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Overinflating tires..good Idea or bad Idea?

8K views 29 replies 18 participants last post by  05wkguy 
#1 ·
I tried doing a search on this but didn't find anything. I wanted to know how safe is this. I was doing some research over the Internet on overinflating tires to get better MPG's and better handling, some say it's good some say it's bad. The bad things I heard about it was that overinflating tires makes the tire narrower on the road surface, thus less traction. The Jeep calls for 33 psi on all tires, I upped to 35 psi. Should I be concerned that if the air of the tired get hot from driving or weather and it expands, that the tires will burst? So far, I feel that the tires are harder and at one instance I was making a sharp turn and it felt that the Jeep slipped from underneath me, but then again that could have been the left over snow from the other day. Should I take them back down to 33psi?
 
#5 ·
I tried doing a search on this but didn't find anything. I wanted to know how safe is this. I was doing some research over the Internet on overinflating tires to get better MPG's and better handling, some say it's good some say it's bad. The bad things I heard about it was that overinflating tires makes the tire narrower on the road surface, thus less traction. The Jeep calls for 33 psi on all tires, I upped to 35 psi. Should I be concerned that if the air of the tired get hot from driving or weather and it expands, that the tires will burst? So far, I feel that the tires are harder and at one instance I was making a sharp turn and it felt that the Jeep slipped from underneath me, but then again that could have been the left over snow from the other day. Should I take them back down to 33psi?

Reread your original post. Overinflating your tires means less rubber on the pavement - that means WORSE handling. Bad idea. Especially on wet, snowy, or icy roads.
 
#15 ·
This is not correct. Narrow tires are actually better in the snow. They can easily cut through it, and reach pavement. The analogy I have always heard for this is a knife works better than a spoon to cut butter. Less contact patch equals mores driving force downward, which is much better for snowy conditions.
 
#6 ·
Let’s put this into numbers. The jeep weighs 4000 lbs. Each tire is supporting 1000 lbs. At 33 psi, each tire’s contact patch would be 1000 lbs ¸33 lbs/in2 = 30.3 square inches. I don’t know what the tread width is, but in any case it won’t change. Let’s say it’s 9 inches wide. So to find out the length of tread on the road it would be 30.3 in2 ¸ 9 in = 3.36 inches. That means that at the recommended pressure each tire has a strip of tread 3.36 inches long in contact with the road. This is what the engineers designed for with the stock setup. So this is the best compromise of efficiency and handling.
Long story short, at 35 psi, the strip would be 3.17 inches long. The rolling efficiency might be better but cornering and braking would be worse.
IIRC the max tire pressure is 45 psi. At that pressure you would only have 2.5 inches of tread length contacting the road. Cornering and braking would be really bad and you would be risking a blowout if you ran over something sharp.
 
#11 ·
Hahahah. Thanks guys! I'm no professor and I don't think I'm a nerd (although I have nothing against nerds). I started in cartography 30 years ago and we had to use spherical trig in those days to calculate great circle distances etc. Couldn't do it today to save my life, but I like using math to try to solve problems when I can. I started thinking about this when there was another thread about airing down for the trail or beach.

There are lots of physics students here....I actually thought someone brighter than me would find a mistake with my numbers....;)
 
#12 ·
I'd be more concerned with getting an accurate tire gauge. You might think you're running 33 or 35 and actually be running significantly more or less because so many gauges suck. I'd say instead of worrying about 2 psi, invest in a good analog gauge if you don't already have one.
 
#13 ·
I got a thread on this about tire pressure. It has a calutor on what to put in the tire based on the max psi. I got max psi of 51 on my 20 inch tires. I was putting in33 psi way to little I put 38 psi in it rides great I got better mpg. Just look for the tire psi calutor
 
#14 ·
i have mine at 35psi, at 33 they were visibly more flat on the ground, MPG was up by .5-1mpg with the adjusted inflation
 
#18 ·
It almost sounds like if tire pressure is more of a matter of taste than concern, if not seriously overinflated.
 
#20 ·
2 lbs is not going to make that much difference. Tire pressures also change by about 1 psi for every 10 degree change in outside temperature.
 
