A Primer on “Stops” Mod, Bushing & A-arm Theory

[Lago Blue]

Suspension of Disbelief, Part 1.d. cont'd.

2.) Where: Along the Front Sub-Frame to A-arm Hinge-Axis.

1. Because a good deal of the car’s driving character is riding on these most heavily loaded hinges, and that their continued best operation is essential to several other better than stock, mod outcomes; this will discuss stock hinge-axis construction, operation, failure and some of its’ effects. This contends, notwithstanding that the engine and trans-axle do over-hang and heavily burden these hinges, but rather that instead, early bushing failure simply results from the production design of their installation alone, and which actually allows them to be over-loaded by over-travel; and from an entirely different direction.

2. Two of the necessary three fixed chassis points in the tripod required to suspend the massive partly spinning wheel/brake/hub assembly in a front fender-well are the bushings which form the subject hinge. Acting together, their not insignificant job includes responsibility for two-thirds of all:

a) Fixing of the wheel position in 3-dimensional space;
b) Guiding of vertical wheel travel; &
c) Transmitting loads to and from the tire’s contact patch.

Considering these critical tasks, any play along this hinge axis is, in my view, unacceptable. This argues the stock A-arm installation can be economically improved without subtracting from the original design, but by adding to it, netting benefits in all three above areas and more. Across these two hinges, the majority of all thru-puts to and from the tires (acceleration, steering, cornering and de-acceleration/braking) take place. They should therefore, be as good as any found elsewhere, yet typically even the driver’s door hinge has less play. This hinge should approach the operating properties of the ball-joint. Otherwise what is the point of a strut-pivot so precise at the apex of the A-arm, if held at the base by a hinge so loose? The mod is an attempt to elevate them towards that goal.

3. Driving the car will be more enjoyable and stay that way longer if the mod is installed when the hinge bushes are replaced. The "Stops” mod, by altering the way the original A-arm installation functions, will prevent both early bushing failure and A-arm slide-travel. That this approach, this old but up-dated idea is perhaps still valid, may be borne out by a few observations as follows:

a) Despite even all the last model changes in the 1994 re-think that begat the RS2, the original bushings remained to carry approximately 30% more torque than previously;
b) Even today, few marques have moved away from these metalastic type bushings to any of the now several alternatives. For instance, as one does not see suspension originally hinged on poly bushes on either high-production street or purpose-built race vehicles, a question might be why has neither field embraced them?
c) While polyurethane is seen in production vehicles as bump-stops, motor-mounts and anti-roll bar link bushes, it is not seen in tasks involving rotation. In fact some poly bushing sets for our A-arms contain thrust-washers (similar in location to the subject mod’s “Stop”-washers) made of a material that is not polyurethane.

I believe the above illustrates learned folk consider the OE metalastic type of bushing still to have some merit here; that the Boges in particular, at least while still relatively new during the Porsche build program, must have been deemed adequate and that complete substitution of the OE bushing with other materials and construction may not offer the benefits the Boges plus the mod do. The issue here is ours become far less capable too quickly. The “Stops” mod aims to add durability and precision to the way the A-arm is hinged to the car, preserving the “new’ in new bushings by significantly reducing their core’s loading.

4. Note below, the contrast between how well the hinge-axis bushings are aided and supported in handling these first two tasks vs. the third:

a) Primary Arc-Travel. During vertical suspension movement, they provide a small (approx. max. 20 degree) arc of rotation. For illustrative purposes this arc is less than three and a half minutes of clock-face minute-hand sweep, a small zero-friction movement. The rate of steel bushing shell rotation relative to its core is restrained by the Mac. strut main-spring and damper. These limit the speed and displacement of rotational force that would otherwise be felt by the bush cores alone and ensure they operate inside their elastic limit. This construction will with the mod, allow the bushings to function undiminished indefinitely. Factory proper bushing installation ensures the cores are at rest with full weight on wheels.

b) Cornering. New OE bushing function is also quite strong laterally (i.e. 0.7G cornering) because there is a mid-core steel tubular support to spread the load evenly, and the steel shell directly in contact surrounding the outer rubber core to contain and back it up. Any outer shell rotation serves to further tighten the rubber core, re-centering and resisting further displacement of the bushes' metal center. Also in part because there is so little movement in what is almost a pure lateral load, (average load <200lbs. lateral / bush @ 0.7G), it is working almost solely against this, the bushing’s strongest flank. This construction will also with the mod, allow the bushing to withstand lateral loads undiminished indefinitely. Bushing core is at rest laterally when the car is not cornering.

