Polyurethane suspension bushings, a good or bad thing? - MR2 Owners Club Message Board
 
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post #1 of 20 (permalink) Old February 17th, 2011, 11:47 Thread Starter
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Question Polyurethane suspension bushings, a good or bad thing?

I installed polyurethane bushings in my 1991 SSM MR2 from the company "Prothane" a while back because alot of people gave good reviews so I never really researched it or looked into other options. I am now researching them and finding alot of mixed opinions about their effectiveness and if they actually hurt handling more than help. Ride comfort and squeeking don't concern me as this is a car for autox.

There seem to be 2 camps about polyurethane suspension bushings.

Camp 1: Polyurethane suspension bushings are good

-They reduce unwanted camber changes from soft or worn out oem bushings
-They are quicker to react to driver inputs than oem bushings due to the increased stiffness.
-It makes the car "feel" really good and solid and I can't "feel" the suspension binding

Image from Energy Suspension FAQ about polyurethane bushings:


Camp 2: Polyurethane suspension bushings are bad

-They are too stiff and do not allow your suspension to move through its normal range
-They artificially increase "spring rate" or wheel rate by making it harder for the suspension to compress/move, and they do so in a non-linear way.
-They bind and can prevent alot of suspension movement/travel

Good website for Camp 2:
http://www.crystalridge.net/cars/bushings.htm#Poly
Another
http://www.gnttype.org/techarea/susp...uspension.html
And another
http://www.elephantracing.com/techto...nefriction.htm

Examples of Polyurethane being a poor choice:
Front strut rod identical to the 91/92 mr2 front strut rod snaps in half on multiple 240Zs after using polyurethane:
http://forums.hybridz.org/index.php/...ots-of-photos/

Rear toe arm gets malformed (vendor says it was due to dyno strapping down but also talks about bushings)
http://my350z.com/forum/brakes-and-s...mber-arms.html

A very telling quote from the link above: "For those who insist on the urethane bushings, I recommend you check your suspension for bind by removing the shock/spring, then moving the suspension up and down, it should move relatively freely through its full range of motion."

I will be doing this "test" on two mr2s this weekend, one will poly bushings installed, the other on stock bushings.

Different Durometers can also be used to tune suspension characteristics so putting in stiffer bushings can alter suspension characteristics.
http://forum.miata.net/vb/showpost.p...94&postcount=9

Also an important note, I am talking about using prothane bushings that move across a multi plane axis. I realize that polyurethane bushings are ok to use on single axis bushings such as sway bar bushings. Here is a diagram of the MKII's suspension:



I also realize that some polyurethane bushings brands aren't as stiff or as high durometer as others, it seems Prothane is the stiffest, followed by Energy Suspension, then super pro poly but I have been unable to find the durometer of each bushing to know for sure.

So what does everyone think of the use of polyurethane bushings to replace oem suspension bushings for autox?

Last edited by Levi; February 17th, 2011 at 13:23.
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post #2 of 20 (permalink) Old February 17th, 2011, 12:26
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Subscribing cause Ive done this dance before (pros vs cons) and never got anywhere.
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post #3 of 20 (permalink) Old February 17th, 2011, 12:59
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Yes, binding of poly bushings is/can be bad. Is it as bad as stock rubber? Probably not. Poly bushings still keep bending in check IMHO. (especially camber)
From experience it's still an improvement but going to rod end susp. is the way to go of course. Poly bushings need to be greased well for them to work freely and not bind. Many people forget to grease their bushings from time to time. (especially sway bars) Just my 2 cents...

Steve, any insight on this? You have way more experience in this area I am sure.
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post #4 of 20 (permalink) Old February 17th, 2011, 13:09
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I'm not Steve, but I've had polyurethane bushings in every car I've owned, ever.

I'm not sure how people are having polyurethane bushings bind. Both Prothane and ES have a metal pivot bushing that allows everything to move smoothly. Personally, I grease the crap out of my bushings with some EXXON Synthetic High-Temp/High-Pressure, Waterproof Grease (~$12-15 at your local auto store).

The place most people have binding/parts tearing issues is the sway bars. The "D" shaped bushing is usually too large for the U bracket that holds it to the chassis/car. The hole around the swaybar deforms and the bushing binds once installed. The solution is easy - grind/shave off the flat side of bushing until it fits inside the U bracket. Grease as above and don't worry.
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post #5 of 20 (permalink) Old February 17th, 2011, 13:16
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Its good to see someone else looking at this issue, and doing their research on it. I have brought up the 240z strut rod breakage issue before as an example, and have mostly been ignored. Levi, I'm sure given the research you appear to have done that you have already read my thoughts on this subject, but I will re-state them here just for the sake of the thread.


