15 more PSI = 7% better MPG

[gone7]

Originally Posted by cbaber

Did you not read the whole point of this thread? Going from the factory spec of 30 PSI to the maximum sidewall pressure of 45 PSI I gained 7%. That is my own personal experience and many, many, many others have had the same result or even better. That isn't even over inflating, so there is still more to gain if I decide to over inflate the tires.

Clearly, your conversation with Marvin confirmed the fact that center wear on radial tires because of high pressures is a myth. But the vibration issue really struck me as odd. Tire/wheel vibrations happen when tires are not properly balanced. I don't see how increased pressure would create a vibration, unless the quality of the tire allowed it to deform under stress. Now, the entire point of the thread was to discuss if over inflating beyond the indicated sidewall pressure could have these results.

If you have a tire that deforms at 60 PSI, it's probably the worst quality tire in the world. As many have advised me in this thread already, modern tires are capable of handling more than their indicated max pressure.

No, you missed the point. First, the max presssure is not the reference for overinflating; it is the owner's manual or the door sticker for the car. 2-4 psi increase can be used for heavier loads, but not more. The tire manufacturer's max is for the buyer to know if it fits within the auto manufacturer's specs. We must not use the tire max as a reference for what is good for the car. Your tire pressure is way overinflated for the vehicle design.

Second, center wear on overinflation is not a myth. It is just not as big a deal as with bias tires.

Third, vibration happens all the time in a moving automobile. It happens from imbalanced parts, such as in a worn engine or drive shafts that become unbalanced. Tires/wheels that are not properly balanced can also contribute significantly. All these sources shorten the cars life. But overinflated tires will exceed the design parameters of the springs and suspension parts, and that causes much more vibration that was never intended by the manufacturer.

Fourth, a tire must never, ever be inflated more than the maximum rating of the tire in any application.

[gone7]

Originally Posted by niky

Not to mention the fact that it points out that higher pressure is necessary at higher speeds.


Have you ever dealt with vibration damage? I have. What it equates to is changing your suspension bushings maybe a few hundred kilometers earlier (like 120,000 kms instead of 125,000) and tightening your body bolts.

One should note that modern cars are built for stiffer tires (as reflected in the higher door sticker pressures indicated versus older cars), and suspension design nowadays takes into account stiffer tire carcasses and higher inflation pressures.


Wet issues... so-so. Actually... it's dry performance that suffers, particularly in terms of braking. Hydroplaning lessens at higher pressure.


There is no doubt that running very high pressures is bad for the suspension and if you run them high enough, the tire will experience greater wear. But there is no universal "too high" pressure. Some tires run fine over maximum sidewall. Some run crappy and experience center tread wear even below maximum sidewalll, even if you run them at the "proper" door pressures.


All tire carcasses will happily accept higher than maximum sidewall... though again, as some of us have pointed out, the diminishing returns beyond a certain inflation pressure (again, there is no universal pressure at which this happens) mean that you have to experiment to find the best compromise.

Higher tire pressure will be obtained by driving faster. The tire heats up more and the pressure will increase. I am not dealing with more than US highway speed limits here. Driving on the Autobahn is counterproductive to increased fuel savings, overall.

Extreme vibration causes much more damage than a few bolts and bushings. It can actually break welds and cause stress fractures. Besides, the average driver never checks for loose bolts.

The point, as I understand it from my research, for stiffer unibody construction techniques was to create a lighter automobile and to be able to compensate for less metal in the design. I doubt very much that the utilization of unibody was to use stiffer tires.

Hydroplaing effects increase at higher pressure.

Actually, the universal tire rating is the door sticker.

Most people are doing their own calculations, but failing to calculate damage to the vehicle. And that is the problem.

[gone7]

Or there is the other possibility that the tire and auto manufacturers are involved in a conspiracy to:
A. Use more fuel,
B. Reduce their individual and collective liability,
C. Confuse the buyer,
D. Sell more cars,
E. Sell more tires,
F. Sell more repair parts,
G. Make money on repair labor,
H. Or all of the above.

[fbov]

Granted, most of my tire pressure experience comes from competition. I was an autocrosser and did my fair share of track events. Increased tire pressures were mandatory to prevent bead seal blow-outs, and the accompanying mayhem that results (Allstate's not far off). Competitors would mark their tread edge to measure roll, and adjust pressures accordingly. Mine ended up at exactly the front/rear weight ratio - equal size contact patches front and rear were optimum. I ended up leaving the pressures where I liked them, roughly 42 front, 35 rear regardless the sidewall rating. End of anecdote.

