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Jim wrote:
>Here's the problem as I see it:
>The subframe is mounted to the body in a number of places by what amount to
>thick (~1cm) rubber washers. The mount points of the subframe are captured
>between a pair of these washers and then a large bolt, with a washer under
it's
>head, is passed thru that sandwich and into the body. Originally this bolt
>tightened to a stop that left the washers slightly compressed.
This is true, but there is a spacer sleeve that this gets tightened against.
This sleeve is trapped by the washer and the frame mount boss.
>With the subframe captured between the 2 compressed rubber washers, the
>effective spring constant was pretty high, making this connection resilient,
>but quite stiff. Today, however, the 2 washers have shrunk a bit, so they
may
>no longer be under any compression. This completely removes any stiffness
from
>that joint, and it may even be loose.
Well the lower one is supprting the body (all of the weight is on the lower
rubber bushing) and the upper is keeping some of the motion limited. Having had
the body off of several of these cars, I've seen this.
>In addition, the rubber parts are probably harder now, so that, if the joint
is
>loose, the parts may "rattle around" against rather harsh stops.
The rubber parts might shrink some, but they are still trapped by the
bolt-washer-sleeve combo, any movement might be very small at this point unless the
inner part of the rubber bushings are worn. This would allow lateral movement
too.
>So, as I see it, a 1 cm rubber mount that shrinks by 2mm may only cause
about 2-
>3mm of body sag, but that 2mm of compression set could completely remove all
>preload from the mounts, making them MUCH looser than they were originally.
With all the load on the lower bushing, I think that if it shrinks that much,
it would have more upward movement than downward movement, after all, the
weight and gravity seem to be working on keeping the lower bushing in
compression, making it more likely to be compressed to your higher number.
>Let me know what you think of this. I suspect that I'll want to see if I can
>replace any more of the bushings in my sun belt 69 with ones from some of my
>scrapped out, rust belt cars.
Jim, this is very well thought out, and if anything you'd want to replace the
lower bushing half rather than both. I think one could build an spacer to
place under the lower bushing to make up for wear and shrinkage. This would
tighten the clamping force of the 2 bushing halfs, and keep the rubber isolation
that was originally there when built. What do you think about this?
Bob 65 Notch S w/ Sunroof
69 Square AT-the baby suburban
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