joey.lisano
New member
Bushings: rubber vs. polyurethane
Note
I intended the article to leave out brands/manufacturers. Inadvertently, I did use the term Group N; however, I wanted to make sure I clearly differentiated between the stock rubber and the harder Group N rubber.
Introduction
Talking about bushings... on a Friday night. That's how profound this revelation is. Where did this originate? Uh, I think I may have to blame A-man again. (Hi
) He really likes Group N bushings, especially for the lateral links and trailing arms. On the other hand, I thought polyurethane bushings were stiffer and thus superior in performance. I trust A-man's judgment, so I was left with a conflict of interests. I had to settle my inner debate.
A-man linked me to some old posts by williaty on IWSTI. In addition to his Group N bushing installation write ups, williaty talks about why Group N bushings are superior to polyurethane bushings for stuff like lateral links and trailing arms. There was a lot of talk about polyurethane bushings causing binding. Instead of just agreeing with williaty's assessment, I wanted to know why. First, I had to define what binding was and what causes it.
Binding
As the wheels move up and down, the bump stops determine the suspension's full range of motion. Within that range of motion, the suspension's movement should be smooth. Binding is when the motion is not smooth. If the suspension binds, gets stuck for a moment, and then snaps out of it, the suspension will be twitchy and thus unpredictable; the car could suddenly understeer or oversteer, and I'm sure that ranks up with hydroplaning on the 'oh crap' scale. This mean less grip and less driver confidence. Well, we don't want that. So, what causes binding? It could be a range of motion issue, such as lowering the car so much that the ball joints and tie rods are at extreme angles and can't rotate properly. It could be a physical contact issue, such as the front sway bar hitting the tie rod, or wide wheels/tires hitting the strut/coilover/spring or fender. It could be that a linkage's movement is limited by the bushings. This is our area of concern.
Axes of Rotation
In the MacPherson strut, multilink rear suspension on the GD, there are two lateral links and one trailing arm connected to each knuckle. This causes a primary and secondary axis of rotation in those bushings. As the knuckle moves up and down, it rotates in an arc about the trailing arm pivot and the lateral link pivots. From the lateral link bushings' perspective: as the knuckle rotates around the lateral link pivot, it also moves front/back with the trailing arm. From the trailing arm bushings' perspective: as the knuckle rotates about the trailing arm pivot, it also moves right/left with the lateral links. This relationship creates a primary and secondary axis of rotation. Here's a picture of the front bushing in a trailing arm.
front trailing arm bushing
What are the consequences of this? The bushing must allow for rotation in the primary axis as well as some rotation in the secondary axis. Rubber can do this because it can compress and elastically deform. Sounds good, but the stock bushings are too soft and cause poor dynamic alignment. As enthusiasts, we want response and driver feedback. This is where Group N bushings fill the need. Why not polyurethane? Polyurethane is stiff, so stiff that it's almost incompressible. This may sound good initially, but it can cause problems in this situation. Because polyurethane is so stiff, it does not allow for secondary axis rotation. This lack of rotation in the secondary axis can cause binding. It's not a guaranteed problem, but it is a possibility. It's difficult to predict because everyone has different ride heights and coilovers/struts, which leads to different suspension geometries and ranges of motion.
Friction
There is more to the story though. In addition to issues with secondary axis rotation, polyurethane bushings also cause static friction. Why? It has to do with how the bushing reacts to the link's rotation. For polyurethane, the inner or outer housing will rotate. This is why polyurethane bushings need grease. Even with a lot of grease, there is still a good amount of static friction to initially overcome because the weigh of the car is sitting on these suspension bushings. Even if the coefficient of friction is low, there is still a huge normal force. Once the static friction is overcome, the metal rotates easier since the coefficient of sliding friction is lower than that of static. Because of static friction, the bushings will not want to turn and react to small undulations in the road; consequently, the ride quality will be rougher. The rough ride quality is usually blamed on the polyurethane's stiffness, but friction actually plays a hidden role. Also, if the lubrication runs dry, the sliding friction will wear the bushing away. This is why it's recommended to clean and re-lube bushings once a year.
polyurethane bushing
On the other hand, rubber bushings act as torsional springs. As the link rotates, a rubber bushing will wind up and develop a reaction torque. The bushing wants to be at its neutral position. This is why you always torque bushings down at ride height; that ensures the neutral position is at ride height. Because rubber bushings twist, there is very little static friction. They are able to react to small undulations in the road and thus feel more comfortable. The fact that they feel more comfortable doesn't necessarily mean that their performance is worse.
rubber bushing
How do you know if you have a lot of static friction? Remove the weight off one wheel by jacking the car up and slowly letting it back down. Measure the ride height. Now compress that corner and slowly let it come back up. Measure the ride height again. In theory, both ride height measurements should be equal, but they're probably not. A difference of 0.25" would mean low friction, whereas a difference of 1" indicates a lot of static friction. A large difference means the ride height and alignment are difficult to precisely measure and lack repeatability because there is such a wide range of neutral positions. If you try to corner balance a car with a lot of static friction, it'll feel like it doesn't want to move.
