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How to pick a starting tire pressure

xXDavidCXx

Autocross Champion
Location
AZ
Car(s)
2017 GTI SE DSG
I've been meaning to follow up on this topic since I have specific numbers I run and that I suggest, but there is a reason, and I don't always articulate the reasons, and neither do many others.

So with that said. Here is what I think should be considered when choosing tire pressure.

All things being equal, each of these conditions may require/result in more or less tire pressure
ConsiderationMore tire pressureLess tire pressureThe relative order of importance
G's expected (need to support the load on the tire) and vehicle weightX4
Stiff tire sidewall stiffness & load ratingX1
Soft tire sidewall stiffness & load ratingX1
wide tires compared to OEMX7
surface with less grip (sand/dust, wet, cold)X5
Surface with more grip (concrete, dry, hot)X5
The driver is aggressive and brakes hardX6
The driver is timid and a newbX6
Stiffer springsX3
Softer springsX3
Shocks in race mode (especially with more low speed bump)X8
Shocks in drive home or low grip modeX8
Stock camberX2
max camberit depends onthe other settings2

I don't consider tire temps across the tread very useful for most of the VW group, because most of us have such little camber that I guarantee the tire temps will look all off and you will spend precious time between runs trying to "maximize" tire pressures for the best grip, when you just don't have enough camber to utilize the full tread width evenly. Trust me, I've done this, it's a waste of time.

So the real answer is it depends. ;)

Other things to consider.

Understeer is often a result of the front tires getting overloaded and not having enough pressure in the tire to resist the load imposed. When this happens the tire gives up, and effectively the grip is gone, until you slow down and reduce the load on the tire.

If you are running lower pressures and find the front won't turn, add 5 to 10 psi to the front tires and see what happens.

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MonkeyMD

Autocross Champion
Some of this I learned from trial and error, like about high grip concrete & more pressure. Unfortunately, the location we have divisionals has 2 different surfaces in 3 different areas, asphalt on the sides, concrete in the middle. So you have to compromise your pressures a little.

Also noticed on grippier surfaces, have to run more pressure in rear relative to front to prevent understeer. But per your info, maybe I'm just not running enough in the front & getting understeer by overdriving the fronts.
 

krs

Autocross Champion
Location
Reno, NV
Car(s)
MKVIIS R
When I’m trying to find what works I throw a few runs away trying to find what definitely doesn’t work. In my V730’s I went with some ridiculously high and low pressures, just to see, and then moved around from there. That’s usually only the first event or two on new tires though.
 

Roald

Go Kart Newbie
Location
Ohio
Car(s)
GTI mk 7 2017
This really gives more context to the numbers that get posted. Really appreciate you taking the time to do this!
 

DarkArrow

Go Kart Champion
Location
OC
Car(s)
'18 R
I'm not sure things like this should be so generalized. For example:

Wet/dusty/dirty roads should have a higher tire pressure to reduce the tire footprint and reduce the chance for hydroplaning.

Driver is aggressive and brakes hard should have a lower starting pressure because the heat from the tire scrub and the brakes are going to increase air temps in the tire, resulting in a higher hot tire pressure.

Just my .02.
 

xXDavidCXx

Autocross Champion
Location
AZ
Car(s)
2017 GTI SE DSG
I'm not sure things like this should be so generalized. For example:

Wet/dusty/dirty roads should have a higher tire pressure to reduce the tire footprint and reduce the chance for hydroplaning.
Obviously I didn't cover every possible scenario, even wet, did I mean standing water or just wet?
Driver is aggressive and brakes hard should have a lower starting pressure because the heat from the tire scrub and the brakes are going to increase air temps in the tire, resulting in a higher hot tire pressure.

Just my .02.

What are your starting conditions? Did you already have good tire pressures?

We have to speak to assumptions, so my assumptions are load based. The more load you impart on the tire, the more pressure is needed "all other things being equal", as stated at the start of the chart.

Sure, if you already have high pressures but low heat, this will not work.
 
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