dallascajun

I've read through some previous threads on this, and just went through my own little episode, so thought I'd post my experience and some of the knowledge gained:

I had some new wheels installed on my car last week. Since the rears were gone but fronts still OK, I decided to just put some Hankook V12s on the rear and leave the PZs on the front, and then replace all 4 the next time tires were needed.

So after getting the car back, it took about a 1/2 mile to figure out something was wrong. At first I was worried the center lock nuts weren't torqued enough, but then realized I would feel the wheels shaking violently. It then occurred to me that the new rear tires were the cause.

When I got home I noticed that the tire pressure across the corners was 5-7 lbs high, and I thought (hoped) that might be the problem, so I brought them all back to 'normal' pressure. WRONG...that made it WORSE. More to come on that in a bit.

This morning I was driving on one of the Dallas highways and got to the big left hand turn that's a slight double-radius turn that catches many people off-guard and requires some constant steering input, usually at 70mph+. I swore my car was going to hit a wall. The initial steering input gave me quick response, but then there was a 1/2 second lag when the rear end would start it's weight shift, which led to oversteer, and my car almost facing the wall. It took fair amount of small corrections back and forth to get my car where it felt stable, because this motion caused a rather strong oscillation of movement from left to right.

So that episode led me straight to Boardwalk, and I mean STRAIGHT there.

After talking to John Gladwell (Service Tech extraordinaire), several things became quite clear:

1) The Hankook V12 has a MUCH softer sidewall that the Pirelli's, or other factory tire. It just doesn't take the weight shifts very well. He said that tire is decent for people that are looking for a softer, cruising ride and works ok with 996s due to the reduced power in those cars.
2) The P-Zeros up front have much better grip, so steering input through the front PZ would be sharper than the Hankooks can handle. Given the Hankooks weaker grip & sidewall, they would just amplify the difference in grip and the rear would be relatively 'floaty'. This is one of the most basic reasons as to why the front/rear tires should match: equal grip/sidewall limits.
3) After getting my car back with the new wheels, I noticed the air pressure was 7-8lbs high all around, with the TPMS readout verifying. So, I brought everything back to 'normal' when I reduced the air pressure. It turns out that reducing air pressure makes it worse because that change basically makes the sidewalls that much 'softer'. He said it really doesn't affect the contact patch at all, and has negative impacts on tire wear.
4) I knew my Panamera had 2 TPMS settings for Loads (Full/Half), but didn't realize the 997 also had this. When we changed the TPMS settings in the computer to Full, it now showed my tires as 5-7 lbs LOW. Basically, these two settings affect the warning systems in the car to tell you when to add air. By adding air pressure to the tires, it will 'strengthen' the sidewall for this need.

Next Step: Michelin Pilot Super Sports going on tomorrow/saturday on all 4 corners (as soon as BW receives them). I considered just putting 2 more PZs on the rears, but closer inspection of the fronts told us that they really only have a few 1000 miles left on them, and the PZs are known to degrade quickly when they get close to the end. The MPSS has a better tread wear rating, better grip, makes less road noise and maintains it's tread better over the last 1/4 of tread use than the PZ (per John).

Lessons Learned:
1) The Hankook V12 isn't a bad tire across the board, but not the best for this application (305/30). Of course, Any 305/30 (or similar size) is going to be on a sports car, so not sure why such a size is manufactured if it's not quite appropriate for similar applications.
2) Do NOT mix tire types from front to rear due to the relative difference in grip/sidewall strength. After discussing it with John and thinking it through, the simple logic makes a ton of sense.
3) The higher end of recommended tire pressure will not have negative impact on daily (spirited) driving, but will help reduce tread wear.

Edgy01

Occasionally people do this against the guidance of others--and eventually they will learn. NEVER MIX TIRES. The coefficient of friction, and other factors vary from tire type and manufacturer to type and manufacturer. The systems in the cars today cannot sort out the feedback they are receiving. This is particularly a bigger issue for the AWD cars, like all Carrera 4s and Turbos.

