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Unread 04-22-2013, 03:49 PM   #16
jwmbishop
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Quote:
Originally Posted by starscream View Post
No no, for safety reasons, you should never exceed the max psi (cold) at any weight. I think you're confusing max psi with max load psi, which can be two different numbers.
I was taught in school (San Jose State Engineering Department) that tire manufacturers DO NOT set the minimum and maximum PSI - they only set the MAXIMUM LOAD and the PSI needed to properly hold the load and maintain adequately low sidewall temperatures. It is then up to the manufacturer to determine the PSI needed for the load that specific vehicle needs. Maybe the info was wrong - so I will let you teach me using the below examples.

MAXIMUM load 2405 @ 50 PSI (COLD) (stock Rubi tire).
MAXIMUM load 3195 @ 65 PSI (cold) (replacement pitbulls)

That's exactly what it says on my JK tires (and same wording different numbers on the F250, the caddy, The GTO, and the trailer).

Please bold the "MAXIMUM PSI" language in a re-quote for me.

THEN you may want to re-write all of Goodyears "Safe Loading and Inflating Guides" where they give the exact same explanation I was taught and specifically state ALWAYS SET TIRES COLD - AND DO NOT BLEED OFF HOT PRESSURE GROWTH. Who knows they may pay you for correcting them! After all if THEY are wrong they could be SUED as folks COULD die when SUVs have blowouts on the freeway.

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Unread 04-22-2013, 04:18 PM   #17
starscream
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Quote:
Originally Posted by jwmbishop View Post
I was taught in school (San Jose State Engineering Department) that tire manufacturers DO NOT set the minimum and maximum PSI - they only set the MAXIMUM LOAD and the PSI needed to properly hold the load and maintain adequately low sidewall temperatures. It is then up to the manufacturer to determine the PSI needed for the load that specific vehicle needs. Maybe the info was wrong - so I will let you teach me using the below examples.

MAXIMUM load 2405 @ 50 PSI (COLD) (stock Rubi tire).
MAXIMUM load 3195 @ 65 PSI (cold) (replacement pitbulls)

That's exactly what it says on my JK tires (and same wording different numbers on the F250, the caddy, The GTO, and the trailer).

Please bold the "MAXIMUM PSI" language in a re-quote for me.

THEN you may want to re-write all of Goodyears "Safe Loading and Inflating Guides" where they give the exact same explanation I was taught and specifically state ALWAYS SET TIRES COLD - AND DO NOT BLEED OFF HOT PRESSURE GROWTH. Who knows they may pay you for correcting them! After all if THEY are wrong they could be SUED as folks COULD die when SUVs have blowouts on the freeway.
Goodyear Fortera on my wife's GC:

350 kPa (51 psi) MAX PRESSURE.

Cooper A/T 3 on my pickup:

MAX PRESSURE 300 kPa (44 P.S.I.)

Taken verbatim off the sidewalls.
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Unread 04-22-2013, 04:30 PM   #18
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Quote:
Originally Posted by starscream View Post
Goodyear Fortera on my wife's GC:

350 kPa (51 psi) MAX PRESSURE.

Cooper A/T 3 on my pickup:

MAX PRESSURE 300 kPa (44 P.S.I.)

Taken verbatim off the sidewalls.
Gotcha! (and on the Coops on your P/U - those must be the load range C's - 1985 MAX @ stated MAX of 44! - see paragraph three below! Not really a truck tire...)

From Tire Racks site:

Quote:
A tire's maximum inflation pressure is the highest "cold" inflation pressure that the tire is designed to contain. However the tire's maximum inflation pressure should only be used when called for on the vehicle's tire placard or in the vehicle's owners manual. It is also important to remember that the vehicle's recommended tire inflation pressure is always to be measured and set when the tire is "cold." Cold conditions are defined as early in the morning before the day's ambient temperature, sun's radiant heat or the heat generated while driving have caused the tire pressure to temporarily increase.

