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Old 12-14-2009, 02:24 PM   #11
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not to add more variables or confusion, but how do tires allowing for more PSI figure in? my sumitomos have a max rating of 51 PSI so...
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Old 12-14-2009, 02:31 PM   #12
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The maximum PSI of a tire only puts an upper limit on how high you should go. The rest is still done the same way - increase until you reach the max, detect irregular wear, detect adverse effect on handling, or dislike the ride quality. Those are all upper limits.

Lower limits include door jamb label's recommended pressure (assuming original size tires and original weight), adversely affected handling, and irregular wear.

My 3000 pound VW is perfect at 51psi on 195-65R15 tires.
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Old 12-14-2009, 06:40 PM   #13
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Ride and handling are as much functions of the driver preference, suspension condition, and vehicle engineering. Cars like focuses that have both independent and dependent rear suspension variants are going to have different sweet spots. A car with new, high quality struts is going to have a different sweet spot than one with cheap struts with 100k miles on them. Even down to driver preference and perception. some people consider good handling to be twitchy, maneuverable, and slightly unstable. some consider good handling to be balanced and predictable. some mistake good handling for smooth mushy ride.

My moms 02 buick century and lesabre both went from under 30 mpg to 33 avg, 35 peak by going from 35 to 44 psi in all 4 tires. I drove back from the last family gathering and got 190 miles on 1/8 tank (as indicated). My cressida I usually keep around 40 psi, any more and it starts to have lateral traction issues BUT that depends on the tires. One rear pair I didn't take above 35 because they're hard-compound michelins and they might as well be bald for all the traction they have. the set I just got rid of was great about 38-40. My snow tires are 44 before the snow, and 40 in it.

I second the WTF are your rear tires at 38 and front 32 when your car has a 60/40ish weight distribution?! if anything, 38 front, 32 rear.

There's a guy that comes in to the shop I work at that swears high and low that if you take a tire 1 psi over the door sticker the world will end, nevermind that all 4 of his tires have shoulder wear and light cracking in the 1st tread block and just over the bead....definition of underinflation wear.
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Old 12-14-2009, 07:25 PM   #14
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update: down to front 32 back 34 after a little skid
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Old 12-14-2009, 08:14 PM   #15
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If you're just going to disregard the good advice you solicit, then poof away it goes
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Old 12-29-2009, 07:35 PM   #16
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To find your "ideal" pressure, get a non-contact temperature probe. They sell them at harbor freight pretty cheaply. If your pressure is too low, your tire will show that it is hotter on the outer edges than in the center of the tread. If your pressure is too high, the tire will be cooler on the outer edges, and hotter in the center. If the temp across the tread is about equal you are just about perfect. That is how the racecar drivers do it these days. On my dodge truck, you have to adjust pressure depending on the load in the truck. Max pressure is listed at 80lbs. If the truck is unloaded, that can be backed off quite abit by checking tire temperatures.
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Originally Posted by spotaneagle View Post
ok so I went from 30psi all around today(due to wintery conditions, not expected again for a few weeks) to
32 front psi
36 back psi

the difference was from sorta sluggish, crunchy shifting, not really, super good grip, ........to this car is floating around and my gearbox is barely struggling to shift gears...

the car right away gets to a higher mph due to the lower rolling resistance, i can tell right away, my problem is when I approach 34psi front 38back, I loose handling on the highway bigtime

there for this qualifies as a mod that the cow and other would claim as increase speed of car and mpg(how did you word it)

my problem is that any higher on the psi (34/38) makes me have to slow down, downshift, ect around corners, here and there and everywhere, so I think ive found the sweet spot on this car... with these tires

and my one other thing that im wondering about is an increase to say 44psi+ 51 in the case of one of the other saturns on this board... actually beneficial to less rolling resistance in that wouldnt a wider, but hard tire gripping the surface induce this more so than a super hard(from excess psi) thinner tire gripping the surface, has anyone graphed this out.... I would think that finding a sweet spot and being able to handle better, slow down less, and downshift less sound outweigh the benfites of slowing down constantly and a horrid bumpy ride on top of that...

please I want to see this graphed... I think maybe the tires im running toyo spectrum,(which are low rolling resistance tires) may be better at this than other tires due to their wider profile regardless of the tires' size 195/60/15=7.75in wide on a toyo... vs 195/60/15 falkens which would be about 7.25 or less wide

and also the harder rubber in the toyo tires(made to last 80k+ miles) may create the false/psuedo/included result of higher rolling resistance even more so than a softer tire of the same width(probably only measurable by .4 of a mpg.. lol)/if all variables are the same

just something to think about for daily driving/safety of hypermilersss... I guess

