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YOUR RESEARCHTire pressures.......18' wheels
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Run 265/35-18
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I dropped my rears to 42, and have been running like that for a while - with no apparent problems (I run 265/35-18 on the rear - which is the a/s spec) - but I think Thundertub gives the best advice here ....
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but I did wonder why on other cars they recommend higher pressure when running 100MPH+ (Mercedes for example) - so does that mean Porsche only specs for high speed - and does not have a low speed spec (for better ride) ?
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I dropped my rears to 42, and have been running like that for a while - with no apparent problems (I run 265/35-18 on the rear - which is the a/s spec) - but I think Thundertub gives the best advice here ....
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Recommend higher pressure
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but I did wonder why on other cars they recommend higher pressure when running 100MPH+ (Mercedes for example) - so does that mean Porsche only specs for high speed - and does not have a low speed spec (for better ride) ?
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Of course, if you drive like a trophy wife (if you're complaining about ride comfort, you do, admit it!!), then you probably want to stick with Porsche's recommended pressures.
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So the book calls for 44psi on the rear wheels. This makes for a pretty harsh ride. Anyone experimented with lower pressures.........what's the thinking on setting them the same as the fronts......36psi??
i used stock 36/44 for awhile on mine with sport techno 18" rims- 8"" front 11" rear- 235 & 295 tires & X-74 suspension- then settled in on the GT3 settings which is 33/39 i believe.
Porsche spent a LOT of time, money, testing, and energy doing actual engineering studies to come up with those numbers of 36/44 for the 18" rims. The numbers were achieved by actually testing to see which tire pressures produced the best
COMPROMISE
between handling, comfort, tread wear, and safety (and a dozen other factors). If you are a tire engineer and know better, then pick whatever you like. I'm not an engineer, I haven't recently slept at a Holiday Inn Express, and I don't believe that the tire pressures recommended by the factory are part of any subplot between Porsche and the tire manufactures.
I use only N-spec rated tires, only the recommended sizes on the recommended size and offset rims, and only the factory tire pressure recommendation. And I have over 20 years and 70+ DE events (13 years with this car) and a total of 76,500 miles of experience with this specific car and set-up. This is my 17th Porsche over 40 years.
Everything else is just a crapshoot.
Good luck!
Ok, thanks for all the replies.........however.......it just seems odd to me that they went from 36 with the 17" wheel to 44 on an inch larger wheel. Traditionally as tires get larger/fatter the pressure goes down somewhat........as in a bicycle tire uses something like 65psi and most cars use something around 32 - 36 psi. Now we jump one inch larger and need another 8psi! Just seems bloody odd to me is all. Are they using the pressure off the GT?? for the average 996 with fat wheels? If so, perhaps it's not required.
Now, I'm not about to do anything stupid like run them too low but just thought I'd ask the collective wisdom of the forum y'alls opinion..........and yes, I get it that the manufacturer spends lots of time and $$ working these things out.....but???
For the record: mountain bike tires = 35-45 psi, road bike tires 90-110, upright cruisers you wounldn't want to be caught on: anything in between.
I recently increased to 44 in the back and like the effect: firm ride, seems like less oversteer initially. fwiw.
My rear tires have nice even wear after being at 44 psi for 2 years. At the price of tires, that's a good enough reason for me. You get used to the ride.
I think this should be the new qualifier for all 'statements of fact' on this forum. It should replace "IIRC"...
IHRSHIE, but I think it's safe to wander a little from 36/44. Changing weather conditions, day to day, hour to hour, could ruin your best intentions to stick with Porsche-recommended settings anyway.
Full car with 4 passengers? A rare occasion but that would still be only an extra 100 lbs. in a 996: two kids @ 40lbs each and a 20 lb suitcase in the frunk! Not enough to change tire pressures for.
I'd also tend to stick with factory specs ... but I did wonder why on other cars they recommend higher pressure when running 100MPH+ (Mercedes for example) - so does that mean Porsche only specs for high speed - and does not have a low speed spec (for better ride) ? At a AutoX I went to , many owners ran their tires at a slightly lower spec for better grip (but this is also low speed). If you go too low you run the danger of cracking the rim - like the picture above. I dropped my rears to 42, and have been running like that for a while - with no apparent problems (I run 265/35-18 on the rear - which is the a/s spec) - but I think Thundertub gives the best advice here ....
