Another rear tire wear question - worn out in the middle
 

I’ve owned my 4.8is for about 3.5 years. When I bought it at 55K miles it was fitted with Dunlop runflat tires which gave a harsh ride. They also wore down to the cords on their inner edges after about 10K miles.

At this point I did a major suspension refresh replacing the rear trailing arm bushes, the track control arms and the camber arms. The air suspension was set to about 1 inch lower than factory spec and I had a four wheel alignment done. The rear cambers ended up at -2.8 degrees due to limits in the adjustment. The toe ended up at 0.18 and 0.2 degrees - I don’t recall if this was limited by the adjustments or not.

At 60K miles the runlets were ditched and replaced by Falken FK510s. Fast forward 10K miles and I was startled to find the middle of both rear tires worn down to the wear bars while the inner and outer edges were still at 4mm.

I run the rears at roughly 34 psi. Is my high center wear due to the inflation pressure or the alignment? I would have predicted that a toe issue would have worn one edge more than the other, not the middle. Or ore the Fallen FK510s just made from recycled marshmallows? And feedback or advice is appreciated.

Almost all center wearing is a pressure issue (over pressurized) but 34 shouldn't be anywhere close to that. The only other things that I can think of is a bead depth problem if the pressure before was the same and the only difference is tire make, then check it out. Previous stiffer sidewalls and too low of pressure would wear the outer edges while leaving the center not in contact, or conversely weak sidewalls now and low pressure can roll sidewalls in leaving center with the most contact.

Break your beads and see if you have rubber sand inside, if so the you definitely have a pressure problem.

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Thanks for the quick reply. The tires were replaced yesterday with Toyos - the shop didn’t make any comment about the situation other than observing the tires were shot.

that's a shame... one thing to consider: do you use your own pressure gauge all the time? Maybe it's well off the mark. Center wear sounds like overpressure to me

And for the sake of completness... I replaced 2, 4, 6 and 11 in the diagram.

Actually the center wear is more likely caused by under pressure, not over pressure. With too low pressure the centrifugal force changes the tread surface.

Over inflation is a sure sign of center tread wear. Note: driving with one occupant with big fat rear tires and following the recommended inflation as marked in the owners manual and door jam will give you the results you are experiencing with a 4.6 and a 4.8is with 315 width tires. Tire foot print is based on tire width, rim size, and weight on at the wheel. Ignore those who post high numbers as they are running skinny tires on the 6 banger or 4.4 applications.

I run approx 32 psi in the rear after burning through two sets in 20k. These things weigh in at 3600# and will burn off Hugh amount of tread on hard acceleration runs.

Note 2: if you are planning hard runs with lots of hard acceleration out of turns, then bump the rears to 36 psi to keep the tire planted.

Center wear is pressure too high, unless you drive improbably fast.

Pretty confident my pressure gauge is accurate - I also installed an Xtrons head unit with a TPMS using valve cap sensors and the TPMS readings matches my gauge.

Euro tire pressures on the door edge are 2.2 bar (32 psi) front and 2.4 bar (35 psi) rear for a lightly loaded car - this is pretty much what I run. Do US spec cars with 20 inch wheels run the same pressures?

Remember that tire pressure and alignment are compromises between handling requirements, ride quality requirements, load rating, AND tire wear.

If you care more about tire wear than anything else, you probably want a different pressure.

An extreme example of this is my pickup truck. The tire pressure spec is 50 psi front, 80 psi rear. Unlike BMW, they don't do a loaded/unloaded spec. Empty, the truck is about 4,000lbs. on the front axle and 3,000lbs. on the rear. You can probably guess that 80 psi in the rear is totally unnecessary, as long as you aren't trying to tow heavy. Net-net, I run it at 50psi all around unless I'm towing. Tire wear and ride quality and handling are all better this way. Load rating is less, though.