RLCA Help: Spherical Bearing vs. Poly Bushing

[TeeLew]

Those SuperPro bushings look really nice. They're about twice as expensive as the E.S. version, but maybe good for reason.

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[KellTrac]

I tested that stuff years ago.

No thanks.

1: That's not the right location for poly urethane.

2: In that location, polyurethane isn't really a big improvement over a bonded rubber bushing. Makes more sense as a service replacement, if stocker is bad. But I'll pass

 

[Brian@BMVK]

I tested that stuff years ago.

No thanks.

1: That's not the right location for poly urethane.

2: In that location, polyurethane isn't really a big improvement over a bonded rubber bushing. Makes more sense as a service replacement, if stocker is bad. But I'll pass

So what you're saying is we need someone to come out with a K-member and tubular control arm that removes the off-axis joints...

 

[Norm Peterson]

That’s what I got! If you are talking about how far the poly protrudes out, it’s about 1/2inch.

That wasn't quite where I was going, but the fact that the poly extends well out past the outer shell does give it more flexibility than if the outer shell went all the way out to a flanged end-of-poly design. I wouldn't be surprised if limiting the length of the outer shell like that drops the off-axis stiffness down to 25% or so of what it would be with a full-length shell.


Norm

 

[fatbillybob]

Does the PU bushing allow rotation of the center metal in the outter metal shell? If the whole thing is bonded like rubber then the spherical that freely rotates is way superior to not add any rate into the equation.

 

[Norm Peterson]

Does the PU bushing allow rotation of the center metal in the outter metal shell? If the whole thing is bonded like rubber then the spherical that freely rotates is way superior to not add any rate into the equation.

I've never known polyurethane to be bonded to either an inner sleeve, an inner ball, or the outer shell. There's a fairly tight fit at the shell, I guess theoretically some sort of interference fit, that keeps the poly from rotating at that surface.

You do get a little stiction, depending on lubrication and the sleeve to poly 'fit'.


Norm

 

[Brian@BMVK]

Does the PU bushing allow rotation of the center metal in the outter metal shell? If the whole thing is bonded like rubber then the spherical that freely rotates is way superior to not add any rate into the equation.

It's not bonded and it is instructed to heavily grease the sleeve that inserts into the PU. It is intended to allow rotation between the torqued sleeve and the PU bushing. Obviously with some off-axis motion it'll have some stiction but nothing anywhere near what the stock bonded bushing does.

 

[SAY WHAT]

I would say this video some what applies. I would say the super pro bushing lets the arm move more freely than the stock bushing but maybe not as much as a bearing.

Time stamps for the lazy :20 rubber 7:20 for poly 7:50 spherical bearing

 

[moffetts]

Unless I live on the bonneville salt flats, I am not going to go through the considerable trouble of replacing the stock bushings with.... another bushing. Not gonna happen.

 

[Norm Peterson]

I would say this video some what applies. I would say the super pro bushing lets the arm move more freely than the stock bushing but maybe not as much as a bearing.

Time stamps for the lazy :20 rubber 7:20 for poly 7:50 spherical bearing

Good as far as it goes, I guess, but he's overlooking an important part.

Where he's kind of wobbling the bearing-equipped arm side to side and rotationally - that's the sort of off-axis movements that Brian and I are talking about (this is commonly referred to as "bind"). He doesn't try the same wobbling movements on either the rubber or poly bushed cases, and doesn't even mention it in connection with them (there might be a reason with an agenda involved). And those are the movements that polyurethane does not accommodate well without specific detailing of its shape, constraint by the outer sleeve, or the presence of some intentional "voiding" in the poly itself.


Norm

 

[SAY WHAT]

Good as far as it goes, I guess, but he's overlooking an important part.

Where he's kind of wobbling the bearing-equipped arm side to side and rotationally - that's the sort of off-axis movements that Brian and I are talking about (this is commonly referred to as "bind"). He doesn't try the same wobbling movements on either the rubber or poly bushed cases, and doesn't even mention it in connection with them (there might be a reason with an agenda involved). And those are the movements that polyurethane does not accommodate well without specific detailing of its shape, constraint by the outer sleeve, or the presence of some intentional "voiding" in the poly itself.


