Author Topic: Tire Pressure - Why? (Read 2198 times)

Ben · « **on:** August 25, 2007, 02:00:48 PM »

The vehicle manufacturer recommendations for tire pressure on both my vehicles calls for lower pressure in the front than in the back (30F/35R and 60F/75R). This has always seemed counter-intuitive to me since barring a big load in my truck, most of the weight on both vehicles is in the front.

Anybody know the reasoning for this?

Paddy · « **Reply #1 on:** August 25, 2007, 02:02:42 PM »

What kind of vehicle? Are you talking different size tires front/back?

Ben · « **Reply #2 on:** August 25, 2007, 02:20:07 PM »

Isuzu Trooper 4WD and F-250 2WD. Same tires all the way around on both.

drewtam · « **Reply #3 on:** August 25, 2007, 02:27:03 PM »

The only thing that comes to mind is that the lower tire pressure will give more handling, and create more understeer. Understeer is what every car is designed for.

Drew

Paddy · « **Reply #4 on:** August 25, 2007, 02:30:39 PM »

Are you getting the recommended pressures from the decal on the door?

grislyatoms · « **Reply #5 on:** August 25, 2007, 02:37:34 PM »

I think you got that backwards, drewtam.

Understeer is when the rear end sticks to the road better than the front end. "Plowing"
Oversteer is when the front end sticks to the road better than the rear end. "Fishtailing"

The low pressure will make the front end stickier, but the term is oversteer, not understeer.

Which doesn't make any sense in a pickup, as they tend to oversteer (no weight in the back) anyway, hence BenW's question, I believe.

Ben · « **Reply #6 on:** August 25, 2007, 03:25:42 PM »

Affirmative on the truck being specifically perplexing to me grisly.

Riley, yes from the door. I understand there is often a difference between what the tire manuf. recommends and what the vehicle manuf. recommends. Just trying to figure out the engineering reason on the latter.

Paddy · « **Reply #7 on:** August 25, 2007, 03:34:28 PM »

Engineering reason. That leaves me out. I just keep tire pressure equal in all tires (measured cold), toward the top of the specs.

mgdavis · « **Reply #8 on:** August 25, 2007, 04:37:44 PM »

Higher weight rating for the rear axle? They need to ensure that, if you happen to put a huge load in the vehicle, the tires are inflated to the proper pressure to carry it. CYA on their part.

Firethorn · « **Reply #9 on:** August 25, 2007, 04:56:49 PM »

Quote from: mgdavis on August 25, 2007, 03:37:44 PM

Higher weight rating for the rear axle? They need to ensure that, if you happen to put a huge load in the vehicle, the tires are inflated to the proper pressure to carry it. CYA on their part.

That's probably the reason. They do tend to state those pressures for ignorant, 'most purposes' uses for a vehicle.

Quote

I understand there is often a difference between what the tire manuf. recommends and what the vehicle manuf. recommends. Just trying to figure out the engineering reason on the latter.

The vehicle manufacture is recommending pressure based on the expected weight of the vehicle. IE, for proper wear a lighter vehicle shouldn't have as much pressure in the tires as a heavy one. A single tire size can be used for a range of vehicle weights.

The tire manufacturer is generally stating the maximum safe pressure for his tires, IE 'up to XX PSI'. Well, maximum pressure with a substantial safety factor. There was a towing demonstration where they pulled a 747 using a diesel car. In order to get enough grip they filled the back end of the car with a couple tons of sandbags, they also proceed to fill the tires to something like four times the tire's rated capacity to keep them from flattening out too much. They didn't burst, but the engineers told the driver that they might. Then again, they were probably new tires, and ruined by that usage. The guy only towed the 747 a couple hundred feet, if that. Still impressive.

Ben · « **Reply #10 on:** August 25, 2007, 05:42:47 PM »

Quote

Higher weight rating for the rear axle?

I can see that for the truck, given the CYA caveat of max potential load versus my weekend warrior average load which is usually empty most of the week. But I have the same thing going on the SUV. Sure, it has a cargo area, but not for the kind of cargo you might have in a truck. Of course the difference is only 5 psi.

Out of further curiosity, any of you guys with passenger cars have a manuf. stated difference for front / rear?

Quote

There was a towing demonstration where they pulled a 747 using a diesel car.

Drifting my own thread, I used to work at the county dump, and one day one of the trash trucks broke down on the scale. One of the other weighmasters decided he could tow it off with his Bronco II. We all thought he was gonna thrash the thing, but he managed to pull a 40+ ton truck 100 yards off the scale.

Tallpine · « **Reply #11 on:** August 25, 2007, 06:15:28 PM »

FWIW, I carry about 55psi (IIRC) all the way around in my 3/4 ton pickup.