NVH
Rubber is a natural damper. It dampens out high frequency vibrations and thus cabin noise. If you want increased performance with as little noise as possible, get Group N bushings. This selection can be seen in Group N engine and transmission mounts. On the other hand, polyurethane bushings transmit everything. They are stiffer and create more noise. For example, if you want to hear your transmission scream, get a polyurethane transmission mount and bushings. Because polyurethane is stiffer than rubber, they'll have a slight edge in performance for a stationary bushing.
Types of Bushings in the 04-07 STI
There are two broad categories of bushings: those that rotate and those than remain stationary. The rotational bushings should be rubber. A hardcore rotational bushing would be a spherical bearing, and only a few dedicated track guys step up to this level. The stationary bushings can be rubber or polyurethane. A flat-out stationary bushing would be a solid mount.
1. rotational bushings (use: rubber, spherical bearing)
- sway bar D bushings
- lateral link bushings
- trailing arm bushings
- front transverse LCA bushings
- rear transverse LCA bushings (commonly called ALK)
- front shifter bushing
- pivot shifter bushing
- linkage shifter bushings
2. stationary bushings (use: rubber, polyurethane, solid)
- engine mounts
- transmission mount
- pitchstop mount
- rear diff mount bushings
- rear subframe to frame (lockdown bolts bottom these bushings out)
- rear diff outrigger bushings
- strut top mounts
- endlinks
- driveshaft center carriage bushings
- transmission crossmember bushings
- rear shifter bushing
- exhaust hangers
- radiator cushion bushings
Does everyone follow this? Of course not. Think of it more as a guideline than a strict rule. Just realize that if you have polyurethane rotating bushings, such as the sway bar D bushings, you need to lubricate them.
Summary
Rubber bushings are always appropriate. Polyurethane should be kept to bushings that do not move. Due to rubber's unique characteristics, it may feel quieter and smoother than polyurethane. This is a natural phenomena and does not necessarily mean that rubber has worse performance characteristics.
Ultimately, everything I read was spot on with williaty's assessment.
A-man is justified in liking Group N bushings. Me, well, I was sorta right: polyurethane bushings are stiff, but they are so stiff that they should not be used in lateral links and trailing arms. I say "should not" because you can use polyurethane and survive just fine. I leave the choice up to you.
Note
I intended the article to leave out brands/manufacturers. Inadvertently, I did use the term Group N; however, I wanted to make sure I clearly differentiated between the stock rubber and the harder Group N rubber.
Introduction
Talking about bushings... on a Friday night. That's how profound this revelation is. Where did this originate? Uh, I think I may have to blame A-man again. (Hi
A-man linked me to some old posts by williaty on IWSTI. In addition to his Group N bushing installation write ups, williaty talks about why Group N bushings are superior to polyurethane bushings for stuff like lateral links and trailing arms. There was a lot of talk about polyurethane bushings causing binding. Instead of just agreeing with williaty's assessment, I wanted to know why. First, I had to define what binding was and what causes it.
Binding
As the wheels move up and down, the bump stops determine the suspension's full range of motion. Within that range of motion, the suspension's movement should be smooth. Binding is when the motion is not smooth. If the suspension binds, gets stuck for a moment, and then snaps out of it, the suspension will be twitchy and thus unpredictable; the car could suddenly understeer or oversteer, and I'm sure that ranks up with hydroplaning on the 'oh crap' scale. This mean less grip and less driver confidence. Well, we don't want that. So, what causes binding? It could be a range of motion issue, such as lowering the car so much that the ball joints and tie rods are at extreme angles and can't rotate properly. It could be a physical contact issue, such as the front sway bar hitting the tie rod, or wide wheels/tires hitting the strut/coilover/spring or fender. It could be that a linkage's movement is limited by the bushings. This is our area of concern.
Axes of Rotation
In the MacPherson strut, multilink rear suspension on the GD, there are two lateral links and one trailing arm connected to each knuckle. This causes a primary and secondary axis of rotation in those bushings. As the knuckle moves up and down, it rotates in an arc about the trailing arm pivot and the lateral link pivots. From the lateral link bushings' perspective: as the knuckle rotates around the lateral link pivot, it also moves front/back with the trailing arm. From the trailing arm bushings' perspective: as the knuckle rotates about the trailing arm pivot, it also moves right/left with the lateral links. This relationship creates a primary and secondary axis of rotation. Here's a picture of the front bushing in a trailing arm.
