Shock mount failure

[david brady]

If the tires stay on the ground, not "getting air" on the Nimitz

in Ca, the max shock force is 4000lb (according to Justin at

Koni), and all of that force occurs under rebound; i.e, the Koni

shocks provide no, or negligible, compression dampening.

IOW's all the forces are in the extension direction. That's not

to say that the initial damage hadn't occurred under compression.

It's just that for it to occur under compression something weird

has to happen, like shocks that are too long, or dumping the

HWH jacks w/o air in the bags. Once the initial damage

has occurred, then normal shock loads add to the metal fatigue

and the ultimate failure.



Kurt Horvath wrote:

I also believe that the stress cracking comes from extension not
compression of the suspension. When my coach was lifted up on the front
end the broken piece pulled away when lowered it you could barely see
the fracture lines on the mounts. I just basically reseated itself in
it's original position. I doubt that anything we could do intentionally
would cause such a fracture. Air in or out of the airbags lifted with
the jacks or not. A constant pressure of 13000 LB is not the same as
13000lb rebounding and extending at 60 MPH. Perhaps dropping the jacks
on empty airbags could do it, when it hits bottom it hits hard, real
hard.



Kurt Horvath

95 PT 42

10AC



--- In "WanderlodgeForum%40yahoogroups.com",
Pete Masterson > wrote:

>

> If I understand correctly, that raises the front of the coach. If
the

> air bags are inflated, it can put 13,000 lbs pressure on the shock


> mount (from the air pressure in the bag) with the maximum pressure


> achieved if the front tires are lifted completely off the ground
(as

> sometimes is the case).

>

> Pete Masterson

> '95 Blue Bird Wanderlodge WBDA 42 (For Sale)

> <http://www.aeonix.biz/BBforsale.html>

> El Sobrante CA

> aeonix1@...

>

>

>

>

> On Mar 10, 2009, at 12:30 PM, Dan Williams wrote:

>

> > Pete, I may be getting in on this late, but how does lowering
the

> > leveling jacks with the suspension still aired up put stress
on the

> > front shocks and mounts?

> > Dan Williams, 88WB38, Jackson, MS

> >

> > From: "WanderlodgeForum%40yahoogroups.com"
[mailto:"WanderlodgeForum%40yahoogroups.com"


> > ] On Behalf Of Pete Masterson

> > Sent: Tuesday, March 10, 2009 12:20 PM

> > To: "WanderlodgeForum%40yahoogroups.com"

> > Subject: Re: [WanderlodgeForum] Re: Shock mount failure

> >

> > There is a sub-frame that holds the shock mount, air bag, and
other

> > parts of the front end. It's a piece that's about 24 to 30
inches

> > long. That piece comes from Ridewell. (Although Ridewell
might claim

> > that Blue Bird provided the specification.) The
speculation is that

> > the Ridewell piece is not thick enough (or otherwise not

> > sufficiently strong) to take the constant hammering.

> >

> >

> > Awhile back on one or the other WL lists, someone reported
that

> > lowering the leveling jacks with the suspension still aired
up puts

> > an unusually large stress on the front shocks and mounts.
That may

> > start the weakening process. However, I know that it was rare
for me

> > to forget to dump the suspension before I leveled the coach.

> >

> > Before the complete failure of my shock mount, I'd noticed a

> > 'popping' noise in the front end when encountering the
porpoising

> > movement that occurs after passing over one of the multitude
of

> > ground subsidence undulations that are frequently found on
the

> > freeways, especially in the SF Bay Area where the highways
are

> > frequently built on fill (near the bay) or unstable clay
soils

> > (nearly everywhere else in the area). Later I realized that
the

> > noise was cause by the flexing of the split (and eventually
broken)

> > piece of the Ridewell sub-assembly. (The acoustics caused it
to

> > sound like it came from the opposite side.)

> >

> > Pete Masterson

> > '95 Blue Bird Wanderlodge WBDA 42 (For Sale)

> > <http://www.aeonix.biz/BBforsale.html>

> > El Sobrante CA

> > aeonix1@...

> >

> >

> >

> > On Mar 9, 2009, at 7:40 PM, Don Bradner wrote:

> >

> >

> > Did your tear look at all like mine (mine looks almost
exactly like

> > Pete's)? Presumption is that the slamming downward when the
wheel

> > bottoms into a pothole or similar is the issue. Countless
slams, and

> > the metal gives way. Note that it is not the frame, but
rather the

> > steel mounted to the frame - that steel is a part of the axle


> > assembly and was presumably not welded by BB.

> >

> > The repair done on mine comes close to matching the original

> > assembly, making it possible/likely that sometime in the next
20

> > years it will fail again. I'm going to try to remember to
stick a

> > camera in there for photos every few months (more often to
begin

> > with!) that it is driven.

> >

> > On 3/10/2009 at 2:22 AM Eric Perplies wrote:

> >

> >

> > My front left shock mount broke just before the Q rally in
08. CCW in

> >> Riverside fixed mine shortly after the Q rally. On my
coach the

> >> shock

> >> mount ripped out a chunk of metal on the frame. A piece
of steel

> >> about

> >> 1/2" x 8" x 6" was welded onto my frame. A hole was
drilled in it to

> >> accommodate the brake air line. The original shock mount
with it's

> >> three

> >> gussets were removed from the broken out piece. That
original

> >> shock mount

> >> was then welded back onto the 1/2" plate mentioned above.

> >>

> >> The welds made during the repair were VERY thorough.
There was an

> >> indication that the original BB welds may not have been
as through

> >> as they

> >> could have been. (One side of one of the gussets had not
been

> >> welded onto

> >> the frame.) That having been said this could not be the
reason for

> >> the

> >> failure. After all the frame on my coach had a section
ripped out

> >> of it.

> >> The shock mount itself did not fail.

> >>

> >> A far more interesting question is: What causes the
failures on

> >> the shock

> >> mounts?

> >

> >

> >

> >

> >

>

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