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Old 09-20-2013, 04:19 PM   #1
tmumolo
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Tire Pressure vs Weight Distribution

Recommended tire pressure on my 2012 Focus is 35psi cold, front and rear; That is based on a fully loaded vehicle; meaning at least two adults front and back plus cargo.
If you drive mostly alone with no cargo or with one passenger it would seem that 35psi all around may be less than optimum for overall ride and handling. ( I'm not talking performance driving) I'm talking every day street and highway driving.
I'm well aware of the 10 degree adds 1psi temperature affect on tire pressure so I am allowing for that. I'm thinking of reducing rear tire pressure 1psi with no load.
When you consider that the front tires always have about the same amount of weight bearing down on them and the rear tires do not; it would seem that making moderate adjustments based on personal preference and weight distribution makes sense.
When I set tire pressure at recommended levels when the ambient temperature is very cold or very hot ; I get a very different ride and performance characteristics.


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Old 09-20-2013, 04:24 PM   #2
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You're overthinking it if you're worried about a 1 PSI change.

Figure out what you want out of your tires (comfort, mileage, performance, whatever) and inflate accordingly. Obviously checking pressure without adjusting for ambient temp will lead to poor results.
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Old 09-20-2013, 04:50 PM   #3
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I wasn't addressing specific tire pressure variations i.e. 1psi, 2psi 3psi, I was addressing the more fundamental question of weight distribution vs pressure. As I said I'm not talking about ultimate performance. It seems that the tire pressure conversation often ignores weight vs pressure considerations.
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Old 09-20-2013, 05:15 PM   #4
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What you have for 'contact patch' is idealizing the weight distribution. you want the same size contact patch all four wheels. The size varies with the weight/vs tire pressure.

On my last car i did some tests by jacking up each corner then taking a tire footprint in paper.
On that Contour SVT the best equal patch was obtained with an eight PSI differential front to back. So the front tires would have 8PSI more than the rear tires whatever the pressures were.
That car was supposed to use 32PSI all around. So I usually was at 41 front and 33 rear.
Without bothering to check my current Focus, I 'guesstimate' the front should be 6 more PSI than the rear as the engine is lighter.(4 vs V6)
I use 39 rear, and 45 front in my Focus.

Last edited by Elizabeth; 09-21-2013 at 02:13 AM.
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Old 09-21-2013, 09:54 AM   #5
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Your logic is impeccable on equalizing contact patch via psi! Doing a test is a great idea but I won't go that far. I'm going to try 35psi front as per Ford recommendation and 33psi rear unless I have rear passengers and cargo then I would go back to 35psi all around. Your pressure choices are a bit too high for my personal comfort factor but I think that's just personal preference as long as you don't go much under Ford recommendations. Thanks for you input. Good stuff.
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Old 09-21-2013, 12:05 PM   #6
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I like really tight fast cornering and high pressures in the tires make that way easy.
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Old 09-21-2013, 02:49 PM   #7
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Tire pressure advice is all about load on the seperate tires.
A bit about speed and alighnment ( camber angle tires like this on the axle /-\)
so if you can determine the exact weights on the axles, or better seperate wheels, you can calculate a pressure for that.

I once got hold of the formula that the tire- and car-makers use and went running with it. Made many spreadsheets for it.
So if you can give the loads and the tire-specifications, I can calculate a save pressure.
From tires the maximum load or loadindex, and the kind of tire to determine the reference-pressure ( Pr)wich is not the maximum pressure of the tire.
XL/Extraload/reinforced need 41 psi Pr and if you dont find it on the sidewall, its a normal car tire ( P-tire in America) with Pr of 35 psi.

Could be that the car maker adviced the Pr of a normal car tire standard , because that is done often after 2000.
Some off road tires need higher pressure because the profile blocks cover a part of the sidewall and this leaves lesser sidewall to flex, so write it if you suspect it to be such a tir.

Before 2000 in Europe , also a normal use advice was given for 3 persons and a little load, wich gave front higher advice then behind.
Then I am not talking about 1 psi difference , for insance 37/32 psi .
could be that if we calculate, it comes to lower pressures also front, because of the generally given Pr advice after 2000.
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