What's new

Question about tire-pressure for your car. (1 Viewer)

Scott H

Supporting Actor
Joined
Mar 9, 2000
Messages
693
Bill, thanks for the reply. I'd point out that I'm not an expert, I have just paid a lot of attention to the topic for a long time, and some working opportunities provided more info. None of my comments were directed to you at all, and overall I agree with your general assessment.
I would add that in my experience I have never chosen to run the rear tires at a higher PSI that the fronts on rear wheel drive vehicles. In fact, the opposite was often the case. Usually I got a lot more bite and the ability to keep the rear traction under acceleration by giving the tire more 'flexibility/give', and it is still the front tires which endure the most sidewall stress in most cornering (I'm not talking track here). This is a little trickier with modern low profile tires with their very strong sidewalls, but still, in common front engine rear drive cars, I almost always settle on even or slightly higher front pressure. Take a rear drive car out in the rain and see how you can break the rear free easier and easier at higher pressure... Note that this take on things excludes a car like the 911, where the engine and transaxle are over the rear tires (when accelerating through a hard turn in a 911 don't dramatically let off the gas
wink.gif
).
I would agree that the best factor is how things feel to a driver intimately familiar with the car and tire wear. However, if you have any doubts or concerns I would suggest going with the auto manufacturers recommendation and increasing by 2 PSI all around for extended highway driving.
Also, I do run different pressures at different times, earlier I noted my best general pressure findings for the Coupe. As I just eluded to, a little softer is usually better for city driving and rough roads, and higher pressures for interstate travels and higher speeds. It is also my experience that sunlight on the tire increases the PSI more than driving does on most commuter passenger car tires. And remember, all tires lose pressure naturally and in a matter weeks, and try to find a gas station that bleeds the water from their compressors so your not putting all that moisture in your tires
wink.gif

I guess my main point was don't go inflate your tires to their maximum because you read it on a board, and I know most wouldn't, but...
wink.gif

------------------
My DVD Library
Runaway production? No thanks. Where I've filmed, benefiting local economies: AL, CA, ID, IL, IN, IA, KS, MN, MO, MT, NV, OH, OR, TX, WA, WY.
[Edited last by Scott H on October 26, 2001 at 01:02 PM]
 

Denward

Supporting Actor
Joined
Feb 26, 2001
Messages
552
Scott, That is an excellent post
icon14.gif
. Your job sounds like a great subject for a movie. But, they'd probably edit out that whole speech about correct tire pressure
biggrin.gif
.
 

Scott H

Supporting Actor
Joined
Mar 9, 2000
Messages
693
Denward, I work primarily in the camera dept on commercials and films. About ten years ago I used to shoot a lot of stuff like this, hence my association with the state police agencies. I also shot a lot of doc style real police shows your familiar with, which took it further (I also shot a lot of tactical footage - real raids, undercover gang work, and stuff where a lot of hairy driving went down). The precision driving is just a serious diversion for a camera guy whose always too distracted by various things
wink.gif

------------------
My DVD Library
Runaway production? No thanks. Where I've filmed, benefiting local economies: AL, CA, ID, IL, IN, IA, KS, MN, MO, MT, NV, OH, OR, TX, WA, WY.
 

Kevin Eckhardt

Stunt Coordinator
Joined
Jun 16, 1999
Messages
185
Real Name
Kevin Eckhardt
Scott:
Yup, I had it reversed. In my case it was 32psi front and 29psi rear. I knew it was different but sometimes get which one is higher mixed up. Thats why I always read the door jamb when I check my tire pressure :) Unlike the guys who did my last tire rotation who just inflated everything to 35psi all around. Sorry to be spreading misinformation.
Kevin
 