c) Torque Transfer. Whereas here (during acceleration, de-acceleration and braking) the production design of the installed A-arm (I believe only by default) allows this task, by far (just by looking at the range of possible fore/aft bush distortion) the largest task of these three, to fall to the core, where it should only see the first two loads above and in my view never need be loaded axially. The production design puts every axial (fore/aft) thru-put, including loads sized similar to cornering (i.e. 0.7G braking) onto the bushing cores alone. There is nothing deficient about the bushings, it is because the stock load-path briefly stops at the bushing cores, and alone, axial resistance is the bushings weakest flank. Without even looking further inside the bushing, any suspension whose fore/aft fixing depends on the A-arm first moving fore or aft from its design position; second, waiting for said travel to take place; and third, the A-arm then striking slide-hammer like, the chassis bracket to which it is already fastened; must be suspect. The bushing’s integrity cannot withstand the near constant fore/aft thrust and displacement without damage, they’re only rest seen briefly during thrust-reversals and vehicle coasting. Not surprisingly, they soon degrade, less able each time to restrain A-arm travel or return it to the fore/aft “on-center” position. The A-arms are then freer with each occasion, to wander where they are pushed to. Consider that with the mod installed, this third task is no longer asked of the bushing’s cores to perform.

That in the first two tasks described above, loads are moderate and carefully taken care of with continuous load-paths from tire to chassis, whereas the last is not, (note how the axial thrust load-path stops at the bushing until the A-arm moves), serves to confirm what the voids bracketing the installed A-arm already suggest; that a deletion or omission must have occurred between the drafting-table and the production-line. This could not have been a conscious engineering decision. Early and total bush failure can be the only expected result.

5. However, even earlier on and perhaps unnoticed, the sinewy response of new bushings is lost. Well before even more visually obvious failure, fatigued bushings may allow a surprisingly large fraction of an inch in also oft undetected range of fore/aft A-arm travel, where this contends there ought to be none. If like play was seen in the nearby ball-joint each of us would replace that part immediately. This argues allowing A-arm slide-travel to develop at all in the first place is the root cause of our early bush failure.

6. WRT hinge-axis failure’s effects, what follows are two perhaps less well known examples of the possible divergent down-stream effects of this failure beyond the more obvious front-end geometry changes, variation and instability.

a) Rear-Steer. This note is regarding the older cars with the reverse-framed front-style suspension; on the rear. The same slide-travel develops here and manifests as requiring corrective steering particularly during both high-speed straight-line driving; and when de-accelerating towards a corner. This occurs in part because any alignment change or variability that occurs back here tends to be magnified, due to its greater distance (and therefore a longer lever-arm) from both the center of pressure and the C of G of the vehicle. Because the mod puts a slight compressive pressure on the re-installed A-arm bushing flanges, the arms are held more steadily than stock brand new; and the back-end trails more solidly.

b) Slack-Time. Because these hinges position the hubs, front to rear A-arm slide-travel that develops here translates directly into slack in the drive-line. Briefly, (e.g. between pulling the car forward; and being pushed by the car) the hubs are effectively briefly unharnessed. This mechanical slack becomes a delay interval which must be taken-up before each and every torque thru-put (either braking or acceleration) can take place. The older cars will display this caterpillar-like behavior at all four corners. Every launch from rest, gear-shift, throttle reversal, and brake application has to re-traverse this space, the drive-line equivalent of turbo-lag, before the car is once again either shoved or slowed by the shuttling A-arms and the huge spinning gyroscopic wheel/tire/brake/hub/CV assembly traveling with it.

As previously “Lift-pause-thump, press-pause-thump.”

While slack will certainly be found elsewhere, this slack is another reason to do the mod. During these, granted brief intervals not a great deal takes place, no torque is transmitted, the wheels are in effect de-coupled, rolling slightly forward or back without the chassis, their fore/aft connection briefly absent until the A-arms again collide with the sub-frame brackets. Mostly annoying, these periods of contact-patch unloading/zero torque/re-loading aren’t likely helpful particularly when traction conditions are marginal. However, under all conditions they continue to dull the car’s response to commands by effectively putting play in your throttling, clutching and braking; and are just an embarrassingly primitive method to both propel and stop the car. Torque may be momentarily shunted elsewhere, but with the mod, need not be. Regardless, any time the hubs are 'travelling', the car is not accelerating, de-accelerating nor braking with those wheels, activities we occasionally try to hasten. This argues said drive-line slack is yet another preventable production design behavior whose elimination yields a better driving experience.

“Stops” make the going, better.

Link to George Bush Jr. era attempt at humour sure to offend someone:
http://www.audiworld.com/forums/audi.../#post18606491

[purplelion77]

I must say I thoroughly enjoy reading this thread, especially as I believe the suspension is Audi's B4 Achilles' heel since it does neither comfortable nor sporty well. I do have my reservations though.

I agree that steering feel is a joke at least on my Audi 80 Avant compared to other cars I've driven. But isn't the fore-aft movement of the wishbone supposed to improve ride comfort? I thought it allows a bit of play when the wheel hits the bump or the pothole in order to absorb some of the energy. So in a way this fore-aft movement is engineered into the suspension in order to soften the thump when the wheel hits the bump head-on. Wouldn't the modification you're proposing have negative effect on ride comfort?

[Lago Blue]

Purplelion77,

1.) Thanks, very good questions and initially, I thought as you. I would even go further re Achilles' heel and say that likely the number one reason folks give up on these cars is they are disappointed with the performance of the suspension. Which is sad given the long history of racing success these cars have had, the suspension would have to have been excellent to not wear out the occupants over some of those long races.

