Air Pressure???

tommudd · **Posted:** Mon Dec 20, 2010 1:42 pm

LibertyCRD wrote:

Matt_ wrote:

:ROTFL:

Wow. Now I come from the full size truck world where its hard enough convincing guys with 1500 series trucks that those E rated Max pressures are for 3500 series trucks with a 4000lb load in the bed...... I never thought I'd see guys with small/mid sized SUVs trying to run Max pressure. That's actually on the verge of dangerous given that braking distance is directly related to you contact patch.

35psi is good for a kj and kk. If you have a bunch of armor/tools/etc then 40ish is good too. 50 is too much... if you need that much pressure in one of these vehicles then you're overloaded.

There's not enough weight in a fully loaded liberty to cause an E rated tire to flex & overheat at 35-40 psi. That's more of a concern for 1500 series trucks hauling Max capacity; and even then, 50psi often more than makes up for the equivalent load published for the stock p-rated tires at 35 psi.

Exactly. That's what I've been trying to tell these clowns for a year now, and nobody listens. If the guys with Ram 1500s don't want "E" rated tires, why on God's green Earth does somebody with a compact Jeep think they need them? All it does is suck your fuel economy, ride rougher, and add unsprung weight. You'll NEVER put a load on a Jeep that will need an "E" rated tire. Ever.

And yes... the PSI on the tire is the MAX you can put in it... it is NOT a recommendation. Geez people... :shock:

No matter if its tires, lifts, whatever some will never listen :banghead:

JeepinJarhead03 · **Posted:** Mon Dec 20, 2010 5:45 pm

Yeah, I think I ran 33.5 to 36 psi in my E courser's depending on what sort of driving i was doing

14psi offroad, 33.5 city expected for the most part and 36 highway

33.5 drew up nice, smoothed out typical city potholes and gripped good in very heavy rain, 36 pushed the sway out of them in heavy wind on the highway

2.5 psi makes a huge difference within the 'correct' range of pressures, over inflated tires = rocks that just beat your suspension to death. When you think of your tires as being part of your suspension, which they are, when over inflated just 10psi is an equivalent of a couple hundred pounds of suspension tension added that the vehicles suspension can't accommodate for and it all gets absorbed into the vehicle. Makes your coils wear quicker, bushings wear quicker, body panels loosen much before their age or the vehicle mileage would predict them to, basically costs you money over the long run

And it's a bunch more money than you'd ever save at the pump

EAGLE01_2002Libby · **Posted:** Mon Dec 20, 2010 6:17 pm

I have been on this site for two years now and i have NEVER heard anyone so freakin rude as this Matt_ person and a couple others in this post. It's obvious that I was not sure, so i posted. When i get horrible responses like yours, it makes me want to leave the site and tell people not to use it because people here are inconsiderate and immature.

All i was asking was if i did something wrong. Now, the only reason I purchased Load E Rated tires was because they had the best tread for what I could afford and they were durable. They were recommended for my Jeep by two different certified Mechanics.

Now, if someone has a problem with me posting a simple question like this then PM me. This is ridiculous that I get such rude answers from adults, when I'm just looking for a simple explanation.

JeepinJarhead03 · **Posted:** Mon Dec 20, 2010 6:25 pm

many people from the 3/4ton market and 32" tire market and above balk at E rated tires when it comes to KJ's and KK's

many of the desired tire tread / design choices we have for our vehicles, especially when wanting something with good sidewalls, end up being E's

does the vehicle weight warrant it, nope, does it cause more wear to the vehicle in terms of suspension, brakes etc, sure..

is it acceptable.. absolutely

I think there's enough info in the thread to help you figure out what PSI your tires need to be at, as for the other stuff, don't sweat it or otherwise let it get to ya, not worth it..

if you feel more comfortable PM'ing someone with questions in regards to finding the right psi or any other concerns on the topic go for it, don't let it drive ya off or anything, sometimes people just don't come across like they meant to.. it's a whole perceptive game I guess

Matt_ · **Posted:** Mon Dec 20, 2010 6:58 pm

EAGLE01_2002Libby wrote:

I have been on this site for two years now and i have NEVER heard anyone so freakin rude as this Matt_ person and a couple others in this post. It's obvious that I was not sure, so i posted. When i get horrible responses like yours, it makes me want to leave the site and tell people not to use it because people here are inconsiderate and immature.

