Lower pressure really does ride smoother

[BrocLuno]

The way to do this is to +1 or +2 the tire size and then adjust the pressure for contact patch and wear on tread. Door frame or slightly lower (do to increased tire size) will get you better ride and better traction

My rule of thumb is to have the actual tread width slightly wider than the rim width. That way you get excellent wear with modest pressures and a very nice ride

Off road in 4x4, might air down to 12 psi for sand, prolly 25 psi for gravel, and only high pressures for nasty rock country. The door sticker is for nominal full passenger load, so is prolly higher than needed for only two on board ...

 

[IndyIan]

Originally Posted By: AZjeff
Originally Posted By: faramir9
Originally Posted By: Eddie
I would never run a full 20% higher pressure for normal driving. In addition to discomfort, there is the safety factor of lost traction with the lower footprint.


That's a plausible argument, but check out some hyper-miler forums to see folks who disagree.


Hypermilers are known to do unsafe things for a few tenths. 20% less contact patch is what it is (almost like 1 less tire on the road..). Always hope Capriracer stops by with some expert insight.

Somewhere around 36-38psi on a car with fairly high profile low performance tires is probably ideal for handling and would give the best autocross times. 38-40psi cold is what I run on the front with slightly undersized tires, and either go up to near 50 psi on the rears to get more spring rate to help the front bite, or you can go down to high 20's on the rear to reduce rear grip but have the rear break away slowly.
Also if you aren't running high performance summer tires, you are giving away lots of grip already. Does that make all season tires suddenly unsafe?

 

[Duffyjr]

When the put the LT's on my Sierra they had them set at 50psi and holey cow what a ruff ride but I left them there for a while but eventually dropped them down to 40psi and it is much better. I guess I don't really care about how the wear I only put 2k a year on it so they'll need replacing long before they wear out.

 

[CapriRacer]

Originally Posted By: AZjeff
……. Always hope Capriracer stops by with some expert insight.



First, the size of a contact patch doesn't change much with a few psi. It's the big changes in pressure where most of the contact patch size change takes place.

And while it is true that a difference in pressure changes the size of the contact patch, the differences aren't proportional - that is a 10% change in pressure doesn't equal a 10% change in footprint.

But dry traction is always a big number, so hypermilers always report nothing negative when they change pressure. Further, when using a higher pressure, the footprint penetrates the water film better, so many folks also report better traction in the rain.

- BUT -

Where the footprint change makes a big difference is at slow speeds or where the water isn't deep enough to cause hydroplaning - and in those cases, people have reported loss of grip at higher pressures.

And since most people - including hypermilers - don't conduct tests when they change pressure (Note: a true test would result in destroying a set of tires!), I recommend people use the vehicle tire placard pressure. I might condone a few psi more especially for older vehicles (especially SUV'!), but I would recommend against using pressures lower than the vehicle tire placard - unless you change to a larger sized tire (load carrying capacitywise!) - and then it's a whole new ball game.

 

[OVERKILL]

Originally Posted By: oil_film_movies
Originally Posted By: OVERKILL
Originally Posted By: faramir9
20% less contact patch sounds scary. But take a look at that link to the Firestone air pressure guide. Note that for the 2005 Corolla with 195/65R15 S-speed rated tires, the spec is 30 psi. For the 2014 Corolla with 195/65R15 S-speed rated tires, the spec is 35 psi. (a difference of 16.67%) Wonder what changed over the course of nine years.
Probably weight.

Not "weight". Higher pressure means it corners better, and it also helps CAFE MPG averages. The Corolla has been evolving into something more sporty.


Well yes, weight.

Curb weight changed.
2005: 2,595lbs
2014: 2,845lbs

So it packed on close to 300lbs. Not saying that your claim isn't also true, it likely is, but the car undeniably got heavier.

 

[Rand]

The cherokee trailhawk specs 36psi all around on 245/65r17 firestone destination AT

The lesser offroad options (IIRC 100lb less or so) have 225/65r17 or 225/60r18 and IIRC 32 or 33psi spec.

On my cherokee
If you inflate to 36psi it will get near 40psi while driving on a hot day.

I used to run it at about 37psi to make sure it stayed@ 36psi or higher overnight.. but now I run about 35psi in it and it still hits 37 or 38 while driving.. and doesnt shake my fillings out quite as much on the terrible roads... FIRST CAR, I have ever run at or under the door pressure.. usually I go +2 or +3 psi.

Not sure why they spec this vehicle with bigger tires so much higher than other optioned models.

 

[faramir9]

Originally Posted By: OVERKILL
Originally Posted By: oil_film_movies
Originally Posted By: OVERKILL
Originally Posted By: faramir9
20% less contact patch sounds scary. But take a look at that link to the Firestone air pressure guide. Note that for the 2005 Corolla with 195/65R15 S-speed rated tires, the spec is 30 psi. For the 2014 Corolla with 195/65R15 S-speed rated tires, the spec is 35 psi. (a difference of 16.67%) Wonder what changed over the course of nine years.
Probably weight.

Not "weight". Higher pressure means it corners better, and it also helps CAFE MPG averages. The Corolla has been evolving into something more sporty.


Well yes, weight.

Curb weight changed.
2005: 2,595lbs
2014: 2,845lbs

So it packed on close to 300lbs. Not saying that your claim isn't also true, it likely is, but the car undeniably got heavier.


I was formulating responses to both of you noting that you are both right. The curb weight increased by 250 pounds, though the load rating of the tires (89) did not change. That might be part of it.

But the big thing is CAFE. Check out the recommended psi for any standard family-type vehicle from 10-15 years ago compared to today. The numbers speak for themselves. Corolla and Malibu went from 30 to 35, Camry went from 29 to 35, etc.

Tire pressure is a trade-off, like most things. Higher psi can give better snow traction ("bite"), slightly better fuel economy, crisper steering, but at the cost of ride comfort and beyond a certain point perhaps dry traction and braking.

So where is the sweet spot? If the psi number listed on the door jamb was based solely on reports from independent engineers, i would treat it as gospel. But it's pretty clear those numbers were biased toward ride comfort till recently, for mainstream vehicles. Today they are influenced by CAFE, in the same way 0w-20 has taken over from 5w-30.

Not to mention some tire models "like" a bit more pressure than others to respond similarly. I went through that with the H727.

On my gently used Corolla, I am comfortable with tire pressure anywhere from 30 to 36 psi, same as I am comfortable using any 5w-30 or 5w-20 SN oil. Winter performance and fuel economy are priorities, so I have tended to run 35-36 psi and synthetic 5w-20. The differences are minimal, in this car with my conservative driving style, I reckon.

But the lower pressure is easier on old achy backs.

 

[oil_film_movies]

Originally Posted By: faramir9
On my gently used Corolla, I am comfortable with tire pressure anywhere from 30 to 36 psi, same as I am comfortable using any 5w-30 or 5w-20 SN oil. Winter performance and fuel economy are priorities, so I have tended to run 35-36 psi and synthetic 5w-20. The differences are minimal, in this car with my conservative driving style, I reckon.
No need to reckon. You save about 2% in gasoline if you use a 5w20 vs. 5w30, and pump the tires up by +4psi. Less drag on the rolling vehicle, less gasoline used, under any condition, your easy driving or not. It's why its that easy for the car makers to gain 2% toward CAFE goals while still selling more and more pickup trucks & SUV's, just spec higher tire pressures and lower oil viscosity.