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996TT Discontinued Tire Sizes & AWD Implications

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Old 10-24-2016, 01:30 PM   #1
TheKane
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Default 996TT Discontinued Tire Sizes & AWD Implications

I thought I'd share recent updates from Michelin and Tirerack on tire availability for those of us who have aftermarket rims. I also had an informative PM conversation with Macster (thank you Macster!). I have three friends who have 996TT's with my exact set-up, and we're all in the same situation. Let me preface this by saying I'm not an expert, and Michelin is notoriously bad at providing customers with accurate tire availability information.

I'm interested in the community's opinions and expertise.

Here we go.........
Michelin has discontinued the PS2's in my size for the rear 315/25-19. They also don't make the SuperSports or Cup2's in the size. Neither does Pirelli or any preferred (speaking personally) aggressive street/track oriented summer tire manufacturer. Michelin has confirmed this from a manufacturing standpoint and so has Tirerack from an availability perspective. Michelin also says they have no plans for a tire replacement in that size.

A note about my setup: I have a 2002 TT with PS9's, anti-swaybars (not sure what brand, previous owner installed), GUARD lsd, etc. I prefer to run with PSM off using mechanical grip, and I actually need to as under aggressive driving trail braking into a decreasing radius corner, the PSM gets very unhappy. Yes, the system has been checked and is operating correctly. The issue is that the system can't handle the wheel size in the front compared with stock calculations. This is apparently a common issue, non-issue.

So I went for the Pirelli's in a size that my local aftermarket shop says isn't an issue from an AWD or viscous clutch standpoint. My concern is the diameter/circumference difference front to rear, and resulting damage to the VC, a less effective AWD system, or power loss due to VC drag. That last is my lowest concern.

Here are the numbers.....

OEM SETUP:
Front
225/40-18Dia: 25.1"
Circum: 78.8"
Revs/mile: 804
Rear
295/30-18
Dia: 25.0" (some say 25.1)
Circum: 78.4" (slightly less than than 78.8" for the front)
Revs/mile: 808 (a few more revs per mile than the fronts, but not much)

CURRENT HRE SETUP (but tire sizes discontinued)
Front
235/35-19
Dia: 25.5
Rear
315/25-19
Dia: 25.2

TIRE SIZE NOW AVAILABLE (Pirelli PZero and others)
Front
235/35-19
Dia: 25.5"
Circum: 80"
Revs/mile: 792
Rear
305/30-19
Dia: 26.2"
Circum: 82.3"
Revs/mile: 770

With the above wheels/tires fitted there is a significant speed difference between the rear and front wheels/tires (fronts turning faster than the rears).

Let's look at this in percentages (thanks again to Macster):

OEM: 804 vs. 808 revs/mile is a speed difference of 0.49%

NOW AVAILABLE: 792 vs. 770 revs/mile is a speed difference of -2.77% This is the number that worries me.

With the Pirelli's in that size, I'm concerned about the numbers above. Here are the concerning real-world symptoms I've experienced:
1. The frunk, after a spirited drive below 80mph, is noticeably warmer than with my previous set-up under the same conditions.
2. The car pushes a bit, and the front wheel drive kicks in a little later. This means that the car is slower to correct course by use of the throttle. I'm concerned about the differential at higher speeds, say 140 to 170mph. It makes sense that the frunk is warmer given how much revs per mile difference there is between the front and rear wheels/tires. That the car pushes a bit also makes sense given the front wheels/tires are turning more revs/mile and since the front end is more lightly loaded the difference in rotational speed between the front driven discs and rear driven discs in the viscous coupling the front wheels/tires are being braked to some extent by the slower turning rear wheels/tires via the viscous coupling. At higher vehicle speeds the speed difference between the front and rear viscous coupling discs could have the fluid temperature elevated way too high. The fluid's working temperature can reach 350F which is plenty hot enough. Even if the fluid can withstand this the sealed bearings in the viscous coupling along with the diff fluid and diff's open bearings and gears that this diff fluid lubricates may be negatively affected.