In what way was i rude and immature? Because I put the laughing smiley? I got a chuckle when i read some of the responses in this thread of people who were actually advising to run MAX psi in E-rated tires.... yes, that is amusing to me.

If you took that personally, then i apologize... it wasn't targeted at you (if there was anything to be taken as targeting).

You're over reacting a bit. Take a deep breath and chill, and don't take the internet so seriously :juggle:

EDIT: And PS, there's NOTHING wrong with running E-rated tires on your vehicle... as long as you put them to the proper pressure for your vehicle (though there are some downsides... but there are benefits too... that's a different thread :lol:

)

striperman36 · **Posted:** Mon Dec 20, 2010 8:27 pm

It would be much easier if someone could just post the formula for calculating air pressure. I'm not thinking the stock air pressure for the Liberty is good for E tires. But we wouldn't be asking if we knew, because you feel like your driving a boat when it set at 35

And if 70 is wrong why is hankook recommending it, 90 pct of max pressure.

Matt_ · **Posted:** Mon Dec 20, 2010 8:34 pm

There's a few ways you can go about it. Do the "chalk" test to make sure your tread is making proper contact with the street.

Another method (primarily used by truck guys who want a good pressure for towing but not too much) is to find out what load the OEM tires are rated for at the pressure specified on the door. Then, for whatever replacement tires you have the manufacturer can provide a load vs. pressure chart. Find the load that matches the OEM load and run that pressure.

You can also take the max GVWR for the vehicle and divide by 4... but I have a feeling that this may give you a lower pressure than desired for the Liberty.

Or, do a combination of the above for redundancy. See if the comparative pressure is good by doing the chalk test afterwards.

From personal experience (and quite a bit with tires I might add), i'll tell ya now that some of the responses in this thread were dead on. Really, anywhere 35-40 is plenty for Liberty.

striperman36 · **Posted:** Mon Dec 20, 2010 8:47 pm

I was thinking 40, I ran that when I tow my boat about 3500 lbs + trailer on the old tires and it was solid.

Thanks for the recap.

Matt_ · **Posted:** Mon Dec 20, 2010 8:49 pm

striperman36 wrote:

And if 70 is wrong why is hankook recommending it, 90 pct of max pressure.

Just took a look at Honkooks site. They say "generally": http://www.hankooktireusa.com/Serv/Infl ... ChildNum=4

"Generally" does not apply to a tire that is over designed for an application. If that E-rated tire was on a 3500 series truck like designed for, and you aren't sure what pressure to run for your fully loaded 3500 series truck, then 90% would apply.

There's some more good info in that web page too. They're reiterating what we've all been saying. Finding the correct pressure is important. Worst case, give Hankook tech support a call and i'm sure they'll help out.

Matt_ · **Posted:** Mon Dec 20, 2010 8:51 pm

striperman36 wrote:

I was thinking 40, I ran that when I tow my boat about 3500 lbs + trailer on the old tires and it was solid.

Thanks for the recap.

I tend to prefer a slightly higher pressure when towing too. Helps tone down any sway. :JEEPIN:

striperman36 · **Posted:** Mon Dec 20, 2010 11:03 pm

Sent in a question to Hankook. I'll post what I get in return.

Matt_ · **Posted:** Tue Dec 21, 2010 9:11 am

Awesome.

What size and model hankook are you running?

striperman36 · **Posted:** Tue Dec 21, 2010 11:05 am

DynaPro ATM RF10 215/85/16

Matt_ · **Posted:** Tue Dec 21, 2010 3:54 pm

Goodyear has a load vs pressure chart.

