Tire Pressures for Star Specs

[DirtE Thirty]

Have Dunlop Star Specs mounted on 15x7 wheels on our E30 project for chump car. Looking for advice on what people are using for cold pressures on these tires for track use. Thanks

[E30Gus]

I target 28psi hot so depending on the temp that day and driving style cold pressure might be around 24psi. I don't have a lot of experience but for HPDE's that is what has worked for me. A long race may be completely different. As the day goes on I usually have to let some more air out as the track temp increases.

[NigelStu]

I believe we were shooting for 38 hot at the Chump Car race at Nelson's ledges. Cold pressures would have been around 30+/- 2 psi. You'll have to adjust a bit depending on the track. For Road America, we will likely start at ~31-32 left side and 33-34 right side.

Where is your first race going to be?

[E30Gus]

I believe we were shooting for 38 hot at the Chump Car race at Nelson's ledges. Cold pressures would have been around 30+/- 2 psi. You'll have to adjust a bit depending on the track. For Road America, we will likely start at ~31-32 left side and 33-34 right side.

Where is your first race going to be?

38? Holy Shit! Maybe I'm doing this all wrong then.

[priapism]

We settled on 35.5 hot for the Azenis on our Lemon, and the Star Specs have the same compound so I'd go in that area. Start at 28-29 or so in the cold morning and they'd get up to 35 or so. 26 seems way low! These aren't race tires, they're street tires.

[akorcovelos]

Damn those pressures seem high. I was running my Toyo t1r at around 28 cold, and kept themto around 29-30 warm. Once I accidentally went to 35 in the rear and the car felt like it was on ice.

[NigelStu]

Testing is really the best way to figure out what works best.


Easiest way to do it is to start high pressures; drop 2 psi each session. Mark the edges of the tires (tire chalk, silver marker, white shoe polish, etc) so you can see how much sidewall roll-over you are getting. Tire pressures and temps across the tires after each session. Look for even temps and minimal roll-over, but in the end go with the pressure that yields the best lap times and best 'feel' for your driving style.


And when I say 'hot' pressure, I mean immediately after coming off the track from a spirited lap, in pit lane - not back in the paddock area. (I will generally see a 1-2 psi drop between a crew guy taking pressures in pit lane vs. me taking them in my paddock spot).


For reference, on Toyo RA-1s (Spec Miata), I'm targeting 38-39 psi hot (usually 1 psi higher in rear for car balance). In our E90 M3 on Michelin Pilot Sports, we target 39-40. Figured the same ball park would be a good place to start and seemed to work pretty well with good temp numbers and wear characteristics.

[E30Gus]

^ and you're on 15x7's?

[Earendil]

I'm pretty sure NigelStu has my tiny bit of experience on the track beat, but here is my little anecdote.

First, I found out that there is "Cold", "Hot", and "Track Hot". By this I mean that I drove 60 freeway miles, and a mile from the off ramp was the track, where I checked my tire pressure. I checked it a third time after my first (and slow) run. Here are the pressures I recorded with a decent gauge.
Cold = 32 psi (36F morning)
Hot = 38 psi (off freeway)
Track Hot = 41-42 psi (after first run)

And let me tell you, 42 made my back end rather sloppy ;)
I dropped it down to 38 rear and 37 front and it handled rather nicely in my inexperienced hands. A guy at the track that races E30's suggested 38-39 for star specs, but I believe he admitted to not having run that tire in a long time.


Edit: I am running 205/50/15 on 15x7 wheels.

[NigelStu]

^ and you're on 15x7's?

Yes, 15x7. 205/50R15 Star Specs

Kosei K1 with 10 mm spacer or TRM C1s

[akorcovelos]

Im running 225/50/16 so im guessing larger dia tires require less pressure. Makes sense since huge offload tires require way lower pressures.

[NigelStu]

^
May be true in some cases. Some tires construction/compounds react differently to different pressures. On the E90, we were running 295/30R18, and still targeting high 30's for pressure. Only way to really know is to test.

When you say car felt like it was on ice skates, was it the whole car or just one end (front or rear)? Also, were you at 35 all around or 30 front / 35 rear?



off-road tires is a whole other game - sort of like comparing baseball to badminton.



*mini thread hijack - is that Blackhawk Farms?)

[akorcovelos]

It was just the rear. Fronts were 29 rears were 35. That was my old toyos though and they were staggered size 245/35/16 rear 215/45/16 front, and mounted on 16x8.5 k1s all around so kinda goofy.

[DirtE Thirty]

We are a Des Moines based team. Our first race is Brainerd in June. Then IA Speedway in August and Road America in Oct. Anyone planning on being at those races? We are a new team with an '89 325i. We have about a dozen Chump/Lemons races between myself and the other car co-owner with other teams. We also have two new drivers to Chump, but with prior autox and track day experience. Would be great to connect with other E30 Chump teams.

So starting at say 30 psi cold all around should be a good starting point for testing purposes?

[priapism]

Im running 225/50/16 so im guessing larger dia tires require less pressure. Makes sense since huge offload tires require way lower pressures.

Offroad tires want lower pressures because it gives them a flatter contact patch and allows them to wrap around obstacles, sort of. They run like 15psi in rock crawlers.

I agree that starting at 30psi cold would be a good place to start. We usually start the day at 29psi or so. It also depends on ambient temperature, track temperature, how hard you guys run and how hard the track is on tires.