I have decided to revert to my "Current HRE Setup" and go with the only tire available in that size, the Toyo Proxes T1R. These tires are less than optimal (loud on the freeway, way less grip than the PS2's/Cup2s/SS's, and have a lot more tread squirm). Hopefully by springtime Michelin or Pirelli will make another batch, but until then, I think Toyo will see a little spike in sales.
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Old 10-24-2016, 01:51 PM   #2
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You always want the fronts to be taller than the rears with the way our AWD system works.. With the rears being shorter, it "pre-loads" (for lack of better term) the viscous coupling due to the rears moving slightly faster than the fronts. With the fronts shorter than the rears, the AWD cannot work as designed and you are going to run into issues with how the car handles and drives as well as risking damage.
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Old 10-24-2016, 03:09 PM   #3
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Dont feel bad, the stock size of 295/30/18 is limited, it used to have more tire brands and now is down to a few!
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Old 10-24-2016, 10:11 PM   #4
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Nitto Invo comes in 315/25/19 also.
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Old 10-25-2016, 01:46 PM   #5
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Anyone have experience with the Nitto Invo's? I missed those since TireRack doesn't sell them.
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Old 10-25-2016, 03:24 PM   #6
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Quote:
Originally Posted by T10Chris View Post
You always want the fronts to be taller than the rears with the way our AWD system works.. With the rears being shorter, it "pre-loads" (for lack of better term) the viscous coupling due to the rears moving slightly faster than the fronts. With the fronts shorter than the rears, the AWD cannot work as designed and you are going to run into issues with how the car handles and drives as well as risking damage.
In debating winter tires I read a lot about acceptable differences in revs/mile, and whether the front needs to be slighter larger than the rear. In my travels through the internet I saw a lot of disagreement and no consensus, with seemingly logical discussions on both sides (some even getting into the weeds on engineering principles).

I can't add anything from an engineering perspective, but I do think that from it a logic perspective it's important to note that Porsche specifies a winter tire for 996 C4s and Turbos that slightly larger than the front. This is a factory recommended specification. I have trouble believing that Porsche would specify a tire size that is either harmful to the AWD or inconsistent with proper operation. If Porsche has something published on this that contradicts, I'd love to read it (before I put on my winter tires!). In the meantime, I'm comfortable with the factory specifications.

Also, for whatever it's worth, I checked with the service manager at a dealer regarding a proposed winter tire size that has slightly larger rears (0.8%) than the front. He approved of the size, in writing.

I don't suggest either of these points are conclusive evidence that ends the discussion, but just more grist for the mill.
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Old 10-25-2016, 03:45 PM   #7
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Quote:
Originally Posted by Rustler View Post
In debating winter tires I read a lot about acceptable differences in revs/mile, and whether the front needs to be slighter larger than the rear. In my travels through the internet I saw a lot of disagreement and no consensus, with seemingly logical discussions on both sides (some even getting into the weeds on engineering principles).

I can't add anything from an engineering perspective, but I do think that from it a logic perspective it's important to note that Porsche specifies a winter tire for 996 C4s and Turbos that slightly larger than the front. This is a factory recommended specification. I have trouble believing that Porsche would specify a tire size that is either harmful to the AWD or inconsistent with proper operation. If Porsche has something published on this that contradicts, I'd love to read it (before I put on my winter tires!). In the meantime, I'm comfortable with the factory specifications.

Also, for whatever it's worth, I checked with the service manager at a dealer regarding a proposed winter tire size that has slightly larger rears (0.8%) than the front. He approved of the size, in writing.

I don't suggest either of these points are conclusive evidence that ends the discussion, but just more grist for the mill.
Porsche like any other car manufacturer makes mistakes from time to time. Highlight examples are, thermal reactors that cooked the 2.7L motors in the early 70s, faulty emission control systems on 993 models that require a full engine rebuild to correct, sealed IMS bearings in the M96/97 motor that fail, etc.

The factory recommended winter tires are not idea for exactly the reason T10Chris states. I do not believe using those sizes will cause damage but it will absolutely cause the front half of viscous coupling to be spinning faster than the rear which is negatively loading the system. It is physics, not an opinion or a belief.

Last edited by Carlo_Carrera; 10-25-2016 at 04:09 PM.
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Old 10-25-2016, 05:11 PM   #8
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As you correctly state, even engineers make mistakes sometimes. I have seen the "facts" about the physics of it argued both ways, although I myself can't add to the fabric of that conversation.

