Rear control arm bushing clearance

[series8217]

Originally Posted by burnout8488

How can the gap be set at all? Tightening the bushing would theoretically push it against the control arm as far as it goes, right? I don't see how there is any adjustment on this bushing as far as where it sits on the control arm stud.

I'd like to do it right as well, even though I'm sure it isn't that critical. Something about shelling out $85 per bushing makes me want to get it right!

The slotted adjustment holes in the outer bracket allow you to slide the outer part of the bushing forward and aft. What's actually being done in this step is setting the thrust preload on the bushing.

Does that make sense? You aren't sliding the inner part of the bushing along the control arm, but rather setting the outer part of the bushing in the right place relative to the inner.

I didn't engineer this bushing, but I suspect that setting the thrust preload incorrectly will lead to premature failure of the "sidewalls" of the bushing (imagining it as a tire).

[cardoc]

Originally Posted by series8217

Torquing that bushing does not affect the clearance. That nut is supposed to be torqued before setting the clearance. In fact, the FSM says to torque the rear bushing nut with the control arm off the car, though you can only do that if you have the angle marked so that the bushing ends up with no preload when at ride height.



I am following the Subaru factory service manual.

I hope you don't feel like Subaru is overthinking their engineering and the way they build your car!

The bolts are to be tightened with the weight of the car on the wheels. The arm will move back and forth through the bushing even after it is torqued, not much, just the total of the "space" left after torquing the nut. The nut stops at the end of the threaded portion, leaving the space fore and aft. It is not meant to be a permanent position of the bushing on the arm since the arm moves with the torque of the drivetrain and jouncing. So, if you follow what I posted, it will be correct. Downside is, it won't work proper if the rear wheels are on the ground and the front on ramps. The weight will be shifted. It has to be level weight. The spacing after everything is tightened is also dependent on whether or not you have other worn parts, such as ball joint, weak springs, strut mount, crossmember bushings or front bushing. It would also be dependent on factors such as bent body or crossmember.

Did you think of any of these other possibilities? Most likely not. So, keep it simple. Get it mounted and realign the car. Don't waste time measuring the bushing position. Its useless on a car with aged parts.

Whenever I explain something, I intend to make it as simple to understand as possible. If I got really technical in explaining every detail, it would make for long post and confused readers. If someone wants technical detail, then I can provide it, but its been my experience that most people don't really understand the mathematics, physics or chemistry involved so I find a simple route.

I apologize if you misunderstood my post. I assumed it was straight forward instruction you needed to finish the job.

[series8217]

Originally Posted by cardoc

The arm will move back and forth through the bushing even after it is torqued, not much, just the total of the "space" left after torquing the nut. The nut stops at the end of the threaded portion, leaving the space fore and aft.

That is incorrect. The nut stops against the inner sleeve of the control arm bushing. It serves to retain the fore-to-aft movement of the inner sleeve as well as prevent the inner sleeve from moving against the control arm.

The control can still move fore and aft when the nut is tight because the inner sleeve is not fully constrained axially to the outer sleeve. Like any bonded rubber bushing, the center sleeve is not (and cannot be) fully constrained axially. Since the inner sleeve is bonded to the outer sleeve through the rubber, it can move against the outer sleeve by deflecting the rubber.

The free-foating castellated thrust bushings serve as progressive stops for the axial motion, and limit toe change under acceleration and braking.

The bushing clearance is adjustable to ensure that there is no thrust preload between the inner and outer sleeves, which might allow the bushing to become overloaded when it reaches the far limit of its fore to aft motion.

[series8217]

cardoc, I understand you have good intentions in your advice, and from a mechanic's point of view it is probably quite sound. However, from an engineering perspective (mine), it's not totally correct and can lead to problems down the road. Since I keep cars for a long time, and use them very, very, very hard, I prefer to keep things as the engineers of the equipment intended, or modify things based on my own re-engineering where improvements are necessary.

[cardoc]

With high mileage, the only way it will be perfect is to change all associated parts on the vehicle. This will be costly.

As an example, my car, which got new rear bushings, gets driven hard. I didn't build it to go fast for nothing. Been driving fast since I was 8. Thank you speed carts. There is no performance change in the suspension now vs replacing all the parts at once.

