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Discussion Starter · #1 ·
Hi Everybody!
I was doing a little web research for Peter/Pyce (he's doing some neat rear suspension Solidworks modeling -- look out for it over the next few days), and came across a really nice Forum explanation for why poly bushings can be troublesome in the rear suspension. Since some of you seem to be interested in the topic, I thought I'd repost it for you all to read. (Please note that I'm just reposting someone else's thoughts, and don't feel strongly about this issue myself!)

Anyway, we'll do it in parts. Halfway through the discussion was this observation from a Virginian named sccaITA16V:
Quote »

Bills comments are right on, and I would like to share my personal experience with bushings.
On my race car I originally used delrin front and rear for the first two seasons, then switched to Mr. Shines bearings. The first thing I noticed was that when I jacked up the car, the suspension immediately fell to full droop, whereas with the delrin setup the suspension in the front would s l o w l y fall and in the rear not drop at all! Obviously the delrin was binding.
On the street, I have seen friends with poly in there LCA and after a few years experience "slop" in the suspension. Upon tear down we discovered the control arm was pivoting around the poly bushing, which allowed the paint to scrape away and rust, and this cycle continued till it loosened up the tolerances so much you could feel slop in the suspention!
just my two cents

....followed by this comment from Vortex regular (and New Englander!!) bobqzzi:
Quote »

By their very nature and design polyurethane control arm bushing cannot function properly. In a normal rubber bushing the steel center sleeve is locked in place and the rubber a tight press fit on the OD. As the arm moves up and down the rubber twists radially. All the motion is in the rubber shearing, not the most precise system, but it works quite well for street cars, and the rubber lasts a good long time. The downside is sideways deflection under cornering loads.
A polyurethane bushing works in a different manner altogether. The center steel sleeve is still locked in place, so it doesn't move. The OD is allegedly fit tight enough so the poly material doesn't move either. So all the up and down motion causes the poly to rotate around the steel sleeve. So in effect the sleeve is a bearing and the poly outer is the race. Given the physical characteristics of poly, and the realities of making it press fit into the control arm, yet still have a perfectly round hole to act as a bearing race, there are two conditions that are usually achieved.
When first installed the sleeve fits very tightly in the poly outer, which prevents lateral deflection under cornering. However, it also binds the arm so badly that it is virtually impossible to move it up and down, this is called stiction, and results in odd, undesirable handling characteristics. This can be somewhat alleviated with a special lube.
After being installed for a while the sleeves typically machine a larger hole..someplace in this process is a "sweet spot" where they actually work okay for a little while. However, as they continue to work, and pick up all sort of lovely "grinding paste" from the outside environment the holes continue to get larger, usually taking on an oblong shape. This, of course, results in a load of lateral deflection when cornering. At some point the outer poly gets loose enough that it starts moving around in the arm itself. After a while this begins to enlarge the hole in the arm-not good.
It is a very, very silly design, and I've removed dozens of sets that have done just what I've described.
As I see it, the alternatives are stock bushings, hard rubber bushings (I don't know if any are currently made, VW motorsport used to have them), or the Shine Racing kit which puts teflon lined spherical bearings in place of the bushings. They actually ride better than poly bushing, although they do transmit more road noise than stock rubber ones. They also last virtually forever.


And, for those of you who'd like to see the discussion in its original context, here's the link:
http://forums.speedarena.com/z...27049
Once again, yours truly has no views on this subject (I can tell you why I hate nylatron/delrin/poly bushings on a 1974 MGB vintage racer, but that experience so scarred me that I've never tried anything like that on any car ever since -- hence I don't know what they're like on a VW!) -- here, I'm just a reposter!

Cheers everyone,
-C
 

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Re: Why (Rear) Poly Bushings Are Bad (Ceilidh)

Ceilidh for president!

I have a question about hard rubber bushings like VWMS bushings. Do they still rotate in sheer like soft rubber or spin like poly? If they do behave like soft rubber bushings, their sheering will generate a larger torque than a soft rubber bushing that tries to resist the suspension movement. Is this torque strong enough to not allow full droop like you quoted above for the poly bushings? Just curious if VWMS bushings and other hard rubber bushings have any downfalls (excluding comfort of course).
Lastly, does anyone know if VWMS bushings are still available for MK4 front control arms (last I heard was that they were made for the MK3 but also fit the MK4)?
 

