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Detailed questions and write-up on aftermarket tire pressures / alignment

EraZorX

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You can see from the photos below that while my front tires are fully touching the ground, it seems like the rear tires are not:
unnamed (2).jpg
unnamed (4).jpg

The dusty but mostly flat road basically has the same effect of "chalking" my tires to check tire contact.

I am running 275 35R18 tires (Ps4s), on 18x9.5+45 wheels (TC105X by WedsSport), at 32 PSI front and 30 PSI rear.
Stock tires have a tire load index of 90, the new tires have a tire load index of 99.
Alignment is shown below:
alighnment.jpg


So, my question is:
  1. Is it considered normal for the rear tires to not site fully on the ground, while the front ones do?
  2. If not, what is the problem?
    1. Is it related to alignment?
      1. Despite the differences in camber for the left and right side of the car, the wear looks quite similar on each side - the difference is between the front and rear.
    2. Is it related to tire pressure?
      1. When tires are overinflated, there is more wear in the middle of the tire - is that what's happening here for the rear tires at 30psi?
      2. Looking at the tire load index table (attached to this post), and following instructions for applying tire load index tables, it seems that if my old tire pressures(psi) were 35/33 for stock wheels at a tire load index of 90, the new pressure for load index of 99 "should" be... 25/26 PSI, which seems crazy low. I've set the pressure to be 32/30 as a compromise for a somewhat lower tire pressure that still seems "sane".
      3. It is very common advice when up-sizing wheels is to increase tire pressure, so it seems like it would make sense for tire pressures to decrease tire pressures. The question is if I decreased it enough.
    3. Is it related to offset?
      1. Based on this tool, the scrub radius did change a bit with the new wheel size, but I'm unsure of the real-world impact.

This set up is intended for street use; I'll figure something else out for track tire pressures.
Car is stock besides the wheels / tire changes. The car is on stock suspension with no lowering springs.

I'm hoping someone on these forums can help me, I tried contacting Michelin, but had a strange interaction with them - they first recommended 25psi all around, then changed stance and recommended door jamb pressure (35/33) when I asked them for their reasoning.



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ayau

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Almost every car in production has more negative camber in the rear than the front. Your alignment sheet supports this. Your alignment also looks normal to me.

Not really related to tire pressure as long as you run the factory recommended pressure. Seems like you're on the lower side. I'd bump the tire pressure back to factory if this is just used for the street.

I wouldn't rely too much on what you see visually. When the surface isn't flat, which most aren't, your camber may look off. The alignment rack will equalize this to give to give you the correct numbers.
 

FlexRex

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Whats bothering you here? Chalk lines not the same on front and rear tires? Is that it?
 
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EraZorX

EraZorX

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Almost every car in production has more negative camber in the rear than the front. Your alignment sheet supports this. Your alignment also looks normal to me.

Not really related to tire pressure as long as you run the factory recommended pressure. Seems like you're on the lower side. I'd bump the tire pressure back to factory if this is just used for the street.

I wouldn't rely too much on what you see visually. When the surface isn't flat, which most aren't, your camber may look off. The alignment rack will equalize this to give to give you the correct numbers.
Yes, but the camber difference between front and rear isn't that large - the left rear tire still appears to have significantly less contact than the front left despite a very similar camber, so it seems like camber isn't what's causing this.
Ultimately, I understand this "isn't a big deal" per say - I'm just curious what would be optimal. The most common (and safest) advice is to run at factory pressure, but my question here is more around what's optimal for the changed tire set up for good tire wear / good grip.


Whats bothering you here? Chalk lines not the same on front and rear tires? Is that it?
Basically, yeah. I'd like to know if that's something worth fixing, or just normal behavior. I didn't chalk my tires before the change, so I don't have that comparison.
 

ayau

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The dirt trail on the front left three almost look like you have 0 camber. You can pick up a digital angle gauge from harbor freight to check. Does the front right look the same despite the alignment sheet saying it should have more negative camber?
 

FlexRex

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I dont think its a good method to use. There is the weight of the engine on front wheels and you turn with them thus scrubbing the sidewalls higher than the rears. Its never gonna be the same.
 
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EraZorX

EraZorX

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The dirt trail on the front left three almost look like you have 0 camber. You can pick up a digital angle gauge from harbor freight to check. Does the front right look the same despite the alignment sheet saying it should have more negative camber?
Yeah, front right looks pretty much similar even with more camber. I'd be concerned if the alignment place had their equipment that far off calibration.

I dont think its a good method to use. There is the weight of the engine on front wheels and you turn with them thus scrubbing the sidewalls higher than the rears. Its never gonna be the same.
That makes sense, I did make a big U-turn before taking this photo, so that could account for the difference. I figured the difference in tire pressure would normalize the difference in the weight over the front and the rear axles, unless that's not how it's supposed to be.
 

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