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YOUR RESEARCHTires and AWD, 4x4 4wd...
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Most viscous clutch/coupling systems have a measured "bubble" of gas installed during the manufacturing process the purpose of which is to delay the onset of coupling action.
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So far my usage has had the front tires replaced with around 40K miles on them when the 2nd set of rear tires wears out.
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Rear Tires are: 305/30/19 (on my 04 coupe i ran 315s - which is the better size for the diff slip, 305 seem to be ok)
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The biggest difference in tyre diameters used by Porsche on OE tyres on the 996 AWD range is just over 0.4" (rear smaller than front).
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Most viscous clutch/coupling systems have a measured "bubble" of gas installed during the manufacturing process the purpose of which is to delay the onset of coupling action.
Run non n-rated tires and have reported no problems
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(Sure, some owners have run non N-rated tires and have reported no problems, but these are not sanctioned by Porsche which I why I stated "N-rated" above.)
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Hello, i didnt manage to find a clear answer here- how fragile is the transmission for ex new tires in the front, and old in the back? (i drive a manual 996-4-99)
For ex Haldex system, you can put old wheels in the front, and new in the back, no problem, as long as you have them exchanged by axel, and DONT put one new tire in one corner. That kills it.
What system do Porsche use? What are the do, and the donts?
The official answer is all four at once. Over 70k this has never been an issue for us as the front/rear wear has been pretty close. When our rears need to go the fronts are always close enough that it's not worth the hassle of doing 2 just to have to come back in a month.
Normally on my C4S and driving aggressively at stop lights, my rears wear out way before my fronts ever did.
I'd replace the fronts the next time my rears needed changing. Usually get a 2 (rear) to 1 (front) ratio
The only time I would not do this is when using track tires, as the front tires would have to many heat cycles and not have the same grip level as the newer rear tires. Wouldn't worry about this on the street though.
Ah, thankyou.. where i live, fourwheeldrive is very common, and i got quite good knowledge about what you can, and what you cant with more common cars.. but Porsche.. no idea, bought it 3 days ago. =) Thankyou for the reply
The biggest difference in tyre diameters used by Porsche on OE tyres on the 996 AWD range is just over 0.4" (rear smaller than front). If you stay within that you shouldn't have any problems.
Personally I use tyres at a rate of around 2 rears to 1 front and do not as a rule mix types. I have no issues changing 2 tyres of the same type at a time.
I calculated on this, and with 225/40-18 in the front, and 265/35-18 in the back, the difference is 0,9%, or 1,7cm for every 360 rotation.
The front is smaller than the rear.
in height this is totally 6mm, and that is the same as 3mm wear. (3 up and 3 down)
i guess im safe.. with the rear halfway to the junkyard, and new rubber in the front.. im about equal in height/rotation.. if my math is correct. =)
996 still has a viscous coupling. speed differential in the tires causes the fluid to heat up and become nearly solid, driving the front wheels. Some people (and I believe the factory) use a slightly shorter front tire to essentially prep the coupling to react more quickly.
Is the 996.2 AWD system different than the .1? 4S different than the plain 4? If the gas pedal was a person I would have been locked up for assault a long time ago, but as mentioned our tires have always worn pretty close to each other and there is no way we could get a second set of rears out of the fronts.
Second that, had an 04 4S Cab and now have an 02 4S Coupe.
Rears wear out 2X faster. I have replaced fronts on both cars 1x, rears 2X.
No issues. I am also running on 08 Turbo 19" rims with spacers on front and back.
Front Tires are: 235/35/19zr
Rear Tires are: 305/30/19 (on my 04 coupe i ran 315s - which is the better size for the diff slip, 305 seem to be ok)
- Michelin super sport
The awd systems are the same. The 4S runs more camber and wider rear tires which probably cause the rears to wear quicker on the inside edge. All though if your pushing a lot in corners the fronts will start to wear at a ratio more aligned with the rears but if you just doing straight acceleration runs and not doing high speed corning the fronts will last forever. I don't drive on the street like I do at the track. At the track I see pretty much 1:1 ratio.