2.) Often the emphasis of so-called (after-market) performance is more towards large amounts of torque production whereas its' intelligent application and guidance closer to the ground usually gets less attention than the perceived necessity to lower the car for perhaps dubious technical or aesthetic reasons. We can absolutely provide ourselves very satisfying performance driving characteristics with only modest engine power and near stock suspension. Certainly this platform has an outstanding ability to distribute torque to the four corners, we should but best then try to use it. I suppose my point with these articles is that the near stock suspension is in fact not fully understood, really very good and certainly deserves our attention if only as a logical starting point. IMHO my two (a B3 & a B4) are suspension-wise at speed quite taught, silent and comfortable. The steering in each is actually one of their best features.

3.) There are some really good ideas steering-wise percolating thru the forum here, but a clean, inspect & "return to base conditions" approach may be where to begin. Topic for another day perhaps.

4.) To your question "But isn't the fore-aft movement of the wishbone supposed to improve ride comfort?"

a) I just don't believe it is supposed to move fore-aft at all, for any reason; period. Would even more aft movement provide an even greater "improvement"? I don't believe so. There cannot be any comfort added by any delay in sending the impact's force to the energy-absorbing equipment, particularly if during this delay, we allow this force to hammer the front of the foot-well area.
b) As it is by the wheels alone we are being pulled ahead, we must encourage them directly up, over & down.
c) Where would the force come from to return the wheel back into position? Because having left, we do have to get back there somehow, and quickly. Further, if the bushing no longer knows where the center-point is, where are we going back to? Besides what about the next bump and the one immediately after that, is the wheel back yet? Do you see what I mean? If the car is travelling forward at speed, hits a bump and we want the wheel to slow down at all, for every amount we slow it down, we must also then have to speed it up later so that it can catch back up to us. This just delays the inevitable. What we best need is to get over and past the obstacle ASAP, otherwise chaos ensues.
d) I believe the tire and the OEM installed strut are the only correct tools to provide what you are asking for. Our comfort here is mostly an up/down thing, not something bushes can deliver alone in any case, the rubber & air in the tire, the well damped spring in the strut, and an "as new" top-mount can, and for that we need perfectly intact bushes that hinge upward immediately really well, precisely because they haven't been torn apart by doing the fore/aft travel thing.
e) I also thought that by providing a more solid load-path for such horizontal impact loads to feed directly through to the chassis might possibly be both noisier and feel-able as NVH. That this has not occurred, is for perhaps a couple of reasons:
i) Any noise that used to take place when the rear-ward sliding tire/wheel/disc/caliper/hub/CV/et al hit the sub-frame bracket no longer takes place since the A-arm which positions all that is already in full contact with the sub-frame prior;
ii) The impacts' force is now more quickly directed strictly up-wards into the tire and strut, items actually sized to deal with such loads rather than waiting for that huge spinning assembly to bounce forward again off the sub-frame bracket before then finally continuing upwards; &
iii) As to ride comfort, I have read that fore/aft impacts are quite detrimental to occupant comfort, however with the mod, impacts are actually felt less as fore/aft and more as bumps as perhaps they should be.

5.) To your point "allows a bit of play when the wheel hits the bump or the pothole in order to absorb some of the energy"

It does allow play, but play cannot absorb meaningful energy in this small distance and therein lies several issues as follows:
a) We want the tire to both get back to the ground and its'"home" position ASAP, so allowing it to be forced backwards, then watching it bounce forwards, then up into the spring, then down is just a lot farther (and slower!) than immediately up & back down directly; and granted as energy is lost in each of those changes in direction, it is lost in the abrupt "corners" of that stock travel path to the chassis as noise and reactive force as opposed to the relative soft transfer to us finally via the strut-mount (if still serviceable!) in the mod-ed path.
b) The bush cores and the thin rubber-faced flange of the bush (the only compliant parts in the stock fore/aft A-arm load-path) can in no way be expected to "absorb" any significant amount of energy compared to say the Mac. strut spring and damper (that is why they are so huge, so they can comfortably turn that impact into a softer push and some heat); &
c) This "bit of play" is the root of our early bush failure as per my write-up above.

6.) To your question "Wouldn't the modification you're proposing have negative effect on ride comfort?"

It certainly has effects plural but none I've observed that are negative nor that degrade ride comfort. I should say it is actually quieter. I believe this is due to in part as follows:
a) Noise: is reduced as other than the unavoidable tire-strike, there is no further horizontal impact seen (save for the rear tire strike) by the vehicle; &
b.) Vibration & Harshness: Again as the remaining impact is more quickly directed from the wheel into the strut, (the only equipment with a hope of absorbing much of any impacts other than previously; us) the impact is more fully absorbed by said spring than previously.

