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Discussion Starter #1
I would intuitively think you would want higher pressures in the front with the fronts being narrower and with more weight on them. Why is the recommendation for higher pressures in the rear?


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All Porsches I've had specify higher pressures in the rear tires.
 

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Own 5 cars with staggered tires....the Macan, an M235, an SLK55, a 996, and an X5. In each case the manufacturer calls for higher rear tire pressure. I've long assumed everything else being equal, lower profile tires require higher pressure. Am I incorrect?
 

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I’ve done some searching and can’t find any defense for the higher rear pressures, so I’ll use conventional logic for performance driving and invert these pressures. We don’t need more understeer in cornering a front engine vehicle. Please prove me wrong....


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I’ve done some searching and can’t find any defense for the higher rear pressures, so I’ll use conventional logic for performance driving and invert these pressures. We don’t need more understeer in cornering a front engine vehicle. Please prove me wrong....


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I'm sure your post will have the Porsche engineers reevaluating the recommmended pressures.
 

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I'm sure your post will have the Porsche engineers reevaluating the recommmended pressures.
Now that's funny! A little sarcastic, but funny. :LOL:
 

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Now that's funny! A little sarcastic, but funny. :LOL:
Yes my reply was a bit flippant. But the OP's question does raise an interesting point. Everyone knows that manufacturers build in a large margin of safety into their vehicles, and that understeer is easier for most consumers to manage than oversteer. Therefore, the mfg recommended tire pressures may favour understeer for liability reasons. So there's a possibility that a different pressure spec might be better for high performance driving by a skilled driver. And I guess that is the thought behind the OP's question.
 

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I’m certainly no tyre expert. However my thinking would be that as tyres get wider carrying the same weight and same pressure, more weight would be carried by the outer edges of the tyre and less in the central section. Therefore a higher pressure would be needed to force the central section to maintain proper shape to wear evenly. As we know under inflated tyres wear the edges most and less in the centre.
 

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My 2cents. Everything else being equal.
A 40 aspect ratio sidewall will need a higher pressure than a tire with a 45 aspect ratio sidewall. The lower ratio tire sidewall cannot flex as much as the higher ratio sidewall, therefore it needs a higher tire pressure when compared to the higher aspect ratio tire.
 

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There is another hint, look at the rear tyre pressures for the GTS and PP, they are 4 PSI more than the other models that are fitted with the same size tyre. :unsure:
 

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In the bike world, lower volume requires higher pressure. If you ran the same 20-25 psi in a skinny road tire that you do in a fat mtb tire, you’d have no support. Anyone want to go back to high school geometry and figure out some cylindrical volumes?
 
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My 2cents. Everything else being equal.
A 40 aspect ratio sidewall will need a higher pressure than a tire with a 45 aspect ratio sidewall. The lower ratio tire sidewall cannot flex as much as the higher ratio sidewall, therefore it needs a higher tire pressure when compared to the higher aspect ratio tire.
The side walls of the front and rear tyres are the same height, well almost the same 1.25 mm difference. For20” wheels front sidewall 119.25mm rear 118.00 mm
 

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Where is Fast Eddie, our local tire expert? Bet he can explain precisely what the story is.
 

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Discussion Starter #18

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Thanks iconoclast - this OP seems to support my thoughts and has some relevant background. I’m just surprised to think that Porsche would exacerbate the understeer with even more pressure in the rear, being a driver’s brand inspired by racing.
You been sold on advertising? Its a SUV meant to haul things and people. Despite "advertising" about 911 DNA, its not remotely resembling a sports car. It IS meant for softroading, snow, bad weather, and hauling. Porsche is Very Very good at marketing.

The Quora article is talking about non-staggered setups. Since the Macan is staggered, the rest the article likely has no relevance.

Cargo is an issue but maybe not the entire issue.

@Caymancouver is more on target. I'm amazed at all the internet experts who know more about these cars than the people that build them. I never heard it before on tire pressures but its common on break-in and oil changes. People who believe the know better? Thats so interesting.

