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Old 10-29-2011, 05:54 AM   #21
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Re: Tire pressure difference

On that issue:
In my experience, increased inflation does not reduce traction until you've gone to a VERY extreme level. The only reduction in traction I've ever experienced: Pickup specified for 35psi run at 80psi, front is fine but with the bed empty the rear has reduced accleration traction in rain/snow. I run the rear at 70 and it's fine.

In the meantime, lateral (steering/handling) traction is increased.

Lateral traction is generally limited by softness of the tires allowing them to deflect and curl sideways, taking the weight off of much of the tread*. That was the original cause of the wheel upsizing trend**; the shorter sidewalls allow less of that. Increased stiffness from increased pressure reduces the sideways deflection, helping keep the tread on the ground, and that theory matches my observations of increased lateral traction in my vehicles.

*: Take a car with tall tires around turns as fast as possible every day and soon you'll start to see scuffing on the sidewall. I used to when I was a dumb kid.

**: The trend has been taken to an extreme, I think even to the point of detriment.
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Old 10-29-2011, 06:59 AM   #22
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Re: Tire pressure difference

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Originally Posted by CapriRacer View Post
If the torque biasing is OK at one pair of pressures, why wouldn't there be another set of pressures that also be OK? - and I would think that there would be a whole range of front/rear pressure combinations - that it would just be a matter of discovering the rules.
Well, I was also trying to say that there are quite a few other issues that effect it as well. But yes, there are "rules" in a way, just like there are rules in stock FWD and RWD cars with open differentials and no traction control. Plus, I am not saying the torque biasing is not OK at a different pressure, it is just different at a different pressure. No matter how smart your AWD system, it doesn't respond the same when certain variables are changed. To a racer, half a psi in one tire or another makes a world of difference. Now, the cars we have aren't set up to run as precisely as a race car, but these changes do still scale up.

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I think you'll find that pretty much every car has built in understeer - that is, not neutral. You can change that without changing anything else by changing the front/rear tire pressure split - but that is not what we are discussing.
Sorry, I think you misunderstood. I meant changes to the handling from the car's neutral settings, not that the car handle's neutrally. I'm aware understeer is built in.

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It sounds like you are not a fan of elevated tire inflation pressures - and it seems like you are advocating NOT using elevated tire inflation pressures.
no, I am not saying that. I am just saying there are minute changes from it.

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Ah.....Mmmmm......Uh....... No!!!

Tire rolling resistance is largely caused by hysteresis - internal friction - and very little is from friction between the tire and the road surface.
ummm.... yes there is friction between the tire and road surface....

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And that quote leaves me cold. Why would the weight distribution move MORE with less roll in the vehicle? Wouldn't it move less? And wouldn't that mean that the electronics in the AWD system would be LESS likely to be triggered?
no, I said it requires greater force to redistribute the weight when body roll is lower.
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Old 10-29-2011, 08:48 AM   #23
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Re: Tire pressure difference

sorry, I have been busy. I run two different types of tires on my subaru. I don't buy 4 new tires when I only need two. regardless of what tire dealers tell you , the subaru does fine without all four tires being the same brand. with that said, I noticed that the front tires had a much higher maximum air capacity than the rear tires. I raised the pressure in the front tires, and not the rear. the car handled fine on the freeway, but not in mountain curves. best way to describe it is to imagine the tire is more crowned with the extra air, and in the middle of a turn, the the crown would roll over, causing the car to dive to the right. the right side must be the drive side, so it would veer to the right, but handle fine in a left hand turn. whether filling all four tires would correct this tendency or not, I am not willing to experiment, as this was a very dangerous experience. I will stick with 32 psi on my subaru, and experiment with higher pressures on my other vehicles.
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Old 10-30-2011, 03:46 AM   #24
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Re: Tire pressure difference

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Originally Posted by mikehallbackhoe View Post
sorry, I have been busy. I run two different types of tires on my subaru. I don't buy 4 new tires when I only need two. regardless of what tire dealers tell you , the subaru does fine without all four tires being the same brand. with that said, I noticed that the front tires had a much higher maximum air capacity than the rear tires. I raised the pressure in the front tires, and not the rear. the car handled fine on the freeway, but not in mountain curves. best way to describe it is to imagine the tire is more crowned with the extra air, and in the middle of a turn, the the crown would roll over, causing the car to dive to the right. the right side must be the drive side, so it would veer to the right, but handle fine in a left hand turn. whether filling all four tires would correct this tendency or not, I am not willing to experiment, as this was a very dangerous experience. I will stick with 32 psi on my subaru, and experiment with higher pressures on my other vehicles.
Mike,

Thanks for the clarification. Your first statements were pretty confusing, and this really helps.