Has there been anything published on the matter, by Porsche or otherwise? I haven't seen anything, but would be interested if it's out there.

In the meantime, IMS bearings be damned, I'm with Porsche's engineer's specifications on this.
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Old 10-25-2016, 05:47 PM   #9
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i have read 5% difference in the revs per mile as the max the Viscous coupling can reliably take. Does not mater if it is biased front or rear for the coupling, just for handling reasons. With the handling in mind, for winter tires perhaps Porsche feels that the change in front/rear bias from summer tires to winter tires has to do with how the power is applied for optimum handling in the mountain passes of the alps.
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Old 10-25-2016, 07:01 PM   #10
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Quote:
Originally Posted by Rustler View Post
In debating winter tires I read a lot about acceptable differences in revs/mile, and whether the front needs to be slighter larger than the rear. In my travels through the internet I saw a lot of disagreement and no consensus, with seemingly logical discussions on both sides (some even getting into the weeds on engineering principles).

I can't add anything from an engineering perspective, but I do think that from it a logic perspective it's important to note that Porsche specifies a winter tire for 996 C4s and Turbos that slightly larger than the front. This is a factory recommended specification. I have trouble believing that Porsche would specify a tire size that is either harmful to the AWD or inconsistent with proper operation. If Porsche has something published on this that contradicts, I'd love to read it (before I put on my winter tires!). In the meantime, I'm comfortable with the factory specifications.

Also, for whatever it's worth, I checked with the service manager at a dealer regarding a proposed winter tire size that has slightly larger rears (0.8%) than the front. He approved of the size, in writing.

I don't suggest either of these points are conclusive evidence that ends the discussion, but just more grist for the mill.
There is optimal situation/sizing for how the system is designed (front > rear)

And there is something that works for a temporary situation (winter/snow) without having to re-engineer the whole thing, while not being the overall best for performance. (front > rear)

With such a small variance, it may never cause damage, but it is going to effect the driving dynamics and how the AWD functions. Maybe negatively loading the rears helps in low traction situations such as snow/ice? Maybe those were the only tire sizes available in the era this car was designed that fit the bill? Would have to ask a Porsche engineer to find out I suppose.
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Old 10-25-2016, 07:13 PM   #11
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I think 5% is way too much. Can't articulate why, just my gut feel. My current setup is 1.4% and I'm looking to bring it in even more. I think with blizzaks on stock turbo twists you can get inside of 1%, with 275s in the rear.
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Old 10-25-2016, 07:33 PM   #12
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Originally Posted by TheKane View Post
Anyone have experience with the Nitto Invo's? I missed those since TireRack doesn't sell them.
I have them. No complaints. I haven't had any other tire on this car so I can't really compare them to anything else.
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Old 10-25-2016, 07:52 PM   #13
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Does anyone that is running AWD and taller rears than fronts have any impressions on if it changes the feel of the car at all compared to before? I am wondering if this is something that is talked about and passed down, but actually has no noticeable/measurable effect..
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Old 10-25-2016, 08:12 PM   #14
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I am currently running taller rears than fronts F 225 45 18 and R 295 35 18, came on the car so I have nothing to compare it to. Except my 99 C4 had the proper stagger (but on turbo sized 19s) and i have run both at Laguna Seca and no major differences. The turbo still seems to drift OK, i can feel the front pulling. Perhaps that is due to the C4 and the Turbo being different animals.
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Old 10-25-2016, 08:16 PM   #15
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Quote:
Originally Posted by T10Chris View Post
Does anyone that is running AWD and taller rears than fronts have any impressions on if it changes the feel of the car at all compared to before? I am wondering if this is something that is talked about and passed down, but actually has no noticeable/measurable effect..
When this subject came a couple of years ago and I had member from a northern European country, I think it was Sweden or Norway, contact me via PM about the winter tire sizes I was running. I run the fronts slightly larger than the rear. He was having trouble with traction in the snow using the Porsche recommended sizes. He switched to the sizes I use and contact me later to say the car felt better.

Take it for what it is worth.

Another person to ask is T2. The member who has put 500k on his car. He daily commutes even in the snow. I do not know what sizes he runs though. Maybe he will chime in.
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