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Re: Why (Rear) Poly Bushings Are Bad (Ceilidh)

do you remember the article where one guy desicribed how to make polyeurathane engine mounts using lowes polyeurathane I want to make them but I can't seem to remember where the article was or who created it thanks
 

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Discussion Starter · #4 ·
Re: Why (groftja)

Quote, originally posted by groftja »
.....I have a question about hard rubber bushings like VWMS bushings. Do they still rotate in sheer like soft rubber or spin like poly? If they do behave like soft rubber bushings, their sheering will generate a larger torque than a soft rubber bushing that tries to resist the suspension movement. Is this torque strong enough to not allow full droop like you quoted above for the poly bushings? Just curious if VWMS bushings and other hard rubber bushings have any downfalls (excluding comfort of course).......

Sorry groftja,
I really don't have any experience with VW bushings. In general terms, all the rubber bushings I'm aware of (or have worked with) rely on torsional shear (among other things, the rubber is "sticky" and can't slip in the way that the poly and delrin bushings are intended to). As for not allowing full droop: again, I know nothing of the VWMS items, but in general rubber bushings can support some load if they're torqued down in the wrong position (a classic error is to tighten the bolts while the car's still up on a lift), which will affect ride height....but the bigger concern there is usually premature breakdown of the rubber if they're "preloaded" in that fashion.
Hopefully someone who's used the VWMS bushings can tell you more!
- C
 

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Re: Why (Ceilidh)

This subject is of intense interest to me as I am about to install the FULL poly rear beam bushings in my Mk2. I understand the points about the poly bushing rotating about the metal pin and causing a lot of friction, however I can't help thinking that with the right kind of lube, and possibly regular grease applications (not that hard a job as I see) this would prevent the kind of friction being described.
Something else that has been raised before is the fact that because the action of the torsion beam necessarily causes some twist in the bushing, the poly bushings will eventually start rubbing and binding against the bracket in which they are located. Again, I think to myself that careful greasing might help prevent this.
I've heard word of mouth testimony that the full poly units are really not as bad as Shine and others make them out to be, and quite frankly, when they cost about $35-40 US compared to the $275 or whatever for spherical units, I'm going to give them a try. Stock rubber bushings are just not an option for me anymore- my car wags its tail like a hyperactive German Shepherd.
Picture a modern performance motorcycle with soft-ass rubber swingarm bushings.....you could not ride the thing due to the inevitable wagging motion once you started putting it through the corners. I know the bike swingarm is different than our trailing arms due to the fact that it moves as one piece only, not two independent wheels one at the end of each arm, but honestly, I have aftermarket springs and a 28mm rear bar....I doubt the range of motion is SO large that the bushings will see a great deal of twisting.
Time will tell I guess- I'm prepared to be wrong
 

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Re: Why (Rear) Poly Bushings Are Bad (Ceilidh)

I have used the vwms rear beam and LCA bushings in corrados. They were cast from the same molds as the OEM bushings using 80 shore rubber. As of a year ago, no more mk2/mk3/corrado front pivot LCA or rear beam bushings could be located. You also had to fully trust your source as the oem part #'s were in fact cast into these bushings as well!
Robert
 

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Re: Why (Rear) Poly Bushings Are Bad (Ceilidh)

Hope this helps some ....
The bushings are actually missing from the model on purpose, so it is easier to see how the outer and inner parts move in relation to each other.
This is just a concept, sort of work-in-progress. The real rear suspension is not exactly as seen here, but the geometry we have now is quite right for this exercise.
Note: The animation is very heavy, so wait until it loads well enough to be smooth and fast.
This is 5 degree roll to the right and then 5 degree roll to the left, for a total of 10 degrees, which is kind of the maximum on street tires, if not two-wheeling.
 

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One Wheel Bump ...

I guess the rolling scenario is not so convincing, so here is a "one-wheel-bump" scenario in which the "body" stays firm, only the right arm moves up and down as if we would have a one-wheel bump. Guess it is easier to catch the bushing movement now.....
Note: Wait until the animation loads to full speed.
 

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Peter, it would be easier for us to see how the inner and outer parts interact on your first example´s animation if you were to keep the "body" (red bar) still and sway the torsion beam up and down, instead of the other way around. That´s precisely why it´s easier for anyone to see how the bushing moves in the second example, you´ve got a steady point from which you can accompany the beam´s support position. That´s if what I propose is at all possible, of course.
Cheers
 

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One Wheel Bump - Zoom In

Zoom-in here, so it is easier to see the bushing's movements....

