VW Toureg Porsche Cayenne owners tire question

[ccdhowell]

I recently put a new set of wheels and tires on my Land Rover Discovery. Nice aftermarket 18" wheels and Pirelli Scorpion AT tires in 235/60/18. I was interested to discover that these tires are factory fitted on some Touregs and Cayennes. If you have these tires, please tell me what air pressure VW or Porsche recommend. Also please tell me what air pressure you are running. I know my Disco is heavier than the other two, but it seems I have to run too much air pressure in these tires; more than I've ever run on an SUV before. I have settled on 40psi front and 44psi rear. The tread seems to be flat, but maybe wearing more in the center, meaning too much air. If you own one of these vehicles with this tire, please step in and comment on this tire in general and air pressure speciffically. Thank You.

 

[CapriRacer]

As a tire engineer, I can tell you that even though a tire appears on another car, the inflation pressure is unrelated. But there is a way to determine - within reason - what the proper pressure should be, but the method involves tire load tables - which are published in a book that gets updated yearly. (In other words, it's not published on the Internet!) Fortunately, I happen to have this book. So here's what I'll need: 1) The original tire size 2) The vehicle manufacturers specified inflation pressure The above 2 items can be found on the vehicle placard, which is usually located on a door post or in the glove box. I can also look this up in a book which summarizes the placard info, but I need year, make and model (I guess we only need the year.) 3) The new tire size - and we already know that 235/60R18. I went ahead and looked this up . There is some variation from year to year on tire size and inflation pressure for the Load Rover Discovery. But the concensus is that you need a minimum of a 104 Load Index. The 235/60R18 Scorpion AT has a Load Index of 107 - So you're good there! Also, my book shows the Porsche as needing 38 front / 43 rear for this size. But interestingly, it also shows that the Porsche weighs MORE than your Discovery! BTW, my book says that size does NOT appear on the VW! Based on this info, it appears you should be running about 3 psi higher than what's specified for the original tires on your vehicle - which I'm guessing is 255/55R18 at 30 / 38 - so your new pressures ought to be 33 / 41. This is a bit lower than what you are using now and might partially explain the wear in the center. But just to be sure, look up the placard info and post it.

 

[ccdhowell]

Thanks for the thoughtful responce. The placard recommends 28psi front and 46psi rear fot the factory 255/55/18 109H Goodyear HPs on factory 18x8 allows. I found that I liked a bit more air in the front (31psi) and a bit less in the rear (41psi) on the factory tires. I upgraded to 18x7.5 alloys and the aforementioned Pirellis. It was certainly an upgrade IMO, those particular GYs aren't very good. I've often wondered why Land Rover would spec such a terrible tire. Anyway, I started the Pirellis at the same pressure I'd come to prefer in the GYs, but the tire seems to be designed for more pressure. I understand that a tire can be designed for a particular pressure range, but have no experience with a tire that prefers psi in this (high) range. What part of construction or design would affect the optimal psi for a tire and vehicle? Just curious. I worked at a Goodyear dealer, as a salesman, in college, and have ever since been interested in tires, their design and construction. Thank you for whatever information you can pass along concerning my tires and tire design and construction in general. Chris

 

[ccdhowell]

Very good of you to respond in such a manor as you did. My trouble with this particular set of GYs is my inability to balance them and have them stay balanced for very long. It becomes tiresome to rebalance tires every 6 weeks or so. Anyway, I understand what you have stated and that I'm 3 steps closer to a possible rollover. In my defence, steering responce is much better now. I don't understand how a reasonably close load index can skew the inflation pressure so much. One reason I bought these Pirellis is because their load index is quite close to the GYs. I've been running 39 front/ 44 rear on the Pirellis, and I've rotated every 3,ooo miles and can't tell a difference from front to rear, just center wear. The tires are marked as 50psi max. I will try what you have recommended. Thank you again.

 

[CapriRacer]