OK, I'm thinking that you have 2 variables (inflation pressure and different tire brand) and are attributing the problem to only one of them. Yes, changing the one can turn the problem on and off, but what happens when you change the other one?

Personally, I think you are correct, but I would be very careful of assigning the cause/effect without either testing the other combination or explaining the circumstances.

Secondly, I think the limited amount of testing you've done on your Subie doesn't warrant a blanket statement that different tires DON'T cause problems. There are plenty of folks who have had problems.

And lastly, I think the clarification means we should avoid further discussions about what some mythical AWD system may or may not be able to do and stick with how real-life systems behave - and in this case, it's clear that changing the balance front to rear on a Subie causes issues.
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Old 10-30-2011, 11:12 PM   #25
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Re: Tire pressure difference

if the tires are the same diametor, it shouldn't hurt the car. I have been driving subarus for 10 years, with over 100,000 miles on my current car, and have never had troubles running two brands of tires. if that is limited testing, so be it. I'm not advising anybody else to try two different brands, but I will continue to do so until someone can give me solid proof that it does any harm.
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Old 10-31-2011, 08:30 AM   #26
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Re: Tire pressure difference

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if the tires are the same diametor.......
I think this is where the problem lies. Even though the tires may say the same size, that doesn't mean they are the same diameter.

Plus Subies are notorious for being picky about differences in tire diameter.

But you do what you want, but if you want proof, read your owners manual.
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Old 11-07-2011, 12:07 AM   #27
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Re: Tire pressure difference

Well after increasing my tire psi from 28 to max of 44 psi, my gas mileage shot up like a cannonball. I went from averaging around 45 or 46 mpg to 57.8 mpg last tank. Most of it was highway driving but this is an incredible difference. I notice the ride is not as smooth. Bumps are harder. Smooth pavement and there's no noticeable difference at all. But the tires just roll so much further and better. It's like there is extra life in them without needing to give the car the gas. I'm killing it on this tank too. My mpg's are threw the roof. These VX's were before their time. I cannot believe cars can't be made today just as efficient if not better. My pocket book has really noticed it too!!!
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Old 11-07-2011, 02:59 AM   #28
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Re: Tire pressure difference

That's over 20%. That's great! It can't all be from just the pressure, though, can it?

Is it worth the stiff ride?

How are traction and handling affected?
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Old 11-07-2011, 05:31 PM   #29
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Re: Tire pressure difference

I fully intend to air the tires on my van up to the max pressure shortly. Have to replace the back two first. Keeping them at 32psi for now, but the fronts are 36psi and I'll bump the rears up to it after I find a better pair of tires. Probably not going to buy new but will look for a pair of Firestone FR380's with similar tread depth to the fronts. Then when all four are spent, get those Kumho ecoSolus tires all around.
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Old 11-08-2011, 10:13 PM   #30
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Re: Tire pressure difference

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That's over 20%. That's great! It can't all be from just the pressure, though, can it?

Is it worth the stiff ride?

How are traction and handling affected?
Honestly I believe I short-tanked myself about 3/4 of a gallon, which would lower my mpg to about 52-53 instead of 57.8 Still, it's a huge jump of about 7 mpgs. I also have been doing some hypermiling technique of turning my car off on certain hills and letting it roll into the parking lot where I park at work and down the hill and into my driveway and garage. It's not a huge gain, but you're talking about an extra free 1/2 mile or so each time I do it. Do that 30 times in a couple of weeks and that's an extra 15 miles and no gas used.

While the ride is stiff, it is only stiff at high speeds like 55-60 on the highway. If the pavement is smooth, then the stiff ride isn't bothersome at all. Is it worth it? Yes. I noticed the stiffness at first but quickly forgot about it after a few days.

As far as traction and handling, I've not noticed any negative effects. If anything it seems to grab better when I go into a corner. I'm surprised by this because it seems you might lose one if not both. That's just not been my experience. I feel totally comfortable driving at the higher pressure, and the higher mpg makes it that much more worth it!
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