The Kumho tires are quiet but squirmy
 

The 2000 S I recently acquired had 4 fairly new tires - less than 6 months old, but the steering wheel shook and balancing didn't fix it. The left front was cupped moderately on the inside so I decided to replace both fronts, which were directional tires in the Formula 1 rain tire tread pattern. My experience is that those style tires develop uneven wear pretty quickly and they get loud pretty quickly too. So I looked at all 225/40/18 tires at Tire Rack trying to find some asymmetric tires rather than directional tires. At least I would be able to rotate them from side to side to even out the wear and try to keep them from getting nasty loud. This is the tire I picked, a Kuhmo.

http://www.tirerack.com/images/tires...plat_ci2_l.jpg

The tires are nice and quiet but I've lost some of the steering responsiveness, which is a pretty big deal since the Boxster is about chassis communication. Now the car understeers more than I like. It's almost like I need to get some squirmy tires for the rear to match the front, lol.

What kind of tire pressure adjustments reduce understeer? I'm running 29/36 right now.

Are the 4 tires matched? Never mix tires on a performance sports car.

How many miles on the new tires? It takes about 200-500 miles to scrub off the mold release tire manufactures use in production, this was covered in the product information you received with the new tires. Did you read it?

I suspect your tire pressures are low. Your cold pressures should be more like 32 front and 36 rear.

Were the tires road forced balanced? Alignment completed?

About a year ago I needed to change the tires that came on the car when I bought it. My tire dealer talked me into Continental DWS. Great traction in all conditions but terrible, I mean terrible for what a Boxster can do. Understeer like crazy and no high speed stability. Fortuneatly they came with a love them or leave them warranty. Well after some intensive on line research, I replaced them after a few months with Kuhmo ASX. Let me tell you about that tire. WOW !!!! They do everything a Boxster can do. Or at least everything I can make a Boxster do. Yes they did have that wiggle squirmy feeling for a short time at first. That went away quickly.

As much as I hate to buy off shore tires ( Go figure that one out. I own 2 Porshes and several motorcycles made off shore.) I realy hate and won't buy Chinese and I'm not to hot about Korean stuff either, but I have to admit the Kuhmo ASX to me is an awsome tire.

Sounds to me like the
 

cupping might have been an alignment issue, did you have that addressed by a Porsche-knowledgeable shop?

Haven't done an alignment yet. I have some hope these tires might improve a little as they wear. Skinny tread blocks and full depth tread can create squirm. I think racers shave most of the tread off tires to improve handling.

Still looking for tire pressure advice to reduce understeer.

Ok Dale, we can probably help you sort this out. Start at the beginning. Are these the tires you just put on the front?
http://www.tirerack.com/tires/tires.jsp?tireMake=Kumho&tireModel=Ecsta+LX+Platin um&partnum=245WR7ELX&vehicleSearch=false&fromCompa re1=yes

What tires are on the rear (exact make model and size)?

As long as the tires are very similar in type, temperature, traction, and wear ratings, even different brands, they should work together pretty well. If there is a big mismatch in tire compound no amount of air pressure or alignment changes will get them to work well together.

I disagree. This is horrible advice. Especially for a performance sports car and a Porsche.

Dale, what are the code dates on the front tires? What about the dates on the rears? This will tell you a lot too.

Tire compounds harden with age. What you have now is a newer tire in the front with rubber as soft as it gets from the factory and tire on the rear which has probably hardened over the years. Not very conducive to balanced handling, besides the differences in how the tires are designed.

I understand everybody's financial situation is different, but mixing tires, in my experience, isn't going to get you where you want to go with the Boxster.

The rear tires are part of a matched set of very inexpensive ($300 for all 4) Sumitomo tires the previous owner put on in July 2010. I understand the risk vs reward question about mismatched tires and was willing to accept it. The new tires are listed as "grand touring" tires rather than max performance and I was willing to accept that too. The most I'm out if I junk them is $230 so it's not the end of the world.

Seeing as how I'm not getting much tire pressure advice I'll try out some different pressures and report back. I like the nice quiet ride and the ablility to rotate tires from side to side. No so good for responsiveness though.

Sorry Flav,
I guess we will just have to agree to disagree. Been doing this for 5 years, street and track. Michelin PS2, Mich Pilot Sport, Sumitomo, Goodyear, Nitto, and Hankook. I currently have 5 sets of tires in various states of shred and 3 sets of wheels.

Dale,
I am sad to say that tire pressure advice wont fix this. Grand Touring All season tires are just far too different from Max performance Sumis to ever be a good match. Your car will feel like it is on banana peels until you resolve this. If you like the Grand Touring A/S tires, put em on all four corners. You will get a nice quiet comfortable ride good in all weather conditions and sacrifice some grip. Maybe a good tradeoff depending on your driving style.

I think that you're not getting much tire pressure advice because adjusting tire pressures can't possibly solve the problems that you have described.

Here is a guarenteed solution to every problem you mentioned:

Step 1: Throw away all four of the cheap Grand Touring tires.

Step 2: Go buy a set of four Michelin PS2's and have them installed at a reputable tire installer. You simply cannot go wrong with these tires.

Step 3: If you want to reduce a little of the inherrent understeer that is designed into the Boxster, then go slightly wider and slightly lower in profile in the front (245/35/18) to get a little better front bite. You need to lower the profile (while increasing the width) to keep the overall diameter nearly the same as the stock tire to avoid problems with your speedo and ABS. Stay stock in the rear (if you increase width in the front and rear then you won't address any of the stock understeer).

Case closed. Problems solved. :)

My 2 cents
when I put new rubber on the rear (M-pilots)
I messed with the tire pressures for about the next week
Why? A 2 pound difference makes a large enough difference
to the way a tire carcass behave...that u should not ignore the/feedback
that the tire is giving you....this goes for any brand of tire
I started @ 31 psi then 33 then 35 (rear)
noteable diffence in how the tire behaved (rolling over/ sidewall flex/ or too harsh/stiff)
and what.sound it was making (tire squeal/chirp)
pick the same road/turn (cloverleaf) & drive it a few times with different tire pressures

I settled @ 34 in the rear
and 33 in the front
your results will b different BUT TRY IT
I've heard good things about the tire ur using
But the person telling me about this tire was using it on a 240Z
and loved how it made the car handle and behave when pushed

Establishing the right tire pressure helps u get the max out
of the tire which means u get the car to handle like it should
And to me, handling is the utmost important aspect of this car
especially when I tackle cloverleafs & on/off ramps.

I remember someone on this forum stating that anyone can go fast
in a straight line, but turns make drivers better (something like that)
Ok...my 4 cents
Chris

Thanks for laying out how you experimented with the pressures. I'm running 29/36 at the moment and I'll try going up a few pounds on the front (new Kuhmo) tires.

The PS2's are too expensive for me. I might go with Hankook Ventus, which is what i have on my Corvette, if I decide the Kuhmos aren't gong to cut it after trying some different pressures. The Ventus tires really work well on the C6 but I have to carry an air compressor and tire plugs since they're not run flats. Good trade off for a better ride though.