Understeer tire pressure bias

[mvanbuhl]

The variables that go into setting up a car for the track get pretty complex for a beginner like me. However, I have read recommendations about tire pressure producing understeer that I find confusing.

I have read on different threads that increasing rear tire pressure increases understeer. I have also read that increasing front tire pressure above 38 psi hot will increase understeer. I'm sure both statements are true, but my question is what is the safest way to increase understeer for a beginner driver?

I realize there are many variables, so here is my current setup heading into Infineon this weekend (should be warm, in the upper 80's):

1) JIC suspension, lowered 1.5". Running full stiff front and 8 dn from full stiff rear.
2) Comptech sway bars - middle position front (can't get my OEM links into the tightest position) and loosest position rear.
3) Advan A048 front, two years old with 2 track sessions, but still plenty of tread and good even wear. R888 rear, brand new. I'm sure mis-matched tires will elicit some opinions, but these things aren't cheap and I am hoping new rear tires will help increase understeer.

Tire pressure is my main dilemma. Based on what I have read and replies to my post on the R888, I plan to shoot for 32 psi hot (starting at 26 cold) for the rears. Sounds like the A048s are round and can run at lower temps as well, so I plan to shoot for 32 psi hot fronts (starting at 28 cold).

The question is, if I feel like I want to increase understeer after a couple of sessions, what should I do with tire pressure? Also, I would appreciate any suggestions on my setup going into the weekend.

 

[nsx2tall]

OPINION:

Very generally and relatively, less tire pressure gives more grip while more tire pressure gives less grip.

Increasing front tire pressure = less front grip = more understeer
Decreasing rear tire pressure = more rear grip = more understeer


Tire pressures are a fine tuning tool. If your set-up is overly biased because of the sways, mixed tire compounds, or lowered suspension geometry, tire pressure adjustments aren't really going to help much.

Just off hand you have room to bump up the rear pressures and see if it gives you what you are looking for. I ran R888 (different car) at close to 40psi hot so you have a fair bit of overhead from 32.

 

[NSXDreamer2]

OPINION:

Very generally and relatively, less tire pressure gives more grip while more tire pressure gives less grip.

Increasing front tire pressure = less front grip = more understeer
Decreasing rear tire pressure = more rear grip = more understeer


Tire pressures are a fine tuning tool. If your set-up is overly biased because of the sways, mixed tire compounds, or lowered suspension geometry, tire pressure adjustments aren't really going to help much.

Just off hand you have room to bump up the rear pressures and see if it gives you what you are looking for. I ran R888 (different car) at close to 40psi hot so you have a fair bit of overhead from 32.

Are you sure ?? This is totally opposite of what I experienced and what i read:

my remedy of understeering on nsx is to increase the front pressure while decreasing rear. (i'm running same pressure front to back right now, as I have a heavily sprung front) The stock setting of 32f 40r is to counter the "build-in" oversteering of early nsx's alignment.

increase tire pressure (to an extend of course) will have more contacts with the road surface. As it has more traction at that axle of the car Thus increase front pressure and/or decrease rear will have more oversteer/less understeer.

there are many different articles of what to do but here's one:
http://www.tirerack.com/images/tires/tiretech/bfg_Gforce/bfGoodrich_gForce_R1_care.pdf
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=58

 

[titaniumdave]

You have adjustable shocks, use those to balance understeer! The tire pressure should be set to optimize traction, then balance the car with other things.

Tire pressures hot, shoot for 34-36 on your fronts, 38-40 on the rears. If you are not hitting those pressures the tires will never really hook up.

If you want to get more traction on the front, soften the shock settings, if you want less traction(more understeer) stiffen the front shocks.

Have your instructor drive your car, see if he can make it preform like you desire and maybe he can help you get to the next level.

Have fun!!!!
Dave

 

[nsxtasy]

I've heard it both ways. Some people say you increase tire pressure for better grip, others say you decrease tire pressure for better grip. I'm sure both are true depending on a lot of factors.

However, in general, I don't think adjusting pressures is the best way to deal with understeer and oversteer.

increase tire pressure (to an extend of course) will have more contacts with the road surface.

That statement is false; the opposite is true. A higher tire pressure REDUCES the size of the tire contact patches on the road. For example, a 3000-pound NSX with all the tires inflated to 30 pounds per square inch will have contact patches totalling 100 square inches; a 3000-pound NSX with all the tires inflated to 40 pounds per square inch will have contact patches totalling 75 square inches.

 

[mvanbuhl]

Thanks guys for the replies! Unfortunately, I am still a little confused.

As nsxtacy and nsx2tall state, I guess tire pressure is too fine an adjustment to deal with under/oversteer. I think Dave makes a good point about maximizing grip as the focus for tire pressure, so I will concentrate on that. I have seen Billy and others state that on the NSX the R888 really doesn't like pressures above 32 - 34, so I will try that zone.