Mike Lenthol

Second Unit
Joined
Jul 28, 2000
Messages
322
I came across an interesting bit of information
today.
Turns out the recommended PSI ratings on the 'door jabs' are simply derived by a
formula based on vehicle weight and stock tires. Replacement tires of same size
of course would have specs pretty close to stock, but not in all cases... and
other size tires would throw off the PSI figure completely.
The formula is pretty simple.
First we take total vehicle weight say 3500lb* and a weight distribution
of say 60/40 front/rear. Simple math: each front tire would have a load
of 1050lb, and each rear 700lb.
Now for example to calculate the 'optimum**' tire pressure for the front tire,
formula: (weight on front tire) / (maximum tire load) * (maximum
tire PSI
)
Imaginary tire of 35 PSI max and a load of 1500lb.
(1050) / (1500) * (35) = 24.5 PSI
*Weight of passengers and other loads should be
accounted for.
** Maximum absorption of shock while keeping the tire in safe operating range
 

Philip_G

Senior HTF Member
Joined
Nov 13, 2000
Messages
5,030
cars? BAH who cares :)
I run my motorcycle at yamaha's reccomended 36PSI.
BTW for those tire pressure conscious (I do salute you, I know better but mostly ignore the tires on my cage.) they do make neato little valve stem caps that you buy for your pressure, they show green for good, yellow for a couple PSI low, and red for way low. Pretty cool for about 40 bucks a set.
 

Henry Carmona

Screenwriter
Joined
Feb 7, 2000
Messages
1,299
Location
San Antonio
Real Name
Henry Carmona
When driving, you want all of the tire you can get in contact with the road. This is what gives you traction. Many people think that lowering the pressure of the tires will put more tire in contact with the road. Actually, the opposite is true. Running your tires at a lower pressure will actually cause the center of the tire rise up, and you will be driving on the outsides of the tire, with less tire touching the road surface(we used to do a demonstration of this at the driving range). The tire surface gives you traction, not the tread in the tire. The tread is there only to prevent hydroplaning on wet surfaces. In dry conditions, tires without tread (slicks) provide the best traction (thus why race drivers run tires without tread).
This info is very important, but do not adhere to it if off-Roading :)
P.S. I usually run my 315's around 12-15 psi offroad and 36 onroad :)
 

Philip Hamm

Senior HTF Member
Joined
Jan 23, 1999
Messages
6,874
Henry,

Why do you change the PSI for off-road driving? I used to do some off-roading and my owner's manual specifically stated to not change the PSI for off-roading, except if I was actually stuck in the sand. And I took that 4x4 Amigo on some pretty hardcore mountain trails and loose sandy beaches with no difficulties whatsoever. Most of the information that I got pointed to the lowering of PSI for off-roading purposes as a common myth that should NOT be followed.
 

Henry Carmona

Screenwriter
Joined
Feb 7, 2000
Messages
1,299
Location
San Antonio
Real Name
Henry Carmona
Im no "Hard-Core" offroader, but i have done some myself and have ridden along in "Hard-Core" rock crawling jeeps.
The lower air pressure in the tires (large tires) helps in many ways, but particularly in that the tire is able to conform to many obstacles and grip much better.
The lower pressure makes it easier for the tire to get over obstacles better than if it was at full pressure.
A general rule is the 25% rule where you air your tires down 25% of their original height. This generally give you almost twice the contact patch which would be like riding on 8 full pressure tires.
Almost every rockcrawler lowers pressure in their tires and many do so in the sand also.
This is basically serious 4 wheeling and i dont do it when encountering basic mud, or loose dirt etc, but i do drop my air pressure when i hit the beach for a few days.
Heres some info i dug up:
About the lowest pressure you can normally use for off-roading is ten p.s.i., but care must be taken not to push the vehicle, especially under hard side loads. At this pressure, flotation in deep sand is excellent. However, steering accuracy suffers badly and the front end will respond in a sluggish fashion. Even with power assist, the steering feels heavy, much like your pump belt is slipping
The lowest pressure I have ever run is 4 p.s.i., and that was at some sand drags at an event in Rosarito. The tire looks damn near flat at that pressure, but it works and you get incredible traction in soft sand or silt. For mud , it's great.
Naturally, you have to completely avoid rocks or any other sharp stuff at this low setting, but I made a dozen 1/8 th mile runs at this setting and my 35 inch BFG All Terrain tires stayed on the 15 X 7 rims
At low settings (10 p.s.i. or less), the stress on the carcass is incredible and prolonged usage will actually break down the metal wire on the inner lip and cause seperation of the tire layers in cheaper tires.
Radials are the only way to go for serious off-road use. I have never - REPEAT NEVER - seen a really good non-radial tire for off-road work.
 

