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Lower Control Arm NORMAL movement is what?


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5 replies to this topic

#1 SVX_commuter

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Posted 25 April 2008 - 06:30 AM

Hello,
I was looking for some assistance. I have noticed that the lower control arm on my SVX moves FORWARD and BACKWARD when I pry on it with a crow bar. It doesn't move side to side or up and down just forward and backward. It moves less backward and more forward. Both side do the same movement.

So then I checked the lower arm on the Impreza and it does basically the same thing. It moves forwards and backwards.

On both cars it does not take much force to move the lower arm so I am thinking it was designed to move this way. I think it must have something to do with a smoother ride.

So now I am concerned about wheel alignment during heavy braking and heavy acceleration. During heavy braking and acceleration the lower control arms will move backwards andforwards.

I check the alignment with the lower moved backward as when the car brakes and there isn't much of a change but when the lower arm is moved forward it makes a very big change in the toe. This would be the case for heavy acceleration. More of an issue on the SVX then the Impreza or any higher horsepower Subaru if the arms are all made this way.

So I just wanted to know if anybody else know if this is normal motion for the lower control arms.

Thanks ! :)

Take care of that Subaru,

John

#2 bulwnkl

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Posted 25 April 2008 - 08:53 AM

Yes, there should be control arm motion longitudinally. The bushings are supposed to flex for ride and damage prevention. Factory alignment settings (are supposed to) take into account the flex and also the countering force of engine power. Remember that axle torque to the drive wheels will try to flex those arms forward as the wheels pull the car along. So, you have offsetting forces at work against each other. Don't flex the arms to do an alignment, just put it on the rack (or drive it into the garage if you do home alignments) and set the alignment.

#3 SVX_commuter

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Posted 25 April 2008 - 12:08 PM

Thanks for the reply. By "offsetting forces" do you mean the wheel torque and force through the bushing to frame?

#4 bulwnkl

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Posted 25 April 2008 - 02:25 PM

More or less. Rolling down the road will tend to make the arm flex backward because the car is 'pushing' and the wheel/road interface is resisting. Drive torque will try to flex the arm forward because as the wheel drives forward the car wants to slow down.

I wasn't trying to say the forces precisely counteract because they don't and of course they change with speed and conditions. I'm just trying to say there's a certain amount of offset there. These kinds of things affect toe as well, and are why at least for a long time rwd cars ran a little toe-in and fwd cars ran a little toe-out. It's not necessarily that way any more.

#5 SVX_commuter

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Posted 09 May 2008 - 11:57 AM

Thanks for your reply.

I have been interested in this lower control movement to see how it affects the tire wear.

I have measured the toe when I rolled the front wheels onto a block of wood. I used a 2x4. I rolled backward so the 2x4 displaced the lower control arm forward. Then I measured the toe. I am surprised at how much the toe changed. I got 1/4 of inch diference from the front to the back of the rim.

So I am thinking that if I am always on the throttle the tires will be running with toe way over the limit.

I want to put a shim in between the control arm and frame to keep it from moving.

Would the STI have this same problem since it is horse power related?

Any comments are welcomed.

#6 jamal

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Posted 09 May 2008 - 03:04 PM

the sti has stiffer control arms and stiffer bushings. The fact that yours aren't brand new doesn't help anything either, especially if they're leaking (rear bushings are fluid filled). The front bushing doesn't flex as much.

I replaced my rear control arm bushing with a whiteline alk. It's a stiffer urethane bushing that also alters the suspension geometry slightly for better front end grip while turning and accelerating.

I really noticed a difference in braking responsiveness and stability due to the added stiffness. It is a bit harsher when you hit expansion joints and whatnot.




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