I wonder if Car Company Engineers read this stuff?
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Old 12-29-2009, 07:54 PM   #17
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There's an interesting experiment on rolling resistance related to inflation here. Basically, for the two cars and their 44 psi max sidewall tires, best rolling distances occurred at max sidewall or a few psi higher. YMMV.
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Old 12-30-2009, 04:01 AM   #18
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Quote:
Originally Posted by dwestmodesto View Post
To find your "ideal" pressure, get a non-contact temperature probe. They sell them at harbor freight pretty cheaply. If your pressure is too low, your tire will show that it is hotter on the outer edges than in the center of the tread. If your pressure is too high, the tire will be cooler on the outer edges, and hotter in the center. If the temp across the tread is about equal you are just about perfect. That is how the racecar drivers do it these days. On my dodge truck, you have to adjust pressure depending on the load in the truck. Max pressure is listed at 80lbs. If the truck is unloaded, that can be backed off quite abit by checking tire temperatures.
Dave
That method is perfect for finding the "ideal" pressure for best traction and treadwear. The OP was asking about the point of diminishing returns or the maximum return on extra inflation for fuel economy.

Also, race car drivers use different tires than road users. Their tires are designed much stiffer to prevent the tire from deflecting and bending under the lateral stress applied during a turn. That way, having even contact pressure across the tread width provides the best traction.

For more common road-use tires, that will provide the best braking traction but lateral traction suffers when the tire rolls sideways. This is why some autocross drivers and some police driving training schools use severe overinflation (far beyond the tire's maximum pressure). I suppose you could measure tread temperature after driving around a circle track at the limit for ten minutes to figure this pressure the same way - the inside edge of the tire will be cooler but the middle and outside will be the same.

The pressure for even treadwear is not the best for fuel economy. A little bit of center wear is ok; once it wears even you have an even contact patch again until the center wears bald. My VW specifies 35psi; I ran it at 51psi (tire's max) and the center wasn't bald until the edges were well past the wear bars, and traction never suffered (except in snow).
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Old 12-30-2009, 01:47 PM   #19
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Wow this debate could go on as long as the debate on religion, except that there is only one right answer to the religion debate

Your going to comprise something, and there are a lot of variables.

For efficiency take a good look at the wheels on a train. That's about as good as you can get with out getting high-tec.
If you want traction you want to have as much (super soft) rubber on the road as possible, that's why drag cars have wide tires. (i suspect rather low pressure too)
Handling I see as more of a preference thing. What feels right to you. I like my car on extra inflated tires, but this time of year i'm driving on sandy roads. I do enjoy some over steering here and there, but i don't like skidding into pedestrians, so i haven't filled them up above the recommended.

Snow traction is a interesting thing to consider. Some arctic vehicles use huge tires with big tread and 4 PSI in them. For road use a very narrow tire is best, so that you don't ride on top of the snow but push threw it.

So how many square in of rubber do you want on the road?
How much in the front and how much in the back?
Do you want the rubber touching the road to be shaped wider in the middle of the tire (over inflated)?
I don't have the answers to any of these questions, and i'm sure there is may conflicting answers to them.
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Old 12-30-2009, 09:27 PM   #20
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Pertaining to "grip" and "psi"...letting psi out of a tire makes it handle better within an acceptable range due to more tread on the ground. This extra tread is NOT from the tire getting wider;instead, it IS from the tire getting longer (like a tank track but not that pronounced). To test this, place a hot dog under the leading or trailing edge of a tire at lets say 44psi. Now with the hot dog firmly against the tire tread either in front of behind the tire...let air out of the tire. The tire will eventually cover or squash the hot dog. You can use your finger if you don't believe me but you should trust me on this one. Of course 1)tire tread design 2)running tire temps 3)road surface 4)weather conditions 4)proper alignment and such will also affect traction.

This is also why adding psi to tires when you are running in very wet conditions aids in reducing hydroplaning by your tires. less tire on the ground...less tire for water to be trapped underneath.

As for WIDE vs SKINNY...you have to remember that on two identical cars with both the WIDE and SKINNY tires aired up to identical psi...the same amount of tread is in contact with the ground. It will just be in a different orientation...one will be short and fat(WIDE) and the other will be long and thin(SKINNY). The reason they use such big tires at such low pressures (I am from Iceland where they use those kind of vehicles on the tundra) is too get as much tread on the ground (ie. traction) as possible. Those huge tires end up almost look like they are a flat tire and must use "beadlock rims" to make sure the tires don't just roll right off the rims. If you are ever stuck in sand...let your passsenger tires down to 10-13 psi and you will have a much greater chance of pulling out of the sand...of course air them back up to travel at normal speeds once your are freed.
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