[Bart]

I have been reading recommended tire pressures on this and other sites. I have been a bit confused as to some of the advice given about higher pressures. My experience has been using cantilever bias ply slicks (very stiff sidewalls) on my race car and not DOT legal tires. I have never tested or tracked any radial race tire or DOT legal tire. So if I am missing something please let me know.



We use tire pressure adjustments to "fine tune" the handling of the car and in my experience lowering the tire pressure generally will provide more grip to the tire that has the lower pressure, unless the pressure is so low that it allows the tire to roll over on the side wall and thus decreasing the contact patch. And if the tire is not rolling over on the side wall adding pressure will provide less grip to that tire.



For example if my car is just a touch loose in left hand corners we will decrease the tire pressure in the right rear tire and thus provide more grip to that tire. It may be only 1/2 to 1 lbs. of pressure to get the results we are looking for. If much more pressure than that is required then we usually go to a spring, sway bar or shock adjustment. The other option would be to add pressure to the right front tire and thus decreasing grip in that tire but that would be counter productive because you want to increase the grip whenever possible and not take it away.



Before anyone asks the car is not a 968 and the pressures we use are not relevant and may make some scared at how low a pressure we start at. We also use nitrogen to help control pressure rise across all the tires.



I did attend my first track day with the 968 last weekend and played with the rear pressures. It was more for a wear issue than tuning the chassis. As I have heard from others these cars do push. Even though I thought my pressure adjustment would cause the car to push more (and it did) I took 2 lbs. out of the rear tires. It seemed to help the car with traction out of a a slow corner that leads to a uphill straight. I started out with 36psi all around which I think I saw that someone was using that DE or raced on either this site or .net. My tires are Yoko ES100 that I have not been real happy with. The did heat up at the track and provide more grip than I expected. I have never run another street legal car at the track so I can't really compare the yokos for track work vs. any other tire. I run Michelins on every other street car I have and the 924S with pilot sports seems to have better grip on the street.



If I take the car to the track again I will most likely start with lower pressures to see what happens. Fortunately or Unfortunately (depends on your prospective) The yokos will be replaced prior to doing any type of track work so a back to back comparison with this tire won't happen.



And all I could think about in between sessions and occasionally on track was I need an lsd, bigger bars, bigger brakes, stiffer springs, race tires..........................



I need to get my race motor finished!!!!!!



FWIW

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[Jim Child]

My rule of thumb for a 968 running street tires on the track is to shoot for around 40 psi hot. Of course this will vary a bit depending on the tires being run, but 40 psi will be close and should only require adjusting a pound or two up or down to get you where you want to be. Depending on how hard you're driving you'll see a pressure increase anywhere from 6 to 10 lbs from cold to hot, so you need to set your cold pressures accordingly and then adjust as necessary immeidately after coming off of the track. If you started at 36 lbs, and were driving as hard as I expect, its likely your hot pressures were way too high.

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[flash]

ok - here's one - i'm being told that lower profile tires need even more air - thoughts?



i now have a 285/30/18 on the rear and a 255/35/18 on the front - i've logged about 150 miles on them now and even pushed them around a bit - currently, per the advice of the local hot shot, i have 40 cold in the rear and 38 cold in the front - they go up about 4lbs under normal driving and a bit more under agressive driving



here's the thing - i think it's loose and light all over - i'm planning on dropping pressure to see how that goes - i'd swear it felt better with 36 in the front and 34.5 in the rear, which is what i hd before the new alignment



unfortunately i commited the ultimate suspension sin - i made a lot of changes all at once, so now i get to sort it out



i increased front camber to -1.5deg from -1.25deg, decreased front castor to 2.8 and 3.2 from 3.2 deg and 3.6 deg, and am running .04 front toe - rear stayed the same as i had before at -2 deg camber and .38 deg total toe - all the specs are well within reason, and i don't think they are responsible for the feel



i've played with shock and strut settings, and it certainly is more balanced with them set a bit stiffer than i had before - with the other size tires, it was pretty bouncy set this stiff - now with the lower profile, things tightened up a lot



perhaps i was plowing before and running soft - perhaps the other tires were broken in nicely - perhaps i am being impatient - frustrating stuff though, especially since the cops have been patrolling my canyon lately

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[Bart]

[quote name='Jim Child' date='Apr 26 2005, 05:59 PM']My rule of thumb for a 968 running street tires on the track is to shoot for around 40 psi hot.  Of course this will vary a bit depending on the tires being run, but 40 psi will be close and should only require adjusting a pound or two up or down to get you where you want to be.  Depending on how hard you're driving you'll see a pressure increase anywhere from 6 to 10 lbs from cold to hot, so you need to set your cold pressures accordingly and then adjust as necessary immeidately after coming off of the track.  If you started at 36 lbs, and were driving as hard as I expect, its likely your hot pressures were way too high.

