I've been trying to dial-in the tire pressure for our small autox courses. I've perused the SCCA autox forums with no great resolve so far. Some say less than 40#, some say 32# cold. I've been running ~31# hot all around, but the car seems to get loose once the tires heat up, yet going down further appears to be going the opposite of others suggestions.

I run a 255/40/17 and 275/40/17 rear on a stock suspension minus sway bars, and these are a dedicated autox set of wheels/tires. I believe they are set at medium front and stiff rear. Eibach springs and either Koni or Tokico shocks are to follow, but not in the immediate future. This is also a daily driver, so I can't get crazy with camber.

Thanks for any experience-based insight.

 

I like my 615's around 45 hot psi. (I know some miata guys run theirs around 50 hot psi)

*I run BSP and use 615's for some practices...255/18's.

 

I run autoX w/ first350 and run the same 255/40-18 RT615's on 18x8 wheels too. I find that 42 psi up front and 35 psi in the rear works best for me. but that's in summer conditions. When it's cold out I would find myself running slightly lower fronts (40 psi or less). RT615's seem to loose their grip quickly when it's cold outside.

 

Lose their grip or not have grip to start? It was cold at our last autox, and my times showed it.

 

Both. Cold= Not so good for the 615s

 

+1 I run 615s as my street tires...it's been around freezing temps in the Seattle area and it can be scary how little traction these have!

 

i think i would try to settle down the rear end with a simple rear sway bar adjustment. start at medium setting then go soft. is the car getting loose INTO a turn or at track OUT???

 

I bought my first set on RT615's for my S14 last June. They were phenominal. I quickly got a set for my Z and they were great until September when temps were cooler in the mornings, that's when I started noticing the weakness of the RT615's. They definately are a summer only tire. When temps are cold out they do not do well at all. In autoX they are decent for cold use only if you can get enough heat in them and keep that heat in but I've noticed that in fall like weather you'll need tire blankets to get the most out of them.

At the end of last summer at an autoX practice, first350, myself and another friend did 2 consecutive runs in each of our cars (6 runs in total each) only stopping long enough to change drivers. We were all running RT-615's and the course was a fairly agressive one (high speed turns, 2 very heavy braking zones and about a 55 second course) plus we knew the course well and could charge in at 9/10th. The 615's held up very well, not requiring any cooling. Even on our buddy's AWD STi they didn't seem to require any cooling although his brakes may have started to fade.

 

Hmm, I'm wondering if I'm running too low after all. When I said "cold weather", I mean it was mid 40s when I had to run. Our group's autox allows a warm-up lap followed by a timed lap, so there is typically 30-40s of driving to get them warmed up a bit. Maybe I found an extreme (low) that worked "ok", but I'll try them in the 40psi range next month.

Thanks for the replies (and others please keep them coming!).

 

I found out first hand in Dec. how bad the 615s were when cold. There was a free on ramp involved and two walls. One on the opposite side of the freeway entrance.

 

- FTL! I've been luck enough to keep the Z off walls this winter...but the rear end does like to swing out!

 

I feel I have good control of throttle-induced oversteer when the car begins to push, but I believe the tires can allow me to go faster than what I'm currently doing. So to answer your question, the car is balanced but will begin to oversteer from loss-of-traction during constant throttle in constant-radius sweepers. Is also prone to oversteer at track out with 32-33psi hot rear tire pressure.

 

Damn, my edit didn't go through. I did not hit the walls, but I ended up in the middle of the breakdown lane. 4 lanes of traffic, 360* turn, 3 cigs later I was able to get back on the road. I made sure to never run the 615s on the streets again, and I never will.

For autocross I've just been sticking to my falken 452s. I'm not competitive enough to really worry about it.

 

I don't have the RT-615s so I'm not familiar with its characteristics. However, I have two sets of r-compound tires (RA-1s & Hankook Z211s) that I've ran on cold weather. I did not get much grip starting at 40psi all around. Once I dropped the rears to around 34-36psi, I normally will find the sweet spot.

Also, any reason why your sway bars are set this way? I have full soft in front & full stiff in rear and mine seems to handle better with this setting than full stiff in rear (much more predictable in the rear).

 

i kinda brought this up earlier, but it got burried. +1 on why? +11111 on why not try changing it?

 

We set the rear end up stiff so that it would allow me to easily induce oversteer. It sounds like you two think I should soften it up a tad?

 

well if your stuggling with it being to loose then............... uh yeah

 

That's the reason I asked the question -- looks like you're prone to oversteer as you stated. Try going to full soft in the rear -- you'll be amazed how much better it'll feel.

 

FWIW, I run 615s but don't autocross, only trackdays. When my sways were first installed, they were set at full stiff on the rear. This is too much sway for a street tire, so yes, you'll over steer since the car will have very little body roll, therefore the tire doesn't provide the traction necesary so there goes the tail. Not only did it cause that, but I also run a Quaife LSD, which needs some load, so basically, everytime I cornered hard in slow sharp corner I would loose some traction because of wheel lift. I then tried the rear sway at both medium and full soft, and full soft works best for me. When I move to Rcomps I will stiffen the car all around though.

I'm running med/soft front and soft rear on Hotchkis sways now, but as mentioned above, for trackdays.

BTW, too much sway in the front will cause understeer because of the same reason.

The things we learn on these boards

 

I run the exact same tires and sizes. (I have -1.3 and -2 camber front and rear, so my numbers might not match your situation)

I run 39#F and 35#R first run. But I also bring the tires back to these numbers just before every run. (hot or cold)

My 615's do much better "cold" than my MX's did. I say "cold" because I live in SW Florida and cold is a relative term.

BTW: I am running a .6" drop with RS-R springs (that's where the -camber came from) and Hotchkis sways full soft rear, and staggered setup between the two middle holes in the front.

And a Quaife LSD.