Finally, though less scientific I think that with the mod, there is somehow a perceivable sense (more than just that there is less noise) that there is just less drama (less force transmitted to the cabin) during these very brief events; and that it may be with just less rapid, frequent and what must be violent changes in direction and speed of the weighty whirling wheel/tire/etc. assembly [as per 5.) a) above] taking place contributes to this perception of calm; relative to previously.

[Lago Blue]

Suspension of Disbelief, Part 1.d.
3.) When: Your Inexorable Next Front-Type A-arm Removal & Re-installation (and 7 reasons it will need doing soon).

This section will (under the pretext of some gentle prodding) expand some ideas previously discussed and intro some terms to describe yet more bushing and A-arm theory WRT early failure’s causes and bush failure from inside the bush itself.

Whether a hastened forced repair or something planned and more relaxed, in either case on the occasion of your next A-arm hinge-axis repair, which is approaching, you may want to have the “Stops” on hand to alter the fate of your next set of genuine Boges. By not postponing the inevitable too long, this task can be a satisfying money and maintenance-saving exercise, particularly for D-I-Y-ers. How long to postpone? Perhaps only as long as it takes to get ready. Unlike strut-mounts handy under-hood indicators, the bushings also lack any similar ability to capture the zeitgeist. Know that they do however share this characteristic, they accelerate towards total failure as they approach it, as the damage threshold is beaten down by increasing numbers of newly eligible loads. So while you may be considering your location on that curve…
Here are 7 reasons why your current bushes are shot:

1) Early Failure & On-Axis Vulnerability.
Early Failure: An inevitable characteristic process of the A-arm installation’s stock production design whereby a too rapid decline in hinge-axis performance takes place, the primary symptom of which is its’ decreased ability to resist fore/aft deflection.

On-Axis Vulnerability: That this is the particular avenue by which early failure begins; and that the hinge-bushings succumb to loads felt along the hinge-axis, is the reason these loads & this axis are being called out separately.

2) Other Loads.
Twist & Lateral Loads: These two (IMO), on-axis twist from bumps and lateral loads from cornering are the only loads the bushing cores should ever see, are (given factory correct installation and a serviceable strut-mount to work with) by themselves of little consequence to bushing longevity.

Off-Axis Loads: These can be quite powerful loads which, although acting diagonally to the hinge-axis (during very slow & zero-speed steering) can deflect the A-arms fore and aft sufficiently, that the hinge thru-put limit is exceeded.

Combination Loads: These are concurrent loads of different types which may combine and despite each being only 2nd order loads, together they can also push the total thru-put over the limit.

3) 1st & 2nd Order Loads. Loads simply divided into those that can (1st order) & cannot (2nd order) immediately cause damage. Initially with new bushes, only a few peak impact-loads (such as a pot-hole taken at sufficient speed) are capable of causing damage alone, however each that does, lowers the damage threshold & eventually all loads cause damage.

4) Continuous Load Paths. Lacking in the stock production design is sufficient load-bearing structure to provide either fore/aft A-arm location fixing, or an unbroken route to allow transfer & distribution of loads from one component to the next, from tire contact-patch to chassis. Such paths must consist of components of suitable size and material, in immediate and constant contact, shoulder to shoulder to effect a near solid and reliable connection. Because the production design of the hinge-axis fore/aft load path is neither strong nor continuous, the certainty of the hinge-bushings early failure can be the only expected result.

5) Thru-put Limit & Headroom.
Thru-put Limit: The instantaneous threshold beyond which further loading causes permanent hinge-axis damage.
Headroom: Space under the above limit curve within which the hinge can accommodate all anticipated loads, maintain a safety margin and operate without damage. That the thru-put limit is being exceeded and that there is insufficient headroom in the stock production design is evidenced by early failure’s occurrence.

6) Bush & Mount Co-dependency; & Asymmetric Failures.
Bush & Mount Co-dependency: Three points are used to fix each wheel assembly to the chassis because two won’t work. Two and a half won’t work properly either. All three are only sufficiently strong enough when they are all perfectly serviceable, as they rely on each other for mutual support and alignment to effect that original steadfastness in fixing of each strut in its' exact design position.

Asymmetric Failures: Even in the absence of a pot-hole strike or similar, if left stock, one bushing will begin to fail first. One can often see evidence of this on removal. It is often the aft-most bush on an A-arm. Weakness in any one bush or mount, must then accelerate the decline in the others, since the total load input remains, but its’ exit through the damaged mount is shifted to the now lessened total sound structure remaining, loading it more than previously.

Similarly, both the other end of the strut and the strut on the other side see more stress once any mount or bush failure has begun. These load-shifts may effect alignment, drive-ability in terms of the quality of the steering and braking feel; and hasten early failure in the remaining components. This is why if left stock, all these bushes should be replaced together. However installing modded ones, they are so much more robust that they could withstand being replaced just in pairs.

7) Preserving the Original Fully Functional Integrity of the Mounts' Internal Bonded Construction.
With this unwieldy term I'm trying to capture what the physical characteristic is (which is quickly lost from stock mounts) that IMHO, gives us the performance characteristic we should best want to maintain in a metal-lastic bushing employed here.

