Rear End Instability?

[Timbo2]

Originally Posted by Flavor 987S

Could be a bunch of things:

What are your tire pressures?
What is the production date of the tires?
Are all 4 tires a matched set?
Are the rears mounted in the right direction of travel?
Are you using too much tire dressing?

Hi Flavor987S. Tyre (English spelling!!!) pressures are 37 rear 30 front. Not sure how I can check manufacture date, but the rears were fitted Oct 2009 (so I guess no - not a matched set). All correct rotation direction. Tyre Dressing!! - what kind of a guy do you take me for?!

[Flavor 987S]

Originally Posted by Timbo2

Hi Flavor987S. Tyre (English spelling!!!) pressures are 37 rear 30 front. Not sure how I can check manufacture date, but the rears were fitted Oct 2009 (so I guess no - not a matched set). All correct rotation direction. Tyre Dressing!! - what kind of a guy do you take me for?!

Your front tire pressures are too low. All 4 tires need to match. Go to Tire Rack's web site, and read about how to determine your tire's manufacturer date. Don't use tires more than 5 years old.

[TWODAN]

wow. i looked on my door jamb at the beginning of the season for the correct tire pressures. mine read 29 front, 36 rears. they were all at 36psi, so i dropped the front two. i believe i have the original conti's on the car with 9K on the odo. who's correct?

[Timbo2]

Originally Posted by Flavor 987S

Your front tire pressures are too low. All 4 tires need to match. Go to Tire Rack's web site, and read about how to determine your tire's manufacturer date. Don't use tires more than 5 years old.

Manufacture's spec is F29/R36. Where are you getting your info, or is this based on experience?

[Beelzy]

I think it could be the tires. I have Michelin rears that I recently had installed, and
they tend to "break loose" more readily than the BF Goodrich tires I previously ran.

[thstone]

Tires should start at factory pressure, 29F/36R. If you're driving hard, then the pressures will go up and the tires will start to slide around rather than grip like they usually do so you might need to bleed off a little air as you go harder. My PS2's feel like they are riding on grease when the pressures get too high and I am pushing the car hard.

Make sure the alignment is spot on.

Have all of the suspension re-checked and tell the tech what happened. A health check is a nothing compared to a performance handling check. Anything worn will expose itself at 115mph in a corner.

Shifting in a corner at 115mph will definitely upset the balance. Get your shifting done before the corner.

[Pat]

I think thstone's post immediately above is the best response.
I've found the factory tire pressure recommendation is not my favorite. I run 34 front, 36 rear at whatever the hot pressure is in my current conditions.
I've also found the rear end gets a bit light at higher speeds. This became noticeable to me around 140 mph in a straight line. Turning while shifting would likely make this more noticeable at lower speeds.

[Timbo2]

Originally Posted by thstone

Tires should start at factory pressure, 29F/36R. If you're driving hard, then the pressures will go up and the tires will start to slide around rather than grip like they usually do so you might need to bleed off a little air as you go harder. My PS2's feel like they are riding on grease when the pressures get too high and I am pushing the car hard.

Make sure the alignment is spot on.

Have all of the suspension re-checked and tell the tech what happened. A health check is a nothing compared to a performance handling check. Anything worn will expose itself at 115mph in a corner.

Shifting in a corner at 115mph will definitely upset the balance. Get your shifting done before the corner.

Sounds like good advice. I'm certainly willing to accept that the driver could be partially at fault. How much do you think that this kind of thing would be masked by ESP in later model Boxsters?

[Pat]

It could mask most of it to average driver.

[jaykay]

+1 ....and try 30 psi upfront and 34 in the rears to bring the back end back under control ...works for me anyway

[Topless]

Originally Posted by thstone

Shifting in a corner at 115mph will definitely upset the balance. Get your shifting done before the corner.

Bingo!
thstone has been getting some good training. When you are going 115 even in a gentle curve the tires are pretty heavily loaded. It's a physics thing. Lifting the the throttle and shifting mid corner is a good way to experience the thrill of going backwards out in the weeds at 115. If this does not make any sense to you, a spring DE or performance driving school might be a really good idea.

[Timbo2]

Originally Posted by Topless

Bingo!
thstone has been getting some good training. When you are going 115 even in a gentle curve the tires are pretty heavily loaded. It's a physics thing. Lifting the the throttle and shifting mid corner is a good way to experience the thrill of going backwards out in the weeds at 115. If this does not make any sense to you, a spring DE or performance driving school might be a really good idea.

Thanks for the advice. I thought that now I have a Porsche I'd be invincible, but in fact I think I need to be more careful than before. Because it does everything so well it's easy to forget exactly what you're asking of it. (But I guess you new that already

[Flavor 987S]

Originally Posted by Timbo2

Manufacture's spec is F29/R36. Where are you getting your info, or is this based on experience?

I guess I am wrong about OEM tire pressures for an 986. On my 987S with 19" inch wheels, OEM spec's are 34F/37R. I tend to run about 1 PSI lower than spec. Michellin PS2's.

Did not realize front tires pressures are so low (30 PSI) on a 986.

[Gilles]

Originally Posted by Flavor 987S

I guess I am wrong about OEM tire pressures for an 986. On my 987S with 19" inch wheels, OEM spec's are 34F/37R. I tend to run about 1 PSI lower than spec. Michellin PS2's.
Did not realize front tires pressures are so low (30 PSI) on a 986.

Flavor, also on the 987S w/19's
I found out that running about -1 on the rears (34/36) the tires wear more evenly, otherwise the rears will wear too quickly on the inside.

.

[Flavor 987S]

Originally Posted by Gilles

Flavor, also on the 987S w/19's
I found out that running about -1 on the rears (34/36) the tires wear more evenly, otherwise the rears will wear too quickly on the inside.

.

I've gotten great tire wear on the Boxster. My first set of OEM Pilot Sports lasted 20,000 miles. The PS2's look to do about the same. Now, the 911 Turbo eats tires. I'm lucky to get 12,000 miles. Same driving style....lots of highway with spirited sprints on country roads. Not complaining.