[right][post="3795"]<{POST_SNAPBACK}>[/post][/right][/quote]





Jim-

According to your experience I was over pressure by 4-5 lbs. hot. And yes I was pushing the car for all I could get out of it. It some cases I was over the cars ability. My first thought during the first session was that street tires SUCK!!!



I do have a couple of other questions......Under braking, how often does your ABS engage during track sessions? How much oil do you consume while tracking the car?



After taking a look around at the other cars that were running and in particular the two that caught up to me I noticed one (a late model Z car) was running slicks and the other (a 911 RS America) was running pilot sport competition tires and both had competition suspension, brakes, roll cages and I assume more horsepower. That made me grin considering they couldn't figure out how to get through the corners fast enough to pass me down the straights!!

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[Bart]

[quote name='flash' date='Apr 26 2005, 06:33 PM']ok - here's one - i'm being told that lower profile tires need even more air - thoughts?



i now have a 285/30/18 on the rear and a 255/35/18 on the front - i've logged about 150 miles on them now and even pushed them around a bit - currently, per the advice of the local hot shot, i have 40 cold in the rear and 38 cold in the front - they go up about 4lbs under normal driving and a bit more under agressive driving



here's the thing - i think it's loose and light all over - i'm planning on dropping pressure to see how that goes - i'd swear it felt better with 36 in the front and 34.5 in the rear, which is what i hd before the new alignment



unfortunately i commited the ultimate suspension sin - i made a lot of changes all at once, so now i get to sort it out



i increased front camber to -1.5deg from -1.25deg, decreased front castor to 2.8 and 3.2 from 3.2 deg and 3.6 deg, and am running .04 front toe - rear stayed the same as i had before at -2 deg camber and .38 deg total toe - all the specs are well within reason, and i don't think they are responsible for the feel



i've played with shock and strut settings, and it certainly is more balanced with them set a bit stiffer than i had before - with the other size tires, it was pretty bouncy set this stiff - now with the lower profile, things tightened up a lot



perhaps i was plowing before and running soft - perhaps the other tires were broken in nicely - perhaps i am being impatient - frustrating stuff though, especially since the cops have been patrolling my canyon lately

[right][post="3797"]<{POST_SNAPBACK}>[/post][/right][/quote]



Considering Jim's comments I would suspect you would be happier with lower pressures. It sounds like your old pressure settings would be closer to the desired result.

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[Dave]

My street tires usually gained about 8-10 lbs cold to hot. Jim's dead on here. Michelin MXX3's = loved about 41 hot all around. Kumho Ecsta MX's = loved about 39 hot all around. I typically had very equal gain on all four corners. Sometimes took first two sessions of the day to get the hot pressure to stabilize as above (usually had to bleed off one or two pounds after second session, then they stayed dialed in).



From there, I could feel them start to get a bit "greasy" if the pressure went up approximately 2 lbs.

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[Jim Child]

[quote name='Bart' date='Apr 27 2005, 03:15 AM']I do have a couple of other questions......Under braking, how often does your ABS engage during track sessions?  How much oil do you consume while tracking the car?

[right][post="3814"]<{POST_SNAPBACK}>[/post][/right][/quote]



I engage my ABS in most corners that require heavy braking, and especially when I'm trail braking and causing the inside rear wheel to get light.



Regarding oil consumption, the last time I was on the track the car consumed 1/8th of a quart over the course of 3 days. I know others who are using more, but as long as you keep an eye on it and keep it full or slightly over full you should be OK.



I see you've discovered how much fun it is to spank 911's in the corners <img src="/forum/images/smilies/968/biggrin.gif" class="smilie" alt="" />

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[Bart]

[quote name='Jim Child' date='Apr 27 2005, 05:49 AM']I engage my ABS in most corners that require heavy braking, and especially when I'm trail braking and causing the inside rear wheel to get light. 