It doesn't matter how much you grease it, most of the pivot points (on the MKII at least) are multi axis pivots. The poly bushing only allows rotation about a single axis, and in order to rotate about the second or third axis the bushing material must be deformed. THATS where the binding comes from.

The only poly bushings left on my car are the rear strut rod to control arm (doesn't really need to flex at all once everything else is rod end), and the front control arms, which are single axis.

My opinion, if you are building a race car, spherical bearings without a doubt. For a high performance street car, the spherical bearings DO present a maintaince issue, but I am still inclined to say you should weigh that against the performance and make the choice between leaving the stock bushings and going to spherical bearings, and only use poly in the few locations where it is actually appropriate.

Last edited by Alex W; February 17th, 2011 at 13:19.
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post #6 of 20 (permalink) Old February 17th, 2011, 13:18 Thread Starter
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From: http://www.crystalridge.net/cars/bushings.htm#Bind

"Lubing does nothing for the binding problem"

The polyurethane bushings are much stiffer and this stiffness is what creates binding because "The suspension's range of motion is restricted by the lack of compliance in the polyurethane bushings"

EDIT: damn Alex explained it much better than me
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post #7 of 20 (permalink) Old February 17th, 2011, 13:19
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Quote:
Originally Posted by mtbmr2
I'm not Steve, but I've had polyurethane bushings in every car I've owned, ever.

I'm not sure how people are having polyurethane bushings bind. Both Prothane and ES have a metal pivot bushing that allows everything to move smoothly. Personally, I grease the crap out of my bushings with some EXXON Synthetic High-Temp/High-Pressure, Waterproof Grease (~$12-15 at your local auto store).

The place most people have binding/parts tearing issues is the sway bars. The "D" shaped bushing is usually too large for the U bracket that holds it to the chassis/car. The hole around the swaybar deforms and the bushing binds once installed. The solution is easy - grind/shave off the flat side of bushing until it fits inside the U bracket. Grease as above and don't worry.

Have you ever tried "dielectric valve lubricating compound(paste)"? ( I think that's the full name of it) It's a little thicker than grease and seems to keep in place better. Had very good luck with it on my alltrac. Great info by the way!
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post #8 of 20 (permalink) Old February 17th, 2011, 13:31
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Quote:
Originally Posted by Alex W
The poly bushing only allows rotation about a single axis, and in order to rotate about the second or third axis the bushing material must be deformed. THATS where the binding comes from.
Agreed.


Quote:
Originally Posted by Alex W
My opinion, if you are building a race car, spherical bearings without a doubt.
Very much agreed, there is nothing else that even compares. I think the question would be what's better for those who do not want to deal with rod ends or for people building sub-SSM class cars for autox.
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post #9 of 20 (permalink) Old February 17th, 2011, 14:52
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Quote:
Originally Posted by Alex W

The poly bushing only allows rotation about a single axis, and in order to rotate about the second or third axis the bushing material must be deformed. THATS where the binding comes from.
This is basically accurate, in most cases.

I use the Prothane bushings on my DP car now. However, they are scheduled for upgrading to spherical bearings.

To further the above point, as long as the suspension component moves on a single axis, poly bushings work just fine generally. They reduce deflection and most other undesirable motions.

As to the issues, I will use the MK1 as an example: The front suspension has a control arm, a radius arm and a strut. The radius arm is rigidly attached to the control arm. Therefore, as the suspension moves through its range of motion, the radius arm forces the control arm to rotate perpendicular to the normal pivot axis. This causes binding that results in restriction of motion. It is this resistance that causes inconsistencies and loss of ride quality and grip at extremes of travel. So when the car is fully loaded in a corner and the outside suspension is fully compressed, a slight irregularity in the surface will cause a loss of compliance and therefore grip.

Even on a perfectly smooth surface, the binding effect will result in greater wheel rates as the suspension compresses and this will also cause the same results.

Back to the MK1 and this time the rear. Note that radius arm has a bushing at both ends, the body mount and the control arm. So the effect is greatly reduced as the additional bushing allows for a greater range of motion without additional restriction. Not to assume there is NO resistance.

The effect can be reduced by keeping the suspension travel out of the extremes and therefore limiting the effect. So, again we have reason to keep the car as flat as possible and to avoid excessive lowering.

