uneven PSI on tires
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Joined: Mar 2004
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From: Midwest, USA
uneven PSI on tires
Hey guys.
A lot of people I knwo probably some of you here are running with lower psi on the rear tires. I want to know the physics behind this. I think it has to do with that the rear tires decides to go first when losing traction, this is true right?
I have even found others using more psi ont he rears thant he front. Please give me some inputs on this subject.
A lot of people I knwo probably some of you here are running with lower psi on the rear tires. I want to know the physics behind this. I think it has to do with that the rear tires decides to go first when losing traction, this is true right?
I have even found others using more psi ont he rears thant he front. Please give me some inputs on this subject.
Honda-Tech Member
Joined: Feb 2004
Posts: 375
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From: South Carolina
Re: uneven PSI on tires (JDMflavor_EJ)
Tire pressures can be used to control the handling of a car. This can be done in a couple of ways.
1) increasing tire pressure increases the effective spring rate of a tire. Think of how when tire pressure is low the tire sags down, just as a car would if it were on soft springs. This is not really the best way to consider or use it however as the difference is very minimal vs actually changing spring rates.
2) tires behave according to how much pressure is in them, because it also effects sidewall rollover, and contact patch. This is the most effective way to control a cars behavior with tire pressure.
Lets say you want a perfect balance, or the same reaction of the tire on each end (back and front) Since the fronts and backs are different weights (or carry different loads in cornering) pressure for each would be different. My CRX is 60% front 40% rear weight, I will not have to run the same pressure in each end to get the tire to behave the "same" Because of less weight in back, you might find less pressure back there.
This is probably not what would cause someone to run less pressure inthe rear though. Another reason for it would be that less pressure=less volume of air, and less volume takes less to heat so the tire reaches optimum pressure sooner.
All of the above is theories and such and I am just trying to give a basic idea of what tire pressures will do. Now to the real reason.
Having low pressure in a tire causes the sidewall to roll over and the tire to lose grip sooner. This would help with rotation into the corner.
Having high tire pressure reduces the size of the contact patch (think of a baloon blowing up it would "round" the tire surface) Having less of a contact patch also causes the tire to lose traction sooner.
As you see both ends of the tire pressure spectrum cause the same behavior. Different drivers use either end to there preference. This is why you see people with shoe polish on their tires, they are reading how far the tire rolls over to determine the optimum pressure. (not rolling over too much, but keeping the biggest contact patch) I happen to use higher pressures in the back of my CRX because it causes it to slide first, and because i feel that it keeps the spring rate of the tire more consistant than a soft "low pressure" sidewall.
I am just skimming the surface or tire tuning here...I hope that clears a little bit of it up for you.
Jon K
http://www.seat-time.com
1) increasing tire pressure increases the effective spring rate of a tire. Think of how when tire pressure is low the tire sags down, just as a car would if it were on soft springs. This is not really the best way to consider or use it however as the difference is very minimal vs actually changing spring rates.
2) tires behave according to how much pressure is in them, because it also effects sidewall rollover, and contact patch. This is the most effective way to control a cars behavior with tire pressure.
Lets say you want a perfect balance, or the same reaction of the tire on each end (back and front) Since the fronts and backs are different weights (or carry different loads in cornering) pressure for each would be different. My CRX is 60% front 40% rear weight, I will not have to run the same pressure in each end to get the tire to behave the "same" Because of less weight in back, you might find less pressure back there.
This is probably not what would cause someone to run less pressure inthe rear though. Another reason for it would be that less pressure=less volume of air, and less volume takes less to heat so the tire reaches optimum pressure sooner.
All of the above is theories and such and I am just trying to give a basic idea of what tire pressures will do. Now to the real reason.
Having low pressure in a tire causes the sidewall to roll over and the tire to lose grip sooner. This would help with rotation into the corner.
Having high tire pressure reduces the size of the contact patch (think of a baloon blowing up it would "round" the tire surface) Having less of a contact patch also causes the tire to lose traction sooner.
As you see both ends of the tire pressure spectrum cause the same behavior. Different drivers use either end to there preference. This is why you see people with shoe polish on their tires, they are reading how far the tire rolls over to determine the optimum pressure. (not rolling over too much, but keeping the biggest contact patch) I happen to use higher pressures in the back of my CRX because it causes it to slide first, and because i feel that it keeps the spring rate of the tire more consistant than a soft "low pressure" sidewall.
I am just skimming the surface or tire tuning here...I hope that clears a little bit of it up for you.
Jon K
http://www.seat-time.com
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Joined: Feb 2001
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From: Santa Rosa
Re: uneven PSI on tires (JDMflavor_EJ)
also, if I may add,
I ran a tad bit higher pressure in the rear because the rears (In my FWD) didnt heat up as much ( and heat in a tire increases the PSI) and therefore didnt increase in pressure once the tire got up to operating temperature.
My goal (for my personal style) was to have all 4 tires at the same pressures when "at temp".
I ran a tad bit higher pressure in the rear because the rears (In my FWD) didnt heat up as much ( and heat in a tire increases the PSI) and therefore didnt increase in pressure once the tire got up to operating temperature.
My goal (for my personal style) was to have all 4 tires at the same pressures when "at temp".
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Joined: Jan 2000
Posts: 23,478
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From: Chicago
Re: uneven PSI on tires (JDMflavor_EJ)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JDMflavor_EJ »</TD></TR><TR><TD CLASS="quote">A lot of people I knwo probably some of you here are running with lower psi on the rear tires. I want to know the physics behind this. I think it has to do with that the rear tires decides to go first when losing traction, this is true right?</TD></TR></TABLE>
I have found that the ITR generally handles best with more PSI in front, but the NSX handles best with more PSI in the rear. What does that do to your hypothesis?
I have found that the ITR generally handles best with more PSI in front, but the NSX handles best with more PSI in the rear. What does that do to your hypothesis?
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urbanlegend21
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Mar 18, 2003 04:41 AM
