Tire pressure and fuel economy RESULTS

[Jim Allen]

As I'm fairly new to this group, I didn't check to see if this topic had been dealt with before. I just looked. Yeeeha! Y'all been down this road before back in May and June! I shoulda known! I see some of the same handles... and arguments. Some interesting data has come out, now and back then. Good stuff to file away. I doubt we'll reach a consensus here (one could say there's no such thing as a "consensus" on physics and science anyway... it's either correct or not ), but it's sure a fun road to ride down... whether your tires are overinflated or not!

[MazdaMatt]

Yes, this one is quite the can of worms. All i have is a few years of experience in racing and tuning tire pressures for heresay, and all "they" have is data saying that underinflation is no good. Believe neither of us till one of us comes up with a repeatable, properly conducted test. I'm hoping to have a G-force meter next year. I'll use it IN MY STREET VEHICLE to prove one of us wrong.

[Jim Allen]

Originally Posted by Denis

Guys, I quit Ecomodders !

People who tell you that WAY overinflated tires dont wear abnormally are dreamers, liars or drive like grandma. Wanna really saves ga ? Remove the brakes from your car -théy're HEAVY - the handbrake will do...

NO, I quit THIS list, serious, evolving discussion does not seem possible here. It's like those HHO lists where people claim to ignore thesecond law of thermodynamics abd make a perpetual motion machine...cause they wanna !

I only wished that serious people here would start another list with engineering and safety minded ecomodder discussions.

Adieu !

Denis
Lion sur Mer
france

PLEASE don't go Denis! It's the nature of the Forum beast. You often have to reach through some "brown trout" to find the diamonds. Sometimes not fun...but they're diamonds and the brown stuff washes off. Second, if you leave, you abandon us to exactly who you describe. Plus, you are the the second "Denis" I've met (as opposed to a "Dennis")! we need your reasoned voice.

[trebuchet03]

Originally Posted by CapriRacer

Sorry, but tires do not conform to the classical physics interpretation of friction: F= uM (gee, I wish I could post Greek letters!!)

Argg... I had a rather lengthy post written up to revise that post to bring in more variables... Mainly, Stiction (one of my favorite engineering words) and mechanical locking - but Solidworks crashed on me and brought down my entire machine :/ If I can re-find the road tire tribology article on ebsco, I'll come back and link it....

In light of that frustration, and Denis' post - I'm going to break my normal operation and abandon ship... My apologies, I'd be equally pissed on the flip side of that coin....

Before the door hits me in the ass

People who tell you that WAY overinflated tires dont wear abnormally are dreamers, liars or drive like grandma. Wanna really saves ga ? Remove the brakes from your car -théy're HEAVY - the handbrake will do...

Personally, I don't see how to directly link driving like grandma and tire inflation I personally don't drive like grandma - unless legally abiding by a speed limit classifies as that (feel free to gloat about breaking the law, however ). Sorry to see you go, however :/

I only wished that serious people here would start another list with engineering and safety minded ecomodder discussions.

Nobody tell him how many engineers are already on this board

Lastly,

But denis, way underinflated tires don't work as well as perfectly inflated tires. So way overinflated tires must be WAY better!

Not way better - as originally posted

...e minor or nonexistent on dry surfaces and wet surfaces with very little water depth.

[Jim Allen]

Originally Posted by MazdaMatt

Yes, this one is quite the can of worms. All i have is a few years of experience in racing and tuning tire pressures for heresay, and all "they" have is data saying that underinflation is no good. Believe neither of us till one of us comes up with a repeatable, properly conducted test. I'm hoping to have a G-force meter next year. I'll use it IN MY STREET VEHICLE to prove one of us wrong.

Well, I have a G-Force tester. Never used it for braking tests but it has that capability. If you and CapriRacer want to outline some test parameters, I might be able to find some time to try it before winter hits. Pretty busy right now, but I can try. You see what rolling stock I have available. The Honda is the obvious choice, though it's tires are on the thin side (jeez, those PROPERLY INFLATED Michelins are wearing out after only 70,000 miles). The newer Ford has fresh meats, but they are 285/70R-17s. The old Ford has fairly new meats too, but it has sucky brakes (sucky from the factory, I mean).

I also have some tire company contacts and I'll see if there are any other studies.

[07b2300]

Wow, what a landslide of info! I'm the original poster.