By means of a careful read-through, of the factory instructions for bushing installation into the arms, one will glean that they should not be forced into place by pressing on the bushing cores, but solely by pressing on the outer shells, otherwise the bushings will be weakened before they are even finished being installed and put to use; and yet once installed, this is precisely what takes place in the course of everyday driving; and which immediately begins their early destruction.

If we are seeking longevity, we must act to better preserve the integrity of the original as-new bushing core's internal bonded steel to rubber interfaces. The bushing is acted upon by energy inputs which result from motion it must allow (rotative), and forces it must resist (compression and elongation), all of which result in heating of the bush core, depending on both the load frequency and pressure.

The energy throughput that the bushing can withstand and still remain undamaged is largely governed by its volume and ability to shed this heat. My theory is that early bushing failure here is indirectly the result of on-axis (i.e. fore/aft) over-travel, but the direct the result of bonding interface and rubber core over-heating. I say this because so many I've removed are either totally un-bonded from their shells and/or metal cores and otherwise in near perfect shape or at least partially un-bonded and partly torn. I view each as a very similarly caused result, both of which could perhaps be explained by bonding failure that is either full; or partial where the then reduced rubber cross-section is suddenly much weaker and very locally fails by ripping the remaining material.

We cannot change the bush (save for using forged arms which allows use of the larger bushes) or easily change its' cooling rate, but we can buy the best bush possible and we can with the Arm-Stops mod, significantly change the job the bushes have to do.

By reducing their work-load in one direction, we can gain capacity elsewhere. That shifted capacity is what I believe gives them the greatly extended service-life I have seen with this mod. We can lower the workload below the energy thru-put limit to the extent that the more typical disintegration of the bushing construction is delayed perhaps indefinitely, as it maybe we even establish some headroom below that limit as described above. Preserving the core construction allows the entire bush to best respond to thru-puts as a single cohesive unit.

Put most simply, of the several loads described above, the mod eliminates the on-axis loads, the largest of 1st order threats. As now both energy thru-put is much reduced, and bush core integrity protected, the service-life becomes markedly extended.

My view on the above: No matter what brand names, rates or money one pins between the ball-joint & the strut-mount, until these more basic functions and vulnerabilities are addressed, as these mods do in a very civilized near to stock fashion, your suspension (and steering and braking etc.) will remain somewhat limited and primitive.

“Stops” - ensure your hinge-axes arrive intact.

[Kieron20V]

Do you work in marketing?

Sent from apollo 13 using tapatalk

[Lago Blue]

If I was that marketeer, I shouldn't miss this opportunity to crassly point towards how one could contact the maker regarding "Stops" mod parts sourcing.

[BorisShakhov]

Big text and no figures... Are we talking about vehicle dynamics?

You should buy few load cells, LVDTs, and make K&C model to understand the problem.

Do you really think that Audi suspension guys didn`t know basics of a bushing design back in 1994?

It`s calling bump stop and they knew it.

My thoughts on your design:
a) Washers on both sides will make torsional stiffness changing unpredictable ("stiction").
b) Outer sleeve/wishbone shouldn`t be in contact with a subrame ( vibration damping)
c) Bump stops should perform progressively. (not hit subframe after 1mm disp)

There is a more simple solution: change bushing design or choose it by specs from other cars.

[Lago Blue]

Boris,

I may have some figures (not mine) for a post to follow. This is not so much vehicle dynamics, as those are but outcomes of this particular issue, which is more about just plain “olde school” mechanics and its’ classical solutions. I do believe simple observation is sufficient here, I have outlined the problems as I understand them though I’m not quite done, but happy to expand any of the above if asked; and to discuss any parallel or competing theories.

How the bushings where chosen, or if anything was left out of the original design, we may never know, but it appears to go back farther than 1994. Was it the accountants? It may be that Audi just carried over pretty much what they had done previously year after year; it may also be nobody was ever hurt as a result of a failed bushing, and so they sold a lot of bushing replacements to those who cared to notice their failure. Perhaps we as a group just did not complain enough? This type of bushing seems to be largely unchanged since the late eighties.

Link below shows a Sachs/Boge bush for the 1983 Audi 4000 Coupe Quattro; & VW Fox Quantum front A-arms (bushing P/N 811 407 181A). This would be the immediate predecessor to our steel-shelled bushes. This type, although being a somewhat older design, appears to have smaller or even near zero side clearance.

