Air Pressure Changes Steering Bias?

Shadow277

Well-Known Member
Joined
Nov 17, 2019
Messages
1,106
Reaction score
335
Location
Arizona
Vehicle(s)
2016 Mustang GT 2012 Corolla
Treat me like I'm dumb.

I thought lower pressure creates higher gripping capacity. Randy Post said inflating rear tires to 50psi and front at 32psi causes oversteer. I thought oversteer was the back having more grip than the rear. Why would a smaller contact patch to the rear creat oversteer? Is it stiffness?

I should I pressure my tires since I've only been to 3 track days? My club recommends running 42psi so the tires don't detract from the beads.
Advertisement

 

GTX

Well-Known Member
Joined
Jun 27, 2021
Messages
67
Reaction score
88
Location
Mid Atlantic
First Name
Ian
Vehicle(s)
2017 GT350
Treat me like I'm dumb.

I thought lower pressure creates higher gripping capacity. Randy Post said inflating rear tires to 50psi and front at 32psi causes oversteer. I thought oversteer was the back having more grip than the rear. Why would a smaller contact patch to the rear creat oversteer? Is it stiffness?

I should I pressure my tires since I've only been to 3 track days? My club recommends running 42psi so the tires don't detract from the beads.
Understeer the front pushes in corners, oversteer the rear comes out in corners.

The higher pressure will work to reduce grip in the rear inducing oversteer.

You should monitor your tire pressure at the track. I usually start the day around 30 PSI. Starting at 42 PSI, by the end of the session you’ve got to be at 50 or more unless you live in arctic conditions. I venture to say that isn’t safe. Hit a curb or have an off and your tire might explode.

I have run PSS and now Supercar 3Rs at 30psi and haven’t experienced any separation from the wheel or detachment from the beads.
 

Ewheels

Well-Known Member
Joined
Apr 5, 2019
Messages
1,002
Reaction score
998
Location
SoCal
First Name
Eric
Vehicle(s)
2018 GT PP1
Vehicle Showcase
1
Randy Post said inflating rear tires to 50psi and front at 32psi causes oversteer.
This scenario would give the front tires more grip...meaning the rear end will kick out first = oversteer.

Oversteer: rear tires come loose first. Rear end swings around and you spin out.
Understeer: front tires come loose first. You turn the wheel but the car keeps going straight


Very general rule of thumb is 1 psi for every 100 lbs of car. So 3800 lb mustang would mean start around 38 psi hot and adjust from there
 

K4fxd

Well-Known Member
Joined
Apr 2, 2020
Messages
4,298
Reaction score
2,382
Location
NKY
First Name
Dan
Vehicle(s)
2017 gt, 2002 FXDWG, 2008 C6,
Or in Nascar terms, Push equals understeer and loose equals oversteer.
 

NightmareMoon

Well-Known Member
Joined
Nov 22, 2015
Messages
3,961
Reaction score
2,522
Location
Austin
Vehicle(s)
2016 Mustang GT PP
Vehicle Showcase
1
Tires won't perform if the pressures are too high OR too low. There's a sweet spot.

So increasing the rears to a very high number (well above the sweet spot, like 50psi) causes them to loose grip and not perform. You could also do that by lowering pressure too low (below the sweet spot) but that's a more dangerous way to do it, because yeah if taken to far they could definitely come off the wheel.

Increasing heat also increases pressure. So a cold tire at 32 PSI will start to heat up as you use it on the track, that hotter air increases pressure, maybe 6 or 8 PSI even. On the street the same thing happens, but the heat and pressure rise is less, maybe only 2-4 PSI. On a rainy day, possibly not at all.

Heat will eventually stabilize depending on how hard you're driving and so pressures will stop rising too. Depending on the sweet spot of your tire pressures, you might target a lower cold pressure to account for the pressure rise from heat as you're driving on the track.

Taking notes to how high the pressure get in relation to your sweet spot pressures will help you figure out how much air pressure to set when the tires are cold and you're about to go out on track. When you're done for the day, you want to let the tires cool down fully and then set the pressures to your normal daily driving pressures (usually 32psi for mustangs)
 
OP
OP

Shadow277

Well-Known Member
Joined
Nov 17, 2019
Messages
1,106
Reaction score
335
Location
Arizona
Vehicle(s)
2016 Mustang GT 2012 Corolla
Understeer the front pushes in corners, oversteer the rear comes out in corners.

The higher pressure will work to reduce grip in the rear inducing oversteer.