On the first animation it is not so easy to see those things, as the purpose of that piece was not related entirely to this topic. It was built to observe the camber and toe changes during roll.
 

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Re: One Wheel Bump - Zoom In (pyce)

Quote, originally posted by pyce »

On the first animation it is not so easy to see those things, as the purpose of that piece was not related entirely to this topic. It was built to observe the camber and toe changes during roll.

Nice diagrams Peter! Solidworks?

If I am looking at this the right way, then the outside-rear tire goes toward positive camber while the inside-rear goes toward negative camber?
This assumes that there is a compressible bushing in there, correct? Does a racing bearing (like Shine) constrain the motion of the trailing arm to just a simple rotation around an axis which parallel to the stub axle, rather than the offset rotation we see here?
Can you make a diagram of the same rear-end with the shine bearings or equivalent?


Modified by phatvw at 2:15 PM 8-15-2005
 

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Re: One Wheel Bump - Zoom In (phatvw)

Quote, originally posted by phatvw »
If I am looking at this the right way, then the outside rear-tire goes toward positive camber while the inside-rear goes toward negative camber?

Correct. If you observe carefully, you could even see the toe changes. Those are more difficult to see as the perspective is strong enough to skew them, but they are there.
Solid Works can not twist in the way needed for this, and its Dynamic Works package does not simulate rubber bushings, so it is all done manually in a separate package that allows global deformation.



Modified by pyce at 2:28 PM 8-15-2005
 

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Re: One Wheel Bump - Zoom In (pyce)

Thank you SO much for making these models visible. While I understand the limitations of poly given the nature of the torsion beam's range of motion, I think I'm going to try them anyways, and if the bushings get chewed up or the suspension action ends up being wonky, I'll just replace them.
 

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Re: One Wheel Bump - Zoom In (Mr Black)

Quote, originally posted by Mr Black »
Thank you SO much for making these models visible. While I understand the limitations of poly given the nature of the torsion beam's range of motion, I think I'm going to try them anyways, and if the bushings get chewed up or the suspension action ends up being wonky, I'll just replace them.
You can show the people the water , but you cant make them drink it .
Great job peter http://****************.com/smile/emthup.gif Bob.G
 

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Re: One Wheel Bump - Zoom In (rracerguy717)

Mistakes are very important part of any learning process. Real life experience is something no text or animation can replace.
 

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Re: One Wheel Bump - Zoom In (pyce)

Quote, originally posted by pyce »
Mistakes are very important part of any learning process. Real life experience is something no text or animation can replace.
Wow. Pyce just described my entire life.

Excellent work on animation.
 

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Re: One Wheel Bump - Zoom In (rracerguy717)

Quote, originally posted by rracerguy717 »
You can show the people the water , but you cant make them drink it .

Ya ya, I know, the only reason I'm willing to do it is because I have had other people (who seemed inteligent and credible) report that there is a tangible improvement without any of the dreaded bind and destruction that theoretically should happen with a poly bushing.
Hey if they suck, I'll provide photo proof and testimony and then the matter can be sealed forever
 

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Re: One Wheel Bump - Zoom In (Mr Black)

Why don't you apply the appropriate stiffness and damping values to the constrained connections? It appears to me the the bushings mount at the OD is not constrained to be concentric with the pin. It appears to be constrained as a shpereical joint, when its intention is to be purely rotary. I am aware the bushing has a stiffness's in this direction, but it is quite high especially with poly. The stiffness's in the rotational direction is close to zero, therefore the bushings job is to convert most of those conical and neutating forces into more rotational displacement. This is the bushings intended purpose, it holds the components in a certain geometric locations, and resists deflection in certain directions. It is the reactionary force applied by the bushing the bends the axle. In your animation it appears the tail is wagging the dog.
It is also important to note the magnitude relationship between the stiffness's' of the torsion axle vs the bushing. For example, why are the bushings tilted at an angle? Im going out on a limb here but i would bet that upon full calculation of Von Meiss' combined stresses on the axle, with one wheel displaced, that the pin is oriented so that the forces applied to the bushing are as close to pure moment as possible.
The solid-works is sweet BTW.


