Shock mount failure

[Pete Masterson]

I'm inclined to agree that the left shock mount was a less robust component than is ideal, possibly due to a Ridewell error or miscalculation of stresses. The tear starts at the top and works down. That suggests that the force causing the stress is the fully extended shock absorber. This is in the opposite direction of stress caused by lowering the jacks when the air bags are deflated. Indeed, in that situation, the bumper stops ought to be taking the brunt of the weight rather than the shock mount, if the shock is of the correct length.

I noticed the 'popping' noise (that apparently was indicative of impending failure) whenever Itraversed dips or undulations in the road. Unfortunately, many freeways and highways in the SF Bay are built on fill or unstable clay soils, and undulations caused by ground subsidence is common and occurs frequently. Even a small bobble, crossed at 50 or 60+ mph results in a rather severeporpoisingincident. With the frequency of these undulations in the area with the heavy, fast moving traffic, it is rarely possible to spot the undulation and take protective action before you're upon it. (It's so perverse, that one lane may be affected while other lanes are not.) Indeed, there have been several studies (over the years) that suggest that the condition of Bay Area highways may be costing drivers $billions each year in additional repairs to their vehicles. Naturally, living in a near-bankrupt state, there is little chance that there will ever be significant improvement.

So, my feeling is that the most likely source of stress comes from super extension of the shock, rather than compressive stress caused by lowering the jacks before the suspension is aired up. I note that the shock can't be longer, since then it would not be able to compress sufficiently in the opposite direction. Further, between the time of the first repair and the second failure, my coach did not go through all that many jack raise/lower cycles and I've rarely lowered the jacks without sufficient air in the suspension. (I'd say never, because I have a 'get ready to go' procedure that I'm quite careful to follow -- but I admit the possibility of a distraction or memory lapse where lowering the jacks without air in the suspension is possible once in a while.) In contrast, I probably had many porpoising events for each jack lowering with or without air in the suspension.

I also speculate that the second failure after the first repair I encountered is due to the particularly large number of porpoising events encountered in Bay Area traffic (and the generally poor condition of many California freeways). Of course, if the first repair were more robust, I may not have had the second failure. I further note that it did take a dozen years and more than 125,000 miles before the fracture was first found. While we should expect better, that's quite a few miles on the chassis without a failure, so one might conclude that the part is only just short of being "strong enough."

Pete Masterson

'95 Blue Bird Wanderlodge WBDA 42 (For Sale)

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

El Sobrante CA

"aeonix1@mac.com"

On Feb 18, 2009, at 7:45 PM, david brady wrote:

Greg,

I don't think the axle tilt puts undo stress on the shock. If you
consider the steer axle in full droop on one side and full bump on
the other. The angle of the axle is roughly 5 deg to the horizon,
and this turns out to also be the angle of the shock.
(inverse sine of (8 inches / 96 inches)). The rubber bushings
in the shock can easily comply without transferring
excessive forces to the mounts. I'm sure Ridewell did this basic
calculation. If this were the cause then we'd be seeing similar
failures on the drive axles. The cause could be shocks that bottom
before bump stops (shocks too long), folks retracting their
HWH jacks w/o first airing up the suspension, folks raising the
front off the ground w/o first dumping air, or simply and inferior
Ridewell design. After year 2000 or so, I was informed by
the engineering staff at Ridewell that all their suspension systems
underwent finite element analysis and subsequent redesign;
consequently the LXi uses a different shock mount system than
the WB's. Could be that Ridewell addressed an inherent weakness.

David Brady
'02 LXi, NC



timvasqz wrote:

I think the crack was there from off-camber HWH jack parking in the
past. looks like the temper of the weld proved a brittle spot that
under normal conditions would not have issue. Then the new shock
absorbed more bounce but reacted equally agressive causing much more
stress on the mount. I think the mount post should have less strength
and be a point that can bend at failure. the tripod gussettes are a
bad idea.

I think problems arise when the bus air bags are dropped to be
leveled but the spot is too off camber. the HWH push one side up
and tilt the axle too much. at that point raising the front is
effortless and and the 30 foot lever post all the stress on the
tucked tire.

If I was to repair that mount I would remove two bolts and make a
trapazoid shape cut and replace the plate with a butt weld then build
the post mount back no better than before.

If you lose an airline and must drive. there is a screw set in the
maxicam to turn off the parking brake. you can then plug the airline
and fold it over then tape it.
Greg ofTim&Greg
94ptca

..--- In"WanderlodgeForum%40yahoogroups.com", "Don Bradner"
...> wrote:
>
> I'm the latest victim of a driver's side front shock mount failure.
It took out the brake line when it went. I was not far from home on
the way back from Q, so I limped on in (I know, in retrospect it was
too dangerous, but I kept a lonnnng following distance!)
>
> I've used the local Detroit dealer in Arcata (Trinity Diesel) a
couple of times now for service, so I called them, and the service
manager said he would swing by for a look, no charge. He ended up
removing the shock and the air line, and said that he would come by
the next time he was this way with a new airline. A new shock will
have to be ordered (the top bent/caved in) and they will schedule an
appointment to get it welded after they have that. Shock was 17
months old.
>
>
> Don Bradner
> 90 PT40 "Blue Thunder"
> My location:www.bbirdmaps.com/user2.cfm?user=1
>

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