PCA SDR

Not sure if this post belongs under Technical, but anyways...

I recently changed the rear tires only on my Boxster. Is it ok just to change the rears without changing the fronts? Will this cause problems?

Also, has anyone noticed anything strange driving on I-5 North in the area around (before and after) the Palomar Airport Road exit? I've driven that area in both Porsche and non-Porsche cars, and it feels strange - like the surface is unstable and the car is constantly slipping a little bit (but not out of control or anything.) If you look at the surface of the highway, there's a lot of waviness to it, so that might be some explanation. Just wondering if anyone else has had this experience.

I have noticed that same effect in that area - you're not crazy!

It's okay to just change the rear tires (both of them), but it is
best to keep the same brand and model as on the front. The
ABS will be okay with any tire combination, but the car's handling
will not be as good with different models and aspecially brands.

Hope this helps,

Paul.

A bigger question is why you needed to change only the rears. I have gone through at least six (seven? eight?) sets of tires on my Boxster, and they have all worn very evenly on all four tires.

I do notice the wear first on the rears when on the track, as the car gets noticeably "loose" when the rears wear down into the harder layers of the tire (even if there is about 30% tread left). However, the fronts have worn down the same amount, so replacing only the rears at this point would give me a car with a LOT more understeer (something most Boxsters do not need).

If you are actually getting significantly different tread wear between the fronts and rears, maybe a change to camber and/or toe-in on the rears would help.

I have my rears set at maximum camber (around -2 degrees) and almost zero toe-in (you want a small amount to avoid some "twitchiness" on the highway). My front tires are at about -0.5 degrees camber, and zero toe-in. These settings have consistently produced very even tread wear on all four tires.

And yes - there are areas of our highways where pretty much any wide+soft tire will feel "twitchy". This phenomenon is called "groove wander". Rain grooves cut into the road surface are the culprit. Note that your tires are always compressing and decompressing where they contact the road surface - if the grooves are angled at all with respect to the line of travel, wide and soft tires will "squirm" to one side or the other as they encounter the grooves during this compression and decompression. The wider and softer the tires, the more pronounced the effect.

Kim,
You get even tire wear because you are using the front tires more than the average driver, and having a good time at it as well.

On a street driven 993, 996, or Boxster you are lucky to only have to replace 2 sets of rears to one set of fronts.

Certain driving styles will also increase rear tire wear.

While everything Kim said is completely true for his usage , the fact he has gone through 6, 7 8? sets of tires on a 2000 Boxster, shows his usage isn't 'normal street driving'

I found on my Boxster S (mostly street usage) the rears did wear quicker than the fronts and if I hadn't hated the Pirelli tires, may have just changed the rears as well. If you really know what you are doing, you can adjust the handling characteristics of the car this way.

What kind of usage do you have for your car, street, track a mix? and how many miles did you get out of the tires and what kind were they?

A good 4-wheel alignment probably isn't a bad idea as Porsche doesn't do the best job at the factory at even getting them even, let alone the proper alignment for agressive driving like Kim does. A street car wouldn't need quite so agressive a setup. Talk to the shop to get an appropriate alignment if you feel you need one to improve handling or tire wear.

For a normal street car, you are probably fine with about any similar rear tire and alignment, only at the limits of adhession will you experience changed or possibly strange handling which hopefully doesn't happen on the streets.

Jad wrote:What kind of usage do you have for your car, street, track a mix? and how many miles did you get out of the tires and what kind were they?

If you were asking me, Jad, the answer(s) would be: about 62,000 miles so far. I have gotten between 7000 and 8500 miles per set of tires usually - I "could" get 10,000 or a bit more, but at around 30% remaining tread, the tires get hard and greasy, so I don't try to get every mile out of a set.

My first set (Continentals that came with the car) lasted around 11,000, but I did hardly any track work with those. I then went through 2 sets of Bridgestone S02A's - got around 8000-9000 out of those. I then switched to BFG T/A-KDs and am on at least my 4th set today, getting between 7000 and 8500 miles per set.

Until December, 2004, the Boxster was my daily driver - I put around 13-15,000 miles a year, plus 7-10 Autocrosses per year. Since December 2004, my usual commute is via air between San Diego and San Jose, so the Boxster is a hangar queen between Autocrosses - probably now doing around 3-5000 miles per year, mostly weekend driving. I expect to get fewer total miles out of my current set of tires, given the ratio of Autocross events to street driving now. (I did point out to my wife that a set of tires should last a year now, instead of six months. )

I recall the Continentals wore somewhat unevenly, but since then the Bridgestones and BFGs all wore evenly. I had the alignment changed to the current settings part way through the first set of Bridgestones.

Thanks for the replies - they are all enormously helpful.

[quote="Jad"]What kind of usage do you have for your car, street, track a mix? and how many miles did you get out of the tires and what kind were they?[/quote]

My driving is strictly street. I had about 24,000 miles on the car and the tires were the ones that came with the car, Michelin Pilot Sport 17 inch.

I am taking my car to Black Forest for diagnosis. When I described the problem to a guy there, he also suggested the 4 wheel alignment.

