_
_ HalfDork
8/13/19 5:36 p.m.

Last event, I ran 38psi, all the way around. That was cold. Things heated up and we were afternoon run group. Checked temps before our group, nothing changed. 

Ran three b2b runs, third run felt greasy. Saw everyone else airing down. I checked my pressures- 45psi. Aired down to 30psi. Came home, checked psi, 36psi. 

Does any of that seem normal? I checked it with two different gauges as well. 

WonkoTheSane
WonkoTheSane SuperDork
8/13/19 5:49 p.m.

It's primarily a function of how much water was in the air you filled up with.  I've had as much as 8 psi on new tires with a"fresh" fill from a compressor without a dryer on a rainy day..

alfadriver
alfadriver MegaDork
8/13/19 5:51 p.m.

Before you air down, you need to figure out if you even need to- which is to chalk your tires before each run.  The more you do it, the more you will know how much to adjust tire pressures on hot tires.

BTW, going down to 30psi on hot tires seems way too much.  Then again, my tire data is over a decade old now, and I'm aware of tires that like to work at low pressures...

IMHO, that was too much, BUT repetition, repetition, repetition.  Doing as many autocross runs as you can has benefits on many, many levels- driving, set up, prep, etc.  Do it as much as you can, and take notes.

NermalSnert
NermalSnert Reader
8/13/19 5:52 p.m.

I'd never thought about humidity in the fill air until now-Excellent!

NickD
NickD PowerDork
8/13/19 5:52 p.m.

I'm running 225-width 200tw tires on a 9" wheel and reasonably set up suspension, so my data may be incompatible, but I air waaaaay down on my Miata. Like, 27psi front, 22psi rear

Dusterbd13-michael
Dusterbd13-michael MegaDork
8/13/19 5:54 p.m.

Umm....

I set my sm7s to 40psi last event. Never check afterwards. Looked like the right amount of roll based on scuffing

 

Im not sure im a good enough driver to notice a difference in the tire pressures....

slowbird
slowbird Reader
8/13/19 5:58 p.m.

Wait, you mean the sidewall isn't *supposed* to be used as part of the gripping surface? I thought it was like, bonus treadwidth. cheeky

jimbbski
jimbbski SuperDork
8/13/19 6:09 p.m.

How much air you need in a tire will vary based on the tires construction, profile, wheel width its mounted on, the cars weight, well I can go on from here but you get the idea.

  Back when I first started to auto-x  (Mid 70's) there were no street legal "race tires".  A 60 series was as low as you could get and 15" wheels were the largest available and a 7" wide rim was considered, well wide.  

I  ran pressures as high at high 40's psi with a hot pressure over 50 psi. All this to keep the tire from rolling over.  As wheels got wider and tires lower and the sidewalls stiffer the pressure requirement  dropped.   

 

I now run pressures cold of 27-29 psi on my road race car which uses DOT legal tires in size 205/50X15 on a 7" wide wheel.

poopshovel again
poopshovel again MegaDork
8/13/19 6:15 p.m.

Honda-Tech has really gone downhill.

Knurled.
Knurled. MegaDork
8/13/19 7:07 p.m.
NermalSnert said:

I'd never thought about humidity in the fill air until now-Excellent!

IMO that is the real advantage of nitrogen fill.  It's not the getting the oxygen and argon and other gases out, it's getting the moisture out.

mazdeuce - Seth
mazdeuce - Seth Mod Squad
8/13/19 7:23 p.m.

I gain about 8psi on track, but only 5-6 psi autocrossing and much of that comes from the day getting warmer. I usually start 2ish psi below my goal temp and adjust as needed after the first run. 

_
_ HalfDork
8/13/19 7:28 p.m.

In reply to alfadriver :

Yeah I chalk, that really works. Although, when i aired down to 30psi, I had better grip and less sliding effect, the downside was immediately noticed though, that sidewall rollover made the Miata inconsistent until the tire settled into place. 

wlkelley3
wlkelley3 UltraDork
8/13/19 7:51 p.m.

Did the chalk method and trial & error to final tweak the pressures. 205 RE71's on a 6.5" 15" wheel on a NB1. Front is 28, rear is 26 hot. Since the car isn't a daily anymore I just leave it there all the time.

Javelin
Javelin MegaDork
8/13/19 8:01 p.m.

Staggered RE71R on staggered 18's on the Boxster. Set them at 34/35 F/R before first run and maintain that by airing down if needed every run. If the car gets pushy and doesn't want to rotate, I'll let the rears come up to 36-37. Yes, that little bit of change makes a wild difference on my car.

alfadriver
alfadriver MegaDork
8/13/19 8:51 p.m.

In reply to _ :

So you are really close, then. Just a little low, it seems. 

KyAllroad (Jeremy)
KyAllroad (Jeremy) UltimaDork
8/14/19 7:30 a.m.

My old CSP Miata with 275 hoosiers on a 9' wheel does best aired down to 21f/19r psi.

Current STR MX-5 with 245 RE-71s on 9" wheel is a bit heavier and seems right at 24f/22r. 

From cold I usually have to drop 5-8 psi in 2 psi increments until pressures and heat stabilizes around the 6th run (3 each on co-drive).

The cars with proper camber really benefit from lower tire pressures.  Running 38 psi would leave me skating around like a pig on ice skates. 

GameboyRMH
GameboyRMH MegaDork
8/14/19 8:16 a.m.

I don't simply air down over the course of an event. I tune tire pressures over time and then stick to a particular cold pressure I've settled on. On Z1SS I found that the best pressure was 42F/27R. Recently I've been running used R-comp semi-slicks and the much stiffer sidewalls on those need a lot less pressure, low 30s front and low 20s rear.

(these are all 195~205 wide tires)

iceracer
iceracer UltimaDork
8/14/19 1:46 p.m.

7 psi. was normal increase  on Hoosiers when I ran track.

 There is a saying, "Never air down a hot tire"

Read it all the time on tire sights.

Eh, what do they know.

 

wvumtnbkr
wvumtnbkr UberDork
8/14/19 3:19 p.m.

Why use the chalk on the side of the tire?

 

Shouldn't you be going by tire temps?

 

If the tire is rolling over, shouldn't you add camber, not add pressure?

 

I always use tire temps to set my pressures and camber.  I get great grip and great tire wear.

 

At a trackday, I start at 27psi cold and end up at 34 hot.  225 on an 8 inch wide wheel.  2400# rwd car.

GameboyRMH
GameboyRMH MegaDork
8/14/19 4:39 p.m.

If the tire is rolling over, ideally you should first try to fix it with camber. If you can't get any more camber, find that it isn't helping, or don't want to adjust it at the moment and you want a quick band-aid, then you add pressure.

poopshovel again
poopshovel again MegaDork
8/14/19 5:05 p.m.

Aren’t pyrometers crazy cheap nowadays?

wvumtnbkr
wvumtnbkr UberDork
8/14/19 6:11 p.m.

In reply to poopshovel again :

Yessir, they are.  Cheaper than a set of ruined tires.  More like cheaper than 1 ruined tire.

WonkoTheSane
WonkoTheSane SuperDork
8/14/19 10:07 p.m.
Knurled. said:
NermalSnert said:

I'd never thought about humidity in the fill air until now-Excellent!

IMO that is the real advantage of nitrogen fill.  It's not the getting the oxygen and argon and other gases out, it's getting the moisture out.

Yeah, it kind of blew my mind when I first read about that, too :)

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