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The N Specification is a Porsche system of approved tyres, first introduced in the 1988 Model Year – it means that Porsche have worked with the manufacturer to produce an optimal road tyre for a particular model and wheel size combination, and tested it to their satisfaction at the time.
Whenever this process is performed for an individual set of approved tyres, the N spec number will increase. Often this will coincide with a model change, but can certainly occur within a model's lifetime, and thus a bewildering chart of approved wheel and tyre combinations (for summer and winter use) is produced and published twice a year in Christophorus, the Porsche company magazine, and in TSBs issued to OPCs.
Importantly, Porsche recommendation is matching brand, type and correct N spec ALL ROUND, however you can mix "the same N rated Tyre Brand on different axles Only" See the next post below for more details.
Since the first N rated list was produced subsequently updated TSBs have been issued, updating the list. N rated Tyres are a recommendation and not mandatory for Insurance purposes, all cars need to have 'E' marked tyres.
Technical Service Bulletin Quote [TSB]
If new tyres are to be mounted or the tyres of one axle are to be replaced, tires of the same make, the same type and with the same specification code must always be used on each of the two axles. If tires are replaced on one axle only, the different tread depth from that on the other axle can cause a noticeable change in the familiar handling. This is especially the case if new tires are mounted on the rear axle.
This effect decreases with increasing tire mileage. When replacing a tire on an axle, make sure that the tread depth of the new tire does not differ from that of the other tire by more than 30 %. They are also rather proscriptive on getting a round result when fitting tyres. In order to optimize smoothness of rolling, it is appropriate - and necessary in individual cases - to mount the tire in a certain (favorable) position with respect to the wheel (matching).
Matching (uncontrolled and controlled) is explained below:
Uncontrolled matching: Turning the tire on the wheel by 900 or 1200 if necessary in order to achieve an acceptable value with regard to rolling smoothness (true running, imbalance and weight distribution of balance weights).
Controlled matching: With a balancing machine with matching program. In most cases, this produces an even better result with regard to the rolling smoothness (true running, imbalance and weight distribution of the balance weights) than can be achieved with uncontrolled matching.
UK
Not all manufacturers produce (or did produce) all sizes, and for the UK, winter recommendations are ignored, which are aimed towards countries with mandatory European winter driving regulations.
Manufacturers continue to produce the same sizes in multiple N specs on the latest Porsche cars, although sometimes you have to search hard to get the correct one. Higher (and some lower) N spec tyres will not have been tested on older cars, although will work as well.
Same non-N rated tyre
Sometimes manufacturers produce identical size / models with and without the N rating. The N spec tyre will be different in some way, compound, tread grooves, sidewall construction, whatever, but it may not be apparent.
Porsche did all this testing for performance reasons, to ensure your car handled the way the factory intended, there are good guidelines here to follow. If you change sizes at all, you (are on your own and) need to be aware of the change in the ratio of front to rear rolling diameters, because a significant change will adjust the ABS and/or the 4WD.
The logical calculation of rolling diameter from the tyre section / profile is not accurate in this regard, so be warned, you may need to check with the manufacturer.
The same tyres in different sizes are often at different N specs, which is a totally normal stage of the tyre development process.
Whenever this process is performed for an individual set of approved tyres, the N spec number will increase. Often this will coincide with a model change, but can certainly occur within a model's lifetime, and thus a bewildering chart of approved wheel and tyre combinations (for summer and winter use) is produced and published twice a year in Christophorus, the Porsche company magazine, and in TSBs issued to OPCs.
Importantly, Porsche recommendation is matching brand, type and correct N spec ALL ROUND, however you can mix "the same N rated Tyre Brand on different axles Only" See the next post below for more details.
Since the first N rated list was produced subsequently updated TSBs have been issued, updating the list. N rated Tyres are a recommendation and not mandatory for Insurance purposes, all cars need to have 'E' marked tyres.
Technical Service Bulletin Quote [TSB]
If new tyres are to be mounted or the tyres of one axle are to be replaced, tires of the same make, the same type and with the same specification code must always be used on each of the two axles. If tires are replaced on one axle only, the different tread depth from that on the other axle can cause a noticeable change in the familiar handling. This is especially the case if new tires are mounted on the rear axle.
This effect decreases with increasing tire mileage. When replacing a tire on an axle, make sure that the tread depth of the new tire does not differ from that of the other tire by more than 30 %. They are also rather proscriptive on getting a round result when fitting tyres. In order to optimize smoothness of rolling, it is appropriate - and necessary in individual cases - to mount the tire in a certain (favorable) position with respect to the wheel (matching).
Matching (uncontrolled and controlled) is explained below:
Uncontrolled matching: Turning the tire on the wheel by 900 or 1200 if necessary in order to achieve an acceptable value with regard to rolling smoothness (true running, imbalance and weight distribution of balance weights).
Controlled matching: With a balancing machine with matching program. In most cases, this produces an even better result with regard to the rolling smoothness (true running, imbalance and weight distribution of the balance weights) than can be achieved with uncontrolled matching.
UK
Not all manufacturers produce (or did produce) all sizes, and for the UK, winter recommendations are ignored, which are aimed towards countries with mandatory European winter driving regulations.
Manufacturers continue to produce the same sizes in multiple N specs on the latest Porsche cars, although sometimes you have to search hard to get the correct one. Higher (and some lower) N spec tyres will not have been tested on older cars, although will work as well.
Same non-N rated tyre
Sometimes manufacturers produce identical size / models with and without the N rating. The N spec tyre will be different in some way, compound, tread grooves, sidewall construction, whatever, but it may not be apparent.
Porsche did all this testing for performance reasons, to ensure your car handled the way the factory intended, there are good guidelines here to follow. If you change sizes at all, you (are on your own and) need to be aware of the change in the ratio of front to rear rolling diameters, because a significant change will adjust the ABS and/or the 4WD.
The logical calculation of rolling diameter from the tyre section / profile is not accurate in this regard, so be warned, you may need to check with the manufacturer.
The same tyres in different sizes are often at different N specs, which is a totally normal stage of the tyre development process.