i often hear one way or the other, seems that many people don't agree on this topic. especially when it comes to street tires.


i am mainly regarding autox, where high rotation and faster transitions are evident.

my autox car is a integra running in sts.
koni/gc set up 380f 450r
stock sways
205/50r15 azenis rt-615

i listed my car just as an example but seems as theoris will pertain to most fwd hondas.

DISCUSS

 

I run 36psi in the front and 41psi in the rear on Dunlop Direzza Z1's . I'm running Buddy Club N+ spec coilover 10k front 14k rear. I'm looking at increasing the rear spring rate thought. This in a RSX type S too. I too am running in STS.

 

i also find myself running higher pressure in the back, but i find the car is twitchy and wants to snap at higher speeds. which last weeked ruined my chance at a class win.(car spun out right before the last gate)
lost to a rsx



Modified by climb_on at 3:42 PM 4/4/2008

 

I think alot of folks will either use high pressures to get rotation on a milder setup, or will use low pressures to make rotation more predictable on an aggressive setup.

 

It really depends on the car, suspension, and your driving style.

On my 00 Civic SI I always ran higher in the rear than the fronts with 350/400 spring rates, Koni Yellows, stock front and Comptech Rear sway with -2.5 f and -1.5 rear camber, 0 toe. Usually ran 40f 45r.

On my 91 Civic Si if I ran higher rears I would get snap oversteer all the time which was scary to drive. Setup was SPSS3 shocks, 400/450 springs, HF front sway, Suspension techniques rear sway, -2.8 camber, -2.0, 1/8 toe out front and rear. So I ran 41 front and 38 rears.

With my 93 Civic Si I run low front and high rears since the car is mostly stock. Some cheapie Ebay coilovers, stock shocks, stock front sway bar, 0 toe front and rear.

 

Yeh, I normally run higher pressures in the rear to create oversteer..It can get really twitchy at times though. Softer/less aggressive rear setups can benefit from this.

 

To clear things up, the simple answer is this:

As long as you are in the 30-40 psi range, higher psi=less grip

Just think about it, drag cars run like 12 psi to get a lot of grip. Also, auto-x'ers put the chalk lines on their tires to check for the tire rolling over...why?...because they want to run the minimal pressure they can without the tire rolling, which would decrease the surface patch area of the tire.

So if you give the rear a higher pressure, you are inducing less understeer, or maybe some oversteer. And if you give the fronts a higher pressure, you will induce more understeer, or less oversteer.

Certain tires like certain pressures when at temp for optimal grip, which will usually require you to start the rear at a higher psi since it doesn't do much, and doesn't heat up as much as the fronts do. So to get the front and rear both to the optimal heated pressure, you have to start the rear at a higher pressure to begin with.

Again, that is the SIMPLE answer, not turning this into a super technical thread about optimal tire pressures

 

reason for bringing up the subject is that i have spoken to both chirs from red shift motorsports and andy hollis who have both compete in the national level in sts.

both running 91 si's, and both using lower pressures in the rear to create rotation. chris was saying he was running in the higher 20's.

 

It's all preferance. Some people run high in the back (compared to the fronts) and some run low in the back (compared to the fronts).

As I said in my above post with my 91 Civic Si I had to run low compared to the fronts or I would get snap oversteer on slaloms and sometimes downhill offset camber turns, it was really sphincter inducing cause you know it was coming

I like the progressive break away of lower air pressures in the back (compared ot the front) as when the car started to oversteer it was easier for me to correct and I could drive at 10/10ths all the time, whereas higher in the back (compared to the fronts) I probably could only drive 9/10ths due to the fact of the back being so twitchy.

It all depends on setup, driving style, the car and too many other things. That is why non-competiton runs or testing days are a great way to figure out what works for you and your car for a particular course.

The other thing that influcenced me was talking to Chris Janusz (one of Chris Shenefield's customers) one time at the DC Tour. I was talking about how I couldn't drive my 00 Civic Si 100% all the time since the car was snappy. At that point I was running 42f/47r, and the rear bar on full stiff. He said you will drive faster if you put your car at comfortable settings. So I backed the air pressure down and put the sway on medium. And sure enough I was able to drop about 2 seconds on a 70 second course.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by fireant &raquo;</TD></TR><TR><TD CLASS="quote">I think alot of folks will either use high pressures to get rotation on a milder setup, or will use low pressures to make rotation more predictable on an aggressive setup.</TD></TR></TABLE>

Perfectly stated

 

Man, sheeee-it.

Using tire pressues to adjust the balance of a car in a class like STS that allows shock, sway, spring, and alignment modification is retarded.

Maximize the grip from the tires then adjust the above mentioned items to get the balance you desire. Never undercut your available grip.