For the reasons indicated above, It is also normal to experience "hot" tire pressures that are up to 5 to 6 psi above the tire's recommended "cold" pressure during the day if the vehicle is parked in the sun or has been extensively driven. Therefore, if the vehicle's recommended "cold" inflation pressures correspond with the tire's maximum inflation pressure, it will often appear that too much tire pressure is present. However, this extra "hot" tire pressure is temporary and should NOT be bled off to return the tire pressure to within the maximum inflation pressure value branded on the tire. If the "cold" tire pressure was correctly set initially, the temporary "hot" tire pressure will have returned to the tire's maximum inflation pressure when next measured in "cold" conditions.

A tire's "maximum inflation pressure" may be different that the assigned tire pressure used to rate the tire's "maximum load." For example, while a P-metric sized standard load tire's maximum load is rated at 35 psi, many P-metric sized standard load performance and touring tires are designed to contain up to 44 psi (and are branded on their sidewalls accordingly). This additional range of inflation pressure (in this case, between 36 and 44 psi) has been provided to accommodate any unique handling, high speed and/or rolling resistance requirements determined by the tire and vehicle manufacturers. These unique tire pressures will be identified on the vehicle placard in the vehicle's owner's manual.
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Unread 04-22-2013, 06:19 PM   #19
starscream
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Quote:
Originally Posted by jwmbishop View Post
Gotcha! (and on the Coops on your P/U - those must be the load range C's - 1985 MAX @ stated MAX of 44! - see paragraph three below! Not really a truck tire...)

From Tire Racks site:
That third paragraph of the tire rack site is essentially what I said. I'm glad now you realize there are two different numbers and you may not have been taught everything correctly in school.
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Unread 04-22-2013, 08:51 PM   #20
222Doc
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one thing you have take in here too. First off the op has 8"rims on a 12.5 wide. This SOP to run anywhere from 25-28 psi on this tire/ rim set up. it aint in no book. Its a great set up off road and an ok set on the road. Leaning more to an off road set up. C rated work better on Jeep, but its getting harder to find them as well. Many C rated tires are weak on side wall. Why I would only use Kevlars.

Tire rack and the Book, a school would say you need more rim. Some may not even mount this set up as its out of all specs. But a 10 rim on a 12.5 "they would say yes to". Off road you can not air down enough to even be worth trouble of air down. Tires dont even start to get soft enough until about 12psi. the rim of a 10? would get beat to pulp on the rocks.
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Unread 04-23-2013, 02:07 AM   #21
jadatis
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Quote:
Originally Posted by jwmbishop View Post
I was taught in school (San Jose State Engineering Department) that tire manufacturers DO NOT set the minimum and maximum PSI - they only set the MAXIMUM LOAD and the PSI needed to properly hold the load and maintain adequately low sidewall temperatures. It is then up to the manufacturer to determine the PSI needed for the load that specific vehicle needs. Maybe the info was wrong - so I will let you teach me using the below examples.

MAXIMUM load 2405 @ 50 PSI (COLD) (stock Rubi tire).
MAXIMUM load 3195 @ 65 PSI (cold) (replacement pitbulls)

That's exactly what it says on my JK tires (and same wording different numbers on the F250, the caddy, The GTO, and the trailer).

Please bold the "MAXIMUM PSI" language in a re-quote for me.

THEN you may want to re-write all of Goodyears "Safe Loading and Inflating Guides" where they give the exact same explanation I was taught and specifically state ALWAYS SET TIRES COLD - AND DO NOT BLEED OFF HOT PRESSURE GROWTH. Who knows they may pay you for correcting them! After all if THEY are wrong they could be SUED as folks COULD die when SUVs have blowouts on the freeway.
Registered to this forum to give more information about tire-pressure.
End 2007 I got hold of the formula that the European tyre-makers use to determine the advice pressures , and went running with it.
Learned myself Excell to make spreadsheets for it, and translated a few to English to go worldwide with it.
Came to the conclusion by reacting on several American fora, that there are a lot of loose ends at the way it is calculated , especialy in America, but also in Europe the maximum load of some tires is given to high ( low aspect ratio tires and Off-road(looking) tires with profile that cover a lot of the sidewall).
Planted a lot on my skydrive that belongs to my hotmail.com adress with the same user name as here.
https://skydrive.live.com/?cid=a526e...E092E6DC%21128
Dont have a Jeep , so wont introduce myself in a seperate topic.
Also I am not a tire-specialist, but think I know more about this pressure-part then many in the field.

