My IS250 comes stock with 225/45/17 (95V?) tires. I had a perfectly new set of 215/45/17 91Y Falken FK510 sitting in the basement that I had mounted. Now I'm trying to get the tire pressure dialed for comfort. Do I have to go higher than the door sticker numbers because I've gone down in load rating and the carcass is softer?
Probably.
Some manufacturers publish data showing load rating per pressure applied. (35 psi has greater capacity than 30psi, as you imply).
They based this upon maintaining a distance from the ground to axle center line to maintain proper tire section profile. That distance is different than the unloaded radius of course.
I'm not going to say it has no effect, but I would think the change in width would have a bigger effect (and is probably a big part of the reason for the reduction in load rating)
I've done that way before. Take the tire max load rating @ x pressure
(example: 2100lbs @ 51psi)
Now use your corner weight of the vehicle. Say 1200lbs.
Now figure out your ratio. 1200/2100=.571 or 57% of max load rating.
Then multiply by your rated pressure. 51 x .571 = 29.121 or 29psi.
I saw this method a long time ago on a Ford Ranger forum and I've had good luck with it and gotten even wear on my trucks.
In reply to Patientzero :
I've had good luck on the racecar using that method, and going both directions. Had to jump between tire sizes mid race and a little napkin math led to decent tire wear.
I think you'd be surprised at how little it changes. With obvious possible caveats, it takes about X psi to suspend your car's weight inside a tire. For a while I went from a stock 255/50-17 to a 275/35-20. I did a few tiny burnouts to check the contact pattern and the factory pressure was spot on.
The load rating of the tire is based primarily on the amount of pressure it can hold which is directly proportional to sidewall integrity (not to be confused with stiffness). The load rating is also based on heat during testing, but the max load rating is at max pressure which you'll never really see. The max pressure/weight is also saying nothing about being properly inflated. If you pump them to the max AND put their max weight on the tire, that is not a guarantee that they will be properly inflated for the contact patch. That just means they blew them up, tested them, and the temps didn't get too high.
The tire itself does almost nothing to support weight, it's the air inside. An E-range tire doesn't hold weight because it has stiff sidewalls, it holds weight because it can hold a lot of pressure... BECAUSE it has stiff sidewalls.
Do a tiny burnout. Not a smoky, crazy, destructive burnout, just enough to put rubber on the pavement. Take a look at the streaks you leave on the pavement. Even pressure across the patch? Good enough.
The correct pressure is the pressure that keeps the contact patch even and doesn't cause too much flex in the sidewall and generate heat. I think you'll find that the pressure on the sticker will be spot on for a very wide range of tire selections.
Curtis73 (Forum Supporter) said:I think you'd be surprised at how little it changes. With obvious possible caveats, it takes about X psi to suspend your car's weight inside a tire. For a while I went from a stock 255/50-17 to a 275/35-20. I did a few tiny burnouts to check the contact pattern and the factory pressure was spot on.
The load rating of the tire is based primarily on the amount of pressure it can hold which is directly proportional to sidewall integrity (not to be confused with stiffness). The load rating is also based on heat during testing, but the max load rating is at max pressure which you'll never really see. The max pressure/weight is also saying nothing about being properly inflated. If you pump them to the max AND put their max weight on the tire, that is not a guarantee that they will be properly inflated for the contact patch. That just means they blew them up, tested them, and the temps didn't get too high.
The tire itself does almost nothing to support weight, it's the air inside. An E-range tire doesn't hold weight because it has stiff sidewalls, it holds weight because it can hold a lot of pressure... BECAUSE it has stiff sidewalls.
Do a tiny burnout. Not a smoky, crazy, destructive burnout, just enough to put rubber on the pavement. Take a look at the streaks you leave on the pavement. Even pressure across the patch? Good enough.
The correct pressure is the pressure that keeps the contact patch even and doesn't cause too much flex in the sidewall and generate heat. I think you'll find that the pressure on the sticker will be spot on for a very wide range of tire selections.
makes sense. any tips on how to do a burnout on an AWD automatic IS250?
In reply to twowheeled :
Chain it to a tree and soak the tires in tire shine. Ken Block style.
Vegetable oil?
You really only need one front and one rear to spin and I'm sure you have open differentials
Please make sure to report back with a video of said burnout.
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