#22 ·
Take it off, submerge it in a tub of water and look for bubbles...you might have a leak at the air valve. If it's a really slow leak, you may not see anything. Dish soap and a little water around the valve might help show any leaks too.:thumbsup:
 
#26 ·
I disagree in that Jeep wants to keep the ride soft and comfortable and so will give a recommended number with that as a primary focus, with a measured pressure only a few PSI higher I believe you will get higher performance, better MPG and "feel". Just don't stray far from recommended.
 
#27 ·
All MFR are also using the inflation figure for the OEM tires. Some other times work better with a little more or less. With GY Fortera triplecraps, they were rough at 32, and so were the Destination AT tires, so I ran both at 30 since the ride was too harsh and bouncy any higher. With the Cooper AT3, I run 33, and it is far smoother than either of the others at 30.
 
#28 · (Edited)
Not trying to be rude, but I actually have to disagree with bobharly, although his math and theory are absolutely sound, they apply to objects that are solid, not made of pliable rubber. Now I'm not an engineer or anything but i have been autocross and road racing for a while, and used to be a drag racer before that.

Tires are contructed in a way with different "stiffness" in different areas. Steel belted radials (all street tires now) are designed to be softer in the sidewall than in the tread areas. This is why when you add a few pounds of air pressure,you can see the tire get taller, however, if you were to measure the contact area between a tire at 33psi and then the same one at 35psi, you would need some very high quality equipment to measure the difference, less than 2%. What happens in the real world is that when a tire's sidewall rolls over, you actually have an effect of "jacking up" the inside of the tire. What I mean is that because the tire is designed to try and keep itself flat across the tread(from inside to outside),when the sidewall rolls it is lifting the inside of the tread up, actually removing up to 1/3 of the contact patch from the road. This is why we always raise tire pressures up to around 38-40 psi for racing. So you lose a tiny bit of contact front to back but gain so much in the left to right, when cornoring. Keep in mind that at these pressures, we are also driving the cars quite hard and will add alot of heat, still blowouts are almost unheard of.
Next something different happens when you accelerate or brake;there is weight transfer to the front or rear of the vehicle. When you brake, you are increasing the pressure on the front tires by 2-3 times over that of static. Extra weight causes the tires to squish down and you gain your contact patch back,plus more. Same thing on the rear tires when you accelerate, and this still happens,less drastically,at light brake and throttle applications.
So yes, you technically should lose grip when adding pressure,but in the real world, things happen differently than on paper.

Some other things to remember: we are talking about differences in pressure of 2-4 psi (for street use), not the 20-25psi difference like in racing or off-road environments. Your typical racing slick for drag racing is NOT a radial,it is a much softer bias-ply, designed that way for absolute maximum traction in a straight line, and run at about 15psi. Off-road tires are designed to be aired down to around 6-9psi, from the typical 30-35psi they would use on the street. If you were to run 15 psi or less in your street tire, your chances of doing damage to the tire from overheating go up drastically. You can't compare the two for the prupose of saying that you should or shouldn't do. The reason you CAN apply that to autocross situations is because we use street radial tires all the time.
Also, snow is different,trust me, i live where snow on the ground is a reality for 5-6 months of the year. Narrow tires to cut through the snow is better,because any contact patch on the ground is better than a big contact patch on snow.

(almost done guys, lol)

So adding 2-3 psi will not put you in danger of blowing out a tire, it generally offers a little better gas mileage, but does affect the ride quality. I would never recommend running a tire at 40+ psi all the time, unless you are a glutton for punishment, lol. You put yourself in more danger by running a tire underinflated than overinflated.
 
#29 ·
No offense, 05wkguy. I like thoughtful answers. I was just under the truck recently and while down there I could see the tire wear, or lack of wear on the inside edge, which would support the idea that when the tire is "rolling" on it's (out-)side edge, it is losing it's contact patch on the "jacked up" inside edge. I also like the observation of weight transfer, as the air in the tire does compress, even when pressurized. I should thank dcmski for resurrecting this old thread. I just learned something. ;)
 
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