"811" series bushing

To your points:

a) Hinge Stiction. The material chosen for the “Stops” is very stiff, has a very low friction co-efficient, lower than polished steel; and the static and dynamic frictions are nigh on equal. This means that similar to the ball-joint, any drag is predictably low, relatively steady with increased load, and changes of direction or stop/go transitions are near seamless. Certainly the small negative clearance of the installed “Stops” must add some anti-rotational drag and I'm sure it does. However just as a new ball-joint adds its stem displacement resistance to arm rotation drag, I view it as similar in scale and inconsequential. Particularly as without “Stops”, the drag of the OEM bushing’s rubber-faced flange against the sub-frame bracket, looking at the severe wear we have all seen that takes place on the worn face of the bushing's flange; this must be higher.

b) Sub-Frame Contact. The “outer sleeve/wishbone” must be not just in contact with, but more properly connected to the sub-frame somehow in order to best both propel and slow the vehicle. I do not view the stock production design where the rubber bush cores are exposed to axial tension as either sufficient connection of the A-arm, or that this core exposure to tension was ever the correct route through which the majority of acceleration and braking forces should be routed. This is only the stock new condition until normal use quickly first weakens, then renders the bushing cores apart at which point contact becomes intermittent and worse. I do however think that sufficient rubber in compression is a fine alternative, which is the "Stops"-modified condition. The mod puts to better use, more of the entire circumference of the bush flange-facing in a non-destructive manner. As nothing the hub is carrying should be vibrating, no further extra damping beyond the bush-facings now held by the installed "Stops" very firmly but not quite solidly, in axial compression; is therefore required and I experience no sensation of any NVH increase over stock. In fact, as explained in response to Purplelion above, I contend it is quieter than stock.

c) Progressive Action. The small fore/aft negative clearance of the installed “Stops” puts the rubber-faced flanges of the Boges into slight compression, so there is no empty space for uncontrolled travel; and no fore/aft arm displacement takes place without immediate and rising resistance from the bush flange faces themselves and the always in contact (via the “Stops”), sub-frame brackets.

Consider that :
i) The success of the "Stops" mod illustrates that a "better" alternative replacement bushing simply may not be required;
ii) Just the replacement of your current bushes with original Boges re-attached with the "Stops" mod, stops one from having to make further replacements; &
iii) If you prove me wrong and they don't quite last forever, you will have enjoyed the increased performance for years to come and they will -still- have saved you sufficient time and money to search for that perhaps mythical unicorn of a cheaper, superior and adaptable alternative replacement bushing.

LB

[Lago Blue]

"Stops" End the Cyclical Nightmare of Forced Hinge-Axis Repairs

Just a little cheerful encouragement:

Although almost forgot,
Oft too hurriedly fought.
Burning nightmare or not,
New bushes were quickly shot.

Didn’t drive as it ought,
Just alignment t’was thought.
But then failed the MOT.
This occurred quite a lot.

Your current bushes are shot.
“How could they be not”?
But then, you’ve been taught!
Will the -next- be for naught?

Mod’s a fix well-wrought,
It pays for being bought,
Effects welcome brought
Beyond bushings kept taught.

“Stops” wander at WOT,
Knock-knocking at knots,
And bushes from bashes caught
In holes big as pots.

Spare arms and forethought
Mod and bits best be got
Guard against getting caught.
“Stops” bushing distraught.

Decade since mod re-thought,
Bushes yet as new bought.
If better steering’s sought
Stock plus mod may besot.

What?
Cheers!

[A1QSHIP]

Mono ball & be done with it.

[Lago Blue]

Hi A1Q,

Taking the opposite view, this will go through some text-book disadvantages to choosing sphericals or as you call them mono-balls (M/Bs) for use in this application; and my experience-based reasons favouring "Stops".

M/Bs would perhaps be my last choice after several others, as they would be a misapplication of that type of bearing due to as follows:

a) In general, M/Bs are often more correctly used to accommodate conditions such as single-point attachment where both multi-axis misalignment and on-going changes to that alignment take place; neither of which we have nor want here (note the requirement to also attach the second inner A-arm trunnion disables the M/Bs' primary feature), so the penalty in employing them would be that we would also then suffer the resulting reduced service-life because (in comparison with say a plain bushing) their load capacity is necessarily reduced in order to incorporate the ball & socket, the feature (which allows them to accept smaller loads but from more directions) we will not use. Being an abbreviated sphere in shape, they therefore present (relative to other types), a much smaller area with which they could support either of our largely pure axial and radial loads, wear is therefore accelerated relative to other choices;

b) Further, M/Bs on thru-bolts in the stock location (oriented like the stock bush) wouldn't take axial loads as well as they would otherwise since the thru-bolts themselves occupy the area which would have been most perpendicular to; and therefore best able to withstand said axials. Just like the stock production design (without "Stops"), this orientation puts the highest loads seen in this application onto the M/B's weakest flank - it's axis (which lies perpendicular to the shaved ends of the sphere). As we do in fact have significantly higher axial loads here than radial; and that these loads are very rapid onset and reversing direction all the time as discussed previously, each reversal works to increase axial bearing clearance. The ball is forced to further enlarge the socket open at both extremes of travel with every traverse, the increased wear that will take place would mean they'd require even more frequent replacement; and they may also become noisy with that wear;

c) M/Bs would have to be adapted both to fit the A-arm; and to correctly install in the sub-frame;

d) Each A-arm's two trunnion's M/Bs would have to be set-up to share the axial loads equally, not a simple task; or alternatively, one M/B would have to be paired with a floating bushing on the other trunnion, which in either case ought to immediately make clear, the lack of any engineering requirement beneficial to us for using a M/B at all here, in both cases they would be an unfortunate, retrograde and unnecessarily complex choice;

e) M/Bs would have to be sealed or again likely replaced even more often due to the harsh environment;

f) Each extra hinge-axis repair occasion (due to more frequent bearing replacement) may also disturb the alignment, which would require it be re-checked to confirm/adjust;

g) Any small curb-strike may more likely damage the wheel/arm/sub-frame if the resilient original bush is replaced with the unyielding M/B;

h) M/Bs are more expensive to begin with; and all of the above would add further to that; &

i) To be fair there are certainly advantages to M/Bs, I just can't think of any that help here.