No offense to engineers in general, but engineers design cars, the technicians have to deal with the aftermath. Most times it is the technicians dealing with engineering faults that come up with the "fix" for what the engineers created. I have an advantage over a lot of techs in that I understand the engineering design and know how each part functions as an assembly and what needs to be done in aspect of either repairing the problem at hand or putting it back to "new". It all depends greatly on what's in your wallet.

[1 Lucky Texan]

I couldn't fault anyone for following the FSM, but, the procedure cardoc outlined should allow the mount to assume a neutral position when the car is in a neutral 'stance'. Even his advice about the use of level ground instead of ramps is aimed at assuring the forces on the mounts are minimal. I did not use OEM when I changed mine, but the prothane instructions were almost identical to cardoc's.

I doubt it would be possible to detect a meaningful difference from either approach, but you have to do what you're comfortable with.

[cardoc]

Originally Posted by 1 Lucky Texan

I couldn't fault anyone for following the FSM, but, the procedure cardoc outlined should allow the mount to assume a neutral position when the car is in a neutral 'stance'. Even his advice about the use of level ground instead of ramps is aimed at assuring the forces on the mounts are minimal. I did not use OEM when I changed mine, but the prothane instructions were almost identical to cardoc's.

I doubt it would be possible to detect a meaningful difference from either approach, but you have to do what you're comfortable with.

That's what I think is going on. He is working on the arms with the car on jack stands or ramps and its skewing everything. The Subaru guidelines state the wheels need to have the weight of the vehicle on them before torquing the nut and bolts and this cannot be accomplished on stands or ramps as the wheel assembly will move toward the front of the vehicle when the weight of the car shifts to the rear. Physics.

[1 Lucky Texan]

Originally Posted by cardoc

That's what I think is going on. He is working on the arms with the car on jack stands or ramps and its skewing everything. The Subaru guidelines state the wheels need to have the weight of the vehicle on them before torquing the nut and bolts and this cannot be accomplished on stands or ramps as the wheel assembly will move toward the front of the vehicle when the weight of the car shifts to the rear. Physics.

yeah, after dropping the car and bouncing it a few times (maybe I should have rolled it forward and back 4-5 feet - dunno about that) I squeezed my gut under the car with it flat on the garage floor to final torque the bolts.

[series8217]

I checked it again with all 4 wheels on the ground after going for a drive (with everything tight of course) and the clearance is no different. However, I did check more carefully and found that the aft clearance on the outside is a good deal less (> 1mm) than the inside.

It's likely that the control arm or crossmember is ever so slightly tweaked. You're right, going to need some more parts. I put a control arm on my list of parts for my next order from the dealer. I'll have to look closer for deformation of the crossmember where the front control arm bushing bolts in.

Something probably got bent gradually lap after lap at the last RallyCross Enduro; I do remember now a certain corner that was pretty rough on the pass side.

Originally Posted by cardoc

That's what I think is going on. He is working on the arms with the car on jack stands or ramps and its skewing everything. The Subaru guidelines state the wheels need to have the weight of the vehicle on them before torquing the nut and bolts and this cannot be accomplished on stands or ramps as the wheel assembly will move toward the front of the vehicle when the weight of the car shifts to the rear. Physics.

You are correct that the nuts and bolts fastening the inner sleeves of the bushings need to be torqued with the suspension settled to normal ride height. This is to avoid torsional pre-load of the rubber bushings.

Your comment that the wheel assembly will move toward the front of the vehicle when the weight shifts to the rear is not totally correct if you're careful about it .

It's actually the BODY that will move, and in this case that's an important distinction even though the relative motion would be the same. I ensured that the body motion did not impart a thrust load on the front wheel assembly by locking the rear wheels with the parking brake (and chocking them tightly), so the rearward displacement of the body is limited by the rear suspension's limit of deflection. I jostled the chassis on the ramps until it settled on the rear in this fashion. This way there is minimal thrust load on the front wheel assembly, so I could measure the thrust clearance with the front elevated. The fact that this worked is evident by my thrust clearance measurements this evening with the vehicle on the ground being the same as with the configuration I measured them as described above.

Physics!