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