Dr. No

Glad you didn't wreck, and thanks for sharing your experience. Everyone gets tempted to save a few bucks now and then, so it's good to be reminded of the hidden costs!

Edgy01

We had a guy in the local PCA try and do something like that. The tire dealer was a smart guy, and told him it would lead to trouble. The first road trip he took his turbo was in trouble. If he tried to go over about 60-65 mph the thing grew more and more unstable. Rather than always being in front and leading the pack, he had to drop out and follow meekly along and was lucky to keep up.

Ultimately, he went back to his tire dealer, tail between his legs, and the tire guy took back the tires and sorted things out for him. This can be extremely dangerous. If in doubt, go review your owner's manual again.

stronbl

OP - did you purchase the new tires from a Porsche dealer or some other tire shop? The reason I ask is that whenever I have purchased tires from Porsche dealer, they have always verbally warned me about mixing any tires, even from the same manufacture and even same model. As an example, I had to buy 2 new Pilot Sports for the rear and had the original PS's on the front. My service guy actually made a note in the my car database that he informed me I should buy 4 new tires but that I declined and purchased only 2. Since these were identical tires and wear on the fronts was very low (only an N1 vs N2) I figured I was ok. 5 track days and 12 heat cycles later, no issues. When I switch to MSS's I'll go for 4 new ones.

Thanks for your post, it is a good reminder on the subject.

alexb76

Glad you didn't wreck the car. Expensive lesson indeed, but a less nonetheless. MPSS, or S04 tires are the ones to get if in the market now. RE-11 if you track. Pzero (not rosso) is also a great tire.

Kinda good that we got an *actual* report of someone trying to mix front and rears as for SOME REASON folks have asked this many times before and no matter what we've all said, some have tried to justify doing so.

San Rensho

Never mix tires, and the best advice is don't even replace the rears with the same tires. Get symmetric tires, rotate the rears by remounting them left to right when the inevitably start to wear on the insides to extend the mileage, and just get 4 new tires when the rears wear out.

And why get extremely expensive Michelins or B Stones when Sumi's or Hankooks are 1/4 the price and have almost indistinguishable performance on the street?

dallascajun

I did get them from the dealer, but no warnings from them beforehand. BUT, when I spoke with the technician today, he made a comment that he wished he would have talked to me first. Apparently it's one of those things that most P-people know about, but few know the details or reasoning behind the mixing of tires.

The way I see it is that it's much better that I learned this lesson with my wallet than with an accident and subsequent injury to myself and others.

Dr. No

They should have known better.

jason74

I'm not sure that your initial problem of violent shaking could be attributed to sidewall unless tire was grossly out of balance or defective. Also, new tires come with the very slick coating that needs to be worn off before pushing limits. Could either of these help to explain the dramatic symptoms you experienced?

No HTwo O

Silly on many levels. Silly to mix tires. And silly again to push new tires until scrubed-in and the mold release worn off. Glad Dallascajun is A-OK and can write about it.

user 72902

+1 and in addition to mixing tires I 'm shocked a Porsche dealer would sell non-N rated tires.

No HTwo O

I've had non N rated tires (Kumho) installed by the dealership before. No problem. And I don't infer the OP had his tires installed by a dealership, but maybe he did. I did not read (scan) it that way.

alexb76

BTW, if you haven't already bought the MPSS, check out Bridgestone S04 (replacement for RE050), it's a fantastic tire and value. MPSS or RE11 if you track.

user 72902

Understood. I put Kumohs on my Cayenne S and I like them much better than the N rated Continentals (2 sets) and the Cayenne is very sensitive to tires. They are holding up better, quieter, and handle as well or better. I've used the various non-N Bridgestone Pole Positions for years on my Carreras and they have been great so I'm not an advocate for the N rating as being superior. I decided to give Michelin a try after a bad set of Bridgestones on my Mercedes; can't remember the model but they slid allover the road whenever it rained. I've had two sets of Sumitomo on my SLK and they have performed well.