What are the consequences of this? The bushing must allow for rotation in the primary axis as well as some rotation in the secondary axis. Rubber can do this because it can compress and elastically deform. Sounds good, but the stock bushings are too soft and cause poor dynamic alignment. As enthusiasts, we want response and driver feedback. This is where Group N bushings fill the need. Why not polyurethane? Polyurethane is stiff, so stiff that it's almost incompressible. This may sound good initially, but it can cause problems in this situation. Because polyurethane is so stiff, it does not allow for secondary axis rotation. This lack of rotation in the secondary axis can cause binding. It's not a guaranteed problem, but it is a possibility. It's difficult to predict because everyone has different ride heights and coilovers/struts, which leads to different suspension geometries and ranges of motion.
Friction
There is more to the story though. In addition to issues with secondary axis rotation, polyurethane bushings also cause static friction. Why? It has to do with how the bushing reacts to the link's rotation. For polyurethane, the inner or outer housing will rotate. This is why polyurethane bushings need grease. Even with a lot of grease, there is still a good amount of static friction to initially overcome because the weigh of the car is sitting on these suspension bushings. Even if the coefficient of friction is low, there is still a huge normal force. Once the static friction is overcome, the metal rotates easier since the coefficient of sliding friction is lower than that of static. Because of static friction, the bushings will not want to turn and react to small undulations in the road; consequently, the ride quality will be rougher. The rough ride quality is usually blamed on the polyurethane's stiffness, but friction actually plays a hidden role. Also, if the lubrication runs dry, the sliding friction will wear the bushing away. This is why it's recommended to clean and re-lube bushings once a year.
On the other hand, rubber bushings act as torsional springs. As the link rotates, a rubber bushing will wind up and develop a reaction torque. The bushing wants to be at its neutral position. This is why you always torque bushings down at ride height; that ensures the neutral position is at ride height. Because rubber bushings twist, there is very little static friction. They are able to react to small undulations in the road and thus feel more comfortable. The fact that they feel more comfortable doesn't necessarily mean that their performance is worse.
How do you know if you have a lot of static friction? Remove the weight off one wheel by jacking the car up and slowly letting it back down. Measure the ride height. Now compress that corner and slowly let it come back up. Measure the ride height again. In theory, both ride height measurements should be equal, but they're probably not. A difference of 0.25" would mean low friction, whereas a difference of 1" indicates a lot of static friction. A large difference means the ride height and alignment are difficult to precisely measure and lack repeatability because there is such a wide range of neutral positions. If you try to corner balance a car with a lot of static friction, it'll feel like it doesn't want to move.
NVH
Rubber is a natural damper. It dampens out high frequency vibrations and thus cabin noise. If you want increased performance with as little noise as possible, get Group N bushings. This selection can be seen in Group N engine and transmission mounts. On the other hand, polyurethane bushings transmit everything. They are stiffer and create more noise. For example, if you want to hear your transmission scream, get a polyurethane transmission mount and bushings. Because polyurethane is stiffer than rubber, they'll have a slight edge in performance for a stationary bushing.
Types of Bushings in the 04-07 STI
There are two broad categories of bushings: those that rotate and those than remain stationary. The rotational bushings should be rubber. A hardcore rotational bushing would be a spherical bearing, and only a few dedicated track guys step up to this level. The stationary bushings can be rubber or polyurethane. A flat-out stationary bushing would be a solid mount.
1. rotational bushings (use: rubber, spherical bearing)
- sway bar D bushings
- lateral link bushings
- trailing arm bushings
- front transverse LCA bushings
- rear transverse LCA bushings (commonly called ALK)
- front shifter bushing
- pivot shifter bushing
- linkage shifter bushings
2. stationary bushings (use: rubber, polyurethane, solid)
- engine mounts
- transmission mount
- pitchstop mount
- rear diff mount bushings
- rear subframe to frame (lockdown bolts bottom these bushings out)
- rear diff outrigger bushings
- strut top mounts
- endlinks
- driveshaft center carriage bushings
- transmission crossmember bushings
- rear shifter bushing
- exhaust hangers
- radiator cushion bushings
Does everyone follow this? Of course not. Think of it more as a guideline than a strict rule. Just realize that if you have polyurethane rotating bushings, such as the sway bar D bushings, you need to lubricate them.
Summary
Rubber bushings are always appropriate. Polyurethane should be kept to bushings that do not move. Due to rubber's unique characteristics, it may feel quieter and smoother than polyurethane. This is a natural phenomena and does not necessarily mean that rubber has worse performance characteristics.
Ultimately, everything I read was spot on with williaty's assessment.