Chris, Let's start at the beginning: Land Rover's inflation spec of 28 / 46 is extremely unusual. Even though the 255/55R18's are Extra Load, the rear inflation spec is 5 psi above the load limit inflation pressure. This says to me that Land Rover is juggling the pressure to get a particular vehicle handling characteristic - probably understeer. So let's look at the inflation pressures you were using - 31 / 41. Increasing the front pressure decreases understeer, and lowering the rear does the same. Now while I personally prefer a crisper steering response, I increase the inflation pressure on all the tires equally, with the idea of not changing (much) the overall understeering characteristics of the vehicle. But changing the tire size from a 109 Load Index to a 107 Load Index moves us closer to the saturation point of the tire. Perhaps I ought to explain that. Emergency maneuvers, tend to be in areas of the tire were the tire's response to steering input is non-linear. This makes the vehicle's response less predictable. It turns out that the more heavily loaded a tire is, the less linear it is and at some point, the tire doesn't respond at all to input - it is said to be "saturated". So vehicle manufacturers have been gradually oversizing tires to stay more in the linear region of the tire. So looking at all that has been done: 3 steps toward less understeer - on an SUV this is not a good thing - a sideways SUV tends to rollover. But your current tires are wearing in the center - but is that only on the rear tires? Needless to say, tires that are highly inflated (or lightly loaded for the inflation pressure) tend to wear more in the center. But looking at the whole vehicle - front and rear - is the wear more or less even? Remember, Europeans aren't interested in the tire wear - grip is their thing. Perhaps that is why you didn't like the Goodyear's? Also, tires are designed around a combination of load and inflation - which is what tire load tables are about. Put another way, The basic construction of a tire may be designed to a maximum load / inflation combination, but the wear/ride/handling characteristics are more or less independent of that. Which brings me back to your current tires. I am very concerned that you seem to be 3 steps further away from understeer on an SUV. Of particular concern is the rear tire inflation. If I've done the math correctly, the equivalent inflation pressure for your current set up is 31 / 49. Hopefully your tires are marked as 51 psi, but this is still way high. What I'd recomend is going to a larger Load Index, but your rims are going to be a limiting factor.

 

[gtx510]

quote:

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Originally posted by CapriRacer: As a tire engineer

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do you work for Conti?

 

[CapriRacer]

quote:

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Originally posted by gtx510: do you work for Conti?

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No. So does that mean you find my posts unbiased?

 

[gtx510]

huh? You would think that I'd find your posts biased, not unbiased? No, actually, I'll hoping your tire posts will be well informed. (they appear to be) imho, I think LR is doing the BMW thing and trying to tune the chassis with the tire pressures. Less air in front = understeer, and lawyers like people to hit headfirst. I'm just guessing NC tire companies here. Kelly-Springfield?

 

[ccs v2.0]

Not questioning your vast knowledge Capri, but wouldn't increasing the tire pressures up front (39/44 psi, a net difference up front of +9 psi) induce understeer, instead of oversteer? I understood the rule of thumb to be incresing the stiffness in one corner would increase the grip in the opposite, diagonal corner (to a point, we're only talking 9 psi). If true,in this case raising the pressures up front and softening the rears would bias the grip to the back. Am I that far wrong?

 

[CapriRacer]

quote:

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Originally posted by ccs v2.0: ....but wouldn't increasing the tire pressures up front (39/44 psi, a net difference up front of +9 psi) induce understeer, instead of oversteer?....(to a point, we're only talking 9 psi).....Am I that far wrong?

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No, you're not wrong, but there are a couple of things that make this an unusual situation. First, I'm only considering an emergency maneuver - like a lane change - rapid steering input followed by another rapid steering input in the opposite direction. Increasing the tire pressure causes the tire to react quicker. So increasing the front pressure reduces understeer initially (on turn in). The vehicle then reacts to the steering input - front first, then the rear reacts. Reducing the inflation pressure in the rear tends to make the rear of the vehicle wash out if the steering input is strong and rapid. It's usually not the first steering input that causes the problem - It's the second one in the opposite direction. It's the combination of load shifting - which is higher with lower inflation pressures - and the slower reacting tire that results in a sudden increase in forces (the technical term is "jerk"), and tires don't like to be "jerked" around (Sorry, I couldn't resist!!) Put a different way, if you can supply enough side force to get the tire to start sliding, for practical purposes the game is over. So you don't want the rear to be the first to break traction. Second, the pressures we are discussing are way high. Anything above 36 psi for a Standard Load tire and 41 psi for an Extra Load tire is beyond the intended operating parameters (that is: intended by the tire manufacturers) Outside these limits and tires can do some strange things, so I caution eveyone to be very careful operating in these regimes. You mentioned 9 psi as not being very much pressure difference and I thoroughly disagree. Usual operating ranges for passenger car tires is 26 psi to 35 psi - a range of 9 psi. And while I'll recommend to folks that they use 3 to 5 psi above the placard pressure, I'm referring to ALL the tires - front and rears. When it comes to pressure differential front to rear, this becomes a more sensitive issue so I don't like to see movement more than 2 or 3 psi. BTW, this is a thoroughly confusing subject and even though I've been through the discussion many times, I still get be fuddled. And the worst part is that there are situations (combinations of spring, shock, sway bar, center of gravity, steering geometry, etc.) that change the net result (vehicle in control / vehicle out of control). I tend to stay very near what the vehicle manufacturer recommends. They've spent a lot of time and effort and have resources that I can't match. I trust that they have done their job - and even if they didn't, I don't think I know enough to overrule their expertise and experience.