What?s better when your dealing with rear upper and lower control arms, spherical bearings or poly bushings? Also, what are the pros and cons of an adjustable rear upper control arm? Thanks!
Spherical bearings vs Poly bushings
3v_Dawson
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I am also interested to hear some fresh input on this. Here are some thoughts / basics:
Adjustable UCA is needed only to correct pinion angle, which is usually only important if you have an aftermarket one-piece driveshaft (stock two-piece driveshaft is pretty tolerant of pinion angle)
I have never seen a spherical joint in a UCA, but I guess here is such a thing. It is important that the UCA can rotate so the left and right wheels can move somewhat separately when one hits a bump and the other doesn't.
Main advantage of spherical joint is that it allows the arm and the axle to rotate freely as the suspension travels. This is relatively important on cornering (road track applications) but not so important in a straight line (not very important for drag racing). The main disadvantage is that the hard-core ones are steel on steel which transfers a lot of noise, vibration, and harshness (NVH) into the chassis and the cabin. There are hybrid designs available (J&M?) that have a steel ball inside a poly cup that are supposed to be a good compromise. Also you only need the sperical joint on one end, so the other can be poly and provide some isolation.
Main advantage of poly bushings is NVH isolation; generally at the expensive of some rotational freedom.
I use my car on the road track so I like the LCA's with the hybrid spherical joint on one end. I am not sure if I also need a UCA that has the ability to rotate, I think the one I have not does not allow much rotation (I did not pay attention to that when I got it)
Adjustable UCA is needed only to correct pinion angle, which is usually only important if you have an aftermarket one-piece driveshaft (stock two-piece driveshaft is pretty tolerant of pinion angle)
I have never seen a spherical joint in a UCA, but I guess here is such a thing. It is important that the UCA can rotate so the left and right wheels can move somewhat separately when one hits a bump and the other doesn't.
Main advantage of spherical joint is that it allows the arm and the axle to rotate freely as the suspension travels. This is relatively important on cornering (road track applications) but not so important in a straight line (not very important for drag racing). The main disadvantage is that the hard-core ones are steel on steel which transfers a lot of noise, vibration, and harshness (NVH) into the chassis and the cabin. There are hybrid designs available (J&M?) that have a steel ball inside a poly cup that are supposed to be a good compromise. Also you only need the sperical joint on one end, so the other can be poly and provide some isolation.
Main advantage of poly bushings is NVH isolation; generally at the expensive of some rotational freedom.
I use my car on the road track so I like the LCA's with the hybrid spherical joint on one end. I am not sure if I also need a UCA that has the ability to rotate, I think the one I have not does not allow much rotation (I did not pay attention to that when I got it)
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Thanks jbert for your reply! That all sounds really helpful. Hopefully more people do reply so we can both get new info on this topic, even though it sounds like you know your bearings and bushings. Lol I daily my car so comfort matters but performance comes first. I like to drive ?spiritedly? on the road now but later I want it to be a street strip car so handling matters but I?m mostly going for the straight line set up I guess.
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The sole function of the UCA is to maintain the pinion angle and prevent the axle from rotating on its axis. Therefore the best set-up there is poly bushings at both ends. The UCA needs to have an adjustable length if the ride height is altered, as this will alter the pinion angle from stock. Lowering springs will also shift the rear axle towards the driver's side, requiring an adjustable panhard bar to keep it centered laterally.
The LCAs maintain fore-and-aft axle location and merely need enough freedom to move in an arc, though a little lateral movement at the axle end is required during hard cornering. You could either do poly bushings at both ends, or poly bushings at the chassis end with spherical bearings at the axle end. Unfortunately spherical bearings transmit more axle noise into the cabin and require frequent greasing to prevent premature wear.
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I'm talking about rotation in the other direction, around the car's end-to-end axis. When the right rear wheel hits a bump and the left one doesn't; or when taking a hard corner and the body leans; the rear axle has to rotate; and if the UCA prevents it from doing that, then you can get binding and undesirable things like snap over-steer, or so I am told.
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None of the axle motions happen in a perfectly straight line - everything moves in 3-D arcs. So kinematically - meaning you're considering geometry alone - none of the four locating links pivots precisely about the bolts that attach it to the axle or to the chassis. Instead, all eight pivots (two each for the LCAs, the UCA, and the PHB) need to allow their links to rotate in all three axes as the suspension moves. Ideally, and the way just about anybody who isn't working at the OE level would be able to analyze the suspension, would have every pivot back there being a spherical.
Tools for analysis at the OE level are orders of magnitude more capable (and orders of magnitude more expensive and needy of more input data) so let's just say they can predict how much bushing distortion is occurring and how much the axle location would change as a result and leave it at that.
When compliant bushings are introduced at the ends of the links instead of sphericals, distortion of a soft-ish bushing material is intended to accommodate that 3-D rotation. This is what allows Ford to use what I call 'cylindrical bushings'. The main rotation is about the attaching bolt center, as you'd expect. The smaller other two mainly occur within the bushing material. When the material is soft enough, this happens without generating any appreciable amounts of force or resistance and everything is fine . . . as long as they aren't subject to too much force. Should too much force happen, the axle may no longer be under adequate location control. Like wheel hop.
On the aftermarket side, off-axis rotations are generally assumed to be small enough that firmer bushing materials can be used. This would be for more positive axle location, with suppression of wheel hop or at least minimizing that being the main intent. But since nearly all cylindrical bushings are about equally stiff in all the other directions as they are in the 'good direction' that resists wheel hop, now you're generating "unexpected" forces as the bushing distorts to accommodate those off-axis movements/rotations.