I would not change the recommended tire pressure because of a harsh ride (you may need to change tires at your next opportunity instead). We get to 80mph without even realizing it...
I run much lower then recommended. I can't deal with the bumps on our roads, just way to bumpy. If I plan on driving long distance on the highway, I'll boost it up a bit...
Yes, this. Anyone who frequently autocrosses can tell you that Porsche's spec alignment & tire pressure suck for performance. They're very clearly intended to produce safe, boring, predictable understeer to keep trophy wives out of ditches. Rear tire pressure/alignment is, in my experience, pretty close to optimal- Porsche's compromise here is to favor excellent grip at the expense of tire wear to prevent the rear end losing grip before the front. Front tires at OEM spec, however, are very undertuned. Rather easy to increase air pressure and test on the skidpad until optimal. You don't even need a real skidpad, just find a parking lot and drive in circles; keep adding more pressure 2psi at a time until you find the sweet spot. Wider than OEM tires helps, too, as does as much negative front camber as you can get (it still won't be enough without camber plates or adjustable lower control arms). And yes, I did stay at a Holiday Inn Express once. Several times, in fact!
Honestly, I'm always amazed at people complaining about firm ride or tire noise. You do realize you're driving a Porsche, not a Cadillac, right? Of course, if you drive like a trophy wife (if you're complaining about ride comfort, you do, admit it!!), then you probably want to stick with Porsche's recommended pressures.
I remember reading several tire pressure threads where people running the rear tires with LESS than 44 noticed more even wear. They said running at 44 caused the center to wear out faster?
If 36/44 is for trophy brides, what to real drivers use for the street and highways??
yea pulling rubber over aluminum is hell on wheels
wanna have some fun with tire pressure....think of your HOT pressure as your "ending" point and back up from there. In my case I want my hot pressure to be at the optimal tire performance(which for my tires is 46 psi)
factory is 36 rear...I run 39-40 cold...my car is pretty light and its cold here most of the year, I cannot seem to get the heat (and thus the pressure) up in the tires if I start lower - so I am over the factory recommendation but a few laps and they settle right in the sweet spot...they have the "BEST TRACTION" in the sweet spot
so I run them high on the street too....
when they are cold she can be a little bit tail happy....not at all when they warm up a little - I might be silly for runny such presure but i have spent just enough time reading to be dangerous - that's the way I roll !!
FROM TIRERACK:
Disadvantages of Underinflation
An underinflated tire can't maintain its shape and becomes flatter than intended while in contact with the road. If a vehicle’s tires are underinflated by only 6 psi it could lead to tire failure. Additionally, the tire’s tread life could be reduced by as much as 25%. Lower inflation pressure will allow the tire to deflect (bend) more as it rolls. This will build up internal heat, increase rolling resistance and cause a reduction in fuel economy of up to 5%. You would experience a significant loss of steering precision and cornering stability. While 6 psi doesn’t seem excessively low, remember, it usually represents about 20% of the tire’s recommended pressure.
Disadvantages of Overinflation
An overinflated tire is stiff and unyielding and the size of its footprint in contact with the road is reduced. If a vehicle's tires are overinflated by 6 psi, they could be damaged more easily when running over potholes or debris in the road. Higher inflated tires cannot isolate road irregularities well, causing them to ride harsher.
However, higher inflation pressures usually provide an improvement in steering response and cornering stability up to a point
. This is why participants who use street tires in autocrosses, track events and road races run higher than normal inflation pressures. The pressure must be checked with a quality air gauge as the inflation pressure cannot be accurately estimated through visual inspection.
here is some pretty solid advice...- unless you wanna "play around" and the only way to play is to monitor tire pressures like every single trip and see where they move...Porsche recommends the best average for the most average driving on the most average conditions with the average load and average...well you get it....
it is possible to do better, but you better put in a lot of readings....and be willing to change it not only daily, but hourly...or more frequently
IMO if you wanna error from factory
lower is the worst way to error
....
