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Understeer due to tire/wheel size


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I just experienced this first hand. went with fatter tires in the rear and experienced noticable understeer. I have a 2005 997 carrera 2. I upgraded my 18" 997 carrera III wheels (235/40/18 front and 265/35/18 rear) to new 19" Sport Design wheels with 235/35/19 front and 305/30/19 rear. the 305 rears is what Porsche puts on the 997 GT3 so I figured it would be okay instead of the 295/35/19 on the Carrera S (although they go with 19x12 wheels - does wheel width make a difference as long as offset is okay?). I went with Bridgstone Poetnza E050 tires in the rear which are porsche approved and because the tire center was out of matching Bridgstons for the front I went with Toyto Proxy T1's in the front.

now I am experiencing massive understeer. The Carrera S 19" come with 295/30/19 instead on a 19x11 wheel, is this my problem? Since my new rears are 19x11 do they need to be 295 instead of 305's in the rear.

Some people have suggested adding some width to the front tire will reduce the massive understeer - changing the front from 235/35/19 to 245/35/19 but Porsche does not provide an N designation for this front tire size. N0, N1 etc are porsche's stamp of approval on tire size, tread pattern etc for each brand of tire.

Can anyone help. Should I go wider in the front, or cut back in the rears or will getting bridgstone fronts to match the rears fix the understeer?

PS - I had no idea tire/wheel size made such a difference in handling.

Here are some before and after pictures -

post-19578-1188872730_thumb.jpg

post-19578-1188872952_thumb.jpg

Edited by ferris
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I just experienced this first hand. Can someone provide any input or has anyone else experienced this. I have a 2005 997 carrera 2. I upgraded my 18" 997 carrera III wheels (235/40/18 front and 265/35/18 rear) to new 19" Sport Design wheels with 235/35/19 front and massive 305/30/19 rear. the 305 rears is what Porsche puts on the 997 GT3 so I figured it would be okay (although they go with 19x12 wheels - does wheel width make a difference as long as offset is okay?). I went with Bridgstone Poetnza E050 tires in the rear which are porsche approved and because the tire center was out of matching Bridgstons for the front I went with Toyto Proxy T1's in the front.

now I am experiencing massive understeer. The Carrera S 19" come with 295/30/19 instead on a 19x11 wheel, is this my problem. Since my new rears are 19x11 do they need to be 295 instead of 305's in the rear.

Some people have suggested adding some width to the front tire will reduce the massive understeer - changing the front from 235/35/19 to 245/35/19 but I Porsche does not provide an N designation for this front tire size. N0, N1 etc are porsche's stamp of approval on tire size, tread pattern etc for each brand of tire.

Can anyone help. Should I go wider in the front, or cut back in the rears or will getting bridgstone fronts to match the rears fix the understeer?

PS - I had no idea tire/wheel size made such a difference in handling.

Here are some before and after pictures -

I wouldn't sweat that NO, N1 rating..just put a quality tire on and you will be fine. The problem is going with a 245 tire on a 8" wide rim, you can do it but it won't be optimal.

Dave

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Two different brands of tyres can give you this problem to.

I second that...IMMHO I wouldnt worry about the 'N' but would definetly have the same brand and "thread design" in all four wheels. Different brands will have a direct impact on the car handling.

Good luck,

Gustavo

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thanks for the feedback. I am going to go with 245/35/19's in the front as it seems to be a better match to the 305/30/19 rears based on online tire/wheel diameter calculators. I am hoping this will offset the sever understeer I am experiencing. will let you know findings.

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forum members DVell and Loren had some great insight into this. Dvell mentioned that the GT3's suspension geometry is setup for the 305/30/19 rear while the carrera 2 is setup for 295/30/19 in the rear -even though they seem like the same car structurally. This, and the fact that there is more tire in the rear while the front stays the same causes the understeer. They both suggested going back to stock 235 front and 295 rears. Altough 245 up front will help neutralize the understeer somewhat.

Thanks for the insight.

Edited by ferris
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one word, tread pattern! (or is that two) :thumbup:

I put Bridgstone Potenza PE050 245/35/19 up front to match same model in rear with the SAME tread pattern and day and night difference - no understeer, handles like a Porsche again. the 245's up front match the 305 rears in almost the same ratio as stock 235/295 combo on the 19's.

calculate those tire circumfrances, offsets to match what those Stuggart engineers formulated from the factory (using online tire size calculators), but don't forget about matching tire tread pattern front and rear. Different model tires can make a world of difference.

