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caster setting normal?


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14 replies to this topic

#1 efseiler

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Posted 08 August 2014 - 11:54 AM

Is it normal for two wheels on the same axle to have opposing caster settings?

 

I noticed (on a Forester we recently acquired) that the rear wheels have opposite caster settings (one is negative and the other positive).

 

I'm concerned cuz one of the struts appears to be shot and I'm wondering if it's cuz of that setting and also if it may be adversely affecting the LSD.

 

 

Thanks!

 

--Damien



#2 grossgary

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Posted 08 August 2014 - 12:09 PM

LSD should be fine.  being viscous i'd think you'd feel it if it is heating it up enough.

many tired Subaru rear VLSD's function as open differentials over time, so there's a 50/50 chance there's nothing to compromise in the unlikely event t was symptomatic.

 

someone more familiar can comment on strut/alignment/caster.



#3 1 Lucky Texan

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Posted 08 August 2014 - 12:52 PM

I'd say there's no reason the sides shouldn't be mirror images of each other so, there is something bent/broken/worn causing the issue, or an incorrect part might have been used for some past repair.



#4 efseiler

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Posted 08 August 2014 - 07:32 PM

I don't know if it's a viscous one.  It's a 2001 Forester S...it says 'limited slip differential'.

 

I read that they align wheels like that on purpose to affect handling characteristics but I was wondering if it could also affect the suspension/axle adversely.



#5 Fairtax4me

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Posted 08 August 2014 - 11:51 PM

Caster is the angle of the centerline of the steering axis. The angle off vertical from the upper ball joint or strut mount to the lower ball joint.

On the rear this would be from the upper strut mount to the trailing arm bushing, but its not a measured angle during an alignment since the rear wheels don't steer.
Caster being off will not be visible to the naked eye without there being very obviously damaged suspension parts, mostly the trailing arms.

Camber is the angle of tilt of the top of the wheel towards or away from the vehicle. Camber being off can cause horrible tire wear, and can cause the car to lurch towards the side the wheels lean towards when hitting large bumps at highway speed. It will not cause drastically different wheel speeds from side to side, thus no effect on the differential.

#6 efseiler

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Posted 09 August 2014 - 06:56 AM

On the rear this would be from the upper strut mount to the trailing arm bushing, but its not a measured angle during an alignment since the rear wheels don't steer.

Caster being off will not be visible to the naked eye without there being very obviously damaged suspension parts, mostly the trailing arms.

 

I don't think the strut is severely damaged like that...although the technician suggested that the mounts may be worn/damaged.  It just doesn't support the wheel much more any longer...I guess they are gas shocks and the air leaked out from one of them cuz of a bad hit or something....

 

One of those struts is leaning way forward while the other is leaning completely the other way...that's what I was trying to say.



#7 heartless

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Posted 09 August 2014 - 08:16 AM

the struts on a Subaru should appear to be almost perfectly vertical - if they are visibly/obviously leaning one way or the other you have some pretty serious issues going on with the other suspension components



#8 efseiler

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Posted 09 August 2014 - 05:21 PM

Oh I see...yeah...it's subtle.  I think that setting those ideally depends on vehicle weight, tire dimensions/composition/tread design, spring constant, strut characteristics, torque arm length, drivetrain characteristics, etc. etc.

 

 

They didn't fix it but they gave me a free engineering lesson! 

 

 

I guess that what my kind of money buys these days...  :lol:

 

 

 

Cheers!

 

--Damien



#9 Fairtax4me

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Posted 09 August 2014 - 05:55 PM

If one side of the car is sitting lower than the other you may have a weak or broken spring.

Gas shocks don't hold up any extra weight. The gas only controls aeration of the oil in the shock to prevent foaming during actuation of the strut.
The springs are what hold up the weight of the car. The shocks/struts simply dampen movement of the suspension to keep it from bouncing.

#10 efseiler

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Posted 11 August 2014 - 06:55 AM

Well then why did the vehicle clearance return to its normal value when he lowered the vehicle from the lift?  No that doesn't make any sense cuz those shocks have some intrinsic lift associated with them (apparently).

 

 

Don't those gas shocks work in a similar way to the ones that support the rear hatch?



#11 1 Lucky Texan

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Posted 11 August 2014 - 08:31 AM

Well then why did the vehicle clearance return to its normal value when he lowered the vehicle from the lift?  No that doesn't make any sense cuz those shocks have some intrinsic lift associated with them (apparently).

 

 

Don't those gas shocks work in a similar way to the ones that support the rear hatch?

 

 

When I replaced the struts on my wife's car, It was the first time I handled items like that. The gas that is in them is a very minor amount of pressure and the rods of the un-assembled struts are easily pushed down by hand. Of course, the oil is resisting that too, but the gas pressure is minimal compared to those springs - like 1:300 ratio.



#12 Fairtax4me

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Posted 11 August 2014 - 11:02 PM

The gas is under pressure but not nearly enough to support the weight of the vehicle. Only about 100-150 psi, but the volume of gas is very small, and its contained in a very small chamber compared to the size of the strut. The force of the gas acting on the strut rod is low enough that you can usually compress the strut by hand.

Gas lift struts for a hatch are tiny and contain pretty much only gas. The ratio of the volume of gas to the size of the strut is much higher, thus the force acting on the strut rod is higher.

#13 efseiler

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Posted 12 August 2014 - 05:57 AM

The ones I ordered came with the rods secured in by a clasp.  I don't know how much pressure there is in them but I suppose it could be quite a lot.  It looks like a pretty good product...even though it's from China

 

I heard about a model of Subaru that had pure gas struts that were actually adjustable with no springs so maybe in this model they copied some of that idea.  It certainly is technically feasable as anyone has seen that the hydraulics on excavators, for example, work perfect and can easily toss around thousands of pounds of weight, job after job.

 

 

 

I'm just trying to figure out as much as I can before I go ordering parts...unfortunately I have to grope around in the dark (like most scientists).

 

 

 

--Damien



#14 heartless

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Posted 12 August 2014 - 06:44 AM

even new struts can be compressed by hand with enough effort. in fact, the KYBs I ordered for my car came with instructions to compress them and let them extend 2 or 3 times before installation to make sure things are in good working order. a small block of wood against the rod end saves the hand.

 

Hydraulics, such as on an excavator, are a completely different beast, however. those require a fairly large resevoir of hydraulic oil and a pump...bare basics are: the pump adds oil/pressure to the cylinder to extend, oil/pressure are released from the cylinder to retract.



#15 1 Lucky Texan

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Posted 12 August 2014 - 07:19 AM

as mentioned above, the gas is inside typical struts just to keep foaming under control - maybe to help limit pulling in moist air too?

 

Typical car struts aren't made to operate lying down or upside down and, like mentioned above, you are instructed to cycle them before installation help move the oil back to where it belongs after shipping.






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