You should monitor your tire pressure at the track. I usually start the day around 30 PSI. Starting at 42 PSI, by the end of the session you’ve got to be at 50 or more unless you live in arctic conditions. I venture to say that isn’t safe. Hit a curb or have an off and your tire might explode.

I have run PSS and now Supercar 3Rs at 30psi and haven’t experienced any separation from the wheel or detachment from the beads.
Last Saturday it was mid 80s outside. Started at 29psi and ended at 31.

I think I get it now. Less contact patch in the rear means the front has more grip so the tail will slip out.
 

GTX

Well-Known Member
Joined
Jun 27, 2021
Messages
67
Reaction score
88
Location
Mid Atlantic
First Name
Ian
Vehicle(s)
2017 GT350
Last Saturday it was mid 80s outside. Started at 29psi and ended at 31.

I think I get it now. Less contact patch in the rear means the front has more grip so the tail will slip out.
More air to start, the temps/pressure will rise exponentially.

I am surprised you only went up two degrees in a twenty minute session in 80 degree weather, regardless of starting psi.

You got it!
 

Norm Peterson

corner barstool sitter
Joined
Jul 22, 2013
Messages
9,011
Reaction score
4,672
Location
On a corner barstool not too far from I-95
First Name
Norm
Vehicle(s)
'08 GT #85, '19 WRX
Last Saturday it was mid 80s outside. Started at 29psi and ended at 31.

I think I get it now. Less contact patch in the rear means the front has more grip so the tail will slip out.
Less contact patch area isn't exactly the issue - it's things like the uniformity of loading on the contact patches (which can vary considerably from being uniform), and over-inflation tends to focus contact patch loading more toward the center of the tread.

Tire Contact Patch Pressure Distribution.JPG



There's also the fact that higher inflation pressure effectively makes the tire treads stiffer, which reduces the amount interlocking that happens between the tread surface and the roadway's aggregate surface roughness under vertical load. This is one of the components of overall tire grip and ultimately slip angles while cornering.

The Cliff's Notes hints regarding pavement roughness would be to consider autocrossers' comments regarding concrete vs asphalt lot surfaces in the dry or talk to any motorcycle rider who ever had to lay his bike down on the low side. Or any kid who skinned a knee falling off of a bicycle or skateboard.

Best picture I could find right away - just substitute street or track surfaces for runway.

Roadway surface texture.jpg



Norm
 

TundraOnKings

Well-Known Member
Joined
Jun 7, 2021
Messages
338
Reaction score
389
Location
PNW
First Name
T
Vehicle(s)
2020 Mustang GT Prem PP1
I don’t think I’ve ever seen it mentioned online, and I had no idea, nor did my track buddy that’s been tracking his 911 for 13 years. Until my last instructor showed me.

So he showed me a neat little trick on my GoodYear SuperCar3’s….there is a GoodYear “wing” emblem, on the side of the tire. For track, you adjust your pressure on each tire until you’re scrubbing the tip of the wing.
So I checked Michelin’s - same deal, there is a marker, but it’s a little “balloon man”.

This seems so very simple to me, and last track day after adjusting the first session I was spot on, and the car felt amazing.

Not sure if this is just common sense that everyone knows, but it sure seems like it’s a really good starting/reference point for anyone adjusting pressure for track.

My instructor said tire MFG will also put an “arrow”, I just googled it, and this looks like he was correct.
CBDD86FE-EB35-41ED-9008-D3E5CD66FAF6.png
 
Last edited:
OP
OP

Shadow277

Well-Known Member
Joined
Nov 17, 2019
Messages
1,106
Reaction score
335
Location
Arizona
Vehicle(s)
2016 Mustang GT 2012 Corolla
I don’t think I’ve ever seen it mentioned online, and I had no idea, nor did my track buddy that’s been tracking his 911 for 13 years. Until my last instructor showed me.

So he showed me a neat little trick on my GoodYear SuperCar3’s….there is a GoodYear “wing” emblem, on the side of the tire. For track, you adjust your pressure on each tire until you’re scrubbing the tip of the wing.
So I checked Michelin’s - same deal, there is a marker, but it’s a little “balloon man”.

This seems so very simple to me, and last track day after adjusting the first session I was spot on, and the car felt amazing.

Not sure if this is just common sense that everyone knows, but it sure seems like it’s a really good starting/reference point for anyone adjusting pressure for track.

My instructor said tire MFG will also put an “arrow”, I just googled it, and this looks like he was correct.
CBDD86FE-EB35-41ED-9008-D3E5CD66FAF6.png
Is every tire like that? I have Firehawk Indy 500s.
 
Advertisement

 
19 - Diode Dynamics - 1
Advertisement
Top