Back to the understeer issue. I am running my suspension at full stiff front and 8 down from stiff rear. My sway bars are middle position front and loosest position rear. What else should I do?

 

[docjohn]

Thanks guys for the replies! Unfortunately, I am still a little confused.

As nsxtacy and nsx2tall state, I guess tire pressure is too fine an adjustment to deal with under/oversteer. I think Dave makes a good point about maximizing grip as the focus for tire pressure, so I will concentrate on that. I have seen Billy and others state that on the NSX the R888 really doesn't like pressures above 32 - 34, so I will try that zone.

Back to the understeer issue. I am running my suspension at full stiff front and 8 down from stiff rear. My sway bars are middle position front and loosest position rear. What else should I do?

soften up your front shocks..stiffen up the rear..tire pressures should be adjusted to be whatever you feel is optimal for you but the ranges given above are fine..

 

[mvanbuhl]

Hmmm, I continue to be confused. I thought the following was true about suspension stiffness:


Front Springs Stiffer: Front roll resistance increases, increasing understeer or reducing oversteer

Rear Springs Stiffer: Rear roll resistance increases, increasing oversteer or reducing understeer

 

[CL65 Captain]

Hmmm, I continue to be confused. I thought the following was true about suspension stiffness:


Front Springs Stiffer: Front roll resistance increases, increasing understeer or reducing oversteer

Rear Springs Stiffer: Rear roll resistance increases, increasing oversteer or reducing understeer

Nope. The stiffer end will generate LESS grip in general.

But you are thinking too simplified:
1. Tire camber and toe
2. Tire pressures
3. Anti Roll bar size/stiffness
4. Springs
5. Damper - compression/rebound
6. Aero (wing/splitter/undertray, etc)
7. OAT
8. Driving style

As you can see, there are A LOT of variables and ways to adjust the handling.

The best suggestion we can offer you is to learn to drive the car as is.

You may think it understeers, but I could hop in your car on the track and make it oversteer. Let's put it this way. I took a 1997 Toyota Camry with all-season Dunlop S6000 tires out at Mid Ohio (thanks Grandma) and could get it to rotate by trail braking, and drop throttle oversteer. The same thing applies to any car. In the NSX it's more difficult to catch the back end when you purposely start throwing it around. But if you learn to drive the car, you can make the car do what you want. Only after you learn to drive the car should you start playing with your setup. It is easy to want to start changing things to make the car go faster, but the truth is that the loose nut behind the wheel is the most important part to make a car go faster. Ask the guy driving the Porsche 911 GT3RS at Mid Ohio that had to point by the Camry all afternoon.

BTW: I know you are going to ignore this advice and still play with your settings and tire pressures, but hell we all know that tinkering and playing with knobs is half the fun.

PS: An oversteering NSX is not a fun car to drive as it's not an easy car to catch the rear when it starts to go.

 

[SFNSXguy]

Previous post is dead on!

Great example of this: Thirty-five years ago Bob Bondurant gave me a ride around Sears Point in one of his school's (well prepared) Datsun Z cars.

First lap he just drove around to get the tires warmed up a bit.
Second lap he says "Here's a badly oversteering car" and did a lap quickly with the back constantly hanging out.
Third lap begins he says "Here's a badly understeering car" and did a lap about the same pace as the preceding one, but this time the car pushed severely into into each turn.

It was all about where, when and how the throttle was applied.

The fourth lap was fastest of all and the car exhibited near neutral balance.

It's all in the driver's touch.

 

[Hrant]

Harry, good comment. So was the understeer induced by early apex whereas the oversteer too hot going into a turn? I don't think the Z car back then had enough torque.

 

[SFNSXguy]

Harry, good comment. So was the understeer induced by early apex whereas the oversteer too hot going into a turn? I don't think the Z car back then had enough torque.

Believe it or not, Bob induced the under/over steer just by changing his entry speed and the minor car attitude change he induced by flicking the steering wheel. "Car control" no?

 

[dquarasr2]

Let me see if I can add to the confusion, or clarify. :smile:

Tire Pressures: Increase or decrease pressure to change characteristics?

Depends. On what? On where you are relative to optimal. If you are already at optimal grip, increasing OR decreasing tire pressure will decrease grip at whichever end of the car you are adjusting. IN GENERAL, decreasing pressure will have a more dramatic effect than increasing pressure a similar amount. Increasing pressure will increase "feel", up until you've gone too far. Increasing pressure will allow less sidewall flex making it feel more responsive. When you've gotten the pressure up too high, the contact patch will crown and elongate longitudinally. Then grip goes away dramatically.

Springs: Adding stiffness will decrease relative grip at the end that is stiffened. Why? Less weight transfered front/back makes the end not stiffened carry more of the weight = more grip at that end. So, increasing the stiffness at that end decreases the relative grip at that end.

Sway bars: similar to springs, to a point. Some cars, like certain front-drive models, roll over so far invoking huge camber change such that increasing swaybar stiffness actually increases grip by reducing camber change, but again, in general, increasing swaybar stiffness decreases grip at that end.

