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      12-09-2012, 05:14 AM   #1

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Tire Pressure for non RFT on E90

When changing tires from RFT to conventional or non RFT winters, do you adjust the tire pressure from the recommended on the door jamb? Was just wondering if the pressures that are recommended are specifically for RFT and whether I should run a little higher pressure as conventional tires have softer sidewalls.
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      12-09-2012, 12:42 PM   #2
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I would check the tire manufacturer's recommended PSI, making sure there isn't a large discrepancy from BMW's recommended.
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      12-09-2012, 12:51 PM   #3
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Lots of variables, and preferences.

If you're running a narrow sidewall on 18's 19's, you're going to want to go higher PSI to maintain a civil ride. 36-<40 psi isn't too uncommon.

Thicker sidewalls (like 50'series aspect ratio) you can get away with 32psi-34psi.

Next time you fill up, air up the tires to 36psi or so. Drive a few days, and if you want more a more compliant ride, air them down to 34psi. On non run-flats, for daily driving duties, I'd suspect the sweet spot to be around 33psi for something like a 205/50/16, and 37-39psi for thinner sidewalled non-rft's.

If you want turn-in response and less sidewall rollover with total disregard to a civil ride, 40-45psi is where you want to be.

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      12-10-2012, 06:00 AM   #4

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I kept myself busy with calculating tire-pressure, with use of the Formula that the European tyre-makers also use . Came much to know about tires and pressure in time surfing the internet. I am not a tire-specialist though.

But a RFT bares a lot of weight on its construction , and then especially on the sidewalls.
I estimate that when under pressure about 20% of the maximum load is bare by the construction and the rest (80%) by the pressure.
When the pressure is gone the tire deflects , I estimate again, about 2 times as much and bares about 80% of the maximum load on its construction .This makes possible to ride for about 50 miles with a lower speed , before they get that much damaged, that they also give up.
Read a topic somewhere that those tires have 2 kinds of rubber in the sidewall wich react different to heat, and yust like a bi-metal thermometer streches the sidewall .

A normal car tire bares about , again estimated by me, 5% on its construction and a tire with low aspect ratio ( further AR = the 40 in fi 235/40 ZR18) about 10% . And there is another thing for Low AR tires , and that is that I discovered that they are to low calculated in their maximum load by the tire-makers, and they know it, and that is the reason why the advice- pressures are kept verry high nowadays.

RFT tires , I think, are calculated right , because the deflection thing is better handled, because that is what they are desighned for.

I conclude from all this that you have to keep the pressure for your non RFTtires a lot higher to prefent damage and accidents by that.
Not getting damage is the only criterium for tire-pressure advice for the tire-makers. But most think these pressures are determined by severe testing.

If you give the Gross Axle weight ratings ( GAWR) of the car and the data of the tire ,written on the sidewall, I will calculate a save pressure for fully loaded , as the tire-makers also give nowadays. for normal use I have to know more, like empty weight and weight division Front/Back, and how you use it ( number of persons and loading).
Then I will calculate it wiht my own formula wich is saver then that of the tire-makers, and add an extra for low AR tires if they are.

Greatings from Holland
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