Replacement tire pressure question??

[K20FA5]

Our 4Runner had Dunlop GRANDTREK AT20 P265/70R17 on it from the factory. I recently replaced them with Michelin DEFENDER LTX M/S 265/70R17.

The Dunlops were 113S and the Michelins are 115T

My question is about the pressures. If I run the Michelins at the pressure on the door sticker, 32psi, the contact patch doesn't fully touch the road, the outside edges of the tread don't touch the road. If I lower them down to 30psi, then they do touch. They are also kind of squirly at 32psi, much better at 30psi!

My question; is running 30psi too low for the tires? The old Dunlops were 300 B B and the new Michelins are 800 A A, so they can handle the heat better than the Dunlops. Am I hurting anything running them that low? I'm guessing that the higher load rating is part of what is contributing to this contact patch issue?

 

[dubber09]

I think the best place to get correct answer is to ask Michelin.

I'm in a similar while not quite boat - had Nexen CP 671 factory installed tires and replaced them last Sat with Sumitomo Encounter HT. Sticker in the car calls for 35psi, Nexens were ok with that while Sumis show very noticeable bulge as if they were under inflated, exp on the fronts even with 36psi.
I think I'm gonna run 36psi in the back and 38psi in the front to prevent chewing up the tires. Max tire pressure for the tire is 44psi and they only 'gain' 2-3psi when hot during driving.
It is also important to have overall diameter of all 4 tires as close as possible on AWD cars.

 

[CapriRacer]

First, read my webpage on the subject of load tables; Barry's Tire Tech: Load Tables

Short version: Regardless of the tire standardizing organization or the tire manufacturer, tires behave the same. The load tables are ways to describe what happens and they differ ever so slightly. So there is no difference between a tire designed to The Tire and Rim Association's load table (113S) and ETRTO's load table (115T).

That means the placard pressure is applicable regardless.

You may ask: What about the fact that the Michelins don't completely touch the road surface? That is intentional on Michelin's part. I am not privy to what Michelin had in mind when they decided to do that (and neither are you), - and it may just be that their tire does something different that other tires - but it doesn't have anything to do with the different in standardizing organization or the Load Index. or the speed rating)

So what might be their intention about not having the shoulders touch? It might be that when cornering a Michelin deflects more, so full contact is made when it is needed and not when it is not needed (when the tire is going straight ahead). They might have done that for rolling resistance purposes.

Are you doing any harm using a lower pressure? You're going to get worse fuel economy. The steering will be less crisp. The wear will be more rapid and more towards more shoulder wear (less center wear) - but this affect is fairly small and other factors will easily overwhelm that.

On the other hand the difference between 32 and 30 psi isn't all that great, so if you feel horribly uncomfortable using 32 psi …….

 

[JLTD]

CapriRacer I have always appreciated your expertise on tires. But I have to ask after reading your post: what if the same exact tire was on a heavier or lighter vehicle? If it was on a pickup that was towing, (my F150 also said 32psi) the tire would be greatly compressed, be underinflated for the weight carried, and generate a lot of extra heat. I am speaking from personal experience on this one. How about on a smaller SUV - that same tire also at 32psi - would have a tiny contact patch, and the lighter weight might not be enough to roll the tire to full contact with the road. This seems to go along with the inflation/load tables posted on your site.

Your site also says that manufacturers adjust tire pressure based on weight, so in the pickup example above, adjusting your tire pressures based on your load/towing vs empty makes sense and goes along with this concept.

I've always read that the best inflation pressure for a tire is with a full contact patch. That is, with the tire touching the ground all the way across; if overinflated just the center will touch, as in K20FA5's case, and if underinflated, the center will not touch and the edges will wear quickly, which is another thing I've experienced on more than one used vehicle I've purchased.

 

[dubber09]

It's not just wearing edges when under inflated it's also a lot of flexing in sidewall (bulge area) causing heat and mechanical stress on the tire.
Having tires at diff overall diameter on AWD car can cause transfer case and/or differential damage due to wheels spinning at diff revolutions per mile.

 

[CapriRacer]

JLTD,

First, the original question was replacing tires of the same size. The fact that they had different load indices is an artifact of the standardizing organizations. The tires will behave similarly regardless.

