In reply to accordionfolder :
I've done some track time at NCM as well and I was FAR more comfortable on SM7s than I was on my RE-71's (even though the RE-71's were on a better car. If you haven't tried them, pick up a set of take-offs for next to nothing and give it a shot. 9" wide wheels are a skosh wide for them but not bad at all and should feel very crisp.
In reply to Tom1200 :
The operating temp range for r888r's according to toyo is between 160 degrees F and 220 degrees F. I'm seeing that on REALLY hot days where I'm getting after it (RoadAtlanta) - but on warm to cool days I'm lucky to see 140-150 - which in the time it takes to come off the track may have cooled and had been in the lower operating range - but I can tell you by the seat of my pants they're not fully hooked most of the time. When they're on - they're golden, but the window has been realllly narrow in my experience.
The 200tw street tires that I've used for autocross (Bridgestone RE71R and BFG Rival S) come up to temp fairly quickly, but they get used up quick. They also overheat easily on hot days. This is with a C6 Z06 though. They are quite grippy overall.
At 1700lbs with driver and moderate power I'd say they could work for you, but if you tow the car to the track I'd just get some dedicated R-Comps like Hoosier R7s.
Cold tires are indeed edgy, while 125-140F range isn't stone cold you are correct in that they are also going to be inconsistent (you already knew this). For racing I tell people they need to get comfortable with running hard on cold tires (read stupid comfortable with oversteer) but for track days I actively discourage this because there's no prize for "outlaps".
If it where me, as I mentioned, I'd go to a narrower tire for two reasons; they'll come up to temp easier, they'll reduce the frontal area of the car and so it might be possible to eek out another mile per hour or two.
Let us know how the new tire choice works out.
Is the video from a warmer day? The car did not seem to be sliding like the tires were cold. Or at least watching your hands I did not see you having to make a lot of corrections.
In reply to Papabear :
Yeah - that day was "ok" - not the most hooked it's been, but I definitely wasn't fighting the tires as much as others. My car is setup to understeer at the limit - you can see it a bit in the high speed sweepers - I have more lock than I should trying to get it to turn in.
In reply to accordionfolder : I'm intrested in seeing where all the tire discussion goes. I'm hoping to get mine out to a track soon.
Very interested in the cold and hot pressures. Too much cold pressure will do as you describe, especially on a tire with a VERY stiff sidewall on a light weight car.
In reply to racerfink :
True. Accordionfolder, what sort of pressures are you running?
I have a thread somewhere asking about that, I've tried everything from the toyo recommended 27 cold (for a car my weight) to the forum recommended 35 cold. All yielded similar lukewarm results. For the most part less pressure was better.
An interesting comment I heard from an engineer on the Nissan NPTI team from the 1980s: He said one time that someone gave him grief about running so much front toe-in. He said that they had to do that in order to keep the tires hot, else they'd cool off too much in the straightaways and not stick on corner entry. Granted that this was on a 200 mph car carrying ~10,000 lbs downforce!
In reply to accordionfolder :
IMHO even 27 cold is probably too much pressure for an EXO. I usually start at 25 psi cold on our NA (2250 w/driver), and I'm rarely putting air in. I'm at work and can't check my notes, but IIRC I ended up ~28 psi hot on Toyo RRs. Was around the the same on NT-01, which should be pretty equivalent to R888R.
I think the advice to go to a narrower tire might be a good one, but I would try it before committing. There is a better selection of 205/50-15 as compared to the 225/245 sizes.
I'm running 25 hot on my car (1762lbs with me in it).
That's the internet for you I suppose - I've tried a bit lower, but was told in my previous thread here that I was going too low on tire pressure. I was planning on calling a race tire reseller, part of my problem with R-comps is the buy in is steep for the sizes of wheels I have on hand, but if I can get some decent 13" a7s for pretty cheap that wouldn't be the worst experiment.
Keep in mind I'm using a bias ply tire. I too tried researching via the internet and got answers that were a bit all over the map. I finally asked my tire guy; along with selling to amateur idiots such as myself they also do Pirelli World Challenge.
I still had to experiment slightly; I went as low as 22 hot but the tires deflected alot and the lap times weren't any better.
Additionally take into account driving style; I'm a trail braker and I tend to be Mr Oversteer, which tends to put more heat in the tires. The Datsun is also a leaf spring car and that works the rears as well.
Just for reference: in my 780lb w/driver Formula car I run 11 hot and in my Showroom Stock Miata (NA chassis) I think I ran the 27-28 range. (it was 25 years ago)
Autocrossing various Miatai, if the car has sufficient camber to stay off the sidewall, I run the pressures way down. My CSP NA was at 21/19 psi. My STR NC is at similar on RE-71s and when I took it to NCM is was happiest at about that as well.
As light as your exo is I'd think 18/17 would be a good place for tire pressure.
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