Tire pressure difference

[mikehallbackhoe]

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.

[madnessspirit]

That is because your subaru is AWD.

[CapriRacer]

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

[theholycow]

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.

[brucepick]

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.

[madnessspirit]

Originally Posted by theholycow

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.

[CapriRacer]

Originally Posted by FIND

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!

[trollbait]

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.

[madnessspirit]

Originally Posted by CapriRacer

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.

[CapriRacer]

Originally Posted by FIND

....

Find,

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

Originally Posted by FIND

....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.

Originally Posted by FIND

....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.

Originally Posted by FIND

....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.

Originally Posted by FIND

....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.

Originally Posted by FIND

.... 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!