Here's video of a tire test rig replicating the forces tires see during heavy cornering. Remember- forces upon tires are dynamic and weight transfer during braking and maneuvering can drastically increased the load on a tire. Running too low of pressure can cause tire roll-under, where the tire rolls off the rim and the sidewall makes ground contact. This is not only causes a dangerous lack of grip/control, but also damages tires and rims. And yes, it actually does happen for real. A friend of mine took a nail through the sidewall of his 996TT due to this, and I have video of my tires deforming during hard maneuvering, with the tread moving roughly 1" back and forth with relation to the rim (and this is with stiff-sidewall high-performance tires & 10psi over Porsche spec pressure). You can even see it a bit in my avatar, though I'm using stiffer racing tires in my avatar than I had when I took the video.
You Tube
There's science to it, as well. Overinflating a tire by a few psi does generally have less impact on performance than underinflating by the same few psi. So if you're in the ballpark but unsure, it's always better to err high than err low. Here's a good chart showing some typical tire pressure/grip curves:
iOVSY.jpg
For more of a layman's explanation:
3F6A3.jpg
For basic straight-line driving, like most cars see most of the time, you'd select pressures for maximizing tire life, and pick the pressure that lays the tire flat like the tire on the right. Under maneuvering, though, weight transfer under braking, acceleration and/or cornering can double (or more) the downforce an individual tire sees- very evident in the racing video ivangene just posted. In such conditions the "overinflated" tire will squash flat and give good traction, while the "properly" inflated tire will deform under load like the underinflated tire on the left, and the poor trophy wife will end up in the ditch. No reason a Porsche should ever run on underinflated tires; you paid for the performance, why cripple it?
Internet gansters, gotta love'm.
Not one post here illustrates why these rims are cracking in the exact same area. I see posts talking about tire damage and even one that states the rim comes in contact with the road however no road rash is near the cracked area of the photos posted nor my photos of my cracked 996 rims.
My guess is me having to install new rears yearly (10+ years) and a rear engine car wears the rims down over time. I've owned/own other vehicles with aftermarket rims and stock that have never cracked no matter what tire pressure and "profile" was involved.
Actually, guys who are serious about tire pressures at the track use a pyrometer to determine at what pressure the tire is heated uniformly across the width. That indicates maximum contact patch.
Highly unlikely. There are thousands of rear engine Porsches driving around with wheels that are 30+ years old (I own one myself). There are also plenty of daily driver 964's and 993's that are 15 to 20 years old that have no wheel breakage issues.
The damage is almost certainly caused by impact with a pothole or something like it that could cause a very sharp hit to the very low profile tire, which then transmits much of that direct energy to the cast aluminum wheel. Over time, the wheel just fatigues and eventually cracks after another hit.
Getting back to the discussion, I personally run the factory numbers on my 996. The car feels perfect for the street environment it was built for.
Honestly, I thought it was implicit; rollunder doesn't crack rims like blunt force trauma does. Every little bit helps if you hit the sharp corner of a pothole, etc, at high speed, but honestly, 2 or 4 psi probably isn't going to make a whole lot of difference if you're hitting with enough force to crack or bend a rim. Wheels with low-aspect tires are especially vulnerable because there just isn't a whole lot of tire to absorb it. If your road conditions are so poor that you're frequently damaging wheels, 17" with higher aspect tires will fare much better.
Prior to getting my new tires I was running 40/32 on my worn Bridgestones because it just felt so harsh (I know, I know, it's not a cadillac). After putting on the new Michelin PS2's on Saturday I put them at 44/36 and they feel great. I wonder if it's because they are a propper Porche approved (N-spec) tire with the correct load ratings, etc.?
I'm hoping you meant to say 36/44, not 44/36. Normal convention for stating tire pressures is Front/Rear. Just a nit, but we all want to be on the same page here.
I keep hearing this, and some people even say there's a TSB. But the manual still says 36/44, and the most recent TSB I've actually seen with my own eyes (6/11, supercedes bulletin Group 4, #13/08, 5/08) says 36/43, of all things.