I should note that this is a lesson I was well schooled on in my experience with the X1/9. The rear a-arm is a one piece a-frame with the pivots at each end nearly 45 degrees from parallel. I knew of lots of people using delrin bushing in these pivots and fracturing the mounts as a result. I found even the eurathane bushings cause significant binding. So I used the OE rubber bushings when everybody else was using aftermarket. Nobody believed me when I told them. I didn't offer up an explanation.

-Steve
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post #10 of 20 (permalink) Old February 17th, 2011, 16:45
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I don't see nor feel any problems on my mr2

The only problem you discraibe are those found in other car makes, with different suspensions, unlike ours. that first image is no wear near what are suspension looks like.

Really, the broken bushings are mostly to a poor install, on a really old car. what did you think it was going to happen.
That 350z had aftermarket arms, that looked super soft. I could bend them by had looks like.

I'll say in the end poly bushing works for our cars, but not on other cars.
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post #11 of 20 (permalink) Old February 18th, 2011, 14:08
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Quote:
Originally Posted by XHead
I should note that this is a lesson I was well schooled on in my experience with the X1/9. The rear a-arm is a one piece a-frame with the pivots at each end nearly 45 degrees from parallel. I knew of lots of people using delrin bushing in these pivots and fracturing the mounts as a result. I found even the eurathane bushings cause significant binding. So I used the OE rubber bushings when everybody else was using aftermarket. Nobody believed me when I told them. I didn't offer up an explanation.
Amazing the parts that people will throw at their car "because everyone else is doing it", without stopping to examine if it is actually having the desired result.
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post #12 of 20 (permalink) Old February 18th, 2011, 14:17
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Quote:
Originally Posted by mr.ac
I don't see nor feel any problems on my mr2

The only problem you discraibe are those found in other car makes, with different suspensions, unlike ours. that first image is no wear near what are suspension looks like.

Really, the broken bushings are mostly to a poor install, on a really old car. what did you think it was going to happen.
That 350z had aftermarket arms, that looked super soft. I could bend them by had looks like.

I'll say in the end poly bushing works for our cars, but not on other cars.
Uhh, the 240z front suspension is virtually identical in design to the 91 MR2, and they are know for breaking strut rods if poly bushings are used on the strut rod. Perhaps its because they cars are older, or under built, but I suspect its only a matter of time before some of these issues start appearing on our cars.

As far as a "poor install" goes, how the installation will have any effect on this issue I have no idea.
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post #13 of 20 (permalink) Old February 18th, 2011, 15:13
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Quote:
Originally Posted by XHead

I should note that this is a lesson I was well schooled on in my experience with the X1/9. The rear a-arm is a one piece a-frame with the pivots at each end nearly 45 degrees from parallel. I knew of lots of people using delrin bushing in these pivots and fracturing the mounts as a result. I found even the eurathane bushings cause significant binding. So I used the OE rubber bushings when everybody else was using aftermarket. Nobody believed me when I told them. I didn't offer up an explanation.

-Steve
What is the reason a manufacturer would design the bushing to mount off parallel from the pivot axis? I could see in the front suspension radius arm setup that by its nature it must have a range of motion beyond the pivot axis but on a a-frame rear setup as described above it would not. Does it have to do with the direction of force being fed into the mounts?
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post #14 of 20 (permalink) Old February 18th, 2011, 22:00
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Quote:
Originally Posted by spiggs
What is the reason a manufacturer would design the bushing to mount off parallel from the pivot axis? I could see in the front suspension radius arm setup that by its nature it must have a range of motion beyond the pivot axis but on a a-frame rear setup as described above it would not. Does it have to do with the direction of force being fed into the mounts?
There are always three issues that affect why a manufacturer designs things the way they do. 1. Cost 2. Packaging 3) function.

In the case of the X1/9 and MK1 MR2 the rear suspension pickups are in almost the identical locations but the individual components are slightly different.

The X has a true, once piece, rigid A-Arm, the MR2 has a control arm and radius arm. In either case, to have the two ends of the control arm pivot in the same axis, either they have a very odd angle or the front pivot occupies the location of the oil filter.

That odd axis will create a lot of undesirable bump steer. Also, the non-parallel axis will actually result in less bushing deflection.

-Steve
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post #15 of 20 (permalink) Old March 5th, 2011, 00:11
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Little late to the thread, but just wanted to share some of my experiences with bushings. Not on a mk2 though...

I can absolutely say that they do not "increase" the spring rate. They change it. Depending on the layout of the bushing and directon of deflection, the spring rate can increased or decreased. The non linear part is spot on.

On my SP car, I used soft factory bushings in the radius arms and Poly in the lateral ones. This limited the binding while giving the least play during cornering and transients.