So now my reaction here.
I would like to discuss further with mr Bishop , mayby by personal message or by mail ( cripted already my E-mail to prefent spamm), think I can learn more from you. Edit: reading back I see tha starscreem seems to know more about tires too, so for you the same story.

But to react to your reaction.
The @pressure that is written in your example is almost always used on C-load/6ply tires and up, and is not the maximum pressure of the tire.
In the formula sheet it was called the reference-pressure, is the pressure at wich the maximum load can be caried riding up to maximum speed of tire, or if lower 160km/99m/h, without damaging the tires by driving alone.
If you read "maximum load xxxx lbs AT yyy psi(cold)" then the yyy is always the reference-pressure ( furter Pr) ,saw it written also maxloadpressure.
TRA ( Tire and Rimm Assosiation) even allowes 10 psi above this for higher speed and more maximum load at low speed, and better riding quallity for LT tires , and even 20 psi for Truck-tires.
But for Jeep tires this is not needed, they are that oversised that a low pressure can carry the load savely.
On the other hand Jeeps have often those off-road ( looking) tires with the large profile blocks that cover a part of the sidewall, so that sidewall may not deflect that much as the tire-maker used for their calculation of the maximum load, I calculated that you have to substract mayby even 20% of the maximum load printed on the sidewall.

To cut my long first post short, If you have C-load/6ply (Pr=50psi)tires or up, you have to use higher pressure for the same load , then a normal P-tire with its Pr of 35 psi for american tires ( Pmax 41 to 50 psi only printed on the sidewall.

If you want me to calculate an advice-pressure with my own system, give me the GAWR's or real weighed axle or wheel weights in the way you drive the car. From the tires the maximum load and Pr or kind of tire , and I will calculate the lowest pressure at wich no tire damage happens. Above that is allowed , but if you go to high, comfort and gripp gets lost to much.
Or try it yourselfes with one of my spreadsheets, there are a lot English on my skydrive between the Dutch.

Last edited by jadatis; 04-23-2013 at 02:35 AM..
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Unread 04-23-2013, 03:11 AM   #22
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Been looking back and found topic-starter Droptop306 his data.
Thoug you cant yust devide curb weight by 4 , I filled it in in my camperRV tirepressure-calculator, wich can do herebecause speed wont be above 99m/h, with a reduction of 20% to the maximum load, because you probably have those offroad-tires or those with the looks of it .
here the direct examples-map with the pdf and the spreadsheet, so you can chanche yourselfes.
https://skydrive.live.com/?cid=a526e...E092E6DC%21836
Can not include a pdf as picture so made a gif of it to show here.
Mind that this is with some reserve for , pressure-loss in time, unequall loading R/L, misreadings of weight and pressure-scales, and what I might have forgotten.
But it can still go wrong if you put on those metal plates
So important is that you know the exact weights and this is often misyudged.
wranglerjeepcamperrvtyrepressurecalculator.gif  

Last edited by jadatis; 04-24-2013 at 03:30 AM..
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Unread 04-23-2013, 04:45 AM   #23
Droptop306
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Ok. Came to work this morning on 28lbs. Rides a lot smoother with bumps. Still seems to be walking. I'm thinking they did not do my alignment correctly or something else is bad. This has become a headache
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Unread 04-23-2013, 06:29 AM   #24
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Quote:
Originally Posted by RockyClymer View Post
There is always the good old chalk test. Make a chalk mark across the tire, drive fwd and then see if the entire tread width is making contact with the road surface...adjust accordingly. At 50 psi your JK probably drives terribly!
Do a mud/dirt test. Hell, you could even do with a gravel test, but please!...Don't do a damn chalk test, that's for wieners.
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Unread 04-23-2013, 08:59 AM   #25
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Your lift will effect the caster. Nothing else excepting Drive Line angles. IF the lift had/has adjustible arms or brackets that correct this. Then if adjusted correctly caster should be in the ball park of 4.2+*. The other option would be cam bolts to adjust this, not an ideal way to set it. (most people do not like this )