Norm

Yea its possible he is trying to push the bushings. All I can say, is ill be going forward with the install this week and hope to have it done by the weekend. Ill have @Bluemustang evaluate it since he would most likely know what to feel for and his car has the steed rlca I believe. If I'm not happy with it ill just change it further down the line.

 

[Brian@BMVK]

Good as far as it goes, I guess, but he's overlooking an important part.

Where he's kind of wobbling the bearing-equipped arm side to side and rotationally - that's the sort of off-axis movements that Brian and I are talking about (this is commonly referred to as "bind"). He doesn't try the same wobbling movements on either the rubber or poly bushed cases, and doesn't even mention it in connection with them (there might be a reason with an agenda involved). And those are the movements that polyurethane does not accommodate well without specific detailing of its shape, constraint by the outer sleeve, or the presence of some intentional "voiding" in the poly itself.


Norm

The OEM bushing binds about several axes: the rotational axes and the off-axis motion because of the orientation of the fastener relative to the wheel & centerline of the car. The combination of the two creates a huge amount of wheel rate in the suspension. It's really quite remarkable that the car works at all with this design.

 

[fatbillybob]

The OEM bushing binds about several axes: the rotational axes and the off-axis motion because of the orientation of the fastener relative to the wheel & centerline of the car. The combination of the two creates a huge amount of wheel rate in the suspension. It's really quite remarkable that the car works at all with this design.

Brian,

Does the front lower control arm bearing also have this off-axis bind?

 

[TeeLew]

I don't think the spring rate of the bushing in its normal use is really the correct thing to criticize. Holding a bushing in a vice and showing that it springs back into position is just a demonstration of what it was designed to do. All the springs of a corner add up to make a single rate. Probably 1 reason why the stock springs are soft is because of the collection of springs which contribute to the total. This is just one of them. Bonded anti-roll bar bushings are another example. These bushings are meant to do this. This *reduces* friction in the system because there's no sliding to produce this motion. The off-axis quality of the motion probably makes the spring rate non-linear, but that's just a guess.

When you change to either a poly bush or monoball, you reduce the wheel rate of the corner because you've eliminated a spring. You do increase the static & sliding friction, though, some small amount. The poly bush probably falls somewhere in the middle of a 'tight' and 'loose' spherical bearing. Both of these joints require some sort of relative bearing/race motion, which introduces friction. It appears the SuperPro bush rotates the inner sleeve for vertical motion & deflects(similar to the rubber bush) to accommodate the off-axis. The spherical is free to rotate about all 3 axis'.

So why have either over the stock rubber if the vertical spring rate isn't inherently negative? Lateral deflection, the 'squish' of the bushing when we put side load on the contact patch, is what we're really trying to eliminate. For the tension link, the bushing deflection gives us undesirable steering and camber movements.

The deflection is the spherical is basically nothing. It's going to be the best option with respect to performance. The deflection of the poly bush is going to be a lot less than the stock piece. I really don't have a handle on the relative stiffness of the bushings, but I would guess the poly is 4 or 5 times stiffer, so you'll get a lot less of the undesirable effects. The poly might have a little less NVH, but that doesn't seem to be meaningful for any option. The other important difference between the two is maintenance. The spherical will need to be replaced from time to time when our gets loose. The poly bush will probably last quite a bit longer if lubed properly. The stock one would last pretty much forever.

 

[Brian@BMVK]

You do increase the static & sliding friction, though, some small amount.

Not at all sure what you're talking about here.

The criticism of the vertically acting rate is absolutely valid for the following reasons: 1) it's not purely in twist about the bolt axis because of the off-axis joint and thus is a progressive rate spring in the suspension, 2) it adds rate in jounce and also resists the spring in down travel. That means that relative to the tire, it acts a bit like a swaybar, affecting both wheels in the same direction in roll, while also still adding significant wheel rate in two wheel bumps. This means that it not only adds ride rate all the time, but also limits inside tire load in cornering. What that difference means in the real world is very significant, and something you need to experience even if you understand what it is doing. There is simply nothing good about having that bushing there except good NVH and cost. For performance and ride, it is hot garbage, and Ford should be embarrassed that they "engineered" that.

@AlbertD

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