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Time for me to learn about tire pressure..a little help please.

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YeloPony

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Discussion starter · #1 ·
Of course I know tire pressure is.. but what affects is, and what effects does it have... here's my situation..

I have Nitto Invos 255/35 & 275/35 18x9, 18x10, they are 280 rated.. I have a semi-aggressive alignment for AX that is still good for DD... max psi says 40PSI on the tire.

I looked recently and had only 24.5PSI in the Driver Front, and 34 or so around the others, so I put them all at 36.5 on Sat Night.. Drove Sun Morning 2 Hours(5am -7am), parked the car and by 1ish I was about to do my AX Runs, I checked and my rears were up to 38psi, fronts were higher too, like 37ish. I'm fairly certain (based on what I see on NASCAR) that heat build up the PSI...

my questions what should I run and what will I feel in driving to know I need to check or adjust it, I notice that a lot of the time (usually early morning and cooler temps) my wheels feel like they are out of balance at 65-75mph, it's very slight vibration, that I don't feel when it's warm or if I've got the tires heated up real good... also I believe these tires 'feel the road' a bit more than some. [BTW: The tires are new, and balanced, alignment is recent] Is this normal for tires like this?

Any info related to this is appreciated.. Thanks!
 
#2 ·
My R888's are rated up to 48 PSI, as I recall. I set them to 36 PSI when cold, and through trial and error, I have found this to be the best temp for me. Last AX event I was at, I had a buddy check them with a pyrometer and found exactly the same temp across all four tires after my run. I have not done this after hours of DD use, but unless I have an issue, I will keep them there.

I generally recommend one start the tires at about 12 PSI below max when cold and see how they feel over the course of driving.

Jazzer :)
 
Discussion starter · #3 ·
Jazzer The Cat said:
My R888's are rated up to 48 PSI, as I recall. I set them to 36 PSI when cold, and through trial and error, I have found this to be the best temp for me. Last AX event I was at, I had a buddy check them with a pyrometer and found exactly the same temp across all four tires after my run. I have not done this after hours of DD use, but unless I have an issue, I will keep them there.

I generally recommend one start the tires at about 12 PSI below max when cold and see how they feel over the course of driving.

Jazzer :)
Click to expand...
Sound's good they are all set to 36.5 as of yesterday... So 28 would be 12 under 40...I'll check again.. does it sound right that I have a little movement with them? I know they are starting to wear on the outside edge a bit, and it may cause that too? but it's funny how they get heated up and feel great!

What would make them ride worse, over or under PSI?
 
Discussion starter · #4 ·
So just looked at my tires and they are 50PSI Max so yeah 24.5PSI was probably pretty bad and some caused some of the wearing issues..

Image


Here's the specs on my alignment

LF
-2.0* Camber
3.2* Caster

RF
-2.3 Camber
3.7 Caster

Total Toe -.14

LR
.3* Camber

RR
-.4* Camber

Total Toe -.17*
Thrust Angle .17*
 
#5 ·
I dont know if it will help any, but I read an old trick in something like road and track or such to calculate your desired pressure; I have yet to try this so I dont know if it actually works but conceptually it makes sense. (maybe Jaz or someone can correct me since i'm still learning myself?)

Take some white shoe polish and run white lines across all 4 tires and partially up the side walls, then go out and driver around at your desired pace and turning speed, making sure to keep the number or lefts and rights equal etc. and monitor the wear.
none on center = too much pressure
none on outside or inside corner = too little pressure
I would think that extremely agressive alignment or bad alignment would totally destroy this idea though.
 
Discussion starter · #6 ·
GidEup said:
I dont know if it will help any, but I read an old trick in something like road and track or such to calculate your desired pressure; I have yet to try this so I dont know if it actually works but conceptually it makes sense. (maybe Jaz or someone can correct me since i'm still learning myself?)

Take some white shoe polish and run white lines across all 4 tires and partially up the side walls, then go out and driver around at your desired pace and turning speed, making sure to keep the number or lefts and rights equal etc. and monitor the wear.
none on center = too much pressure
none on outside or inside corner = too little pressure
I would think that extremely agressive alignment or bad alignment would totally destroy this idea though.
Click to expand...
Yeah, it works, most guys mark their tires on the first run, if the marks are not being worn then your pressure is too high.. I haven't done that yet, probably next time, right now I just need to know what it should be normally :)
Thanks!
 
#7 ·
For the line trick, that's basically it, but the important part isn't that the marks are wiped off the face of the tire (because they will be no matter what pressure you're at), but where the marks are rubbed off to on the sidewall. Basically, you want to rub off the marks right to the edge of the treadface, and no further. If the mark is still on the treadface, you have too much pressure. If it's rubbed off on the sidewall, you don't have enough pressure.

Now, that's not exact. If you still have marking on the treadface, that could also be due to having an improper camber setting, not just pressures.

That's why it's a lot better to use a probe-style pyrometer, and check the temps. You want to check the inside, middle, and outside of each tire after your run, and compare those. Ideally, you should see a slight gradient of temperature, with the outside just slightly cooler than the inside (this is because the outside of the tire cools more due to airflow). If the center is hot, that means too much pressure. Center cold means not enough. Outside hot means too little negative camber, inside hot means too much negative camber.

And if you're having camber issues, remember that this is a strut setup, so you're losing negative camber under compression. To combat this, you can get a little bit of dynamic camber gain by setting a high caster - basically this causes the camber to increase the further you turn the wheel.

Looking at your settings, I would suggest more positive caster, and adjust camber per temps. I'm running 4.5 degrees of caster, but that's partially due to the fact that I can NOT seem to get ANY camber... I'm maxing out at something like -0.5 degrees on both sides. That could be incompetence on the part of the alignment shop, though, I haven't bought an angle guage to see what I can do myself.
 
Discussion starter · #8 ·
Thanks Rev, that's some great info I'll need to soak it up.. but given the markings on my tires (Above) there is some marking on the sidewall so that means it was too low or too negative?
 
#9 ·
Hrm... it'd be a lot easier to explain in person.

Basically, there's the treadface, that you want to maximize exposure to the pavement. Then there's the sidewall, which you don't want scrubbing on the pavement. You want the mark to stay on the sidewall, but not on the treadface. If it's still on the sidewall that means you at least have enough pressure, if it's too much, you'll still see some of your markings on the treadface.

Having too much negative camber would also leave marking on the outside sidewall. Being that you're in a Mustang, I *really* doubt you have too much negative camber, though.
 
Discussion starter · #10 ·
ReverendDexter said:
Hrm... it'd be a lot easier to explain in person.

Basically, there's the treadface, that you want to maximize exposure to the pavement. Then there's the sidewall, which you don't want scrubbing on the pavement. You want the mark to stay on the sidewall, but not on the treadface. If it's still on the sidewall that means you at least have enough pressure, if it's too much, you'll still see some of your markings on the treadface.

Having too much negative camber would also leave marking on the outside sidewall. Being that you're in a Mustang, I *really* doubt you have too much negative camber, though.
Click to expand...
That makes sense.. Thanks Man.. next AX they'll be some mustang guys w/ experience around I'll chat with them too..
 
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YeloPony
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