Modified by Vr6Fidelity at 6:02 PM 8-16-2005
 

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Discussion Starter · #20 ·
Re: One Wheel Bump - Zoom In (Vr6Fidelity)

Hello Vr6,
Excellent points and questions! You've hit the nail squarely on the head, in highlighting a basic problem for those attempting to use poly bushings in the main trailing arm bushings: as you point out, the poly "tries" to convert the general forces into rotational displacement, whereas the geometry prevents such simple rotation. The result is extreme binding (see quotations in first post) and rapid degradation in field use.
If you don't mind, we''ll use the text of your post as a framework for addressing these issues, as the discussion might be of interest to the non-engineers following this thread. We'll start from the top:

Quote, originally posted by Vr6Fidelity »
Why don't you apply the appropriate stiffness and damping values to the constrained connections? It appears to me the the bushings mount at the OD is not constrained to be concentric with the pin. It appears to be constrained as a shpereical joint, when its intention is to be purely rotary.....

Absolutely correct! The bushings are constrained as a spherical joint, and the cylindrical shapes are drawn in to demonstrate the distortion that a cylindrical bushing must undergo in normal use. Track-based Golf/Jetta racers successfully use spherical joints in the main trailing arm pivots, and the OEM vehicle employs large rubber bushings that can easily accommodate the deformation -- and both bushing types (OEM rubber and track-based spherical) work well in the real world. Poly, on the other hand, has some problems...
Quote »

..... I am aware the bushing has a stiffness's in this direction, but it is quite high especially with poly. The stiffness's in the rotational direction is close to zero, therefore the bushings job is to convert most of those conical and neutating forces into more rotational displacement. This is the bushings intended purpose, it holds the components in a certain geometric locations, and resists deflection in certain directions.....

And herein is the primary problem with poly (in this particular application): unlike OEM rubber, the poly has "quite high" compressional stiffness (i.e., it doesn't like to be squeezed!); and unlike spherical, it readily allows rotation only along a single axis -- which in this situation is not aligned with the (changing) axis about which the suspension wants to rotate. The result is extreme binding: whereas the OEM and spherical bushings allow the trailing arms to move as they are geometrically constrained, the poly tries to force them to move in a direction at odds with the basic geometry. When that happens, something has to give....
Quote »

....It is the reactionary force applied by the bushing the bends the axle. In your animation it appears the tail is wagging the dog.

I'm not sure that Peter would quite describe this model as one of the tail wagging the dog(!), but if we were to keep with this imagery, we have the body of a chihuahua connected to the tail of brontosaurus.

On one of Peter's modeling experiments (unposted), he wanted to see what what happen if he had the bushings "force" a simple rotational motion. The result is exactly what Vr6Fidelity predicts: if the trailing arms are to move at all -- that is, if the suspension is to do what suspensions are supposed to do: let the wheels move up and down -- the beam (i.e., the "twistbeam" connecting the two trailing arms) must bend. In fact, it has to bend a lot. So is this what happens?
Now, all those of you who have ever hefted a VW rear twist beam will know that it's a substantial piece of metal. Moreover, it's a substantial piece of metal arranged as a large-dimension "C" channel -- one that readily twists, but which strongly resists bending along any plane. When this roughly 4-foot giant crowbar meets up with a puny poly bushing, the result is pretty much what one would expect: the crowbar remains essentially straight (one can imagine it uttering a scornful laugh), and the bushing cries uncle.
Sorry, got carried away with the imagery there.
More prosaically, for suspension movement to occur with a poly bushing (or any bushing that tries to constrain the Golf/Jetta trailing arm to move in a single plane), either the bushing has to compress, or the twist beam has to bend. If push comes to shove, the beam is going to win (hands down), but in practice the bushing gets out of its quandary by refusing to play the game: because it resists compression, and because the beam won't significantly bend, the suspension in effect locks up and refuses to move. That is why the person quoted in the first post reported that his wheels would not drop when he jacked up his poly-fitted car, and that is why poly bushings (in this suspension) pivot yields such a rock-hard ride: it's not only that the poly transmits more impact harshness than does OEM rubber (note that the poly is reported to ride more poorly than do spherical bearings, which absorb NO impact harshness whatsoever), but more critically, the poly bushings don't allow the wheels to deflect upwards when encountering road bumps.
Now, lest anyone feel attracted to the "stiff, race-car!" implications of a suspension that won't, well, suspend, please reread the second quotation in the first post: under repeated impacts, with the poly bushings fighting the twist beam, eventually (reportedly fairly soon) the poly loses: it begins to break down, passing through an interval where it has degraded enough that it kind of sort of approximates what the OEM rubber bushing does for 100k miles, and then passes onwards into a noisy, clanky, loose-fitting mess.
Quote »
....It is also important to note the magnitude relationship between the stiffness's' of the torsion axle vs the bushing. For example, why are the bushings tilted at an angle? Im going out on a limb here but i would bet that upon full calculation of Von Meiss' combined stresses on the axle, with one wheel displaced, that the pin is oriented so that the forces applied to the bushing are as close to pure moment as possible.

