kennyboy

Just bought 2 new pzero assymetrics for my C2 to replace the badly worn ones already on. The replacements are exactly the same tyre as already on.

Took the car out of my favourite stretch immediately hoping to enjoy some extra confidence on the twistys. But what I got was the complete opposite!

The car now feels all over the place at the back end - not loosing grip but feels like jelly, can't 'feel' the back of the car like before and if anything feels like it's gonna take-off at the back on high speed bends, feels wallowy - completely removes any confidence of cornering above 80mph or cornering under throttle, very disconcerting!

I'm hoping the issue is down to one or both of these:
1. The guy at the tyre garage says I need to go back to get the nuts tightened again after 100 miles - I've done about 80 now.
2. The tyres need a lot of bedding in.

I started at 34psi as recommended by guy at tyre place then went up to 36 as porsche recommend and no improvement.

Has anyone experienced similar? - as it stands at the moment I daren't do anything on the road!

Maurice - I've posted this seperate to the kinematic discussion we had as I'm thinking that whether that needs doing or not, the new tyres have still had a dramatic affect on handling on their own. :-(

Steve Weiner-Rennsport Systems

Maurice:

What you are feeling is totally normal and precisely why we NEVER replace 2 tires on a 911.

I noticed this same phenomenon 25+ years ago and have always replaced all 4 tires when the rears become worn.

I hate to tell you this but,.............................replace the fronts and you will be happy. Find someone to buy the front tires you currently have.

kennyboy

Thanks Steve, looks like I better get back to the bank!

dbelleperc

You also must know that you have to break in new tires.

Tires are comprised of many layers of rubber, steel and fabric. Due to these different components, your new tires require a "break-in" period to ensure that they deliver their normal ride quality and maximum performance. As tires are cured, a "release lubricant" is applied to prevent them from sticking in their mold. Some of the lubricant stays on the surface of your tires, reducing traction until it is worn away. Five hundred miles of easy acceleration, cornering and braking will allow the mold release lubricant to wear off, allowing the other tire components to begin working together.It is also important to note that your old tires probably had very little tread depth remaining when you felt it was time to replace them. As any autocrosser or racer who has tread rubber shaved off of his tires will tell you "low tread depth tires respond quicker." Don't be surprised if your new tires are a little slower to respond (even if you use the exact same tire as before). Their new, full depth brings with it a little more tread squirm until they wear down.

kennyboy

Thanks David

Interesting insight - everything you say makes sense and does indeed feel like the situation I have. I think 'tread squirm' describes what I am experiencing perfectly - and hopefully some lubricant is still hanging around because if not then 'tread squirm' is causing me to get from a to b around 20mph slower!

A question for Steve and David then - given Dave's comments, do you think I still should get the fronts replaced or perhaps (preferred) I should wait until the tyres are broken in properly before splashing out any cash?

dbelleperc

I'm also saying that many Porsche 911 owners have replaced the rears twice or three times before the fronts, without thinking twice before.

Christer

wait until the rears are driven in, then if the problem is still there - replace the fronts...you should need around 150 miles or so before you make a decision...IMHO

Chuck A.

Another point (maybe an obvious one) is to check the tire pressure when the tires are cold. If they have been driven when you check them, they will read higher because the air inside them will have expanded due to heat.

Monique

The tire pressures you mention are much lower than what is recommended for my car.

Front 2.5 bars, rears 3.0 bars... about 36 psi fromt and 45 psi rears.

True about needing to wear in your new tires.

993Maineiac

No, for the NB cars Porsche recommends 36lbs on all four.

Al

nile13

David is corect - give it 200-300 of very mild driving to scrub off the mold release compound. Report afterwards. Be careful in the meantime, they'll bite you when you least expect it.

Monique

The RS is NB. Pressures are recommended as 36 front (225/40/18)and 45 rear (265/35/18). No kidding.

mike cap

Kenny,

I replaced my rear tires last winter after 8,000 miles of service over two years. 18" on my 1997 C2. They are Bridgestone S02's. The fronts were like new (maybe 25% worn), so I replaced just the rears. No problems at all you described. I would give it a little time or check for another problem. Alignment?

Tire pressures for a 993 C2, 18" are 36 psi front and 44 psi rear. Even though there has been volumes of discussion on this matter, I use the recommended pressures.

STLPCA

Different pressure recs for 17" & 18" tires - that's the confusion. However, as discussed in many threads, the factory recs are not ideal for optimum handling.

mike cap

Dan,
What FR and RR pressures do you use on the street, for 18"?