The binding, although a pain, can be largely tuned out with a bit of work. The bigger problem is hysterisis. This is caused by sticking. The large forces involved cause tons of friction, and this directly impacts the dampers ability to work properly.

One of the weird things was looking at the installed rate of the sway bar. The rate increased significantly with Poly bushings, and the affect was pronounced on the smaller bars.

The last point is in regards to the comments about the sway bars. I found the single largest contributer to hysterisis, by far, was my sway bar. As most would guess, cleaning and lubing helped significantly. What most wouldn't guess was how fast it would creep up again. In order to keep it up to par, I ended up having to clean and re-lube it every month. In fact, all the bushings were similarly effected. So, the maintenance "penalty" for going to spherical bushings isn't always as bad as you might think.

Further study revealed that the hysterisis was more or less proportional to the sway bar rate. Partially as a result I ended up running without a swaybar (which, near as I can tell always works better on the drive wheels, and sometimes better on the other end too). I know one top SP car currently running is using sealed bearings to mount his swaybar in an effort to have his cake and eat it too.

In summary, umm.... use spherical bushings, and spend your time looking at your sway bars. If you really don't want to do that, change out 7, 10 and 11 with Poly and don't worry about the rest.

O
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post #16 of 20 (permalink) Old March 14th, 2011, 10:38 Thread Starter
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Finally had some time to actually test the difference between stock vs poly bushings. I pulled up on the hub with the strut/spring removed from the car to see the effect of the bushings. The front was slightly harder to move with the poly bushings but the rear was MUCH harder to move, I struggled to move it more than an inch or two. I made vids showing it as well.

Poly bushings:
http://www.youtube.com/watch?v=zQb6mQ32QBs

Stock bushings:
http://www.youtube.com/watch?v=eNBYKlMLhvw

I'm gonna be replacing the strut rod bushings both front and rear with oem bushings so hopefully that will eliminate most of the resistance to the suspension. Will make another video once I get that done.

Last edited by Levi; March 14th, 2011 at 10:41.
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post #17 of 20 (permalink) Old March 19th, 2011, 12:38
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This occurred to me as well, so I used TRD bushings on the radius arm and poly in the lower control arm to body mounts (mk1). I still need to do the test like you did in those videos, so I'm curious to find out how effective my little half-ass plan is.

Unfortunately for mk1 owners, they don't sell the stock bushings independent of the control arms and even the supposedly harder TRD rubber bushings have been discontinued for years. So it's quite a bit more money to replace your bushings with rubber. If anyone knows of an alternative I'd be interested to hear it. I don't know if mk2 owners have the same situation.

Levi - forgive me if you mentioned it in the video (I didn't have sound) - Were the stock bushings original/old or fresh?
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post #18 of 20 (permalink) Old March 19th, 2011, 13:05 Thread Starter
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Quote:
Originally Posted by thirdvector
This occurred to me as well, so I used TRD bushings on the radius arm and poly in the lower control arm to body mounts (mk1). I still need to do the test like you did in those videos, so I'm curious to find out how effective my little half-ass plan is.

Unfortunately for mk1 owners, they don't sell the stock bushings independent of the control arms and even the supposedly harder TRD rubber bushings have been discontinued for years. So it's quite a bit more money to replace your bushings with rubber. If anyone knows of an alternative I'd be interested to hear it. I don't know if mk2 owners have the same situation.

Levi - forgive me if you mentioned it in the video (I didn't have sound) - Were the stock bushings original/old or fresh?
The ones in the video were old bushings, not rotted through or anything but your standard worn 100k bushings.

To the best of my knowledge you can't buy MK2 oem suspension bushings unless you buy a whole new control arm or strut rod (~$350). So its pretty similar to the MK1 situation.
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post #19 of 20 (permalink) Old March 20th, 2011, 10:13
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bump informative thread.
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post #20 of 20 (permalink) Old March 20th, 2011, 10:25
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Quote:
Originally Posted by Levi
Finally had some time to actually test the difference between stock vs poly bushings. I pulled up on the hub with the strut/spring removed from the car to see the effect of the bushings. The front was slightly harder to move with the poly bushings but the rear was MUCH harder to move, I struggled to move it more than an inch or two. I made vids showing it as well.

Poly bushings:
http://www.youtube.com/watch?v=zQb6mQ32QBs

Stock bushings:
http://www.youtube.com/watch?v=eNBYKlMLhvw

I'm gonna be replacing the strut rod bushings both front and rear with oem bushings so hopefully that will eliminate most of the resistance to the suspension. Will make another video once I get that done.

Thanks for posting those. That was a real eye opener.
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