tire pressure overview please

[Fleetwood]

Looking for some detailed info on tire pressures and the resultant handling changes. I'm pretty old school and have been stuck on the 29psi front and 34psi rear recommendation for about 40 years. Believe its past time to improve and update my knowledge

I've always been very conscious of tire pressures and ensure they're correct for road trips and after long garage spells. I check them after most track sessions, probably because everyone else is doing it. I have even increased the pressures a few times at drivers skills events (autocross type practice) in an effort to get more rotation (sliding or drifting??) but not really sure if I made a large enough change to impact things

My interest was peaked on this subject when I checked the recommended tire pressures for my brides new to us 2017 Mercedes wagon which wants 42 psi/48Psi (front/rear).

So with huge improvements in tires, suspension, etc, how should my tire pressures be evolving and what exactly are the resultant changes from increased or decreased pressures as they relate to air cooled pcars?

Thanks ahead for any knowledge and input. 1979 slightly improved SC

[Bill Verburg]

Originally Posted by Fleetwood

Looking for some detailed info on tire pressures and the resultant handling changes. I'm pretty old school and have been stuck on the 29psi front and 34psi rear recommendation for about 40 years. Believe its past time to improve and update my knowledge

I've always been very conscious of tire pressures and ensure they're correct for road trips and after long garage spells. I check them after most track sessions, probably because everyone else is doing it. I have even increased the pressures a few times at drivers skills events (autocross type practice) in an effort to get more rotation (sliding or drifting??) but not really sure if I made a large enough change to impact things

My interest was peaked on this subject when I checked the recommended tire pressures for my brides new to us 2017 Mercedes wagon which wants 42 psi/48Psi (front/rear).

So with huge improvements in tires, suspension, etc, how should my tire pressures be evolving and what exactly are the resultant changes from increased or decreased pressures as they relate to air cooled pcars?

Thanks ahead for any knowledge and input. 1979 slightly improved SC

While I'm sure that there is a magic perfect value for each tires pressure, it will be different for each setup to to the myriad of variables involved.

You need to experiment, it's cheap and easy to do

in general
for less over/more under raise the front pressure and/or lower the rear

for more under/leas over do the opposite

what the changes in pressure do is change the tires slip characteristic and as a consequence the pneumatic trail but most people only care about the effect not the mechanism

[jadatis]

I can make a pressure/axleloadcapacity list for your tires.
Can include reserve for unequall load, but for racing you want to go to the edges for best gripp.
Yust say how you want it, I made spreadsheet for it in wich I can give percentage of calculated loadcapacityfor the pressure, and also can give for higher or lower max speed used.

Need from you, max speed you wont go over for even a minute.
And you have to determine the axleloads in your used loading 99% accurate (by weighing, estimating is dangerous.
Determining the axleloads is the most tricky part, and your responcibility.

From tires I need next, read from sidewall.
1. Maximum load or loadindex

2. Kind of tire to determine the reference-pressure.
I think for you 2 choices.
Standard Load AT 35 or 36 psi.
XL/ reinforced/ extraload AT 41 or 42psi.

3. Speedcode, less important, every tire Q and above speedcode, I use maxload given for reference-speed 160kmph/99mph.

I figured out the maxload/ maxspeed relation.
Rule of tumb every 10kmph/6.3mph different max speed, 1 loadindex step different, lower speed> higher loadindex, higher speed > lower loadindex.

[PeteKz]

Originally Posted by Bill Verburg

in general
for less over/more under raise the front pressure and/or lower the rear

for more under/leas over do the opposite

Isn't that the same thing?

I know what you meant.

[Bill Verburg]

Originally Posted by PeteKz

Isn't that the same thing?

I know what you meant.

??
less/over/more under are saying the same thing
more/over /less under is the opposite of the above but again is 2 ways of saying the same thing

[PeteKz]

I know what you meant to write, but re-read it carefully. You actually wrote:
"for less over/more under raise the front pressure and/or lower the rear
for more under/less over do the opposite"

It should have read: "for more under/less over, raise the front pressure and/or lower the rear.
for more over/less under do the opposite."

[Fleetwood]

Thanks for the input. Did have to read Bill's answers several times to get the concept straight and think I've got it. More oversteer/less under is raise front pressure and/or lower rear pressures with the opposite ...More understeer/less over is lower front pressure and/or raise rear pressure.

So higher pressure rears...less sticky (or lower pressure fronts, more sticky) brings more oversteer. Please confirm I've got this correct .

Now please get into "tire slip characteristics" and the "pneumatic trail" thoughts

Also Bill, please discuss the cheap and easy to do experiments. Is this the tire chalk around the tire sidewalls and then do some autocross type trials to check the contact patch?

Thanks ahead for the education

[Bill Verburg]

Originally Posted by Fleetwood

Thanks for the input. Did have to read Bill's answers several times to get the concept straight and think I've got it. More oversteer/less under is raise front pressure and/or lower rear pressures with the opposite ...More understeer/less over is lower front pressure and/or raise rear pressure.

So higher pressure rears...less sticky (or lower pressure fronts, more sticky) brings more oversteer. Please confirm I've got this correct .

Now please get into "tire slip characteristics" and the "pneumatic trail" thoughts

Also Bill, please discuss the cheap and easy to do experiments. Is this the tire chalk around the tire sidewalls and then do some autocross type trials to check the contact patch?

Thanks ahead for the education

Chalking gets you in the right neighborhood which you seem to be in any way

30 to 36 psi
experiment just means try a pressure say 34 cold in all 4 and drive the car note what you like or don't like

then raise one end or the other by say 2 is the better or worse in your opinion

repeat till you get the ride and handling you want

om the street there is a pretty wide range of what works, for track use there range narrows considerably and you measure hot. For my car and my driving style there is a specific # front and another rear that gives the best feel and grip

for street you would most likely never get anywhere near this critical point so just go w/ what feels best to you.

If you can't get the handling you want from adjusting tire pressures then you start looking at wheels/tires, and suspension changes.


after that you start losing mass from the ends of the car.

[pmax]

Data point 67F day ... 37R 34F on the targa , factory settings.

[wayner]

to piggy back on Bill's excellent advice, once you are in the handling range as per his advice, on the street you are really then just tuning for tire wear vs. comfort.

(softest max = most comfortable but inside and outside tread will wear)
(Hardest max = improved handling but centre tread will wear faster)

As for autocross, in a safe environment like that, go to extremes and note how it feels.
don't be afraid to go to far and lose performance.

like trying on shoes, always go one size too big and one size to small to know if the ones you have are the best or not.

With any performance tuning, you want to figure out what problems you are trying to solve. if you aren't experiencing any problems, then you don't need upgrade-itis, the setting and upgrades are good. If you pick up the pace on track (or slow down on the street for touring,) then you may experience problems that require adjustment.

[Paul Shugert]