another issue with lifts that are 3+ is the drag link and track bar start to get at steep angle. This makes thins much more twitchy. Many raise the track bar and do a drag link flip.
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Unread 04-23-2013, 10:16 AM   #26
jwmbishop
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Quote:
Originally Posted by starscream View Post
That third paragraph of the tire rack site is essentially what I said. I'm glad now you realize there are two different numbers and you may not have been taught everything correctly in school.
Not really. Read it carefully - that value is there ONLY for if you need to exceed the standard load index value PSI of 35 (but NEVER exceed the actual load value!!!!)!

Here's an example - Caddy DTS weighing 4850/4=1212 per tire. Load index 98 (1625). All's good at 32 psi placard right (index value is @ 35) so no further guidance is required right? Not really - assume a (Y) rated tire (180+ mph sustained) on a car I want to play with at the local road course or proving track. At ~120, centrifugal force crowns the tire and at contact patch is seen as overinflated (narrow and long footprint), while driving straight not a problem and can be good as rolling resistance is lowered - but when entering a corner it rolls off the tread and onto sidewall, making the car "push" like a shopping cart full of 2x4s at Home depot requiring a hard brake - I don't WANT to hard brake - I want to maintain my entry speed! So inflation to 40 is needed FOR PERFORMANCE (to stiffen that sidewall) - not load! SO an adjustment to tire PSI is NEEDED in this application (or the governor needs set below the cars capability to make it legally sale-able in the US - stock setting is 135) - a PSI exceeding the load PSI becomes REQUIRED. Hence a MAX stamp on the tire - and again NOT an absolute MAX - it's a MAX cold! (If a specific need goes OVER that Cold Max - a step up in Index rating becomes required - giving up straight line performance for cornering stability)

On a Wrangler (and on your CG and P/U) I can NOT for the life of me figure out a need to EXCEED the LOAD PSI (as paragraph three explains) - and LT tires are load rated (not Indexed) so it comes back to using the load index value and adjusting PSI down accordingly via the math I provided above!

What I was (and still am) refuting is you statement to NEVER EXCEED MAX. Thats FALSE. You set at MAX COLD and drive 100 miles it WILL BE OVER THE STAMPED MAX. NEVER EVER AIR DOWN (assuming you NEED the 41 of course). ALL tires are designed to accept the heat growth - and the heat growth is dependent on how hard the tire is worked - as long as you are not over the weight max - it is NOT a problem to see a Indexed tire grow pressure over that max! (the eagles on Cup Cars are maxed at 41 - and the engineers recomend a 35-40 cold - yet it is QUITE common on super speedways to see them come off track at over 50!).
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Unread 04-23-2013, 11:33 AM   #27
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Mine came with adjustable upper control arms. I guess the shops don't correct these. I do know that the offroad shop said they installed factory rims and tires to do the alignment then re installed my larger ones. I know nothing about this and suspensions so took there word as the bible.
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Unread 04-23-2013, 11:35 AM   #28
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Just do the dirt test and call it a day.
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Unread 04-24-2013, 03:11 AM   #29
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Though the temperature plays a big part in eventually damaging the tire( sidewall) the repeated stain could be the main cource, and temperature that goes with it can be a side-effect.
What I am trying to make clear is that for instance , if the sidewall bends to much and to many times a second on the north pole, where the rubber can never reach the high temperature, it still gets damaged by the reapeated strain.
At normal speed every part around the circle of the tire deflects and flexes back again about 10 to 20 times a second.
For lower speed the deflection may be more then for higher speed, and this is used for trailer-tires so the maximum load is more for that lower maximum speed of tire, then a tire with same Pr and sises in LT , wich has a higher maximum speed of tire.
On Truck tires and some LT tires there is an aditional loadindex printed with a lower speed-code-letter, and is also determined by the more allowed deflection for lower speed , and so larger surface on the ground.
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Unread 04-26-2013, 06:17 AM   #30
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Got a question. The bar that connects to the ball joints. Should I be able to move that back and forth by hand or should that be tight.
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