Does control arm bushing composition affect ride quality or NVH?

[fljeeper]

I just put a complete set of Metalcloak CA's on my rear axle. The Jeep is a DD. I put it through a challenging obstacle course last weekend. I can discern no ride quality difference. I guarantee there is an articulation difference. Before I installed the new CA's I compared them to stock. The stock had less misalignment range and required much more force to even begin to misalign.

 

[Jerry Bransford]

The stock had less misalignment range and required much more force to even begin to misalign.

Requiring more force for a joint is not a factor where quality of design goes. What may feel stiff and hard to move is nothing for an axle to move around with total ease. A good example are Currie's Johnny Joints. They are anything but easy to move around by hand but when installed it's a 100% and complete non-issue.

 

[jjvw]

In this case, the main thing that's keeping that impact force from being directly transmitted to the frame is the control arm bushings. But this force doesn't twist the joint/bushing, it compresses it. I guess that's what the shake table video was meant to show, if it wasn't so hokey.

What do we know about the elasticity of various control arm bushings in this case? Not much other than butt dyno data, I gather.

But does the control arm bushing contribute to NVH in this case? Yes.

The telegraphing of impacts through the arm and into the frame certainly exists. This is where it would be interesting to know how the factory arms compare to something like a heim joint (an independent test with an unbiased standard of measurement would also be useful). How much difference between two types of connections does it take to feel a difference? Most of our options have some amount of cushion. How different from each other are they in that regard?

This is very similar to the arm angle debate that often shows up in the long arm vs short arm threads. How much change from stock is enough to become noticable?

I know what my experiences were with stock arms at stock height up to 4" of lift, then with mid arms and Johnny Joints at various lift heights. Shocks and tires stayed the same until very recently. There were minor changes on the street after the arm change, but a difference in NVH wasn't one of them.

 

[jjvw]

I just put a complete set of Metalcloak CA's on my rear axle. ... I guarantee there is an articulation difference. Before I installed the new CA's I compared them to stock. The stock had less misalignment range and required much more force to even begin to misalign.

Did the shocks stay the same? What was the difference in travel before and after?

 

[jjvw]

Requiring more force for a joint is not a factor where quality of design goes. What may feel stiff and hard to move is nothing for an axle to move around with total ease. A good example are Currie's Johnny Joints. They are anything but easy to move around by hand but when installed it's a 100% and complete non-issue.

This would be the difference between a 250lb man and a 4200lb Jeep exerting it's force on something. An effort that is difficult for me is barely noticable to the Jeep.

 

[fuse]

The telegraphing of impacts through the arm and into the frame certainly exists. This is where it would be interesting to know how the factory arms compare to something like a heim joint (an independent test with an unbiased standard of measurement would also be useful). How much difference between two types of connections does it take to feel a difference? Most of our options have some amount of cushion. How different from each other are they in that regard?

That's where it would really help to have some hard data. In theory this stuff matters. In practice, all the retail bushings for TJs may have comparable elasticity.

Let's talk track bars for a minute, though. Say you're not driving straight when you hit that pothole, but you hit it with the outside corner wheel while you're in the middle of a turn. Now that impact force from the pothole is no longer aligned with the frame of the Jeep. Some component of the force is going to be aimed across the body. The bushings in the control arms will resist that force a little, as will the springs and some of the other suspension components, but keeping the axle in place horizontally is the track bar's job.

Now a component of the impact force is aimed directly down the long axis of the track bar. The only thing that really moves in that direction is the track bar bushing, since everything else is pretty well locked down. So now the track bar bushing is responsible for damping some of the force from the impact, rather than transmitting it to the frame of the Jeep.

 

[jjvw]

...

Let's talk track bars for a minute, though. Say you're not driving straight when you hit that pothole, but you hit it with the outside corner wheel while you're in the middle of a turn. Now that impact force from the pothole is no longer aligned with the frame of the Jeep. Some component of the force is going to be aimed across the body. The bushings in the control arms will resist that force a little, as will the springs and some of the other suspension components, but keeping the axle in place horizontally is the track bar's job.

Now a component of the impact force is aimed directly down the long axis of the track bar. The only thing that really moves in that direction is the track bar bushing, since everything else is pretty well locked down. So now the track bar bushing is responsible for damping some of the force from the impact, rather than transmitting it to the frame of the Jeep.

So much of this can be endlessly analyzed on paper. Let's keep going!

If (assuming for now) the telegraphing of NVH is a real concern with the control arms, is it also a concern with the track bars? What might the ends look like? Should the design of the track bar be consistent with the design of the control arms?

