the tyre and therefore grip. Most of us don't measure temps (we're not in F1!) so it's about feeling for the grip and how the car is moving. Hence the range of pressures given - personal preference, driving style etc. 

I'll say no more now and wait for an expert to give the right answer!

Edited July 27 by David JLM

[mso]

another non-expert but active racer view from me.  i agree with David JLM.

I think all other the racers i know, would run ZZS between 24-27 hot, so quite a narrow range (and tend to start 21-23 cold to get there - also if you are going to run at a decent pace you tend to start the rears lower than the fronts because the rears heat up by more than the fronts, especially left rear on most circuits).     Id say letting them down to 18 hot is definitely too low and going to get too much movement which wont feel ideal.   

The driver coaches, who tend to advise  and are all hard-bitten career-racers will tell you that the tyres "flop around" if too cold, though I have to say its not as clear-cut to me as is it to them..   What i would definitely say is that the tyres run much more consistently nearer the upper range, because with lower pressures you get a more noticeable warm-up change as they get hotter and firmer in the first few laps.   And if you run them higher./too high you find at the end of a race that your tyres start to go away compared to the other drivers and your times drop off - presumably because the tyres get too hot.

Because the ZZS is fairly low profile the advice falls into a narrow range within which I dont think you can go far wrong.   The real fun comes with the higher profile tyres like the Toyo 888 (because the taller profiles get much more movement/ "flopping around") and you get conflicting advice and wide preferences from 25-34 hot even amongst the front-runners.

 

[GulfSeven]

Thanks chaps. I'll try running higher pressures next track day ans see how it feels!

[RobManser]

Thanks for all the answers. 18psi all round worked well today on my drive down to Goodwood and back. Slightly more steering feel noticeable and the car felt a little more compliant.

[StevehS3]

7 hours ago, GulfSeven said:

Thanks chaps. I'll try running higher pressures next track day ans see how it feels!

Same here. I have only ever run 18 PSI cold. I will try higher and take some pressure measurements hot.