On a related topic, how reliable is a Boxster after it has been in an accident? A few months ago, I spun out on the highway and hit the median - the car had to be towed and the front right section completely taken apart and repaired. (It was raining, and I believe the worn rear tires had something to do with the accident.)

I only began to notice this "groove wander" effect post-accident, although it is entirely conceivable that it was there before, and that I didn't notice it then but do now because I've become more sensitized to the driving conditions post-accident.

I've been talking to some friends (not Porsche experts like you all, or even car experts) and they believe that because a "high-performance" car like a Porsche is precisely tuned / balanced / calibrated / etc., an impact accident like the one I had will cause more problems in the future in ways that the service shops cannot see now. Therefore, they don't think it's worth keeping. (My car is a 2003.)

Any thoughts or any experiences driving post-accident Boxsters?

This one is right up my alley - unfortunately.

I spun out my Boxster in May 2002 at fairly high speed, slid backwards over a curb and took out a rather large tree. Damage included basically all the rear suspension parts (from the curb), the rear bumper, exhaust, transmission housing (cracked mount point), and miscellaneous related parts.

Pioneer and West Coast Specialties took a long time to get it all back together (almost 11 weeks), but it runs and looks like new.

Since then, I have competed with this car very successfully in our AX events, taking class wins the last 2 years.

With about 62000 miles on it (about 30000 since the accident), my Boxster runs and handles perfectly. No noises, rattles, shimmies, or other indications that anything ever happened - it feels and sounds tighter than any of the new rental cars that I drive every week, and I drive my Boxster pretty aggressively.

One key item - was there any damage to the frame? If not, then your car should be repairable like new - if done by a good Porsche-knowledgeable shop, using the right parts. Pioneer and West Coast Specialties checked mine every which way and determined there was no frame damage or alignment problems before they started to do any repairs.

I am stunned that you got 24000 miles on the tires (of course, I go through withdrawal if I go a month without a track event). One thing I was thinking of from the earlier posts is whether the rear tires begin to wear faster and faster (vs the fronts) when the tread wear gets past a certain point.

It is hard to say how your car is, a proper repair and it should be fine, a cheap fix and it could be messed up.

24k on a set of tires is reasonable and does not indicate any severe problems, but you noticing the wandering after the accident may suggest things changed at that time. Not necessarily a bad thing, they may have just given you more toe in/out than before which causes the car to wander more. Racers do this on purpose to get quicker response, but it makes it uncomfortable on the street.

I think a GOOD 4 wheel alignment to street settings should tell you the condition and hopefully fix any problems.

My car was repaired post-accident at Amato's in Sorrento Valley. The car looks like new and seems to drive fine except this "groove wander" problem. Based on the itemized invoice, the only frame-related work they did was "Frame Rack Setup and Measure" for $150 (total repair bill was about $10k).

As I mentioned before, I'm planning on having the guys at Black Forest take a look at the "groove wander" issue. The problem is that they are not willing to go on a test-drive so that I can show them the exact phenomena that I experience (on the 5 near the Palomar Airport exit) because that would consume a significant amount of one of their guys' time. Which I understand. So my questions are:

a) Do you know of any highway areas near Black Forest (basically by the 163 or 52 or 805 near Clairemont Mesa and Balboa) that also cause "groove wander"?

b) Are there any good Porsche service shops near the 5 near the Palomar Airport exit? (I think Hoehn is near there - any thoughts on them?)

If it only wanders noticeably in certain county locations, I am not sure I would worry about it - with wide, soft tires, groove wander is going to happen if there are noticeable ridges/grooves/etc. on the highway.

If you are experiencing this in various locations - particularly on what looks like fairly smooth highway stretches, then you might want to follow up.

There are several web postings that reference groove wander (I tried Google) and common factors mentioned were (in apparent order of cause/effect):
width/softness/grip of tires - #1 factor in wander
tire balance (if out of balance - more wander)
toe-in (different toe-in settings can affect wander)
camber (ditto)
caster (ditto)
tire wear - one posting claimed worn tires wandered worse than new ones - I don't know if I buy that...

When I had my toe-in changed (to zero toe-in on fronts and tiny amount on rears), I definitely noticed the increased tendency to wander on the highway. However, I was willing to accept that for the sharper "bite" when turning in on the track. If you aren't going to aggressively track the car, going for more toe-in on the fronts (and rears!) may cure or at least reduce the wander effect.

Let us know what you find.

I bought my car at Hoehn. (Aviara Parkway exit off I-5.) Their service has been fine, but my *perception* is that the guys at Pioneer are more into performance setups than at Hoehn. I had Pioneer do the camber and toe-in setups.

Usef:

Every car in my "collection" wanders in the area of I-5 you're talking about: Toyota, Jeep, Dodge, and Porsches.

My review of the area indicated recent highway grooving by Caltrans (subcontractor, actually) to prevent ponding during rains. The grooves are new, inconsistantly cut, and harsh.

I don't think your car is the issue. I think the highway is the issue.

If you're really not sure, go to Hoehn in your Boxter, ask to test drive a Caymen S, put it on the 5 Northbound from Poinsettia to Canon Road, and see for yourself what the difference is from your Boxter and the Caymen S ........ and have a fun little drive in a Caymen while you'rea at it

Just my 2-cents

Dan