Tire Pressure and Sag
Lately I have been running my tires at 44/42 PSI (F/R), and although I have not done any actual coast-down testing, my impression is that rolling resistance has been reduced by a decent amount.
That's obviously a good thing. I'm a bit worried about how this will affect my drivetrain, though. My car is all wheel drive with a viscous coupled center differential. This means that unlike Subarus with automatic transmissions, my car can't be driven in 2wd by pulling a fuse. As I understand it, with AWD cars it is important to keep tire diameters equal. Get too much difference in rolling diameter between the front and rear wheels, and the center differential will constantly be trying to lock up, damaging tires, fuel economy and drivetrain. I observed this first hand once when I drove a short distance on a donut spare with a whining sound starting around 25 MPH and getting louder as speed increased. The suggested tire pressure for my car is 32/29 PSI (F/R). I have always assumed that this difference in pressure was to equalize rolling diameters between the heavier front end and the lighter rear end by making them sag an equal amount. I may be wrong about that, though. Can anyone tell me to what extent pressure actually affects a tire's rolling diameter? Should I really be concerned about this? My thought is that as I increase pressure, the amount of tire sag will decrease and so will the amount of required difference in pressure. Any thoughts? Thanks in advance. |
cafn8 -
I don't know what year Subaru Impreza this guy has, but maybe this applies to you. Read his website and maybe you can figure out which year/model he has : Quote:
https://bbs.scoobynet.com/driving-dynamics-354/ CarloSW2 |
I would assume the PSI recommendation difference is such so that the car will ride the best. I have seen the same type of recommendation on RWD and FWD cars also, and they don't have to worry about the front or rear tire diameter matching.
|
Quote:
|
I would have to think that there isn't much danger in having mismatched tire pressures. The average person rarely if ever checks their pressures. The engineers at Subaru know and understand this. Tires also go bad and get leaks. This is an unnavoidable fact of owning a car. Having serious damage occur because of this just seems like something that someone would have caught and put measures in place for.
|
Quote:
|
You would need to have a much bigger difference to hurt anything...your tire whether at 30 psi or 40 psi still has the same circumference...just a different shape.
|
Quote:
On the subject of viscous couplers- my other car is AWD (92 Eagle Talon) with a viscous center diff. In the DSM community it's highly recomended to keep all 4 tires the same. Not only the same size, but the same brand too, since there will be differences in actual tire dimensions among different manufacturers. I'm suprised Scubie USA would supply a spare with a different diameter. |
Hi all, thanks for all the input.
Quote:
Quote:
Quote:
Quote:
Quote:
|
I spent a fair bit of time hanging out on the Subie message board
https://www.ultimatesubaru.org/forum/ Learned that 4wd Subies really need matching diameter tires. Gotta rotate them at every oil change, about 3-5K miles. Subie has specs for how much difference is allowed in treadwear between tires. If not tended to you wreck the "differential", or whatever it's called where the front/rear drive trains meet. As for the different pressure spec'd for front and rear, my guess is that the ~10% difference is just what's needed to end up with matching diameter front + rear. Most of the weight is in the front, thus slightly higher pressure there. I tend to believe the post reporting success at 40 psi. That's high enough to avoid lots of tire squish and reshaping as it rolls. Having the fronts just a bit higher psi makes sense to me. I would expect things to go well at 44/42 as you have it now. Would I go higher, like 50/48 or higher? Dunno. My rwd car is 52/50 now but that's my car, not your 4wd Subie. |
All times are GMT -8. The time now is 09:02 PM. |
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2024, vBulletin Solutions, Inc.