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Hi all. Have a question for you all. Mine is a weekend car. I have a ’13 C4S Cab, and recently I had a flat in the rear. Was going to replace that single tire, but looked at the date codes across all, and decided to take the full set plunge to swap from P-zero’s to PS4S (N spec). Am really looking forward to breaking these in. I got them on Tire Rack and had them shipped to Custom Alignment in Mtn View (for those of you in the Bay Area). Consummate professionals, and not a scratch on the wheels where they were done. Highly recommend in this regard.
Now here is the confusing part. Take a peek at the pic of my old Pirelli’s. Top tire had my puncture - was bigger than a nail, a big bolt or something. Top two tires are the rears. I was running at 30f/36r cold, which ended up being 33f/41r when warm. I loved the handling and noise was fine. Only drove on dry roads, but sometimes quite spirited. The fronts (on the bottom) are still in decent shape, but quite curious on the rears. Center section of the tires are almost slicks. I thought, wow, definitely overinflated. The manager there said that he has seen this before and it is due to under inflation. The sidewalls are stiff and will remain rigid, but I should have aired up. What say all you?? Thanks in advance!
Looks good to me. I don't think the tread block in the center section is as deep as the other blocks so when they wear they wear completely down to nothing.
You really need to get a gauge and measure where the indicators are to see how off a tire is wearing.
The mechanism whereby the center of a tire wears out due to under-inflation eludes me.
At very high speeds, the centrifugal force on a tire will cause the center of the tire outward, increasing it’s actual circumference. Watch a AA Top Fuel dragster leave the starting line. It’s tires increase in size significantly. This is the effect of very low tire pressures, soft sidewalls, and very high rotational speed. The same thing can happen to a much lesser degree with a car tire. It’s just the physics of what is happening as the tire rotates.
By running a higher pressure, the sidewalls stiffen and reduce this effect.
Looks good to me. I don't think the tread block in the center section is as deep as the other blocks so when they wear they wear completely down to nothing.
You really need to get a gauge and measure where the indicators are to see how off a tire is wearing.
Thanks much - good idea. I didn't measure these before they were disposed of, but they were really close to the wear bars in the rear. I've heard of other folks on here that go through 2 sets of rears for every set of fronts. Not sure if I'll end up the same, but since the tires were aging out and I didn't want to mix Michelin's and Perelli's, I went with the full swap. Also not sure since mine is a 4S. Thanks
At very high speeds, the centrifugal force on a tire will cause the center of the tire outward, increasing it’s actual circumference. Watch a AA Top Fuel dragster leave the starting line. It’s tires increase in size significantly. This is the effect of very low tire pressures, soft sidewalls, and very high rotational speed. The same thing can happen to a much lesser degree with a car tire. It’s just the physics of what is happening as the tire rotates.
By running a higher pressure, the sidewalls stiffen and reduce this effect.
Makes complete sense... and yes, my dad used to take my little brother and I to Baylands raceway in Fremont to watch drivers like Big Daddy Don Garlits and Shirley "cha-cha" Muldowney. Thanks for the memory.
My last physics class was in college over 40 years ago so pardon my ignorance. Centrifugal forces on a spinning object like a tire will expand it. I don't quite understand the differences in forces as one moves away from the center of the tire to the periphery. If one lowers the tire pressure, the contact patch is increased which I assume means more tire is resting on the pavement and susceptible to a more even distribution of the distorting forces when the tires are rotating. In which case, I would expect more even tire wear at lower tire pressure than at higher pressures. (The last I remember of an application of centrifugal forces in practice pertinent to my profession was when I used a speed vac to ethanol precipitate and pellet DNA in graduate school).
At very high speeds, the centrifugal force on a tire will cause the center of the tire outward, increasing it’s actual circumference. Watch a AA Top Fuel dragster leave the starting line. It’s tires increase in size significantly. This is the effect of very low tire pressures, soft sidewalls, and very high rotational speed. The same thing can happen to a much lesser degree with a car tire. It’s just the physics of what is happening as the tire rotates.
By running a higher pressure, the sidewalls stiffen and reduce this effect.
This makes sense. A bit. I discounted significant physical distortion because 30/35-series sidewalls are pretty dang stiff. But, with a wide-a$$ed (295+) tire there’s a lot of mass in the tread that’s subject to rotational force.
Those big dragster tires are intended to be a ‘spring’ between the road and the wheel barrel lest the barrel just turn on the tire bead. Part of their ‘job’ is to wind up when the hammer is dropped. You need a tall flexible sidewall for that purpose and the size change is a consequence of meeting the requirement to be a ‘spring.’
I was considering normal ‘mixed’ use, not exclusive use on the drag strip
Originally Posted by AnandN
My last physics class was in college over 40 years ago so pardon my ignorance. Centrifugal forces on a spinning object like a tire will expand it. I don't quite understand the differences in forces as one moves away from the center of the tire to the periphery. If one lowers the tire pressure, the contact patch is increased which I assume means more tire is resting on the pavement and susceptible to a more even distribution of the distorting forces when the tires are rotating. In which case, I would expect more even tire wear at lower tire pressure than at higher pressures. (The last I remember of an application of centrifugal forces in practice pertinent to my profession was when I used a speed vac to ethanol precipitate and pellet DNA in graduate school).
Right-^ Somewhat. Consider the corner cases: At low static pressure there’s more force on the sidewall. At high static pressure distortion of the tire puts more force in the center. Now add dynamic distortion due to rotation and pressure change (PV=nRT) depending upon load. So, doing mostly burnouts on cold under-inflated tires causes center wear or driving ‘granny-like’ on over-inflated tires causes center wear. The wider the tire the larger the effect.
The issue here is the conditions under which the tires were operated. I was assuming ’normal’ use. If you’re expending your tires on the drag strip, the autocross course, or various road courses then that’s not normal use. Cold pressure starting point is different for those uses.
Some simple information I found on Michelin’s website. Doesn’t take into account cornering etc. or driving around in tight circles on under-inflated tires. I run comfort pressure at partial load in my C4S and my Michelins seem to be wearing evenly with he rears down to 5/32 nds after about 8k miles on them. No sign of excessive wear at the outside edges despite some hard corners. My Continental winter tires have only 5k miles and seem to be wearing more slowly. They don’t get the same sort of thrashing as as my summer tires. Tread compound may also differ from outer to inner sections of the tire and and may respond differently to inflation pressure.
On an overinflated tire, the center of the tread bears most of the load and wears out faster than the outside edges.
04-07-2020, 01:51 PM
Tire pressure question