No one's mentioned it, so let me. The mass of air inside a tire determines tire load capacity, mass typically measured by "pressure" in a fixed size tire. Sidewalls are marked with LOAD @ PRESSURE, and one must adhere to that pressure requirement in order to have access to that load capacity. In most cases, tire load capacity greatly exceeds car load requirements, giving automotive designers a little latitude. Regardless, increasing tire pressure up to the tire's rating insures you of the most load headroom above what's required.

The Vehicle Certification card tells you were the car's designers wanted the OEM tires set for optimium performance. Usually it's a compromise amoung ride comfort, handling and vehicle load. As someone who has fitted different tires, not OEM tires, it's s guidline. Here's an example:

Tire A is rated at 1200 lb. load at 32 psi
Tire B is rated at 1200 lb. load at 44 psi
The car's VC label spec's 28 psi with a 2000 lb. axel load.

Tire A has 28/32 = 7/8 it's load capacity at 28 psi, or 1050 lb @ 28 psi. It meets load.

Tire B has 28/44 = 7/11 it's load capacity at 28 psi, or 760 lb @ 28 psi. It fails to meet load.

Clearly, if you put tire B on the car and followed VC label tire pressure, you'd be in danger of meeting with mayhem...

From my perspecitve, it all depends on who makes what, and how they spec things. There are very few absolutes in life.

HAve fun,
Frank

[niky]

Originally Posted by ACEV

Higher tire pressure will be obtained by driving faster. The tire heats up more and the pressure will increase. I am not dealing with more than US highway speed limits here. Driving on the Autobahn is counterproductive to increased fuel savings, overall.

The difference is that this higher pressure is due to heat and deformation. And that's bad. Using higher pressure in the first place prevents heat build up. That's good. Which is why manufacturers recommend higher pressures for high speed use. Which is the entire point of the article you yourself cited.

Originally Posted by ACEV

Extreme vibration causes much more damage than a few bolts and bushings. It can actually break welds and cause stress fractures. Besides, the average driver never checks for loose bolts.

Bolt tightening is something you do after a few hundred thousand (insert distance units here). Most people don't keep their cars as long as I do.

And vibration itself, as pointed out, does not happen with all tires, and is a separate item from damage directly due to hard tires.

Originally Posted by ACEV

The point, as I understand it from my research, for stiffer unibody construction techniques was to create a lighter automobile and to be able to compensate for less metal in the design. I doubt very much that the utilization of unibody was to use stiffer tires.

Who said that stiffer unibodies were to compensate for stiffer tires? Manufacturers tune the suspension to compensate for stiffer tires. Better bushings, rubber isolation for the subframes, firmer shocks, shock and spring settings tuned for the tire inflation pressures used.

Originally Posted by ACEV

Hydroplaing effects increase at higher pressure.

citation required

Because absolutely everybody else (and personal experience) says you're completely and utterly wrong:

NPRM on TIRE PRESSURE MONITORING SYSTEM FMVSS No. 138
http://www.tirerack.com/tires/tirete...e.jsp?techid=3

The smaller the tire footprint, the less it hydroplanes. If you can actually overinflate a tire to the point that you lift the edge of the tread, you can decrease hydroplaning even further.

Originally Posted by ACEV

Actually, the universal tire rating is the door sticker.

I have two cars that use nearly the same size of tires and weigh approximately the same. One suggests 26 psi (cold) up front and the other suggests 35. Sounds pretty universal to me.

I've been in this biz for a while, and I've seen door pressure stickers creep up over time. The door pressure sticker is determined by a number of things... the need for the manufacturer to provide a "comfortable" ride (just ask Ford how that went with the Explorer and them Firestones, eh?), the need to meet performance requirements and the need to meet economy requirements.

As the last requirement becomes more important, door sticker pressures have started to creep up. Not because the "universal" pressure has started to creep up, but because the need to meet CAFE/CO2/whatever comes first. To restore ride comfort, manufacturers retune (again) the suspension to compensate.

Originally Posted by ACEV

Most people are doing their own calculations, but failing to calculate damage to the vehicle. And that is the problem.

The damage isn't in the vibration... which can be pretty bad if you're underinflated on the highway... but in the harshness over bumps. Then again, again... it depends on how high you're going.

-

Not that I'm an advocate of ultra-high "10-20 psi over max. sidewall" pressures... but let's put things in perspective, here.

[euromodder]

Originally Posted by cbaber

Consulting tire manufacturers for information is interesting.