:drive:

Thanks to Carlos at Wheel Dynamics in CA. He's the owner and has lots of Porsche knowledge - not to mention a nice guy to do business with. I would highly suggest them if you're looking for wheels/tires. www.wheeldynamics.com

Also came across some helpful understeer oversteer correction tips - one being tire pressure (raising front tire pressure and lowering rear pressure by about 5 psi will noticably reduce understeer.

common tips here - http://www.nyracer.com/overunder.htm

Edited by ferris
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  • 2 weeks later...
  • 2 weeks later...

I was told by a very well respected Michelin rep on a Porsche GB track day to go for the 305 rears in this application (997s) - so I did !

I did however make sure that I changed all 4 tyres for the same brand, as this I would suspect caused the greatest difference in your handling.

Only difference I noticed was a slight squirm under high lateral loads, until the tyre tread blocks settled in to a pattern.

Super grip now, & no sign of understeer yet.

David

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  • 3 weeks later...

Just thought I'd add that the thing with the brands of tyres is highly likely to be more about the compound and structure of the tyres rather than the thread pattern. For example, in the dry, you could just put on tyres back-to-front (thread-wise) and do the same lap times.

When you're talking about uber-high performance tyres and specially if you're using them in the intended manner, different tyres heat up differently and behave differently as they do so. In the OP, the Toyo's probably work best at a different point to the Bridgestones and get there quicker/later, whereas the Yokos is probably more closely matched.

Structurally, different makes/brands do deviate quite a lot, just look at the weights of different tyre makes of the same size. This rigidity/comliance can and will make a difference.

Playing around with tyre pressures _is_ a possible way to 'dial out' this difference providing it's slight.

The best/safest bet is to go with the same brand of tyres all-round, imho. If you're driving a van you won't notice it, but in something like a Porsche, you would.

C

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Just thought I'd add that the thing with the brands of tyres is highly likely to be more about the compound and structure of the tyres rather than the thread pattern. For example, in the dry, you could just put on tyres back-to-front (thread-wise) and do the same lap times.

When you're talking about uber-high performance tyres and specially if you're using them in the intended manner, different tyres heat up differently and behave differently as they do so. In the OP, the Toyo's probably work best at a different point to the Bridgestones and get there quicker/later, whereas the Yokos is probably more closely matched.

Structurally, different makes/brands do deviate quite a lot, just look at the weights of different tyre makes of the same size. This rigidity/comliance can and will make a difference.

Playing around with tyre pressures _is_ a possible way to 'dial out' this difference providing it's slight.

The best/safest bet is to go with the same brand of tyres all-round, imho. If you're driving a van you won't notice it, but in something like a Porsche, you would.

C

Completely agree. I'd recommend the list of changes - sorted by increasing risk of handling 'challenges' is:

1. Match brands/tread patterns. There really is variance in tire characteristics - often not an issue of good or bad, but just difference. Such a mismatch = imbalanced handling;

2. Changing brands while respecting the constraints of #1 (above). Interesting tradeoffs in performance here - weather, head, durability, sticky-ness, rolling resistance. FWIW, I changed out my Pirelli P Zero's to Michelin P2's and couldnt be happier. Absolute stick maybe a bit less - but much more linear breakaway, improved ride quality.

3. Replicate the sizing of your car or, if you want to go with larger tires, go with the sizing of the CarreraS - front and rear. I don't believe there is a geometry change (not certain here) between the Carrera and the CarreraS (both 2wd version for example). Generally I have a lot of faith in those smart engineers over in Stuttgart. The chance that you'll outguess them is not that high;

4. Wheel size/tire size/offsets experiments. This gets tough. So many variables, and probablity of an successful experiment is rather low. I'd try to copy some other expert - racer or tuner based on your goal - rather than rolling the dice yourself.

Beautiful car you've got there though. Enjoy it.

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I was told by a very well respected Michelin rep on a Porsche GB track day to go for the 305 rears in this application (997s) - so I did !

I did however make sure that I changed all 4 tyres for the same brand, as this I would suspect caused the greatest difference in your handling.

Only difference I noticed was a slight squirm under high lateral loads, until the tyre tread blocks settled in to a pattern.

Super grip now, & no sign of understeer yet.

David

David,

You confirmed my suspicions. I just switched from OEM Lobsters w/ P-Zero Rossos to Carrera Sport wheels with Michelins - 235s in the front and 305s in the rear. I too noticed a slight squirm under high lateral loads (still only have about 400 miles on the Michelins).

Andre

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