As long as any off-axis rotations are small enough, it's still not a big problem. Sometimes that's a matter of link orientation in the car and sometimes it's up to the driver to limit the amounts of rotation by the way he drives. About here is where the simple poly bushings really start to get in their own way, where the movements developed by cornering roll working against bushing stiffness can create forces that are big enough to act as a virtual rear stabilizer bar . . . and that imaginary "sta-bar" can have an effective stiffness approaching that of the real one (this is why poly-bushed axle links/control arms aren't the hot tip if you're at all inclined to drive hard through the corners). Most times you'll hear this called 'bind', though it's really a case of 'induced forces'. The joints don't get stuck, they just get progressively harder to distort as required
Spherical pivots accept the off-axis stuff without resistance or developing forces in reaction - they don't care what precise axis orientation they need to be rotating about. Their main downsides are cost, NVH, and maintenance, and in most cases need to be considered 'wear items'. Not that poly bushings aren't 'wear items', but that's a separate story.
Sphericals do exist for both ends of the UCA if you look around a bit (hint: BMR arms, Steeda diff-side pivot). They will almost certainly transmit more noise from the differential back into the cabin.
Another hint: there are a couple of things you can do to poly to make them a bit more like OE rubber bushings without losing all of the 'good direction' stiffness . . . if you have a good idea what you're trying to do and if you're willing to potentially give up some bushing life. But I'm going to leave that as just a hint, at least for now.
Technically, sphericals are going to be better for the straight line stuff than poly ever would be for the corner-carving side. For LCAs, sphericals in one end with poly (perhaps lightly modified) in the other should be sufficient. I'm less sure about the shorter UCA being a good place for poly in a hard-cornering environment, although poly bushings there would work well enough for straightline purposes where you aren't throwing a lot of cornering roll or relative lateral movement at them. (Because the UCA is located above the rear roll center height, roll creates a little relative sideways motion that has to come out over the length of the UCA. Something similar happens at the LCAs, but by being longer the effect isn't as severe).
As a corner-carver (and not into drag racing or hard launches at all), I haven't bothered changing the UCA on my car. The LCAs are poly/spherical (Johnny-Joints, not Heims), and the PHB is still OE but with bushings that have been stiffened a little.
Norm
Tools for analysis at the OE level are orders of magnitude more capable (and orders of magnitude more expensive and needy of more input data) so let's just say they can predict how much bushing distortion is occurring and how much the axle location would change as a result and leave it at that.
When compliant bushings are introduced at the ends of the links instead of sphericals, distortion of a soft-ish bushing material is intended to accommodate that 3-D rotation. This is what allows Ford to use what I call 'cylindrical bushings'. The main rotation is about the attaching bolt center, as you'd expect. The smaller other two mainly occur within the bushing material. When the material is soft enough, this happens without generating any appreciable amounts of force or resistance and everything is fine . . . as long as they aren't subject to too much force. Should too much force happen, the axle may no longer be under adequate location control. Like wheel hop.
On the aftermarket side, off-axis rotations are generally assumed to be small enough that firmer bushing materials can be used. This would be for more positive axle location, with suppression of wheel hop or at least minimizing that being the main intent. But since nearly all cylindrical bushings are about equally stiff in all the other directions as they are in the 'good direction' that resists wheel hop, now you're generating "unexpected" forces as the bushing distorts to accommodate those off-axis movements/rotations.
As long as any off-axis rotations are small enough, it's still not a big problem. Sometimes that's a matter of link orientation in the car and sometimes it's up to the driver to limit the amounts of rotation by the way he drives. About here is where the simple poly bushings really start to get in their own way, where the movements developed by cornering roll working against bushing stiffness can create forces that are big enough to act as a virtual rear stabilizer bar . . . and that imaginary "sta-bar" can have an effective stiffness approaching that of the real one (this is why poly-bushed axle links/control arms aren't the hot tip if you're at all inclined to drive hard through the corners). Most times you'll hear this called 'bind', though it's really a case of 'induced forces'. The joints don't get stuck, they just get progressively harder to distort as required
Spherical pivots accept the off-axis stuff without resistance or developing forces in reaction - they don't care what precise axis orientation they need to be rotating about. Their main downsides are cost, NVH, and maintenance, and in most cases need to be considered 'wear items'. Not that poly bushings aren't 'wear items', but that's a separate story.
Sphericals do exist for both ends of the UCA if you look around a bit (hint: BMR arms, Steeda diff-side pivot). They will almost certainly transmit more noise from the differential back into the cabin.
Another hint: there are a couple of things you can do to poly to make them a bit more like OE rubber bushings without losing all of the 'good direction' stiffness . . . if you have a good idea what you're trying to do and if you're willing to potentially give up some bushing life. But I'm going to leave that as just a hint, at least for now.
Technically, sphericals are going to be better for the straight line stuff than poly ever would be for the corner-carving side. For LCAs, sphericals in one end with poly (perhaps lightly modified) in the other should be sufficient. I'm less sure about the shorter UCA being a good place for poly in a hard-cornering environment, although poly bushings there would work well enough for straightline purposes where you aren't throwing a lot of cornering roll or relative lateral movement at them. (Because the UCA is located above the rear roll center height, roll creates a little relative sideways motion that has to come out over the length of the UCA. Something similar happens at the LCAs, but by being longer the effect isn't as severe).
As a corner-carver (and not into drag racing or hard launches at all), I haven't bothered changing the UCA on my car. The LCAs are poly/spherical (Johnny-Joints, not Heims), and the PHB is still OE but with bushings that have been stiffened a little.
Norm
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