As just noted, a C-section torsion axle is pretty darn stiff under bending loads! But that leads to the next question cited above (and it's a good one): just why, oh why, did VW orient the bushings at such an odd angle?
Well, who knows?
None of us works for VW, and yours truly is just going on Peter's Solidworks models. But we don't need a full stress calculation to see whether it's primarily to align the rotation on one-wheel bump: if that were the underlying reason, we would next have to ask how VW managed to ignore 2-wheel bumps! (Maybe sometime Peter could post up an animation showing how the bushings move on a 2-wheel bump, but for now, one can take a look at the hinges on an ordinary household door : there are typically three, all of them in line, and it should be evident that were one to unbolt one of the hinges and remount it at an angle, then the hinge would tear off the moment the door is swung open). No, the angle of the bushings seems to be associated with something else entirely:
If I (Ceilidh) had to guess, the bushing angle is related to three separate observations:
1) As has been oft observed, the OEM trailing arm bushings are very, very large -- surprisingly large, in fact. Its size allows it to accommodate twisting motions (as discussed above), true, and it might permit better absorption of vibration and impact -- but it also allows significant distortion along the pivot axis. That is, there's sideways "slop" along the bushing, which is forever driving the autocrossers to distraction, and VW seems to have made no attempt at restricting it....

2) As has been oft reported, replacing those bushings with metal spherical bearings leads not only to better handling precision, but seemingly better neutrality and less understeer. That is, Vortexers who switch to spherical (if I remember the posts correctly) often report reduced understeer -- even when springs, shocks, etc. remain unchanged....
3) When the Golf/Jetta IV came out, VW made a bit of fuss about a "Track Correcting Axle", which nobody seems to have understood (has anyone seen an explanation of it? If so, do please post it up!).

In any case, putting #1,2,3 together with Peter's model makes for an interesting observation:
A) Take a look at the red bar in one of the animations. The red bar is a stand in for the chassis of the car. Imagine what happens in a left-hand corner: centrifugal force pushes the red bar towards the right (or, if your physics teachers spent a lot of time convincing you in your youth that centrifugal force doesn't exist (it does in a non-inertial frame, but that's another discussion!!), you can view it as the silver axle assembly being pushed to the left, to effect the centripetal acceleration....).
B) With spherical bearings, the bushings resist the lateral force. But with the large OEM rubber bushings -- which can distort along-axis -- the red bar (i.e., the chassis) can shift to the right, relative to the axle assembly. (Or alternatively, the axle assembly shifts to the left, relative to the chassis.)
C) If the bushings were not angled, but instead simply sat in a simple transverse alignment, the axle would shift sideways, and (to a gross approximation) there wouldn't be much to write home about. But with the angled bushings, look what happens:
D) As the right hand axle bushing shifts leftwards, it also (because of the angled axis) shifts forwards. Similarly, the left hand bushing shifts leftwards and rearwards.
E) The result is an axle assembly that steers the rear wheels towards the inside of a hard corner: that is, the rear wheels turn in a direction that promotes understeer. For a road car, this is a very nice thing! Road car designers (as discussed at length in last year's Shine/Koni/Bilstein thread) are forever wary of paying customers spinning off the road and over high cliffs, and this cornering-induced understeer is a handy tool: with it, you can give the car a little more neutrality at low g-forces (which makes for sharper transitional handling and a nicer balance at normal street speeds), whilst maintaining enough stabilizing understeer under extremis to keep untrained drivers from spinning off.
F) Replace the OEM bushings with spherical, and you don't get the shift -- ergo, less understeer at the limit.
Whoops -- have to run!! Perhaps more next week...Vr6, thanks for the insightful post, and please everybody have a great week. Cheers!
- C



Modified by Ceilidh at 7:52 PM 8-16-2005
 
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