A lot of people think that the rear tires in a FWD car don't do much and are generally "along for the ride". I disagree. I try to maximize the grip from the rear.

In a FWD car with an open diff, it's crucial to keep both front tires stuck to the ground or else you'll get wheel spin and push. In order to keep both front wheels touching the ground you need to limit the roll of the car. If you approach limiting the roll with front springs and front sway bars, you'll get crazy amounts of push and wheel spin. So that leaves the rear the job of keeping the car flat. If you don't have tons of grip in the rear, the car becomes very loose and impossible to drive. The only way to get rid of push and wheel spin is to make the back very stiff and very grippy.

So in other words, if you have the car setup so that the front actually works, the rear is going to have to be maximized for grip. If you limit this grip, you are basically making a pushy wheel spinner.

I usually run a split of about 3-4 psi front to rear. The fronts being higher pressure. My numbers have come from pyrometer testing on skid pads. The goal being a nice even spread on the tread surface (not the same temp accross the tread but I'm looking for my middle temp to be the average of the outter and inner)... especially for the rear since they really only work hard in a lateral capacity unlike the fronts which have to brake and accelerate.

Good luck.

 

So your not compromising tire grip by stagger? You need to use stagger/pressure to cahnge SOME balance...

 

Nope. The front of my car weighs about 600# on each side. The rear weighs around 300# on each side. That's why the tires need less pressure in the rear.

 

it just seems to me that a high gripping rear while providing a very controlled and comfortable set up to drive would also create understeer as the rear would stick and not want to come around as much as you may need.

 

The idea is not to make the back of the car stuck to the ground. The idea is to optimize the available traction so that you can use more bar to make it slide around not un-optimal tire pressures.

Basically, the more rear bar you can get away with the better. If you don't optimize that grip, you have to use less bar wich means more body roll and more chance for wheelspin on exit.

 

Lots of good quality MISinformation here. I would trust Shenefield and Hollis when it comes to autoX Civic STS setup. They seem to do pretty well.

As a general rule though - a rear tire will produce optimal grip on a given day, track surface and temperature at a certain pressure. Any pressure above or below that optimal pressure will produce less grip and therefore more rotation.

The reason why drag guys use low pressures has to do with using the sidewall windup to simulate a little clutch slip. It has nothing to do with autocross and can be safely ignored.

OP BTW with your setup you really could probably go with a bigger rear bar. But if not you will need to do something to get more rotation.

regards,
Alan

 

Hey Alan, where's the MISinformation? I only see a little bit about the drag tire stuff.

Matt

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 00R101 &raquo;</TD></TR><TR><TD CLASS="quote"> Any pressure above or below that optimal pressure will produce less grip and therefore more rotation.

regards,
Alan</TD></TR></TABLE>

ding ding ding.... that is the correct answer to the original question. I have no comments on setup discussion because there are lots of variables (springs, bar, cross weights, etc.)..... but for the original person who posted, if he keeps his setup the same and no other variables (besides typical temperature of day, track surface, etc.).... then raising OR lowering pressures will produce less grip. I forgot when and who, but some older well seasoned racer said, "lowering pressures create a progressive predictable lack of grip while raising pressures create a more instantaneous lack of grip." Granted, I typically raise pressure to get oversteer in the rear when autoxing.

Maybe (just guessing here) you use the different strategies based on the conditions.... say it is very cold out, not much grip due to lack of heat to get in tires....maybe you want to get less grip by lowering pressures...not as skittish. If it is very hot, maybe the opposite...have heat on surface, tire gets up to temperature quick can deal with more instant "slip".... I don't know? All I know is that either direction produces less grip.

 

you guys seem to know your stuff. i really appreciate all your help.

seems the consesus is that below or above optimal pressure for the specific day and track will produce less grip and more rotation.

less pressure provides a more gradual and predictive loss of grip
more pressure provides a quicker loss of grip and more snappy


the itr 22mm sway and beaks reiforcement kit is defnetly a next upgrade for me. unless i research a better choice in the same ball park price.

thanks for the help on the subject

 

He's right, there's a certain pressure at which the tire will have 100% of it's possible grip. If you go over that pressure, you lose maximum grip, and if you go under it you lose maximum grip. Things like the breakaway and the steering responsiveness are what determine how people like to set their cars up. High pressures will help keep the sidewall stiff, especially on a weaker-sidewall street tire. This helps with steering response, but you trade predictability. I personally run a rear-high setup, because I prefer the way the car feels, but it has bitten me in the butt a time or two when the car has suddenly snapped around on me.
:edit: That was on an STS-prepped Ford Probe GT. I haven't run my Integra yet, but I'm guessing that due to the fact that its suspension is much stiffer, I won't need to run high tire pressures to keep my steering response sharp. In the DC4, I may prefer to run lower pressures in the back.