Heat cycled my R’s rear Cup 2 tires out?

[MikeR397]

Was at Gingerman today, ~9th track day on my rear Cup 2s in the R (fronts last 5 days or so and chord so not an issue, and got a nail in a rear after 5 days on last rear set). Anyway, The car was super greasy and I was like Vin Disel hauling bank vault with the unnecessary sideways, under power and just on fast corner entry. Hell, I had to let a camero pass me...

After 2 sessions of sliding, I put my new spare set of rears on and like magic the car was back to being glued to the track. The rears have at least 50% tread left, so my only though is I heat cycled them out? Presuming 9x5 days/sessions that would be 45 heat cycles. Does this make sense? Any other explanation? I noticed the car sliding a little more the last two sessions as well, nothing like today, but it started and I just blamed it on the 90+ degree heat wave, but today was 70 degrees when I was slipping everywhere on the old rear tires.

Thoughts appreciated!

Edit: if they are done, with that much tread left I guess I’ll have to see what this line lock feature is all about during the dream cruise.

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[chedder]

What size tires and tire pressure at the time they were sliding. How much roll over on the sides. I've got 14days on my Cup 2's and still have 5/32" across the tire width, and I push the car to limits often.

 

[Tomster]

I had to let a camero pass me...

How embarrassing

Yes, as mentioned, hot pressures and temperatures? Use a tire pyrometer and not an IR gun.

 

[TDC]

My experience with cup 2’s is below. They were take offs from a 2017 GT350R with 800 miles on them (I have previous experience with MPSS, RE11, RE71R). Car has 2.5 deg front and 2 deg rear camber. Cold pressure around 29-30 front and 30-31 rear.

- first day driving these to the track they tram line badly. After a few sessions they stopped tram lining and stuck real well. (6) 20 minute sessions on them
- second day on track continues to stick real well and set a new PB by 0.5 seconds. (4) 20 minute sessions
- third day on track they stick well for the (3) sessions and then session 4 start to drop off . (5) 20 minute sessions
- fourth day on track each session gets progressively worse. By the fourth session of the day I am 3 seconds off my best time and the fronts only pull about 1G; understeering like crazy. I skip session 5 because no need to pound on the car if the front isn’t sticking and the fun factor has dropped considerably.

Bottom line is I think cup 2’s need 3 to 3.5 deg camber on the front. These tires are too finicky and only work great at the beginning IMO. In contrast I was able to pound on the RE71R’s all the way down to no tire grooves and they still stuck well with track times about 1/2 seconds longer compared with the Cup 2’s. Maybe with more front camber my experience would have been better but now I’m on to Nitto NT01’s and I’ll get to try them out at Watkins Glen 1st week of September.

Makes me think of a quote from Matt Farrah’s podcast, “Everybody tries Cup 2’s once and then never buys them again”

 

[MikeR397]

What size tires and tire pressure at the time they were sliding. How much roll over on the sides. I've got 14days on my Cup 2's and still have 5/32" across the tire width, and I push the car to limits often.

-3 degree camber front and max of -1.1 rear. I usually start at 27-28 cold and get up to 34-36 psi after a session. Yesterday I experimented with lower and higher pressure and it made zero difference. Swapped only the old rears out and car was back to perfect. Again, I estimate the rears had 45 track sessions on them and 50% tread (see photo). Fronts have always chorded for me before end of 5th day and never lost grip, yesterday the fronts I left on had 17 track sessions on them and gripped great.

I presume you mean 14 days on rears? Unless you run Porsche spec with deeper tread, I see no way for 14 days on front! I may get 6 days before chording now with -3 degrees, but still the outsides Will chord and I rotate them too.

I’m fine with all this, just trying to predict the life and my back stock pile of tires ;)

Pic of the rears with 45 track sessions and maybe 500 road miles. These lasted basically through 2 sets of fronts.

 

[MikeR397]

How embarrassing

Yes, as mentioned, hot pressures and temperatures? Use a tire pyrometer and not an IR gun.

At least it was a ZR1 1LE, and I returned the favor after putting my good rear tires back on ;).

I posted readings from pyrometer in another thread I’ll try to find, but I recall rears were close to even temp in and out and only driver front still had big difference on outside being hotter, but I’ve maxed out my camber plates already to get -3 and not ready for notching or bolts yet.

Pressures posted above, but it was not pressure related.

 

[Tomster]

That's the only things that I can think of. Tires too hot or pressures too high.

 

[JAJ]

The 2019 GT350 Supplement has a lot more tire information than previous editions. Recommended hot tire pressures with MPSC2's on both the GT350 and the GT350R are 38 PSI F and 36 PSI R. They recommend -2.2 degrees front camber and -1.6 degrees rear for the GT350R and -2.0 F and -2.2 R for the non-R.

I've got 315/30x19 MPSC2's on the front of my GT350 (with R shocks, springs and bars) and they're working great at 36PSI hot. Lasting well too.

 

[nordique14]

Is the -2.2 rear camber number for the non-R correct? Seems off to me.

 

[honeybadger]

38/36 psi hot seems really hot to me. I run 33-34 max and that works best for me.

I found the best combo for me so far is:

Front: -4.2 camber, 1/16in toe OUT, 34 psi HOT
Rear: -2.5 camber, 1/16 toe OUT, 33 psi HOT

Last track day my tire temps were 171, 167, 162 outside to inside. And somehow I've squeeze (4) days out of a set of Cup2s that were really knackered on day 1 from a rough day at COTA. Definitely on the right track with tire setup.

Ninja edit: important to keep in mind a couple of things: I haven't had an R compound tire last longer than (4) days before (first set for me) and I run a square setup where I rotate to put the tire with the most tread on the corners seeing the most load.

 

[JAJ]

I agree it's odd...

 

[MikeR397]

Honey badger do you have camber plates and bolts to get -4.2? I can only get -3 with my steeda plates and could use more for sure.

I’m still confused how my rears went so slippery after 9 days as apparently no one else is experiencing this, and I don’t see how people are chording rears on track use earlier than that?

 

[honeybadger]

Honey badger do you have camber plates and bolts to get -4.2? I can only get -3 with my steeda plates and could use more for sure.

I’m still confused how my rears went so slippery after 9 days as apparently no one else is experiencing this, and I don’t see how people are chording rears on track use earlier than that?

Yeah. MM plates and BMR bolts. Highly recommend.

And you're definitely not alone. I know numerous folks that heat cycle out the rears. Hell, mine are heat cycled and it's only been 4 days. I'd probably chalk it up to driving speeds, tracks, and temperatures. For example, the tracks back in PNW when I was there were MUCH easier on tires than the tracks I ran on a trip to CA or somewhere like COTA.

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