Tire pressure

[san rensho]

Why does Porsche recommend 7 pounds difference in tire pressure front to rear?

I've only had my car a couple of months and just recently statrted to push it in the corners. On 60-70 mph sweepers I noticed pronounced lift throttle oversteer. I said to myself, this is a Boxster, the mid engine should make the car very balanced and all, but it felt like an air cooled 911 when I got off the gas in corners.

So I put 4 more PSI in the fronts and it felt better. Then I went to 36 front and rear and now it feels great.

Is there a reason for 29 front/36 rear?

[darkstormvx]

I believe the tendency of the boxster is to oversteer and possibly the manufacturer recommended the psi to offer more control in a high speed cornering situation, as well as during daily driving, where turn-in is more valued at low speeds.

Higher pressure promotes more high speed stability, however the tire takes a while longer to warm up. Slightly less grip. Slightly lower tire footprint.
Lower pressure gives you more flexibility and better cornering as the tire warms up more quickly. Larger tire footprint. More grip. Yet wears out more quickly as well.

I assume making the rear more stable and the fronts more nimble and grippy makes the car a better daily driver and helps the tires wear more evenly. I have 19" wheels and follow the manufacturers specs and love the handling.

I'm not sure what you're feeling, but I know if I need to slow down or brake quickly I want to more turn-in ability from that oversteer and grip in the front. Everyone drives differently tho. And you may prefer a different setup. The best setup is the one that works for you that you can feel. The manufacturer has to cater to the needs of the normal daily driver.

[ddb]

Conventional wisdom is that it's safer for the average Boxster driver to have a car that pushes versus one that's loose. Yes, I live in NASCAR country.

[san rensho]

Originally Posted by ddb

Conventional wisdom is that it's safer for the average Boxster driver to have a car that pushes versus one that's loose. Yes, I live in NASCAR country.

That make sense, that the car should push, rather than oversteer, but with lower tire pressure in the front, it seems to induce oversteer. Once I increased the pressure in the front, the oversteer went away.

You especially don't want to oversteer when you get off the throttle, since this is the typical panic reaction that people have when they get into a corner too hot.

[Topless]

Originally Posted by san rensho

Why does Porsche recommend 7 pounds difference in tire pressure front to rear?

I've only had my car a couple of months and just recently statrted to push it in the corners. On 60-70 mph sweepers I noticed pronounced lift throttle oversteer. I said to myself, this is a Boxster, the mid engine should make the car very balanced and all, but it felt like an air cooled 911 when I got off the gas in corners.

So I put 4 more PSI in the fronts and it felt better. Then I went to 36 front and rear and now it feels great.

Is there a reason for 29 front/36 rear?

My best guess is ride comfort. The car feels smoother with 29F. I typically run 32 cold all around on the street.

Yes every car will exhibit throttle lift oversteer without active stability management... even a 63 Ford Falcon. It's a physics thing.

[jaykay]

Originally Posted by Topless

My best guess is ride comfort. The car feels smoother with 29F. I typically run 32 cold all around on the street.

Yes every car will exhibit throttle lift oversteer without active stability management... even a 63 Ford Falcon. It's a physics thing.

For sure.....even worse if you are in a sweeper and lift in a slide/drift, in damp conditions and baked tires.....it will over steer on the opposite lock right quick....don't ask me how I know this It does seem more er em violent with engine amid ships though

[jaykay]

Oh yes I like 31f and 34r but I am not racing the car

[stephen wilson]

I find this interesting, because I was just complaining on another thread about how my 987 Doesn't display any trailing-throttle oversteer. I actually find it a to be a helpful trait, and about the only thing that will kill mid-corner understeer. Is this another difference between a 986 and 987?

The "snap" oversteer is partially due to the mid-engine design, more rear weight & grip due to wider rear tires, and a lower MOI, but I think the biggest reason is the short wheelbase.

[jaykay]

Originally Posted by stephen wilson

I find this interesting, because I was just complaining on another thread about how my 987 Doesn't display any trailing-throttle oversteer. I actually find it a to be a helpful trait, and about the only thing that will kill mid-corner understeer. Is this another difference between a 986 and 987?

The "snap" oversteer is partially due to the mid-engine design, more rear weight & grip due to wider rear tires, and a lower MOI, but I think the biggest reason is the short wheelbase.

I was considering a constant throttle position and then moving to more open as in corner exit. Perhaps you are speaking to the corner entry phase. I too in the 986 can and do get understeer and have not yet properly reliably learned to correct as you have described. I am a beginner. While accelerating in a sweeper, I have noticed the car twich nose out...to understeer when lifting for traffic. It is really pronounced as if the body is flexing....and I can't feel that the rears were sliding, or can't believe it. Would it have something to do with my differential. If I get a chance I will try going back on the throttle to get the car pointed accurately

[stephen wilson]

Sorry Jay, I was more responding to the OP comments than yours. I agree that I can induce oversteer late in the corner with lots of throttle in 2nd gear. You have to be quick with your counter-steering, and straigthen the wheel quickly, or you'll get into a "tank-slapper". With neutral to mild throttle application there is a definate push, lifting off the throttle ( for traffic ) will tend to lessen this push.

[thstone]

With a mid-engine car where the balance is very close to 50/50, the tire pressures would normally be about equal to create balanced handling (neither understeer or oversteer).

However, very few cars are sold with balanced handling - most cars have understeer to avoid an average driver getting into a situation that they can't control very well (oversteer).

Thus, the 29/36 recommended pressure to add a bit of understeer to the handling.

[stephen wilson]

There are many things that affect the press. choices for a manufacturer, ride comfort, tire "responsiveness", weight distribution, heat build-up at sustained high speeds, and possibly the most important, tire load ratings. They have to err on the safe side of a vehicle loaded to maximun GVWR. The Box is already rear heavy, and driver/passenger weight would seem to be biased to the rear also.
Just my WAG, opinions vary!

[jaykay]

Originally Posted by stephen wilson

Sorry Jay, I was more responding to the OP comments than yours. I agree that I can induce oversteer late in the corner with lots of throttle in 2nd gear. You have to be quick with your counter-steering, and straigthen the wheel quickly, or you'll get into a "tank-slapper". With neutral to mild throttle application there is a definate push, lifting off the throttle ( for traffic ) will tend to lessen this push.

No worries we are just talking....I am just trying to learn proper car control. I have experienced a progressively more violent tank slapper just as you describe in the wet. It was a lot of work with no way out...had to let it spin and lockup once I thought I was going in straight line

[stephen wilson]

Yeah, the Boxster can be hard to "catch" smoothly once you get it sideways. It just takes practice, and trial and error to figure it out! (hopefully without hitting anything) The key is to start straightening the wheel before the car is actually pulling back in line. I forget which author calls it "the pause" in a slide, which tells you it's about to snap back. Hard to explain in text! Good luck.

[jaykay]

Originally Posted by stephen wilson

Yeah, the Boxster can be hard to "catch" smoothly once you get it sideways. It just takes practice, and trial and error to figure it out! (hopefully without hitting anything) The key is to start straightening the wheel before the car is actually pulling back in line. I forget which author calls it "the pause" in a slide, which tells you it's about to snap back. Hard to explain in text! Good luck.

Yes! This makes total sense! I will look for the "pause". It sounds like I am holding opposite lock too long......very helpful thanks!

I will try this on the skid pad next month

[Johnny Danger]

32 front / 36 rear works well of me .