Regarding oil consumption, the last time I was on the track the car consumed 1/8th of a quart over the course of 3 days.  I know others who are using more, but as long as you keep an eye on it and keep it full or slightly over full you should be OK. 



I see you've discovered how much fun it is to spank 911's in the corners  <img src="/forum/images/smilies/968/biggrin.gif" class="smilie" alt="" />

[right][post="3821"]<{POST_SNAPBACK}>[/post][/right][/quote]



Yes it was fun!!



I added over 3/4 of a quart of Mobil 1 in one day, about 1 hour of track time. I thought that was a lot...... <img src="/forum/images/smilies/968/huh.gif" class="smilie" alt="" />



Thanks!

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[flash]

this is my thought too - i'm just about to go test that theory

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[flash]

intersting stuff - yes, 40 hot seems best - that's 36 cold on my setup - gives me a bit better contact patch too - may still need to go down a snitch in the back though - i'll play from there a bit over the next couple of days, but it's now close - ended up having to spin my front struts up to 1.75 turns from soft, where i used to run them at 1.25, and the rear shocks to 6 of 12 (may end up at 7) where i used to run them at 4 - car is pretty balanced, but definitely less steering resistance - i have a touch less toe than before, and less castor, so i think i'm going to chalk it up to that

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[mikew968]

Flash:



Is the car understeering? It seems to me you have the right amount of camber in te rear but not enough in the front. I am set up more for te track but have -3 f and -2.5 rear.



Mike

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[flash]

actually no, it's oversteering just a touch, which is fine - that's what i wanted - remember, unlike the hardtop, which is normally 66lbs heavy in the rear, i am actually about 100lbs light in the rear now after all the weight loss mods - this means i can't put as much camber in the front, or i get to watch the back of my car from the front - also, i have the strut brace up there, which limits the camber change, and resultantly means i need less preset camber to achieve the same net results in a corner



it's just light feeling - i'm guessing that i was used to a lot more sidewall flex from the 17s, and now it just "feels" like it skates, though i haven't been able to push it out of shape yet



it's getting better - a little more tweaking yet to do though

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[pickwick]

> I typically had very equal gain on all four corners.



I run 36 all around on the street, but start cold on the track with 36 RF, 35 LF & RR, and 34 LR. They all get to 40 hot about the same time.



And for the record almost never feel the ABS kick in on track; I guess I'm not aggressive enough.

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[Bart]

From what I saw of the pressure gain I had, I will be starting at 32-33 psi all around. Which is what I was using on the street until the morning of the track day.



Thanks!

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[flash]

ok - had dave drive the car - "no complaints" - "handles better than my car"



i think i've nailed it - ended up with 35# in the rear and 36# in the front (cold) - 1.75 turns from soft on the struts and 6 clicks out of 12 on the rear shocks - ride height is dead even on all four corners when loaded - 250# linear springs rear after torsion bar deletion and shock geometry reorientation - progressive front springs with a max of 250# - M030 bars fore and aft with a few cool extras



nice smooth ride and not harsh - corners danged near dead flat in a 45 degree sweep at 85 mph - weight transfer is predictable - steering is incredibly responsive (translation, don't twitch too hard) - scuttle shake is all but gone (this i'll attribute in part to the serious chassis stiffening from the roll bar, and part to the stiffer settings)



pretty darned happy now

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[944_968CS]

[quote name='Dave' date='Apr 27 2005, 05:46 AM']My street tires usually gained about 8-10 lbs cold to hot.  Jim's dead on here.  Michelin MXX3's = loved about 41 hot all around.  Kumho Ecsta MX's = loved about 39 hot all around.  I typically had very equal gain on all four corners.  Sometimes took first two sessions of the day to get the hot pressure to stabilize as above (usually had to bleed off one or two pounds after second session, then they stayed dialed in).



From there, I could feel them start to get a bit "greasy" if the pressure went up approximately 2 lbs.

[right][post="3817"]<{POST_SNAPBACK}>[/post][/right][/quote]



That’s exactly my experience when it comes to ECSTA MX. 39 PSI hot is just perfect, even tried 37 PSI which seams to be little. I run 235/40x18 up front and 265/35x18 in the rear. The weight of my car is 2930 lbs filled up with gas without driver. Regards /Bo P

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[Clem]

Just another data point:

When I run street tires, I typically start at 34 front, 32 rear and like everyone has mentioned shoot for about 40 hot all-around. I find the few extra pounds up front helps cure some understeer.



Good luck,

Jim

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