Strut Top Mount Upper Mount

[driftin jo]

As andy10v says, they cant lock it down that's not what they do, the strut moves on an ellipse
as it turns, the top cap is there just to stop the strut from falling out of the wheel arch when you
jack the car up or go over a serious bump in the road when the wheel leaves the ground.
It does not clamp the strut in place in any way, if it is doing there is something wrong !

[Colesy]

Cheers for the clarification guys, time for further investigation into the clunking noises.

[Lago Blue]

Re: Top cupped washer and it's necessary noise damper ring

The metal cupped washers which sit on top of the strut-mounts on my 7A were originally the same front and rear. The two types of black rings in the photo will interchange; but should not be.

The front noise dampers are the thin black plastic rings originally unbroken (IIRC) when new, shown on the RH side of photo above.

The rear dampers are the thicker rubber ones.

The two types are different for at least two reasons.

a.) the rear ones don't have to turn in use, so they can be in contact with the painted top of the inner fender; &
b.) the front ones may have to deal (i.e. clear) with the extra material thickness of the OEM welded-on strut-bar end fittings; and may come into contact during driving, hence they must not inhibit steering; so they are slipperier.

The rear ones where also originally covered by glued-up open-cell foam caps for further noise reduction.

With an OEM strut-mount, the cupped metal washer is designed to be torqued down onto just the top of the slottted-nut which must sit proud of the inner mount top, and to not otherwise touch the top of the strut-mount. They would bind the steering if they did, and not further re-enforce the mount. With weight on the strut top-mount, they cannot be employed to stop "rattle". Particularly on the front, they must only act when a corner is in full droop; and the top mount has fallen.

If the strut top-mount is damaged, it needs to be replaced. If you get tired of doing that, see Mount-Savers.

The metal cupped washers main purpose is to prevent dislocation of the top-mount, and only when the suspension is fully unloaded. They do not allow for the change in angularity of the damper shaft that occurs with steering and suspension travel, they are sized to avoid that altogether. They simply wait for the suspension to fall. They also serve as top-mount wear indicators for those who choose to read them. The noise damper rings prevent metal to paint contact and promote quiet contact at full suspension droop.

If you could drive without ever having to fully extend your suspension, you could live without them, but when you remove your front ones, you may see evidence that they have touched the fender tops in "normal" driving.

[VAP]

Another option for those running Pro-Mounts, Mount-Savers or 034's density line strut mounts is to use the VW alloy cupped washers. These cupped washers sit 10mm higher than the OEM Audi style, are made from alloy instead of cadmium plated steel and use a "real" rubber insulated damper ring that is more quiet and doesn't mar the paint on strut tower tops the way the OEM hard plastic insulating rings can.

While these sit higher than the OEM units they work better for the afore-mentioned strut mounts that "pull" the OEM cupped washers inside the strut tower bores by virtue of their internal travel-limiting provision.

Pic below shows a side-by-side comparison of the two styles of cupped washers and their accompanying sound insulating outer rings. The VW parts are available thru any VW dealer and sold under the following part number: 1J0 412 319C

[DeS]

Maybe it is possible to use full TT strout mount specification? Washer and a fubber mount looks similar to these used in Audi 80. I have to get one and try fit it.

Forgive for asking one more time. why the original strout mount and washer have the distance between? Wouldn't it be better to have it stiff?

Cheers!

[Lago Blue]

Re: Why OEM; & Why the gap?

These are good questions and I wouldn't mind taking a stab at this because:
a.) they do make you think;
b.) the different functions of the individual parts that make up the entire installed strut-mount assembly are not obvious: &
c.) as to forgiveness, if our cars where too simple, would we still enjoy the obsession of it all, but less?

I think the answer to the first two questions can be had by looking at the paths loads take through the mounts from strut to body. Perhaps 034's handy cross-sectional photo will be helpful:

http://www.034motorsport.com/chassis-components-audi-40008090urq-strut-mount-early-audi-density-line-p-987.html

LOAD PATHS:

Radial loads:

As the strut-mount body takes radial loads from all directions, for steering precision and alignment reasons, it should fit very well laterally, into the metal receiver molded into the underside of the inner-fender. The strut-mount from the dealer is perhaps best shaped to do this job. It's exterior diameter appears to be almost tapered (like a Morse taper) probably to exactly re-centre itself each time it cycles between being compressed; and then released. The amazing thing is that such a large transfer of energy (strut-mounts get hot!) can ever take place quietly across such a small dimensioned part. That the original design makes good use of it's entire exterior surface to do this is perhaps why it can function at all. A non-OEM shaped replacement may not do as well.

Axial loads:

The strut-mount takes axial loads in only a particular way.

a.) Top load pushing down:
The mount's outer body (bonded rubber/steel ring assembly, see cross-section in link) is loaded from above by the weight of the particular corner's fender-receiver bearing down on it's OD and top edge. This is borne out by the fact that the ID of the OEM mount cavity shrinks at the top; in use. The lowest bonded steel ring contains the upper/outer race of the centre bearing.

b.) Bottom load pushing up:
The mount's centre bearing lower/inner race transfers all loads from below, coming through the upper perch, at the shaft shoulder and washer on the damper-shaft, upward into the resilient mount body. The centre bearing lower race is captive under the installed slotted nut.

Save for off-axis loading (see below), there is nothing that is going to load the strut-mount any other way. So the height of the cupped-washer on top of the above assembly, whether a too-close fit, OEM gapped or not present at all, can have no effect on strut-mount function simply because it is not in the load path. As the damper-shaft moves, so moves the upper cupped-washer, purposefully hovering perpetually above, entirely separate from the mount body's upper surface. If one was to shorten the slotted nut's height to bring the cupped-washer into contact with the cavity floor, you would interfere with steering the car as the cupped-washer turns; and the floor does not. Would the damper-shaft get bent, I don't know.

Even if talking just rear axle use, if in contact, it's on top. The cupped-washer will move up exactly as the shaft moves up, away from the mount, and unable to usefully input load into the mount body.

If the mount body itself is not held immediately or directly by the top-nut may be of little consequence as in use, there is no force that can be applied; to push it off.

More importantly, the required toughness or resiliance (or life expectancy) of the strut-mount to all driving inputs is governed only by the remaining residual integrity, first cast into the OEM bonded rubber/steel-ringed body; when new (ignoring heat-cycling). On installation, it immediately begins to fatigue.

If one wishes to try to maintain the highest possible "as new" strut-mount integrity, one must at least prevent over-travel of the mount-cavity floor upwards by installation of "Mount-Savers" into strictly new, never before mounted OEM Sachs/Boge strut-mounts.

If one wishes stiffer strut-mounts, it appears that less resilient "Mount-Savers" may be available. And they do sit in the load path.

If you're following the above you can perhaps visualize how the spherical bearings present in "Pro-Mounts" serve in part to further relax the bonded OEM mount body assembly (over just Mount-Savers alone) by preventing the off-axis mount-floor tilt that must take place throughout suspension travel. One can then also imagine how the original toughness of the OEM mount can therefore be even better maintained due to the reduced mechanical stress and temperature that the body must endure.

I hope I've answered why the gap, but as to your last question I can't really answer if stiffer mounts are "better", that's too personal.