But you are asking a different set of questions:

a) What about lighter or heavier vehicles? Then the placard size/inflation pressure specification should be different as well. (OK, it just might be that the different vehicle manufacturers might do things a bit differently and they might wind up with the same size/inflation specification for vehicles with different weights - but that's a whole ‘nother kettle of fish.)

b) What about pickup trucks and the difference between loaded and unloaded? The vehicle manufacturers will have tested the vehicle at both conditions and adjusted the sway bars, springs and shock damping rates so the difference is minimized and benign - even if they specify different inflation pressures for those 2 conditions.

But there is also the issue of a tire's spring rate - which is more or less the same for a given size, and a function of the inflation pressure. So from a vehicle manufacturer's point of view, specifying a tire size and inflation pressure locks then into a small range of values.

Will there be differences between makes/models of tires? Yes, but that's the best the vehicle manufacturers can do, and without additional information, it's the best we consumers can do as well. Sure, we could tune by using inflation pressure for a small range of properties, but we just don't have the resources to test ALL the affects. In my opinion, the best answer is to follow the vehicle placard pressure for the size listed there.

You said:
Originally Posted by JLTD
….. I've always read that the best inflation pressure for a tire is with a full contact patch. ……


Well, not exactly.

First, the load tables are the best guesses as to how the tires will carry the load and everything else falls out from there.

So what about the fact that the steering tires and the drive tires wear differently? (steers wear in the shoulders and drives wear in the center.) Shouldn't that be compensated for?

And then there are tire manufacturers who pay only a cursory amount of attention to the contact patch and their tires don't achieve full contact at the appropriate load/inflation pressure. But they do pay attention to the load table for structural integrity purposes.

I think the real answer is that the best inflation pressure is the one where the load on the tires is around 85% of load from the load table - BUT - I have to take into account that the front of the vehicle and the back do different things, so I don't think there is a simple answer to this question.

 

[JLTD]

Pickup example: There was a guy on a towing forum that had squirm and instability at placard pressure; he went up to max pressure and those went away....


So it sounds to me like your position might be that for the informed driver, a small incremental change in pressure from the placard could be indicated in some conditions.

 

[CapriRacer]

Originally Posted by JLTD
Pickup example: There was a guy on a towing forum that had squirm and instability at placard pressure; he went up to max pressure and those went away....


But was increasing the pressure solving the source of the problem, or just the easiest way of dealing with it?

If you've ever watched a NASCAR race, they will talk about changing the inflation pressure in their tires, or in drastic cases adding or taking off a spring rubber - but most of the time what they should be doing is changing the spring (and the spring rate!), which they don't have time for. They are just picking the easiest path to get the results they want.

Originally Posted by JLTD
….. So it sounds to me like your position might be that for the informed driver, a small incremental change in pressure from the placard could be indicated in some conditions.


Yes, that's essentially it. You may notice that I may become pedantic about assigning a "cause" to a particular issue (like the above sentence!), and that's so that folks will think about cause and effect - what is causing what - and not glossing over what is really happening.

 

[jjjxlr8]

It hasn't been mentioned, yet, but the footprint shape of a rolling tire is quite different than that of a static tire and it's the rolling footprint that matters!

 

[K20FA5]

I emailed Michelin to ask them their opinion. I wasn't expecting much, so I wasn't disappointed

They basically said that if I'm running the stock size, I should be running 32psi.

 

[dubber09]

No surprise there from Mich.

Personally, I've already been running +2psi on the fronts on both cars with factory tires for 5 years and will stick to the rule with the new ones while it'll be +3 front and +1 rear now.
I had very even wear and no handling or traction issues in the past. Tires didn't seem to heat up above normal or add more that 2.5psi on hot tire either with little extra air pressure.

 

[zeng]

Originally Posted by K20FA5
I emailed Michelin to ask them their opinion. I wasn't expecting much, so I wasn't disappointed

They basically said that if I'm running the stock size, I should be running 32psi.

Michelin is assuming similar stock size carries similar load Index , which is NOT in this instant.
At LI 115, there is no harm running 32psi specified for LI 113.

Edit:Correction
In ERTRO, the Dunlops 113S at 32 psi would be of equivalent load to Michelins 115T at 30 psi.

 

[CapriRacer]

Originally Posted by zeng

……. Edit:Correction In ERTRO, the Dunlops 113S at 32 psi would be of equivalent load to Michelins 115T at 30 psi.


Except the Dunlops are TRA (Tire and Rim Association), not ETRTO and TRA uses a lower pressure for their rating point. If you adjust for the difference between TRA and ETRTO, you get essentially the same answer.