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Old 04-27-2011, 01:55 AM   #1
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Rear End Instability?

Hi, I bought my Boxster in January this year. It's a 2001 986 with 43K miles on the clock. I had a health check done at JMG Porsche Specialists in Christchurch and they found no major problems. As my confidence has grown I've begun to push the car harder and generally it performs and handles very well. however, I recently had a somewhat 'cheek clenching' moment when, whilst changing into top at around 115mph on what I would consider to be a relatively gentle curve, the back end suddenly felt very twitchy. It's difficult to say exactly what was going on but I think that it was beginning to loose traction and understeer. The weather conditions were warm and dry and the road surface was good. I gently backed off to a neutral throttle and it settled down again.

I know that it's difficult to tell from this subjective description, but I would like to find out whether this behaviour can be expected, or if I need to look at tyres/alignment. Tyres are Michelin Pilot's all round with about 4mm left at the rear.

I suppose I'm a bit worried now about the predictability of the cars handling and just how much warning I can expect when reaching the limit. Any comments welcome
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Old 04-27-2011, 02:19 AM   #2
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I find that my 2001 box is very sensitive to the alignment setup. I must have aligned it at least 4-5 times.... I found that each setup gave the car a different character (was varying the difference between front and rear camber). WIth almost similar negative camber front and rear, the rear was absolutely ready to swing out, especially on sweeping bends at higher highway speeds... (but definitely not understeer).

But then the suspension components esp the wishbone/control arms bushings are common failure items too.

Last edited by shlim8; 04-27-2011 at 02:22 AM.
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Old 04-27-2011, 02:52 AM   #3
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My biggest factor personally when it came to stability of the rear of the car was tires. If the tires are a bit old or hard it will degrade the stability of the car immensely. However, if you are trying to dial out some of the understeer try to put more negative camber up front and flatter out back. Also you could get a technobar from pedro, at pedrosgarage.com. from what I understand is it ties the bottom of the suspension together to keep it from squating in the turns giving you better traction.
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Old 04-27-2011, 04:10 AM   #4
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Red face

Thanks for the feedback so far. BTW I meant to say that it felt like it was going into oversteer, NOT understeer
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Old 04-27-2011, 05:23 AM   #5
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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?
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Old 04-27-2011, 05:41 AM   #6
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I have felt a tendency to snap oversteer since I put Hankook EVO tires on the rear of my Boxster S. They are softer than the normal Michelin Pilots and have a greater tendency to get twitchy at higher speeds. It was very disconcerting the first time it happened. Neither Michelin Pilots or Yokohama S Drives we like that but the EVOs are quite twitchy.

I have read numerous articles about GT40s and their dislike of soft tires and tendency to snap oversteer. Since the Boxster is a mid engine car as well, it would follow that it would suffer the same problem
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Old 04-27-2011, 05:56 AM   #7
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Quote:
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?!
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Old 04-27-2011, 06:01 AM   #8
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Quote:
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.
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Old 04-27-2011, 06:24 AM   #9
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tire pressure

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?
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Old 04-27-2011, 06:30 AM   #10
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Tire pressures

Quote:
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?
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Old 04-27-2011, 06:44 AM   #11
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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.
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Old 04-27-2011, 06:57 AM   #12
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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.
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Old 04-27-2011, 07:44 AM   #13
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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.
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Old 04-27-2011, 08:03 AM   #14
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Quote:
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?
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Old 04-27-2011, 08:24 AM   #15
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It could mask most of it to average driver.
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Old 04-27-2011, 09:28 AM   #16
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How every type of sports become easy to you?? Is there anything special...???...
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Old 04-27-2011, 09:29 AM   #17
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+1 ....and try 30 psi upfront and 34 in the rears to bring the back end back under control ...works for me anyway
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Old 04-27-2011, 10:54 AM   #18
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Quote:
Originally Posted by stevenfrank38
How every type of sports become easy to you?? Is there anything special...???...
Hi Stevenfrank, sorry - don't understand: What's the question?
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Old 04-27-2011, 11:34 AM   #19
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methinks mr. frank is a robot, randomly posting mildly relevant gibberish on websites in an automated effort to promote the links in his sig.
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Old 04-27-2011, 12:50 PM   #20
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methinks mr. frank is a robot, randomly posting mildly relevant gibberish on websites in an automated effort to promote the links in his sig.
Ahh, I see. Thanks. I'm surprised by my own naivety.
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