Tire pressure

chet baker

Midwest USA
Aug 30, 2008
I can't take credit for writing this one but I found it interesting

A short guide to tire pressure

firstly, the goal of tuning tire pressure is to gain as much of a contact patch as possible for a given situation and thus increase traction. therefore there is no set tire pressure for any type of driving, size of tire, etc. to find the best tire pressure takes trial and error, period. having said that, there is a ball park figure: 30-45psi. but that's 25% range! hardly exact science.
so now we must assess what kind of driving your car is going to be doing. if it's just a daily driver, well you can forget most the tools and just tune it by feel in corners. if you're gonna get more serious, here's what you need:

"The Tools
First and foremost, you cannot possibly tune and optimize your tire settings without having the proper tools.

At the very minimum you need an accurate and repeatable tire gauge. Don't cheat yourself with a $3 pencil style gauge. Get a dial gauge, preferably an oil-filled one, with a maximum reading of 50 to 60 psig. It should also have a bleeder valve so you can bleed air from the tire while the gauge is still connected. It's also worth getting the protective rubber case or shell. Dropping a gauge can ruin its calibration, and the rubber shell can prevent that.

To be most effective, you also need a tire pyrometer. You can do a lot with just a pressure gauge, but you'll never be able to quickly and consistently tune the tire pressures without temperature data. Many decent pyrometers are available for a little more than $100. The probe types that penetrate the tires are the preferred ones.

"Taking Tire Temperature Measurements
The first rule in taking tire temperatures is that they must be taken when the car comes in from a full race-speed lap. Taking temperatures after a cool down lap is useless. If necessary, the driver should come in a lap early (before the checkered flag) to ensure the tires are race hot. If the driver waits for the checkered flag, chances are he'll get stuck behind slower traffic. The track will be under a no passing rule during cool down, and the tires will cool too much.

Tire temperatures are taken in three places on the tire. First on an outer tread block on a street tire, or about 1" in from the outside edge of a racing slick. Second, in the middle of the tire. Third, on an inner tread block or 1" in from the inside edge of the tire. Often on high performance treaded tires, you'll need to measure the second tread block in from the edge. The outer shoulder block will not retain enough heat for a meaningful reading.

The temperature probe should be inserted all the way in. Whenever taking temps of a treaded tire, be sure to center the probe in the tread block. The edges will cool faster, so it is important to have as much rubber surrounding the probe as possible. Wiggle the probe a little when first pressing it in to ensure it is well seated.

The temperature reading will fluctuate for several seconds. Do not wait for it to be perfectly stable. As soon as it settles to within a couple of degrees F, take the reading and move on. You shouldn't have to wait more than 5 seconds for each reading.

Always measure each tire in the same pattern. Whether it's inside, center, outside, or the other way around doesn't matter. Just do it the same way every time.

Be consistent in the tire sequence also. Generally, you'll want to measure the hottest side first to be sure they're not overly heated. For example, if the entry to the pits is after a series of long right turns, measure the left tires first. Whatever the sequence, be sure to repeat it with every reading."
from Turn Fast!

now tuning can begin! firstly you should start with equal pressures all round to get a idea of what range of psi your car handles best with. so i would suggest driving the same course with no other variables and start be driving it with trie pressure around 30psi then the opposite end of the scale at 44psi (don't be afraid of it! it's well within the limits of the tire. ) and probably 2 points in between this. if you have a tire temperature gauge and a track, this would be the ideal situation.

once you've established what range of all round tire pressure is best, you can move on to fine tuning. but first there are some concepts that must be understood:
Adjustments------------Decrease Understeer ------Decrease Oversteer
Front Tire Pressure -----------Higher ------------------Lower
Rear Tire Pressure ------------Lower -------------------Higher
Front Tire Section ------------Larger ------------------Smaller
Rear Tire Section -------------Smaller ----------------Larger
Front Wheel Camber ---------More Negative ---------More Positive
Rear Wheel Camber ----------More Positive -----------More Negative
Front Wheel Toe -----------Toward Toe-Out ---------Toward Toe-In
Rear Wheel --------------Toe Toward Toe-In --------Toward Toe-Out
Front Wheel Caster -------More Positive -------------More Negative
Front Springs ---------------Soften ---------------------Stiffen
Rear Springs -----------------Stiffen ------------------Soften
Front Anti-sway ------Bar Soften (Thinner) ----------Stiffen (Thicken)
Rear Anti-sway -------Bar Stiffen (Thicker) ----------Soften (Thinner)
Weight Distribution -------More Rearward ------------More Forward
tire tech information
if you are not familiar with any of these terms, get reading! there's a suspension 101 thread in this section.

