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What's up withe the tires on this car?

6K views 25 replies 12 participants last post by  Audioi 
#1 ·
I just got a 2013 AWD Premium with 18's and when you look at the side profile the tires seem to be lost in the wheel wells! It appears this car needs a larger overall tire?? And why the staggered air pressure? 35 front, 40 rear???
Call me inquisitive but...
 
#2 ·
For one thing, they try to keep the speedo accurate by not using huge tires. Putting tires with bigger sidewalls will also make the car more floaty and less grounded to the ground.

Tire inflation recommendations are based upon the weight distribution of the car. The car with no passengers may have a 50/50 weight balance, but not when passengers are in it. BMW recommends less pressure in the front than the back for my M3, so Caddy is not crazy.
 
#5 ·
Higher inflation is used to reduce sidewall deflection on the tires powering the car= better handling characteristics and more contact patch on the roadway.

Because you have a Premium AWD, I'm fairly certain you have the wider profile rims on standard 235 tires all around, so it might look a bit stretched(or not, just a guess).

IE, Premium FE3 trim RWD ats's have staggered tire sizes of 235 front/255 rear.

You still have the pressure differences because this is a largely RWD biased AWD system.
 
#10 ·
According to my onstar report, the car is supposed to have 32 psi front and 35 psi rear, i believe this to be a safety liability thing from GM to promote understeer which is the safer condition for less experienced drivers, 35 psi all around should be more fun, and possibly quicker, i've seen in some BMW places that BMW does the same thing, but people who play harder with their BMW's go 35 all around. and as a side note i've had the car in the dealer for paint buffing (hood scratched from dealer they got car from), steering wheel replaced (stitches unraveled 2 to 3 inches and growing), HVAC tapping calibration, oil change, and HVAC update TSB, so in 6 visits each time my invoice had a line stating tire pressures checked and set to 35 all around.
 
#17 ·
I'm currently running 35 front 38 in the rear, but I'm looking to increase the front pressure to run Square all around


Problem is, even at 35 front, things can get a little jiggly over rough roads. Plus I'm at about 10,000 miles, which means the tires are beginning to harden and are less compliant.
 
#18 ·
Although both my BMWs recommended 32/35, I have been running them at 35 square without any problems. Tires last longer and the ride is well controlled and precise. So, being used to this kind of ride, I have also been running the ATS at the same pressures without problems. Yeah, it might be riding a little stiffer than at the recommended pressures, but I feel there are two benefits that override the occasional jitteriness: 1) More precise handling, 2) Better tire wear.

BTW, on my 325, the right rear tire wore out faster than the other tires. Guys at Tires Choice told me the reason was that with my rear wheel drive, the initial power/traction is directed to that wheel and thus it gets more use???
 
#19 ·
BTW, on my 325, the right rear tire wore out faster than the other tires. Guys at Tires Choice told me the reason was that with my rear wheel drive, the initial power/traction is directed to that wheel and thus it gets more use???
325i has an open rear diff, which is what I'm assuming you have. Power is not distributed specifically to that wheel by design. The tire guys are talking out of their asses if that's exactly what they said.

If by some coincidence you kept accelerating hard and the rear passenger side tire was always spinning, then yes it could happen faster. But first I'd make sure everything else in the alignment was ok, the tire was inflated properly, and there were no structural issues with the tire.
 
G
#25 ·
Changing tire pressure is not really a good way to "tune" the handling of a car, particularly for normal street driving. There is really only one proper tire pressure for street driving under 100 mph and that is primarily determined by the load (weight) on the tire. Too much pressure makes the tread bulge out in the middle, the contact patch gets smaller and you wear out the center of the tire. Too little pressure doesn't support the tread enough and the weight of the car rides on the edges of the tire, wearing that part out. You do get a bigger contact patch though, which is why off-roaders "air down" their tires when they leave the highway. If you are carrying a full load of passengers and baggage, you need to add air pressure because the weight the tire must support has increased. That is why the tire pressure placard on the door has multiple pressures listed. Too little pressure for a given load on the tire can cause it to dangerously overheat as well due to over-flexing of the carcass. Having the proper air pressure for a given load means the tread evenly supports the weight of the car across the whole width of the tire, giving you an optimum contact patch.

The sharpness of the handling, as affected by the tires, is primarily determined by the sidewall stiffness that's built into the carcass of the tire. "Sport" tires have stiff sidewalls and quick handling, even jittery, "touring" tires have soft sidewalls for a good ride but "mushy" handling.
 
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