Calling Tire Engineers

[ChrisNJ]

Have 20" G-Y LS-2's on a new Sierra Denali. I think it has Z60 suspension. Well the ride is a little stiff, but not bad. Coming off a 2500HD it is still a pleasure. What I don't understand is GM says to put 30psi in, which I do. I few weeks ago I did a tow test with a 7-8K# trailer and bumped to 40psi, was suprised how much better the ride felt unloaded. But it couldn't be, right? So after the pull, I dropped to 35 all around and have left it there, with a better ride than at 30, didn't leave it at 40 'cuase it just can't ride better'. Well, going on a trailer weekend again, so i bumped up to 40 again a few days ago getting ready for the pull, and have been driving it a lot with this, and low and beyold, it feels even better! I don't get it. It goes against the tire-pressure rules. These are my first semi-low profile tires.

Ohhh, so my wife just got a similiar Yukon XL Denali, has 20" but are Bridgestone. Door says 32. Kept it there. Getting ready to tow with hers next weekend, so while i was raising mine, did hers too. Here we go again. It rode better than mine in the first place, but with 38-40 it rides EVEN BETTER. Don't get it.

Anyone know why?

And what I mean by ride better, is the tires seem to soke up the terrible potholed roads here in the north-east better. They both handle better, to be expected with higher psi. But they handle the pot-holes better too.

And if this is indeed true, and others share my experience, why did gm put 30psi on my door?

[bryanhunt]

What does the sidewall on the tire say?

[Ben]

Just referencing PSI is out of context, there are other attributes

that also play.

 

20", so guessing very low profile, like 50's or less, right?

 

The lower PSI might be allowing the rims to bottom out on

the tire's bead/sidewall, which translates to harsher ride quality.

 

With the higher PSI, it might not allow the rim to bottom

and soaks up the pothole/bump by bending enough but not

bottom out.

 

Also, with larger dia wheels, there is more unsprung weight,

which is directly related to 'ride quality'.

 

Low profile tires are mainly for performance (the T&T...aka...

toss and tuck) and looks. They are tougher to engineer for

weight carrying. Why a "P" rated tire (passenger rated)

will weigh less and cost less than the next higher class

tire, which is the "LT" (light truck rated).

 

For the longest time, larger dia tires and wheels were not

rated "LT" or high enough for truck usage initially. Now that

the OEMs have re-engineered them, they do come in LT

ratings and +3K lbs ratings. BUT the down side is that they

weigh a lot more than 16" dia wheels. That extra 4" in dia

replaces rubber and air with metal, which weighs more.

 

Unsprung weight is directly related to ride quality.

 

Also affects braking, as the fly wheel weight (centrifugal)

is much higher.

 

They rim width also plays. Too narrow and the bendback

will be high. Wider will have less side wall bend back, so

stiffer and better T&T characteristics.

 

There are many more aspects, but these are the main ones

affecting the question you ask.

[ChrisNJ]

They are 55's. I was thinking the same thing about the sidewall flex issue, it's gotta be that right? I understand all the weights and unsprung issues, but I can't change that. Can only change the psi. But again, if it rides better with more air, then why didn't gm put higher recommended pressures on, assuming gm is after better ride quality? Is there a detriment to the higher psi, iee.. wearing out the center of the tires prematurely?

[ChrisNJ]

oopss, max psi is 44 with a max load of approx 2500 lbs.

[hotwheels57]

GM recommends a lower pressure for liability and a cushy ride. If you use only 30# cold, they will increase to approx. 32-33# when heated. It won't set off the tire pressure systems and it will give "decent" tire wear.

I'm using 35# cold in my 20" LTZ wheels/LS2 tires at the suggestion of DISCOUNT TIRE. Once heated, they increase to 36-37#. It provides a good ride and good tire wear. If towing a heavy load, I'd increase only to 38# which is what GM recommends for the oem 17" tires.

I'm not a tire engineer, but I've seen the results countless times on the roadway following collisions from improperly inflated tires.

 

What does the sidewall on the tire say?

 

Do not increase your tire pressure to the maximum air pressure on the sidewalls unless you enjoy replacing expensive tires very often. That's a maximum rating, not the recommended air pressure for everyday driving. You're risking tire failure and possible injury/death.

When the tire is over inflated, the tire is riding up on the center portion of the tread, leaving less tire contact with the roadway. That lessens braking abilities. When you brake hard, the center portion of the tire heats up more from the friction, leading to potential for catastrophic failure. ABS at least aids in reducing skidding/more friction. In wet conditions, the tire tread is not diverting away the water as designed and you risk hydro planing. In snow, the tread may pack with more snow.

You're essentially lessening the designed traction of the tires in all situations for the sake of ride?

As a side note, I'm in AZ and we frequently see "load" vehicles, usually vans (of aliens or contraband) involved in roll over collisions. Many times there are multiple injuries/deaths. The tires have usually been over inflated to carry the loads involved, which over heated and failed.

[bryanhunt]

I was asking what the sidewall was to see how close to max he was (sometimes tire rating and the recommended pressure aren't even close). In my case I run e-rated tires (which are the oem tire btw) at the oem recommended pressure without any problems (no discreptancy between center/edge wear, etc) but I have known people that have run tires that required higher psi to maintain sidewall stability and inflated to OEM recommendation which resulted in under inflation and extremely high edge wear (not to mention very bad cornering).