The GTS and PP issue someone brought up. While the Macan doesn't have this, the 911 has another set of tire pressures, for driving over 100. I forget the exact number. Crank up the cold pressure for driving at high speed. My guess is they consider the GTS and PP as the "performance" cars and hence crank up the pressure. They probably don't expect many base, S, or Turbo drivers on the Auto bahn doing 140. But thats a just a guess.

Which brings us to "driver's brand" inspired by racing and inducing understeer. I'm not tying to be flippant here but maybe trying to explain history.

The oversteer problem is a MASSIVE problem. They didn't call 930s Widowmakers because it sounded "cool". Its because people died. Once traction breaks lose, most everyone wants to lift. That means you die, or the tail hits the wall. Its counterintuitive that with RWD, you don't want to lift. But if the car understeers, lifting does eventually correct it and you don't plow into that tree.

Its a SUV. Its bought by tens of thousands of everyday people hauling their kids, buying groceries, going to Home Depot, not wannabe race car drivers. Where is the liability. If there is some gravel on the road, snow, whatever, do you really want a car that's going to go into oversteer for a SUV?

You know many people who actually take their brand new car out in the snow and practice with it to see how it handles at the limit? Yet you want to induce oversteer? My guess is they have the lawyers there all over the liability issue. Wife taking the kids shopping, hits the gas, tires break traction, she oversteers, lifts, hit brakes, and does a spin right into a pole. Meanwhile, if she plowed, she might have recovered.

If the answer is for handling, then yes, understeer it is.
 

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Discussion Starter #20 (Edited)
You been sold on advertising? Its a SUV meant to haul things and people. Despite "advertising" about 911 DNA, its not remotely resembling a sports car. It IS meant for softroading, snow, bad weather, and hauling. Porsche is Very Very good at marketing.

The Quora article is talking about non-staggered setups. Since the Macan is staggered, the rest the article likely has no relevance.

Cargo is an issue but maybe not the entire issue.

@Caymancouver is more on target. I'm amazed at all the internet experts who know more about these cars than the people that build them. I never heard it before on tire pressures but its common on break-in and oil changes. People who believe the know better? Thats so interesting.

The GTS and PP issue someone brought up. While the Macan doesn't have this, the 911 has another set of tire pressures, for driving over 100. I forget the exact number. Crank up the cold pressure for driving at high speed. My guess is they consider the GTS and PP as the "performance" cars and hence crank up the pressure. They probably don't expect many base, S, or Turbo drivers on the Auto bahn doing 140. But thats a just a guess.

Which brings us to "driver's brand" inspired by racing and inducing understeer. I'm not tying to be flippant here but maybe trying to explain history.

The oversteer problem is a MASSIVE problem. They didn't call 930s Widowmakers because it sounded "cool". Its because people died. Once traction breaks lose, most everyone wants to lift. That means you die, or the tail hits the wall. Its counterintuitive that with RWD, you don't want to lift. But if the car understeers, lifting does eventually correct it and you don't plow into that tree.

Its a SUV. Its bought by tens of thousands of everyday people hauling their kids, buying groceries, going to Home Depot, not wannabe race car drivers. Where is the liability. If there is some gravel on the road, snow, whatever, do you really want a car that's going to go into oversteer for a SUV?

You know many people who actually take their brand new car out in the snow and practice with it to see how it handles at the limit? Yet you want to induce oversteer? My guess is they have the lawyers there all over the liability issue. Wife taking the kids shopping, hits the gas, tires break traction, she oversteers, lifts, hit brakes, and does a spin right into a pole. Meanwhile, if she plowed, she might have recovered.

If the answer is for handling, then yes, understeer it is.
grim,

So which is it then, (1) Porsche knows best or (2) Lawyers made them do it? I just want the real story and why wouldn't everyone that cares about performance? I realize there are plenty that will never track their cars, but I certainly will.

I posted looking for maybe something that I don't understand about the setup and dynamics that would make it different than what I understand from tracking my vehicles. Why would you inflate the rear so highly? So far all I see is that we should just accept it because either I'm dumb and Porsche knows best or that it is explained by the liability issue.

Next year I guess I will get a chance to test it, but I strongly surmise that even with reversed pressures (higher in the front), the recommended pressures will make it understeer significantly.

Matt
 
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