The OEM tires on the liberty are rated at about 1580lb @35 PSI (extrapolated from Max load of 1980 @ 44)

Per the Goodyear chart (ill post link when I get home tonight) an LT215/85/16 tire has the following load vs pressure:
1495 lb @ 35
1640 lb @ 40
1940 lb @ 50

Based on goodyears published data, to match the load of the stock tires at OEM recommended pressure you'd want to run between 35-40.

To match the max load of the oem tire, you'd want to run 50.

BTW the only chart I was able to find on Google was a light truck tire load for use on RVs from Goodyear. But I know that these values are practically standard across most manufacturers makes/models. There's another Toyo publication that matches those loads/pressures.... ill see if I can dig that up too.

Hopefully hankook gives you their chart. If not, maybe ask for it and post it up as another reference.

Matt_ · **Posted:** Tue Dec 21, 2010 4:16 pm

Actually ignore that stuff about the RV chart. I did find bridgestones commercial light truck radial tire chart and it has the same pressures/loads so ill post that link.

Interestingly I also came across a warning from Toyo that said LT tires are Not recommended for certain applications where the pressure needed to match OEM p-metric rating in an LT tire would cause an over inflation condition that would be detrimental to safety.

In other words, they don't recommend LT tires where you're sacrificing contact patch to meet load ratings. Makes sense..... something else to think about.

onthehunt · **Posted:** Tue Dec 21, 2010 4:24 pm

I run my 215/85 E's at 40 psi. Any higher and they rattle my fillings out

Matt_ · **Posted:** Tue Dec 21, 2010 4:33 pm

I haven't been around the liberty world that long.... is 215/85/16 the desired size in these vehicles?

The thin tire really hurts the load carrying capacity.... if you ran a 245/75/16 you would have a load of 1700 lb at 35 psi which would help smooth out the ride. Its the same height as the 215/85.... but I'm guessing the width causes fitment issues?

audiboy86 · **Posted:** Tue Dec 21, 2010 5:08 pm

The ussualy size for a lifted liberty is ussually 245/70 a few run taller and some wider. But that is usual size for 16 and 17 atleast. I myself run 245/70/17 Cooper Discoverer ST E and you will see atlot of BFG A/T KOs on Moab.

Matt_ · **Posted:** Tue Dec 21, 2010 6:21 pm

Ok just got home. Here's a good chart to go by for LT tires:
http://www.trucktires.com/bridgestone/u ... k_Load.pdf

I just took a look at my wifes Liberty to give you guys another example of methods for determining tire pressure.

Using her 2008 Liberty Sport 4x4 vehicle placard:
GVWR is 5600. This is the maximum vehicle weight loaded to the max with cargo/people/etc.

5600/4= 1400 lb required for each tire.

OR, also given is the GAWR which is the MAX load for the front and rear axles.
Front GAWR = 2750. Divide by 2 for the tires and you need 1375 lb capacity for the front tires.
Rear GAWR = 3200. Divide by 2 for the tires and you need 1600 lb capacity for the rear tires.

Going back to the chart in the link above. An LT215/85/16 you'll see that at 35 PSI the tires have a capacity of 1495 lb which is definitely good, especially for an unloaded Liberty that has a curb weight around 4000 lb? Now if you're loaded to max capacity and want to make sure you're covered for that GAWR in the rear which requires 1600 lb, you can bump the pressures up to 40 PSI which gives you a capacity of 1640 lb per that chart.

Thus, anything significantly over 40 PSI in the aforementioned tire size is likely overkill.

In any event, i would still suggest keeping an eye on the pressures and tweaking them as necessary.

But one thing is certain.... 70 PSI is way too much.

striperman36 · **Posted:** Tue Dec 21, 2010 6:37 pm

Matt_ wrote:

I haven't been around the liberty world that long.... is 215/85/16 the desired size in these vehicles?

The thin tire really hurts the load carrying capacity.... if you ran a 245/75/16 you would have a load of 1700 lb at 35 psi which would help smooth out the ride. Its the same height as the 215/85.... but I'm guessing the width causes fitment issues?

the 215/85's remove the speedometer error for the most part.

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