[ChrisNJ]

I agree with not going to max. Note: on the past two 2500's i've owned, whenever the trucks went to the dealer for whatever, the backs always came back with 80psi in them, per the sticker. I had to ask them NOT to touch them.

[zozmanGMC]

I'm running Bridgestone Dueler H/L Alenzas 275/55/20. 30-35 psi on a truck tire just didn't give me much comfort riding nor tire longevity. Maximizing the tire pressures did. To get a better ride, you're right, inflate them up to the max. My tires are inflated to the maximum 40 psi during the summer at 95 degrees. I haven't driven the truck since June. I went out to check the air pressure today and they're at 39 psi. Maximize the psi when warm, it will be adequate when cold and the tire pressure monitors will let you know if there is a problem.

 

zoz

[ChrisNJ]

thanks randy, so one other that has similiar results as mine. very weird. i guess it's gotta be the the sidewall issue? hoping we'd get a tire engineer to weigh in on this. part of me is still wondering why gm wants 30 in them? certainly makes one less chore. now i can just keep them pumped up all the time. with my prior 2500's e rated tires, had to go from 50 to 70's everytime i'd hook the trailer up. come to think of it i probably never had too, 50 was probably ok.

And hotwheels, i fully get the 'lower pressure for cushy ride', but these tires don't follow that rule, and why i started this thread. I favor cushy ride over handling (loved my prior park ave's and caddies).

And Ben, i can't believe that the 30psi was allowing potholes to contact the bead/rim area???? wouldn't i be denting my rims? aaahhhhaaaaa, maybe your on to something. a big hole would allow the tire to soak it up but the shock wouldn't respond, at least properly, but with more psi, the shock is doing some of the work, so in essance, the tire to shock tuning or engineering isn't right with the 30psi, at least for me anyway.

Do all nbs's have 30psi recommended psi, regardless of rim and tire? this may be it, gm just puts 30 on all of them? but my wife's xl denali is 32.

[svensk]

I run 50 all the time now on my ltx m/s. I can't believe the difference it makes in mileage and performance.

[Ben]

Talking about PSI without the tire size is out of context.

 

Like talking about a guy's weight should be 180lbs with out his height.

 

A 5'5" guy weighing in at 180lbs is on the heavy side, but a guy

6'5" weighing 180lbs is skinny.

 

Then the label on the vehicle is only in reference to the OEM tire.

Here are a few pictures of door labels and their tire PSI recommendations.

But are the tires all the same size? Are the vehicle ratings the same?

 

 

Yes, it can 'dent' the rim, or 'snake bite' the tire, but not necessarily so. It could be just

enough to jar and reduce the ride quality.

 

I run 80PSI on my fronts of the Sub because of the 16x10 rims, which has almost no

sidewall bendback. These tires are not a good choice for such a wide rim and will

not be getting them again (Michelin LTX-MS LT265/75R16E). The tread has too many

sipes and allows for more tread block squirm, which folds over to allow the tread

to feather.

 

This is the other set, but LTX-MS C's on the same rims.

Like these much better, as the tread blocks do not fold over during braking.

 

This is a 'dent' on my Alco Classic 16x10's after a Land Rover jumped a stop sign and my rims

ran along his bumper and hit one of the bolts. Also ruined one of the LTX-MS LT265/75R16E's.

 

Since you have 55's, a much lower sidewall and closer to the pavement before

hitting a pot hole than my 75's (20% taller).

[C & A s Dad]

Chris, I have found the same to be true with my Wife's Yuk XL 1500 with Bridgesone Dueler HL Alenzas in the stock size 16" P series. I was running 35 PSI which is what the GM recommends in the door. When I tow my camper I move the fronts to 40 and the rears to 42, the sidewall states 44 PSI max. When we would unhook the camper and do any running around in the campground or back and forth to town, the truck seemed to ride better, quieter and handled better with the psi increased. I then began running 38 all the way around when not towing and it makes a difference. I have wanted to go even higher but did not want to get premature center wear.

 

I wish I could find a definative answer on if it is ok to run 40 to 42 all the time. It does make a noticeable difference.

[SinisterSS]

How about this...

 

More air in the tires causes the sidewall to be more stiff.

 

This stiffer tire, instead of absorbing the bumps, transmits more of the road loading into the shocks and springs.

 

The shocks and springs now actually see the real road surface and they can better do their job.

 

So in conclusion, would you rather have your tire doing all the suspension work or the actual suspension bits like shocks and springs doing the work?

[SilveradoCJ7]

I would like to add that I have experienced the same thing with my 06 silverado with 20" Chevrolet wheels. Previous experience has told me 30-33 psi gave the best ride on 16in or 17in wheels. My ride kept getting worse on my 06 over the last few months. So I went and added some air to them. They were at 32psi. I went to 36psi. The improvement was imediate and drastic. I thought I was going crazy. bumps were much softer. This post now confirms that I may not be crazy.

 

Tonight I will take them up to 38psi to test that out.