Sorry that you are leaving Denis, but I have to say that this roll down tire lane has been of real benefit, including especially the link to the post on rolldown tests by MetroMPG.

The result for me: I'm keeping my tires at 35 (max inflation on the side of the tire). The tire size I'm using was rated for low RR by Consumer Reports, but sadly the size of my specific tire has 35 max pressure on the sidewall unlike most others of the same model which take 44 psi.

I am no tire expert but I worked at an alignment shop for years and saw many tires with the center of the tread worn out from overinflation. And many many tires separating. Often those were underinflated (which as described above can cause tires to overheat). But too much pressure (as stated somewhere above or on another discussion) can cause fatigue. Rubber doesn't stick to metal well and I for one don't want to "hyperinflate", go over a manufacturers max inflation, and potentially cause separation and an unsafe situation. The max PSI are what the manufacturers have tested the tires to take, and even if they can take a little more, I'd like to have that margin remain as a margin of safety.

So thanks to all the help I am going to keep filling with nitrogen at max inflation and checking the pressure regularly to keep it right at max inflation pressure.

Thanks for the tip on metal valve caps. And for all the fruitful discussion!

[CapriRacer]

Originally Posted by Jim Allen

.........

If you and CapriRacer want to outline some test parameters, I might be able to find some time to try it before winter hits.

........

Jim,

Don't bother. The testing that will prove this is going to trash the tires under braking. The only other way would be to do skid pad testing of lateral G's - but that's going to trash the tires, too!

Not to mention that heat in the tread compound is going to affect the test results, so the only way to get a good test is to have many tires already mounted up and a guy standing by to do the changeover.

[some_other_dave]

Originally Posted by MazdaMatt

I have raised my tires from 36/33 to 45/42. I notice longer coasts, rougher ride and DECREASED cornering and stopping ability on dry pavement.

Interesting. Are you using R-compound tires? Those generally grip better at relatively lower pressures. Hard-compound "real street" tires usually grip better at higher pressures. I do know that the tires I used to run (Yoko AVSes, that'll tell you how long ago it was! ) gave me better lap times at 40 PSI than they did at 36 PSI.

The above was autoXing, which places a significant premium on lateral grip. Braking and acceleration are not as important, so you can still get better lap times with worse longitudinal traction as long as the lateral grip is better.

Other tests I have seen recently (a recent Grassroots Motorsports issue) showed that higher pressures on the less-grippy end of a car (I think it was on the rear of a CRX) will result in more grip to a point--then the grip falls off. The higher pressures generally resulted in better feel (AKA "harsher ride") and less loss of grip than being an equivalent amount of pressure under the "maximum grip" pressure. However, the grip was less consistent, so it was sometimes harder to control. (I hope I don't have that last statement backwards, it's been a month or two since I read the article.)

I talked to some drag racing friends of mine, and they seem to agree that lower pressures let you launch better, so it sounds like for acceleration a lower pressure is better. I haven't seen much of a consensus for braking distances, though.

...Oh, and I have the tires in my CRX just below the sidewall maximum.

-soD

[MetroMPG]

Originally Posted by 07b2300

real benefit, including especially the link to the post on rolldown tests by MetroMPG.

Well, to throw another wrench into the discussion - there isn't really enough data in those tests to make a conclusive decision. Not enough runs done.

[Jim Allen]

Originally Posted by CapriRacer

Jim,

Don't bother. The testing that will prove this is going to trash the tires under braking. The only other way would be to do skid pad testing of lateral G's - but that's going to trash the tires, too!

Not to mention that heat in the tread compound is going to affect the test results, so the only way to get a good test is to have many tires already mounted up and a guy standing by to do the changeover.

Bummer! Yeah, that makes sense. Well, anything that trashes my tires is a no-go fer sure. I'll fall back on my tire contacts and see if I can get some test info on the tires they trashed. That study that was linked to from Goodyear was interesting but it really didn't answer the question of if, or whether, braking performance degraded at the high end of the pressure range, above the 35 psi listed (which I took to be the max pressure rated for the tire). They were looking at the low end pressure... which, truth be told, is the scary end they should be looking at from the safety standpoint related to neglectful car owners.

I think it was you that talked about a "sweet spot." I'll bet there's one that offers the best of all worlds, performance, safety AND fuel economy, and I'll bet it different for every tire/car combination.