Tire pressure difference - Page 2 - Fuelly Forums

Click here to see important news regarding the aCar App

Reply
 
Thread Tools Display Modes
 
Old 10-25-2011, 04:38 PM   #11
Registered Member
 
Join Date: May 2008
Posts: 140
Country: United States
Re: Tire pressure difference

my subaru outback handles funny with any more than 32 psi in the front tires. in the middle of a right hand turn , the car all of a sudden veers to the right. lowering the pressure back down to 32 psi solves the problem.
__________________

mikehallbackhoe is offline   Reply With Quote
Old 10-26-2011, 11:29 AM   #12
Senior Member
 
Join Date: Mar 2009
Posts: 111
Country: United States
Re: Tire pressure difference

That is because your subaru is AWD.
__________________

madnessspirit is offline   Reply With Quote
Old 10-27-2011, 02:14 AM   #13
Registered Member
 
Join Date: Feb 2011
Location: Somewhere in the US
Posts: 34
Country: United States
Re: Tire pressure difference

I'm a little curious about the mechanism behind AWD being affected by the pressure. Any thoughts?
__________________
Capri Racer

Visit my web site: www.BarrysTireTech.com
CapriRacer is offline   Reply With Quote
Old 10-27-2011, 02:41 AM   #14
Registered Member
 
theholycow's Avatar
 
Join Date: Apr 2008
Posts: 6,624
Country: United States
Send a message via ICQ to theholycow Send a message via AIM to theholycow Send a message via MSN to theholycow Send a message via Yahoo to theholycow
Re: Tire pressure difference

If the tires are not at the same pressure that could affect smart viscous coupled AWD systems...but my first thought was just that sending power to all the tires when they are at different pressures, regardless of the system in use, might cause a change in handling just from differences in traction.
__________________
This sig may return, some day.
theholycow is offline   Reply With Quote
Old 10-27-2011, 07:58 AM   #15
Registered Member
 
brucepick's Avatar
 
Join Date: Feb 2007
Posts: 722
Country: United States
Location: Connecticut
Re: Tire pressure difference

Back to the original question - I don't have the answer either.

BUT I've learned that higher pressure give me smoother rolling A couple times I've found myself getting (relatively) poor FE and remembered to check the tires. Bingo! They were all the way down to sidewall max. Put 'em back where I like 'em and FE goes back where I expected.
__________________
Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.

Now driving '97 Civic HX; tires ~ 50 psi. '89 Volvo 240 = semi-retired.
brucepick is offline   Reply With Quote
Old 10-27-2011, 08:50 AM   #16
Senior Member
 
Join Date: Mar 2009
Posts: 111
Country: United States
Re: Tire pressure difference

Quote:
Originally Posted by theholycow View Post
If the tires are not at the same pressure that could affect smart viscous coupled AWD systems...but my first thought was just that sending power to all the tires when they are at different pressures, regardless of the system in use, might cause a change in handling just from differences in traction.
This is it exactly. The center differential in a subaru has torque biasing features, which send a certain amount of power to one set of wheels or another. Things like differences in tire pressure or size will cause different axles to move at different speeds and cause a difference in the applied torque to the road surface as well. Plus then there is the difference in traction that you mention, which changes the torque biasing to the wheels with greater traction, or in other words, the wheels at a lower tire pressure usually.
madnessspirit is offline   Reply With Quote
Old 10-27-2011, 06:12 PM   #17
Registered Member
 
Join Date: Feb 2011
Location: Somewhere in the US
Posts: 34
Country: United States
Re: Tire pressure difference

Quote:
Originally Posted by FIND View Post
This is it exactly. The center differential in a subaru has torque biasing features, which send a certain amount of power to one set of wheels or another. Things like differences in tire pressure or size will cause different axles to move at different speeds and cause a difference in the applied torque to the road surface as well. Plus then there is the difference in traction that you mention, which changes the torque biasing to the wheels with greater traction, or in other words, the wheels at a lower tire pressure usually.
But wouldn't one adjust the pressure at both ends of the car at the same time? I'm pretty sure that Mike wouldn't do one end and not the other.

Mike, we need clarification here!
__________________
Capri Racer

Visit my web site: www.BarrysTireTech.com
CapriRacer is offline   Reply With Quote
Old 10-28-2011, 07:40 AM   #18
Registered Member
 
Join Date: Nov 2007
Posts: 1,852
Country: United States
Location: north east PA
Re: Tire pressure difference

Some manufacturer recommended tire pressures vary front to rear. Not keeping the difference between to two could have been enough to cause of the AWD issue.
trollbait is offline   Reply With Quote
Old 10-28-2011, 12:23 PM   #19
Senior Member
 
Join Date: Mar 2009
Posts: 111
Country: United States
Re: Tire pressure difference

Quote:
Originally Posted by CapriRacer View Post
But wouldn't one adjust the pressure at both ends of the car at the same time? I'm pretty sure that Mike wouldn't do one end and not the other.