Bill Kane

Screenwriter
Joined
Feb 5, 2001
Messages
1,359
what Henry said...

I belong to a 4WD offroad, loosely-knit, club of street-legal 4Xs and SUVs. BFG All-Terrains currently are the preferred tire.

Henry's tire management tips are what we've learned and been taught. For example, running long washboard roads is easier at 20-24psi and if you get speed up to 40-45mph. Light-touch steering is critical here.

Rock crawling MEANS crawling when you're aired down: tires mold around the sharpies so long as forward progress is 10mph or less.

When I travel forest logging roads, or rougher, I air down from 32psi street to 24-25psi. This is still enough pressure for highway driving.

I now have a 12V QuickAir air compressor mounted in my engine compartment and hardwired to the battery.
 

Philip_G

Senior HTF Member
Joined
Nov 13, 2000
Messages
5,030
as you lower the pressure the tires run hotter also I believe. I do air down my motorcycle tires a couple PSI in the cooler seasons so the tires heat up.. faster? maybe.. maybe it's all in my head :D
 

Philip Hamm

Senior HTF Member
Joined
Jan 23, 1999
Messages
6,874
Rock crawling MEANS crawling when you're aired down: tires mold around the sharpies so long as forward progress is 10mph or less.
On the "roads" I used to drive my amigo on back in the day I would guess that my average speed was under 10mph, closer to 5 or 7. Really steep, rocky terrain, in 4L and 1st (or for the fast runs, second) gear. Full pressure, as recommended by the manufacturer, always served me well. Guess if I ever get another SUV and go off road I'll have to check more information.
 

Jay H

Senior HTF Member
Joined
Mar 22, 1999
Messages
5,654
Location
Pittsfield, MA
Real Name
Jay
Cool, on a similar note, I do the same for mountain biking, lower the pressure when on technical rocky terrain (which is practically all of the NE) but not too low where you pinch-flat alot.

I've actually read some studies for road biking that the skinnier the tire, the more contact patch there is (I would imagine due to deformation of the skinny tire) and is actually slower than a wider tire which has less rolling resistance. This is of course ignoring the weight differences where overall, the skinnier tire most likely weighs less than the wider tirer and has a much more effect on speed than rolling resistance. (Probably why the road racers race on skinny tires and not wider ones).

Jay
 

Henry Carmona

Screenwriter
Joined
Feb 7, 2000
Messages
1,299
Location
San Antonio
Real Name
Henry Carmona
Forgot to mention, the only drawback to running low pressure when doing serious offroading is a loss of ground clearance.
But with large tires it isnt that much of a concern.
With my 6 inch lift and 35 inch tires, i still have plenty of clearance running 12-15 psi.
 

CharlesD

Screenwriter
Joined
Mar 30, 2000
Messages
1,493
Does anyone know the details of adjustiung tire pressure to change a car's handeling characteritics such as understeer?

I drive a Subaru WRX which is a blast to drive but will (when off the boost) understeer quite a bit, and I would like to "dial-out" some of that behavior. I am planning to get a thicker rear sway bar and maybe some strut tower braces, but in the mean time if I can adjust the front or rear tire pressure by a few psi and have less understeer it would be fantastic.
 

Users who are viewing this thread

Forum statistics

Threads
356,712
Messages
5,121,166
Members
144,147
Latest member
cennetkaralowa
Recent bookmarks
0
Top