Question on proper mounting procedure for a certain control arm type. 1

My car's control arm bushings consist of a metal sleeve which is surrounded by a cylinder of rubber. I believe this rubber is bonded to the metal sleeve. The bushing is press fit into the control arm.

I used to think that this bushing was free to rotate on the bolt which goes through the metal sleeve when the control arm is mounted to the car. But I have since come to discover that when you torque the control arm nuts down, the metal sleeve stays still and when the suspension moves it actually causes the rubber in the bushing to flex. Therefore this rubber adds to the overall spring rate as far as I can tell.

My question is:
When mounting the control arms to my car, should I torque them to spec while the car is sitting level (or at least while the arm is oriented as if the car were sitting level)? In other words is the control arm bushing rubber supposed to be "un-flexed" while the car is sitting at it's natural stance?

Thank you

 

[GregLocock]

The answer is - it depends. At Ford of Australia almost all of our bushes are torqued up at design height, or kerb, because it (a) reduces the variation in ride height and (b) has given us measurable improvements in some other things in the past.

It is also good for the durbaility of the bushes.

The exception to this would be where a wiley designer has thought about it in advance and actually used that torsional preload. If you ever meet this wiley designer shake his hand... but don't hold your breath! Cheers

Greg Locock

 

[Chumley]

I agree with Greg. Of course, follow the repair method listed in the appropriate factory repair manual. Most will tell you to torque the bushings so that they are neutral with the car sitting on its wheels, unloaded and level at its normal ride height or give other specific instructions. The springs are usually designed to carry the weight of the car. Not the bushings. I've seen more than a few early bushing failures because of unintentional preloading conditions.

On some more sophisticated suspension systems improperly preloaded bushings can sometimes make wheel alignment get a little squirrelly too.

Chumley

 

Thanks guys.
I do have the factory shop manual but it doesn't say anything about when to tighten the control arm bolts. I thought that was odd.

Can you guys tell me why cars are designed this way? What would happen if I milled down both sides of the bushing sleeve enough to fit a flat bearing in there (similar to a strut mount bearing) so the arm was totally free to move on the bolt?

 

[Chumley]

What would be the reason for milling the bushings and installing the flat bearings? Most likely, adding some additional moving parts into the bushing assemblies would eventually cause the old familiar clunking noises of loose suspension parts. If you live in an area where salt, grit and moisture are present, expect some creaking noises soon to follow unless you lubricate and seal the bearings really well which will be tough to do. Remember, they are right down there in all the flying muck. The bushings are popular because of the low maintenance factor due to the minimal moving parts.

Bushings are designed the way they are for noise
and shock isolation. The bushing surface bearing area and rubber hardness are major factors in determining how the steering and suspension "feels" to the driver. Harder rubber gives a more responsive and positive feel but transmits more shock or "harshness" and noise into the steering, suspension and cabin. Softer rubber makes everything feel more mushy but it absorbs more noise.

Engineers spend thousands of test hours trying to get it feeling right. (to them, of course) The guys who design Cadillacs have a very different idea of what’s "right" than the guys who design Mercedes.

Chumley

 

[GregLocock]

They are designed that way because a moving pivot needs lubrication, squeaks, and costs money. A rubber bush is fit and forget (hmm), silent, and relatively cheap.

Some rubber bushes are not double bonded, ie with rubber bonded to both inners and outers. They are even cheaper but can have durability issues. They are also very annoying to work with since the ride height varies as they stick-slip in their housings. One of the big things you do to a bush to improve the durability is to size it - basically by crushing the outer metal in a collet, usually. This precompresses the rubber. You can't size a bush that doesn't have an inner and an outer, unless you do it as you install it - which takes a lot of force. Cheers

Greg Locock

 

Thanks, I understand now.
I noticed that my bushings have a little dot molded on them so you can tell when they are properly oriented inside the control arm.
I'm gonna go re-torque my control arms while they are in the correct position. I've taken apart and re-assembled this car many times and it's probably wrong right now.