Rear tyres are smaller in diameter which makes the rear wheels rotate faster than the front thus transmitting drive to the front. The faster the rear tyres rotate compared to the from, the more torque is transmitted to the front.
Based on what Porsche says (or doesn't say) any combination of otherwise identical N-rated tires on either axle of the AWD 996 models is fine.
(Sure, some owners have run non N-rated tires and have reported no problems, but these are not sanctioned by Porsche which I why I stated "N-rated" above.)
While Porsche doesn't say it, it doesn't caution against it either, and therefore the assumption is one could get by with tires to the wear bars at either axle while brand new tires are at the other axle.
However, this is not a common situation.
On my AWD 996 Turbo I have not yet experienced these extremes. Obviously, the car has had all 4 brand new tires, two new tires at the rear with "half" worn tires at the front.
So far my usage has had the front tires replaced with around 40K miles on them when the 2nd set of rear tires wears out. The fronts could go longer -- I have run them longer in my Boxster -- but they get hard and noisy and car road feel deteriorates and generally just suck.
The 996 AWD system is a viscous fluid coupling. Two sets of closely spaced discs -- one driven by the cardan shaft which in turn is driven by the transmission -- and the other connected to the front diff -- are in a special silicon based fluid. As the car moves down the road and both axles spin because of the tire size difference the two axles turn at different speeds.
Thus the two sets of discs spin at different speeds. This difference results in fluid friction and this heats up the fluid which becomes more viscous. (And the fluid gets pretty hot: I seem to recall a working temperature of 300F+.) This works to transmit torque to the front diff. Nominally the range is between 5% and 40%. I'm not sure at what speed the 5% occurs but word I get from the literature is the 40% occurs at 150mph+.
The AWD system is not the same as the AWD system found on most other cars.
Thus if one gets into a situation where the rear tires are spinning the front tires will not pull the car out of the situation.
There are a number of vidoes that show 996 AWD cars in the snow often doing donuts and while the rear tires are spinning furiously the fronts are nearly stationary with obviously no torque being transmitted to the front tires.
Continued spinning of the rear tires can over heat the viscous coupling and damage it.
(Similar videos of 997 vehicles with the newer/different AWD system do sometimes show the front tires spinning too as there is a wet plate clutch system that is used to transmit torque to the front axle.)
Likewise improper towing can damage the unit. Always the car must be carried on a flat bed tow truck or a tow dolly used.
Believe it or not some tow truck operators are not aware of this. Not many months ago I came upon a 996 AWD model in with a bad viscous coupling. Since I'm always interested in failures of parts that my Turbo shares with the NA 996's I asked for details and the story was the car broke down and the woman called for a tow truck. A standard hook and drag truck showed up and while car owner insisted the car should not be towed with either axle spinning the tow truck operator dismissed her caution and towed the car in to the dealer. By the time the car arrived the coupling was shot. The tow truck operator/business got to buy the woman a new viscous coupling (front diff as the the unit is not sold separately) for something over (IIRC) $7000.
Take the dimensions of the oe spec tires (front and rear) find the overall height difference (in inches) and stay within that spec with other brand tires or different sized tires.
Here's a write up I did for Lambo AWD, which also uses a vicious coupler, to give an idea of how to set up your own mathematical equation:
http://lambodiy.blogspot.com/p/correct-tire-size.html
*tire sizes should be in the driver door jamb.
Most viscous clutch/coupling systems have a measured "bubble" of gas installed during the manufacturing process the purpose of which is to delay the onset of coupling action.
The gas must be fully compressed due to VC fluid heating expansion before the VC begins to stiffen up.
Plus, you will need to disable TC in order to get ANY VC function, except turning tightly, aggressively.
This is a source reference. Confirm exact model applicability in the original material.