now if you have a tire temperature gauge the tuning can be seriuosly accurate. i must point out at this stage that tire pressure is FINE TUNING and should not be used to make up for the short comings of your susspension set up. i.e. if your car is oversteering like crazy, tune your suspension first rather than having vastly different tire pressures at each end of the car to compensate.
now, here's an example of how to analyse your tire temperature readings:

"If you have a temperature probe, you can use the tire profile reading as the primary guide to tell you if the pressure should be increased, decreased, or left as is.

if the centers of tires are 5 or more degrees hotter than the edges, then the tire pressure should be lowered. Try about 1 psi for each 4 to 5 degrees the center is higher than the lowest edge temperature.
if the center temperature is more than a few degrees lower than the edge temperatures, then the tire pressure is too low. Try increasing it by 1 psi for each 4 to 5 degrees the center is lower than the highest edge temperature.
use the guide below for more detailed tire temperature interpretation tips.
TurnFast! • Tire Pressure 3
i can't include the table unfortunately.
turn fast

now your car is on it's way to faster cornering! for your information you should also consider this table of recommended ranges of pressure depending on the set up of your car:
Type of Vehicle, Position and Pressure
Front Wheel Drive Front 35-45 psi Rear 30-40 psi
Front Engine/Rear Drive Front 35-45 psi Rear 30-40 psi
Rear Engine/Rear Drive Front 35-45 psi Rear 35-40psi

when tuning our hondas you have to consider that the front tires do the vast majority of the work, driving, steering, braking. so consider it when tuning your tires.

and for those that worry about the rain:

For both autocross and road racing, increase tire pressures 6-10 psi from what you would normally run in dry conditions. Hydroplaning occurs when a wedge of water develops between the tire and road surface. This wedge can actually lift the tire off the road and eliminate traction. Increasing the pressure rounds the profile of the tire by decreasing the deflection of the tire. This results in a smaller contact patch - narrower and shorter. It also helps keep the grooves in the tread open so they can channel the water out from under the tire."
tire tech info

this should serve as a general introduction into how to tune your tires for the corners. for more in depth info check these links out:
Tire Tech Information - Air Pressure for Competition Tires
TurnFast! • Tire Pressure 4

finally, tuning for the daily driver.
all of this info is applicable to the daily driver also but other factors should also be considered, and thus is why recommeneded OEM tires pressures seem so low compared to what has been considered here.
the lower, the less bumps you feel, simple. but i would not recommend dropping below 25psi as the tires will probably wear unevenly and the car will handle like a sack of spuds.
fuel economy:
the higher the better. if fuel economy is a concern, raise those pressures to whatever your side wall can handle!
off road:
lower tire pressures will increase your contact patch as the tire will deform to the shape of the ground. the softer the ground the lower the pressure. ever seen top gear race to the north pole? their tires on the hilux are ready to fall off the rims (and they do!) but their contact patch allows them to pull through the softest snow.
FWIW I have been doing short Hillclimb and Sprint events with my EK9, I am running standard wheels with 195/50/15 Kumho V700 Ecsta tyres. Medium compound on the front with Super soft on the rear.

I have found 29psi Front and 26psi Rear to be a very good compromise between grip and feel/response and have given great results so far. 7 events 7 course records.
what about uneven tire wear? I am using kumho v700 205/50/15 tires. and my tire wear out in mystic way. if you can divide the tire thread into 3 parts (side, middle, side), so my front tires middle section wears more and it wears one corner of the middle section. I am not sure this is because of tire pressure? or suspension geometry settings? Too much negative camber? too much pressure? too much caster (I am running more than 6' in front).

Another thing I am worried, in this table you state that if you want decrease oversteer you have to move weight forward? I would think that you must to add weight to the rear so that rear tires could get more grip?