Mike, we need clarification here!
Even with an equal and proportionate change between front and rear, it really messes with the torque biasing. Those smart AWD systems are basically just like smart phones... the moment you ask them to do something just slightly different than what they think they are supposed to be doing, they become very dumb. Subaru has always made their AWD systems just a little smart, meaning they have always had a very good torque biasing function, that is why their cars are so popular for rally circuits.

Plus, there are other issues too. Things like body roll, traction and lateral traction are built into the car using a combination of elements like springs, shocks, swaybars. So if you change any of them, it will change the handling characteristics of a vehicle. For an AWD vehicle, they are balanced between the front and back at a neutral setting for everything. When you start changing away from that neutral setting you are changing the handling characteristics of the front and rear. The front is where your turn in weight has to go, otherwise you get understeer. So if you decrease the spring rate in the front and rear of a vehicle, you need to apply more force to the front to bias weight forward and control your understeer. Conversely, you may find it harder to keep enough weight on the rear to control oversteer.

Plus, again, the tires are the first thing that absorbs changes in road surface and you, changing tire pressure changes your overall spring and dampening rate, just like going with low profile tires, suddenly bumps and such are more pronounced. Tires also have a big effect on the lateral forces on your car, just like a swaybar. So a higher pressure tire will not have as much lateral springiness just like it doesn't have as much against the road, therefore body roll characteristics change.

And again, as was mentioned before is the traction issue. Overinflated tires will have slightly less traction, that is how one accomplishes higher FE from them anyways is by lowering friction. Even if those changes in traction are on all 4 tires at the same time, and that change is proportionate weight distribution is not, especially while you are in motion, since your weight shifts depending on how you are accelerating, turning or stopping.
madnessspirit is offline   Reply With Quote
Old 10-29-2011, 04:23 AM   #20
Registered Member
 
Join Date: Feb 2011
Location: Somewhere in the US
Posts: 34
Country: United States
Re: Tire pressure difference

Quote:
Originally Posted by FIND View Post
....
Find,

Thanks for that post, but there are something that don't seem right.

Quote:
Originally Posted by FIND View Post
....Even with an equal and proportionate change between front and rear, it really messes with the torque biasing..........
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.

Quote:
Originally Posted by FIND View Post
....Plus, there are other issues too. Things like body roll, traction and lateral traction are built into the car using a combination of elements like springs, shocks, swaybars. So if you change any of them, it will change the handling characteristics of a vehicle. For an AWD vehicle, they are balanced between the front and back at a neutral setting for everything. When you start changing away from that neutral setting you are changing the handling characteristics of the front and rear. The front is where your turn in weight has to go, otherwise you get understeer. So if you decrease the spring rate in the front and rear of a vehicle, you need to apply more force to the front to bias weight forward and control your understeer. Conversely, you may find it harder to keep enough weight on the rear to control oversteer......
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.

Quote:
Originally Posted by FIND View Post
....Plus, again, the tires are the first thing that absorbs changes in road surface and you, changing tire pressure changes your overall spring and dampening rate, just like going with low profile tires, suddenly bumps and such are more pronounced. Tires also have a big effect on the lateral forces on your car, just like a swaybar. So a higher pressure tire will not have as much lateral springiness just like it doesn't have as much against the road, therefore body roll characteristics change.......
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.

Quote:
Originally Posted by FIND View Post
....And again, as was mentioned before is the traction issue. Overinflated tires will have slightly less traction, that is how one accomplishes higher FE from them anyways is by lowering friction......
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.

Quote:
Originally Posted by FIND View Post
.... Even if those changes in traction are on all 4 tires at the same time, and that change is proportionate weight distribution is not, especially while you are in motion, since your weight shifts depending on how you are accelerating, turning or stopping.
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?

And lastly: MIKE, WE STILL NEED CLARIFICATION!
__________________

__________________
Capri Racer

Visit my web site: www.BarrysTireTech.com
CapriRacer is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
'95 Honda Civic DX hatchback w/ VX tranny pinging cems70 General Maintenance and Repair 8 02-09-2011 02:31 AM
electric l/100km MJ/km Efried Fuelly Web Support and Community News 2 08-25-2008 07:41 AM
Beating the Baro... MAP reading trickery? GasSavers_RoadWarrior General Fuel Topics 4 03-06-2008 01:43 AM
Suggestions/Help/Curious? 96744 General Fuel Topics 6 08-24-2007 08:20 AM
Newby here diaperden General Fuel Topics 4 06-19-2007 12:54 PM

» Fuelly iOS Apps
Powered by vBadvanced CMPS v3.2.3


All times are GMT -8. The time now is 02:06 AM.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2024, vBulletin Solutions, Inc.