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Redundant Tire Pressure Question?

4K views 17 replies 6 participants last post by  wnbamiamisol 
#1 ·
I have had my brand new '15 Wrangler Sport exactly a month.
I upgraded from the factory 225/70R16 on steel wheels to Pro Comp AT Sport 285/70R17 tires on Pro Comp 7005 17x9 rims. To begin with, 2 tires were out of round, yet 4wheelparts installed them, attempting to get a balance with 32 tire weights!?! At 400 miles on the Jeep, they installed 2 new tires in place of the defective ones. I bought an AEV ProCal and recalibrated the speedometer. I am STILL experiencing a rough ride. I had the balance checked at the Jeep dealership (high speed balancing) gratis. I am now up to 800 miles. I have ordered an OME steering stabilizer as per the Jeep dealer's suggestion.
The tire pressure was set to 37 and I have lowered it to 35. So I can eliminate that as an issue for the "bumpy" ride, what would you suggest as a tire pressure?
Stock suspension and I don't want to lift. This will 99.9% of the time be driven on the highway. Suggestions to help get rid of the feel of every crack and bump on the pavement?
Thanks!
 
#3 ·
Googled the tires and found old Loadindex 103 and new Loadindex 121.
Old 1930 lbs maximum load and new 3200 lbs maximum load.
most likely , but check it , both are standard load , wich carry maximum load up to 160km/99m/h AT 35 or 36 psi .
And because the kind of cars in this forum mosly have oversised tires , of wich one tire can bare the totaal Gross axle weight rating, even for the old tires you could do with about 20 psi , only for savety of tires.
New tires can bare about 1.5 times as much , so needed lowest save pressure up to 99m/h is even lower.

Then at 35 psi I would expect some bumping.

If you can give me the GAWR's , and check if tires are Standard load ( AT pressure 35 psi in USA system ,36 EUR) or XL/reinforced/Extraload( AT pressure 41 USA and 42 EUR) , I can calculate a lowest save pressure for you , and a highest pressure at wich comfort and gripp are still acceptable.
Trickysest part in it all is to determine the actual loads on seperate tires.

But also check if you have offroad-like tires with profile blocks that cover a part of sidewall, they are allowed lesser deflection then a road tire, but tiremakers give same Loadindex/maxload for them as road tire.

Added 2 pictures to show what I mean, one is compare and the other one is an even more offroad tire.

Greatings from a Dutch Pigheaded Self-declared Tirepressure-specialist.
Peter
 

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#4 ·
Curb weight is 3785 pounds.
Here is a photo of the tire. Definitely more off road looking.
 
#5 ·
I have had my brand new '15 Wrangler Sport exactly a month. I upgraded from the factory 225/70R16 on steel wheels to Pro Comp AT Sport 285/70R17 tires on Pro Comp 7005 17x9 rims. To begin with, 2 tires were out of round, yet 4wheelparts installed them, attempting to get a balance with 32 tire weights!?! At 400 miles on the Jeep, they installed 2 new tires in place of the defective ones. I bought an AEV ProCal and recalibrated the speedometer. I am STILL experiencing a rough ride. I had the balance checked at the Jeep dealership (high speed balancing) gratis. I am now up to 800 miles. I have ordered an OME steering stabilizer as per the Jeep dealer's suggestion. The tire pressure was set to 37 and I have lowered it to 35. So I can eliminate that as an issue for the "bumpy" ride, what would you suggest as a tire pressure? Stock suspension and I don't want to lift. This will 99.9% of the time be driven on the highway. Suggestions to help get rid of the feel of every crack and bump on the pavement? Thanks!
35psi is way to much!
Try 28-30psi and go from there. Have you done a Chalk test? I run 315/75/16 and for DD I'm at 25-28 psi. The only time I went above 30 was from long road trips but my Jeeps a trailer queen now so not an issue. And a steering stabilizer wound help a bumpy ride. A well set up front suspension doesn't need a stabilizer.
 
#6 ·
Factory, stock suspension, so....
Dealer suggested the steering stabilizer :-(. May just send that back when it arrives Friday.
No chalk test yet. I am having a difficult time finding sidewalk chalk (in the Florida Keys). Agreed that will give me a good idea.
 
#7 ·
Oh, chalk tested. Chalk worn off center of tire with plenty remaining on the outside edge at 35 lbs.
 
#8 ·
To be expected, I suppose, since the high pressure is bowing the tread.

My new tires (Mickey Thompson Baja ATZ P3, 285/70R17) came from the shop at 36 PSI (which is what the stock tires were set at)

Mickey Thompson suggested 42 PSI all around. That didn't sounds right to me at all, but I tried it ...they're the tire experts after all. It felt like I was running on 110 PSI road bicycle tires. Terrible. The guy must have been high.

Aired it down to 29 PSI cold, and it rides great.

FWIW
 
#10 ·
Copy that. D rated tire is for a Yukon Denali XLT. My Jeep is nearly 1/2 that weight.
 
#12 ·
Curb weigt of 3785 is as I recall not the maximum allowed vehicle weight.
GVWR = Gross Vehicle weight rating.
So I did some estimation and think GVWR=5000 lbs and front GAWR 2500 lbs, and rear GAWR 2750 lbs. These are the axleweights that the car-maker uses to calculate the pressure advice.
Can be that I am way off, so correct me then.

But the weight in normal use can be estimated from this curb weight.
I estimate at front axle 2000 lbs empty and rear rest of 1785 lbs.
2 persons of 200 lbs on front seat with gravity point ( about belly button) half between the axles means 200 lbs adding to both axles.
Then some lugage of say 100 lbs in the back so only on rear axle .
Gives Front 2200 lbs and rear 2085 lbs .

For these axle weight I will calculate.
Tires Li 121/3195 lbs would give F23psi R 21psi.

But I lower the maxload by 20%/8 LI steps because of the offroad story.
then LI 113/2535 lbs would give F 29 psi R 28 psi
Then you can drive up to 99m/h with these loads, you probably wont do that.

for up to 75 miles/h the Loadindex can be lowered by 4 steps so gives
LI 117/2833 lbs would give F26 psi R 24 psi.

My idea of savety is to use the middle of 29 psi all around for R/L misbalance.

Or it must be that I am mistaken about the curb weight , then correct me and I will calculate again.
 
#13 ·
Thanks. After the chalk test, I am at 30psi. Seems to have really smoothed out the ride. You are probably pretty close.
I have the rear seat removed and I am alone in the Jeep, weighing 130 pounds. Have added 40 pound rock crawlers.
Going to leave it at 30psi for a couple days to see.
 
#14 ·
Wanted to bump this up as the 4Wheel Parts store has conceded there is definitely a balance issue with the Pro Comp AT Sport 285/70R17 tires on my new Wrangler. As they come with a 60 day satisfaction guarantee, they will get swapped out on Thursday.
My choices are:
Toyo AT2
Nitto Grappler 2
I know same manufacturer. Experiences with either? Smoother ride for a DD?
 
#15 ·
If you decide on the Nitto Grappler 2, will you let me know your impressions. I'm thinking about those for myself as well and don't know anyone with real world experience on them.
 
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