Shocks: This requires a bit of qualification.

Compression: similar to increasing spring rate.
Rebound: If you have a double-adjustable shock, you may not get the results you want if rebound and compression are adjusted with a single mechanism. Some double-adjustable (I think) have separate adjustments for rebound and compression: increasing stiffness of compression decreases grip at the end being stiffened. Increasing stiffness of rebound PREVENTS weight transfer from that end to the other end, which may increase grip at that end being stiffened.

It can be a lot more complicated than at first blush.

CL65 was spot-on in all his comments (and usually is!). When I first started tracking my NSX, I had a problem with braking performance (unique to my car, not NSX's in general). I was constantly finding myself fighting getting the car slowed. It always felt nervous, never planted, always wanting to swap ends. When I adjusted my driving style to compensate for the poor braking performance, braking much earlier and being able to roll on the throttle before turning in, I actually started invoking understeer on exit. I adjusted such that I braked "just right" and ended up with the car being either slightly understeery or oversteery, depending on whether I wanted to rotate the car at a particular turn, or neutral.

All of you track junkies, please correct or corraborate anything I have said here.

 

[stuntman]

I agree that there will be a decrease of grip above and below the optimum tire pressure. But above and below will exhibit different characteristics that can have side effects to them which alone (the side effects) can affect and overshadow an improvement of grip due to their characteristics. Like lower tire pressure and the tire rolling over on itself feeling less confidence inspiring.

In general, for street and R-compound tires, the optimum tire pressures range from 32-38psi hot depending on the tire, track conditions, etc...

Springs: Adding stiffness will decrease relative grip at the end that is stiffened. Why? Less weight transfered front/back makes the end not stiffened carry more of the weight = more grip at that end. So, increasing the stiffness at that end decreases the relative grip at that end.

In general: Increasing the spring rate quickens weight transfer and thus makes the car more responsive - like a gokart, which has no suspension and reacts immediately. Increasing the rate thus works the tire harder and loads the tire quicker and more than a softer spring - which works the tire less and allows the inside tire to do more work. In an over-simplified way - the stiffer you go, the less peak grip you will have. As you eluded to in regards to tire pressure, there are points of no return and exceptions to the rule where too soft and or under-damped can lead to unwanted suspension geometry problems that have overshadowing side effects of their own.

Sway bars: similar to springs, to a point. Some cars, like certain front-drive models, roll over so far invoking huge camber change such that increasing swaybar stiffness actually increases grip by reducing camber change, but again, in general, increasing swaybar stiffness decreases grip at that end.

Swaybars are springs and should be viewed with the springs as the 'total spring' system of the car. Has little to no effect of longitudinal compression and primarily resist lateral compression (cornering). I think what you are referring to has more to do with a strut-type suspension where strut type FWD cars, and even BMWs and Mustangs can improve front grip by increasing the front spring/swaybar rate which reduces body roll. Since struts have no dynamic camber gain, the static camber is the most the tire will ever see. Reducing roll keeps the tire on the ground more rather than rolling over on the outer edge or sidewall. Thus, reducing a geometry problem or side effect is the major issue and is an exception where stiffer does indeed generate more grip. This is why blanket statements are just that.

Shocks: This requires a bit of qualification.

Compression: similar to increasing spring rate.
Rebound: If you have a double-adjustable shock, you may not get the results you want if rebound and compression are adjusted with a single mechanism. Some double-adjustable (I think) have separate adjustments for rebound and compression: increasing stiffness of compression decreases grip at the end being stiffened. Increasing stiffness of rebound PREVENTS weight transfer from that end to the other end, which may increase grip at that end being stiffened.

Shocks/dampers are dynamic and don't really have an effect in steady-state cornering or braking. They are dynamic and affect the car in dynamic transitions (initial braking, accelerating, transition to cornering, etc...). Although the shocks do affect steady state cornering when you throw bumps into the mix.

Adding compression resists the damper from compressing. In a corner (not a quick flick) the spring will eventually compress to the same amount regardless of the compression change, even though it may feel like body roll has been reduced. Increasing compression increases the response and quickens weight transfer - like a gokart, but also can reduce peak grip in these transitions.

It's important to differentiate body roll with weight transfer. They are almost opposites. A stiff car will transfer weight faster because there is no roll. A car that rolls alot, like an old Cadillac will flop over on itself, but isn't responsive and does not load the tire quickly or transfer weight to the tire (to elicit a response) that quickly.

It's also important to consider many factors like suspension geometry, spring rates, tire sizes, damper settings, tire compound, alignment, etc... Everything is a system as a whole and side effects from unknown factors can overshadow oversimplifications of understandings of shocks and springs. This is why engineers get paid a lot of $.

Our NSX is decently well known, and its good to see more and more people tracking them. It's important to list as much info about your given setup before asking a question, to get a more accurate and educated response.


Billy