Whereas "Stops":

- take axial loads over a much broader flat area which, being perpendicular to the hinge-axis, are best oriented to transfer loads and to resist deformation, so they hold up very well for a long time;
- plus the Boges are still silent and without play a decade on (in my case);
- fit right out of the envelope;
- install quickly and easily during the normal factory procedure for stock bush replacement;
- automatically center the A-arm between the sub-frame brackets;
- plus the Boges are not a precision high-tolerance dependent assembly and OEM bushes have no "moving" parts and kept whole by the "Stops", are impenetrable and so these together are less likely damaged by the working environment; &
- are inexpensive, D-I-Y, simple and a decade later still maintenance-free;

Since by comparison M/Bs would require a great deal of effort and extra expense just to approach what has already been achieved with "Stops", should any of the above prove problematic whilst implementing M/Bs, one might then well ask "Why not just "Stop" it and be done with it?"

What “Stops” aim to provide is four (or perhaps eight) less maintenance items to ever have to deal with again, allowing one to move on to other things. They perform better than stock; are an invisible mod that are also apparently without penalty, which is a particularly valuable characteristic on a street vehicle; and did I mention all this at very low cost. To be a worthwhile improvement, any alternative would have to do any of the above; better.

[A1QSHIP]

I was thinking of something like this. Found on front & rear suspensions of many European vehicles. You would need to machine a bushing to fit to the control arms. Less "stiction" with these than with a bushing. I admit to replacing them on a regular basis but usually on high mileage vehicles.


Chris

[Lago Blue]

A Bird in the Hand...

Chris,

Although the addition of "Stops" to the stock hinge-axis does add stiction (except when they don't, see "Further" below) to the otherwise internally friction-less OEM bushings' ability to begin rotation, another couple of reasons, points b) & c) below, as to why in my view, the very small amount of drag they must add is inconsequential, follow point a), which I have mentioned previously:

a) Material Choice. Granted that typically, the static friction between any two surfaces is higher than the dynamic, which may cause a sudden release or jam to motion during stop and go changes, but the purposeful choice of the hard and long-wearing "Stops" washer material here also captures stopped and moving coefficients with steel which are, not only very low but virtually equal to each other, making such transitions near non-events that are seamlessly smooth, silent and imperceptible.

b) Local Leverage. As the "Stops" are located at the inboard (fixed pivot) end of the A-arm which has at least one-quarter of the car's weight bearing down on the outboard end (a lever-arm that is over a foot long, center to center), the "Stops" are working at a certain disadvantage in delaying A-arm design arc travel; &

c) Neighbouring Inertia. The relatively massive inertia of the spinning shaft/CV/bearing/hub/disc/wheel/tire assembly offers a much larger resistance to changes in vertical position or travel direction and simply dwarfs what a pair of small hard anti-friction washers could offer as resistance. So as a result, one just doesn't find these cars suffering from the suspension failing to, or being slow to either compress, re-extend or reverse course between bump and rebound.

Further, although at first it may seem counter-intuitive to say that "Stops" could possibly reduce the stiction of an installed and already friction-less OEM bush, this example serves to test one's understanding of the A-arm slide-travel issue. There are often times when in comparison to the stock installation of the OEM bushes alone, the addition of the "Stop" washers actually reduces both the static and dynamic bush friction. For instance, without "Stops" - the braking loads acting rearward along the A-arm hinge-axis press the bushes' rubber flange face aft into the sub-frame bracket such that it causes that rubber to be worn away. This wear is entirely due to the rotational friction during suspension arm swing and bushing outer shell rotation while braking. This is evident by the circular pattern of that wear seen on bush removal. This friction is worth minimizing as we certainly want the suspension best able to move vertically unimpeded during braking; and grinding material off the bushes during braking (the stock condition!) ought not to be acceptable. "Stops" also serve to correct both these conditions.

To again use our ball-joint as an example, you can appreciate the utility of that single spherical joint acting alone at the outer apex of the A-arm under the strut-base, where its' primary ability, shape nearly a full sphere to better carry load, compact size and light weight make it a more logical and popular choice at that particular location, but not this one.

Notwithstanding all of the above, I'm sure what you propose is entirely do-able, but meanwhile, because I already have here in service for a decade, a quick, cheap and proven solution (granted that I'd like to promote), I must ask why you haven't yet chosen "Stops"?