 

[fuse]

If (assuming for now) the telegraphing of NVH is a real concern with the control arms, is it also a concern with the track bars? What might the ends look like? Should the design of the track bar be consistent with the design of the control arms?

With the pothole example, we've really been talking about Harshness. We should come back to the Noise and Vibration end of the scale at some point. We need to think about what happens when the suspension movements are high frequency, but low amplitude. But let's keep talking about track bars for now.

We know what the OEM track bars look like:


Those OEM bushings are a lot like the control arm bushings (Moog replacement, e.g.):

Could you do better? If the bushing is harder, the axle moves less horizontally, but we'd expect more NVH. If the bushing is softer, we'd expect more NVH. But steering response suffers if the bushings are too soft, because steering input and turning forces move the frame in relation to the axles more if the track bar bushings are softer.

But as we were discussing before, that's not necessarily the case with the control arm bushings. Control arms twist more than track bars do (except for the ball joint on the front track bar). And if the control arms can twist more freely, that could shift some of the suspension forces to other components that do a better job of controlling NVH.

If you look at the Currie front track bar, it has a Johnny Joint in place of the ball joint, but the same OEM type of bushing at the axle end. Presumably the Johnny Joint moves just as freely as the ball joint, but the Johnny Joint adds some extra isolation, because there's some PU around the joint where the OEM bar just has steel. So, maybe there's room for NVH improvement at the frame end of the front OEM track bar.

 

[bobthetj03]

The entire drive train, as well as the axle assy's are isolated from the frame by some fashion via a bushing. the only connection point I can think of in a factory TJ setup that is not isolated from the frame by a bushing is the frame side of the front track bar. Maybe looking at the whole vs. just the control arms would provide some more observations.

 

[fuse]

The entire drive train, as well as the axle assy's are isolated from the frame by some fashion via a bushing. the only connection point I can think of in a factory TJ setup that is not isolated from the frame by a bushing is the frame side of the front track bar. Maybe looking at the whole vs. just the control arms would provide some more observations.

Are there bushings in the steering linkages?

 

[bobthetj03]

Are there bushings in the steering linkages?

Not sure exactly concerning the steering rod, there is a collar bushing that attaches the rod to the frame, and the steering rod has a compression crush zone in the event of a collision.

 

[fuse]

Not sure exactly concerning the steering rod, there is a collar bushing that attaches the rod to the frame, and the steering rod has a compression crush zone in the event of a collision.

I just took a look under the Jeep because I didn't know, but the connection is tie rod, to drag link, to Pitman arm, to steering gear, to frame, without any isolation that I could see. Maybe there's something inside the steering gear?

Anyway, there are more direct connections from axle to frame. But maybe the vibrations through the steering linkages are noticeable if you're driving on the highway with MT tires or something like that.

 

[jjvw]

I'm not aware of any bushings or soft connections between the steering box and the knuckles.

 

[bobthetj03]

I'm not aware of any bushings or soft connections between the steering box and the knuckles.

I would gather that having a soft connection in that series would produce less than favorable steering response, so a compromise needs to made in that area.

 

[jjvw]

I would gather that having a soft connection in that series would produce less than favorable steering response, so a compromise needs to made in that area.

Could be. What about the frame side track bar mount? Why is Currie putting a JJ there but others use a hard connection? I don't think that 5th suspension link can be easily ignored in this particular discussion.

Also note that unlike the front, the factory rear track bar uses rubber bushings on both ends.

 

[bobthetj03]

Could be. What about the frame side track bar mount? Why is Currie putting a JJ there but others use a hard connection? I don't think that 5th suspension link can be easily ignored in this particular discussion.

Also note that unlike the front, the factory rear track bar uses rubber bushings on both ends.

And to add, why does MC use a heim joint there?

Also to add to the discussion, when I changed my front Currie arms to Metal Cloak arms, my butt dyno results were inconclusive.

 

[jjvw]

And to add, why does MC use a heim joint there?

...

Because a bushing (of any kind) cannot be used in that specific application.

 

[fuse]

Because a bushing (of any kind) cannot be used in that specific application.

But then it's interesting that Currie puts JJs at both ends of their rear track bar, and only at one end of their front track bar.

Also, if anyone wants to hook me up with the MTS 320.050 Light Truck Tire Coupled Road Simulator and a half dozen different control arms, I'll get some real data for you.

 

[jjvw]

An interesting question that I don't know the answer to is why did the factory use a single shear tie rod end in that single location. Yet the other three are bushings in double shear.

 

[jjvw]

But then it's interesting that Currie puts JJs at both ends of their rear track bar, and only at one end of their front track bar.

...

It really is! Is it a size issue with the front axle side? I don't know...