I contacted Michelin for info on the max. rated pressure for their Alpin A4, as it isn't on the sidewall, and got a prompt response from them.

I read somewhere that over inflating your tires can wear your suspension faster because of the increased stiffness.

You'll offload more of the work to the suspension.

And the other way around : worn suspension will get bouncy with higher tyre pressure.

Tyres are the car manufacturers' cheapest / cheap-a$$ way to "improve" their suspension ...

[euromodder]

Originally Posted by ACEV

No, you missed the point. First, the max presssure is not the reference for overinflating; it is the owner's manual or the door sticker for the car. 2-4 psi increase can be used for heavier loads, but not more.

Complete b$, sorry ...

Car manufacturers post inflation pressures that are way too low.
It gives smoother riding.

On my previous car, a Volvo V50, they increased the tyre pressures significantly on later versions of the same model, while they are virtually identical in weight.
The tyre pressure was simply too low, because of that, handling was worse than it could be, tyre wear was higher than necessary and uneven on top of that !

Then the majority of customers forget to check for tyre pressure, and it gets dangerously low ...


On paved roads, you can safely run higher pressure than indicated by the car manufacturer.
I'd suggest using the given max. load pressure as an absolute minimum.


Running too highly inflated tyres - within sensible limits - is not the problem.
Underinflation is.
Proven time and again alongside the road ...

The tire manufacturer's max is for the buyer to know if it fits within the auto manufacturer's specs.

Like the episode with Volvo described above shows ... certainly. Not.

[euromodder]

Originally Posted by ACEV

Higher tire pressure will be obtained by driving faster. The tire heats up more and the pressure will increase.

And that's perfectly normal.

Max. rated pressures on tyres are "cold" measurements, and they already take into account that the tyre will be heated up due to driving at its design speed - the kind of speed where few ecomodders and few car owners will ever go.

Guess what, at low pressure, the tyre will heat up significantly more.

[RedDevil]

Raising the tire pressure will influence handling in several ways.

Because of the smaller contact patch the angle of attack between the thread and the road will be less. this will cause less wear and less noise. But the sides will vibrate at a higher frequency, and those vibrations may therefore become audible.
Steering will be lighter, and so will stresses from non-neutral alignment. Your bearings may last longer.

The sidewalls will flex less; in corners the thread will have less sideways travel, increasing stability and grip. I can corner faster by running my tires 7 PSI over the door sticker pressure; there is one corner when I exit the highway where I want to conserve as much momentum as possible, without making the tires squeal. It may not make a big difference, maybe just 5%; but it is there.

The Gen2 Insight is said to be sensitive to sidewind. And in my first months of ownership I experienced that too - a distinctive buffeting rear shake.
It disappeared when I raised the pressure.
Then I had my wintertires mounted by my dealership. On the way home; there it was again. Pressure up, gone for the winter.
Spring came, dealership put the summertires back on and carefully lowered their pressure to just below the sticker pressure... you guess what happened next.
That was the last time.
This known phenomenon disappears if you run the pressure over the posted too-low door sticker value!

When I started reading posts on ecomodder I was sceptical about raising tire pressure, but I gave it a try if only to stave my scepticism with experience. It all went horribly wrong so now I switched to the other side

[cbaber]

Originally Posted by ACEV

No, you missed the point. First, the max presssure is not the reference for overinflating; it is the owner's manual or the door sticker for the car. 2-4 psi increase can be used for heavier loads, but not more. The tire manufacturer's max is for the buyer to know if it fits within the auto manufacturer's specs. We must not use the tire max as a reference for what is good for the car. Your tire pressure is way overinflated for the vehicle design.

Second, center wear on overinflation is not a myth. It is just not as big a deal as with bias tires.

Third, vibration happens all the time in a moving automobile. It happens from imbalanced parts, such as in a worn engine or drive shafts that become unbalanced. Tires/wheels that are not properly balanced can also contribute significantly. All these sources shorten the cars life. But overinflated tires will exceed the design parameters of the springs and suspension parts, and that causes much more vibration that was never intended by the manufacturer.

Fourth, a tire must never, ever be inflated more than the maximum rating of the tire in any application.

I'm pumping my tires up to 60 PSI after work tonight. Thats a whopping 16 PSI over the tires indicated limit. My car will certainly vibrate to pieces, my tires will blow out, I'll wreck due to decreased traction, my fuel economy will get worse, and the center of my tires will wear unevenly. You can follow the progress in my project thread and when your theories are confirmed, feel free to post, "I told you so". Until that time, I'll continue to experiment with tire pressure in the real world, gathering my own data.