[Lago Blue]

Suspension of Disbelief, Part 1.d. cont’d.

4.) How: Control of Fore/Aft A-arm Position via Pre B3 Series Bushings, Post & with “Arm-Stops”

This is to aid our understanding of how the wheels are held steady fore/aft; or not. This is going to focus on just one aspect of bush construction - how the front (style) A-arm’s fore/aft position is achieved and maintained. Two ancient bits of wisdom (2nd author unknown) I’m reminded of regarding this subject are as follows:

A slight adaptation of Murphy’s Law which may apply here would be:
“Given room for something to go side-ways, it will;” &

also altered:
“Sometimes, a part of the past deserves a second chance at history.”

First off let me say I have no direct experience with the “811” series bush (perhaps you do, and I would welcome your comments), so I am perhaps totally unqualified to make the assumptions I am about to go on about, so sue me.

Contrast & Comparison: Previous (B2) “811” vs. Last (B3 & B4) “857 & 8AO” Iterations of A-arm Bush Characteristics.

Older “811” series (1st photo):
- No outer shell or flange;
- Can be installed either end in first and into either end of A-arm trunnion;
- Both ends of both bushes in A-arm are involved in resisting fore/aft arm movement;
- Save for the cone section of segmented pads (end-cone feature) on either end which must begin to transfer immediately, axial load to and from the sub-frame (S/F), it more closely resembles a traditional shock eye bush that would, in that use, see no axial loads;
- The end-cone feature on the strictly rubber end-flanges provide through compression a non-core response, both immediate progressive resistance to fore/aft loading/displacement; and re-centering force to the A-arm;
- Rubber “core” inside the A-arm trunnion is neither in compression or tension during fore/aft loading, leaving it to deal solely with cornering loads;

Most recent “857 & 8AO” series (last attachment):
- Increased side clearance (vs. 811 series) allows slide-travel of A-arm to occur;
- Rubber core is in tension during any & all fore/aft displacement, this tension provides re-centering force;
- Flanged shell provides proper push point & positive stop for installation; & arm travel limit in that one direction;
- Steel core is offset (protrudes less on flanged end) to ensure flange-facing “arrives” at S/F bracket before the un-flanged end of the A-arm’s other bush does;

First photo below is of the “811” series bush which immediately pre-dates our steel-shelled and more uni-directional bushes. Note there is a good deal less protrusion from the end of the steel core to the flange-face with the end-cone feature than latest “857/8AO” series bushes.

The RH side of second photo shows the "811” series bush installed and attached. The reduced end-clearance between the bushing flange-face and S/F bracket due to the “811” series’ greater face-to-face length is a good deal less than what the latest “857/8AO” series bushes provide.

Third attachment is a cross-section of our late style “857" & "8AO” series OE bush (taken from a Meyle HD info sheet) and while not exactly to scale, what it does illustrate can help one understand the construction, its’ differences from the previous style, visualize how it functions and how that differs from the older “811” series; and finally how the “Arm-Stops” mod changes that late style OEM function:

a) Altered profile - This change to the increased side-clearance of the latest two sizes of B3/B4 bushes occurred in conjunction with the introduction of the flanged outer steel shell and other changes. The end-cone feature on both ends disappeared at that time also.

b) Stock operation – the A-arm trunnion surrounding the outer bush shell pushes the flange-face aft under braking load until the facing contacts the S/F bracket. The first of several results is the rubber core is tensioned axially. These tiny interior rubber bands alone are available to re-center the arm; &

c) The altered via “Arm-Stops”mod condition - as above, A-arm axial load is again shared with the outer bush shell, however its’ attempt to travel aft, is immediately restrained by the facings’ pre-existing contact with the “Stops” thrust-washer, the load is transferred directly to the S/F bracket, avoiding passage through the interior rubber core, it remains isolated and protected from these loads. The flange-facing compression relaxes very slightly (because very little movement occurs in the first place); and re-centers the arm.

With “Arm-Stops”, the car is no longer propelled or slowed by the constant tensioning (and the frequent reversal of that tensioning) of the bushes’ interior rubber bands, the change which adds tremendously to their service-life.

Still unanswered, perhaps any of you can help:
- Was it that as kW and weight increased, the older “811” series bush became less adequate?
- How did the “811” series bush wear out &/or fail, if at all?
- Why wasn’t the end-cone feature feature continued on the later series flanges?
- Were the end-cone feature features removed to make space for something else in their place?
We may never know for sure.

Most interesting to me is that some of the capabilities which have allowed the “Stops” mod the success it has achieved, are features the older bushes already provided; but where deleted from our current ones. In particular, the mod reclaims to the newer bushes, the functionality of the last 3 listed items in the “811” list above, to which I must again give credit to the fellow who may be the mod's originator, Keith Anderson (of Anderson Motorsports, (on “Quattro Drive” no less) Chanhassen, MN) who, with his UrQ likely had experience with this older style of bush.

[andy10v]

AM cars sell the old bushes as rally bushes

http://www.amcarsquattro.co.uk/shop/wishbone-bush/