One time I pulled a loaded gasoline tanker (semi) out of a snow drift with a 1973 3/4t Chevy 4x4. Just a little tug - he had had to drive into the deep snow to get around a tight corner and couldn't get out. Another time I pulled a Peterbuilt COE tractor a mile up and down the road trying to get it started for him. Turns out the fuel solenoid was bad rolleyes

Seems to be a weak point in Cummins.

That poor old pickup. Wished I still had it sometimes. Only thing wrong with it was the danged automatic transmission. Sad

Bogie · « **Reply #12 on:** August 25, 2007, 06:15:50 PM »

pulling rolling weight is a lot easier than dragging non-rolling weight. Heck, 10-15 guys can pull an airplane a pretty fair distance without a major to-do...

I'm wondering what I should have the LT tires on my van inflated to... Someone once told me 45psi, but the door says around 32psi...

mgdavis · « **Reply #13 on:** August 25, 2007, 06:24:04 PM »

Bogie:
I'd imagine that the van will handle and ride better at 32psi. Gas mileage might increase a bit if you bump the pressure up, at the expense of increased wear along the center of the tread and slightly less traction. You could always try for a couple weeks to decide if you like it or not.

S. Williamson · « **Reply #14 on:** August 26, 2007, 01:44:46 AM »

Isn't our technology beyond the use of pneumatic tires these days? Why do we still use 'em?

jeepmor · « **Reply #15 on:** August 26, 2007, 02:21:30 AM »

Quote

Isn't our technology beyond the use of pneumatic tires these days? Why do we still use 'em?

What is your alternative? The run flats? Just curious.

I just run the pressure til I get a good footprint on my tires. ie, I put some new tires on my jeep and they inflated them to 35psi like they do all truck tires. Well, it was raining lightly and I pulled it into the garage to find that the tread was only making about a 3" wide footrprint on a 33x12.5 tire. I ran the pressure down until I got a full width footprint. It was about 19 psi when I was done. A bit low for my taste, but it still drove fine. I did not like it this low and ran it up to 25. The footprint changed a bit, but it was still not overly inflated.

Tallpine · « **Reply #16 on:** August 26, 2007, 04:40:59 AM »

jeepmor, those big tires are designed to carry a lot more weight than what your jeep weighs

Firethorn · « **Reply #17 on:** August 26, 2007, 06:23:05 AM »

Quote from: jeepmor on August 26, 2007, 01:21:30 AM

Quote

Isn't our technology beyond the use of pneumatic tires these days? Why do we still use 'em?

What is your alternative? The run flats? Just curious.

Run flats still have air in them. They just have strengthened sidewalls.

I figure it's one of those cases of the old technology just working. I did see one idea that used a bunch of compressed springs and a rubber tread, but haven't heard about it since.

K Frame · « **Reply #18 on:** August 26, 2007, 08:46:26 AM »

I ignore the pressure rating sticker on my Outback and put 35 pounds in each tire.

It's the best combination of handling and traction.

If I go 28 and 30, as the sticker recommends, the steering feels mushy and dead.

Chuck Dye · « **Reply #19 on:** August 26, 2007, 09:30:03 AM »

Run a search on "Michelin airless tire" for http://www.fastcoolcars.com/airless-tires.htm and other sites.

Pneumatic tires (perhaps airless tires, too?) perform best with a specific foot print best described by the surface area of the contact patch. The best way to tune tire pressure-to-load is to know that surface area and the load on the tire then do the math to get p.s.i.

A fun project, if you have a kid who is so inclined, is to measure the contact patches of your car's or truck's tires and multiply by the tire pressure(s) to get weight. The estimate you generate will be surprisingly close to the actual weight. You can weigh your vehicle for free at many state truck scales when they are closed, at the entrance to most recycling centers, and some privately owned truck scales at trucking companies, gravel pits, saw mills and the like, just PLEASE be polite and safety conscious (don't throw yourselves under the wheels of a big rig!)

drewtam · « **Reply #20 on:** August 26, 2007, 10:28:47 AM »

Quote from: grislyatoms on August 25, 2007, 01:37:34 PM

I think you got that backwards, drewtam.

Understeer is when the rear end sticks to the road better than the front end. "Plowing"
Oversteer is when the front end sticks to the road better than the rear end. "Fishtailing"

The low pressure will make the front end stickier, but the term is oversteer, not understeer.

Which doesn't make any sense in a pickup, as they tend to oversteer (no weight in the back) anyway, hence BenW's question, I believe.

Yeah your right. I knew that, must have had a brain fart.

Drew

News:

Author Topic: Tire Pressure - Why? (Read 2198 times)

Paddy

Paddy

Paddy

Chuck Dye