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Old 08-07-2009, 09:54 AM   #1
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Jacked Up, Tires Suspended in the Air. OK?

I'm going to park my trailer at a campground for several months, probably through the upcoming winter.

It now sits on: tongue jack, 2 scissor jack right behind rear axle at axle mounting plate (pushing up j-shaped bent plates, identical to what you see with rear bumper) -- I think this is how it should be done, right?

Trailer weight is completely taken off from the axles or tires. I can see the gaps that my tires are suspended in the air by about a half inch.

So, the axles' rubber cords are twisted opposite to how they were designed, but I'm thinking this would aid for the long-term health, or the flexibility of the rubber, so I'll have cushy ride when I tow again.

???
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Old 08-07-2009, 10:14 AM   #2
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It's said that exercising the rubber is the best thing for the GKN axles, any torsion axle for that matter, but your approach is the same as having them on the rack, ready for delivery, no weight to speak of on the axle.
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Old 08-07-2009, 10:46 AM   #3
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I now figure...

Negative twist from hanging the mere weight of wheel/tire combo must be negligible in comparison to the rubber's capability to handle the sheer weight of the trailer.

And thanks to Greg for pointing out to exercize the axles periodically -- I didn't know this.
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Old 08-07-2009, 11:02 AM   #4
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Originally Posted by Astroboy View Post
I'm going to park my trailer at a campground for several months, probably through the upcoming winter.

It now sits on: tongue jack, 2 scissor jack right behind rear axle at axle mounting plate (pushing up j-shaped bent plates, identical to what you see with rear bumper) -- I think this is how it should be done, right?

Trailer weight is completely taken off from the axles or tires. I can see the gaps that my tires are suspended in the air by about a half inch.

So, the axles' rubber cords are twisted opposite to how they were designed, but I'm thinking this would aid for the long-term health, or the flexibility of the rubber, so I'll have cushy ride when I tow again.

???
I really never tried what you have done, leaving it suspended on scissor jacks. I think that might become unstable in a small quake or high wind.
Maybe it would be better to get some stout wooden blocks to set the frame on for long term. I'd like also to hear what others think about how to leave it setting for long term storage.
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Old 08-07-2009, 11:52 AM   #5
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It's a question of whether or not there is any load on the rubber bars in the axle at rest? My understanding is that at a standard empty weight, sitting still, ie. suspension not being worked.. there is no load on the torsion part of the axle.

Could be wrong... any buddy else have any opinions?

-yakman
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Old 08-07-2009, 12:34 PM   #6
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Do I read that you have the trailer supported on two jacks and the at the front with the leveling post? If that is the case I would add a few more supports to help keep the floor flat. I have dealt with two trailers that both had back-end separation, I have my trailer supporting at six different locations along the length. Keep in mind I am rebuilding parts of my frame so I need extra support to keep everything flat.

I like the idea of unloading the axles. I have always planed on doing that once I replace the current ones.

And then here is one more twist to the conversation. How do you keep the rodents out? Yes all holes should be sealed but I propose added security. Is there away to keep them from even trying to get to the trailer? My thoughts are drifting to ranchers and electrified fences? Any ideas?
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Old 08-07-2009, 01:06 PM   #7
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An electified Airstream, Hmmmm. Just think of the pests that would keep out.

This is a good question though. I'll be working on a 74 Tradewind soon & thought putting it on jacks for the 2 yrs would be a good idea. Will raising & lowering the jacks regularly (twice a month) be enough exercise? Or should I hook 'er up for a spin around the block?
Ricky
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Old 08-07-2009, 01:46 PM   #8
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I have my stabilizers down as well.

I'm not so worried about my trailer shaken off the scissor jacks due to the quake or wind. I'm located in North East this season, and there's no quake, and rarely a hurricane that reaches inland. The trailer would drop for a few inches and land on its tires anyway, even if ever the jacks fail.

Sure, jack stands, wood or cement blocks would be much more stable. But they would be bulky and heavy, and perhaps infested, and they would be troublesome to transport when I need to pick up and travel again. It would be redundant, because I still need to carry at least one scissor jack to do lifting and for roadside repair anyway.

I opted to carry one more, for this long-term jacking up purpose.
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Old 08-07-2009, 01:59 PM   #9
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I don't agree with previous poster's (yakman's) theory that there's no torsion (load) on the axles when the trailer is parked with no luggage on board. Every suspension on vehicles (car, trailer, etc.) has positive stroke and negative stroke (I'm not sure which is which) The stroke to react and absorb to the bump on the road, and the other stroke to extend and actively track the dip on the road. The purpose is to keep the tires contacting the road at all times for conrol and braking.

I had to lift the trailer about 3 inches before tires start coming off the ground, so there's definitely torsion on the suspension when parked at rest. Airstream's suspension has about 5~6 inches before bottoming out. Assuming same modulus of the rubber is working for both ways, the suspension wouldn't droop for 3 inches only because of the weight of the tires, if it did, loading the trailer with water and gear would bottom out the suspension before even setting out for the trip.

6 inches of stroke <- baseline (at rest) -> 3 inches of other stroke

This means the suspension is subjected to about 1/3 of its capacity when parked at rest.
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Old 08-07-2009, 02:30 PM   #10
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This sounds right ... and if the expressed theories are correct about the rubber torsion components "tiring" or taking a set, you may find that in the spring, the wheels are once again touching the ground, as the rubber "relaxes" some of its supposed "set" - assuming that it is capable of "relaxing."
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Old 08-08-2009, 12:29 PM   #11
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... and if the expressed theories are correct about the rubber torsion components "tiring" or taking a set, you may find that in the spring, the wheels are once again touching the ground, as the rubber "relaxes" some of its supposed "set" - assuming that it is capable of "relaxing."
That is correct...up to a point.

As an engineer, I have a hard time subscribing to the "exercise" theories that pop up here once in a while. "Set" is developed as a function of time and compression - time being the huge factor if you compare the amount of time an axle sits in full compression as compared to the time that an axle is being "exercised". Remember, that even though an axle is being "relaxed" during use, it is also subjected to extra "compression" hits just following the "relaxed" cycles.

Also, consider of the use of an axle in an RV. The RV axle is being compressed to almost the maximum stress as compared to, say, an axle on a horse trailer, when the axle only sees the maximum compression (max weight on the axle) when the horses are actually in the trailer. An RV "rubber" based torsion axle (assuming it is standing when unloaded at close to the maximum design rating) will allow maximum shock absorbing (suspension road impact softening) for the minimum amount of time as compared to almost any other axle use application.

Consider that the elastomer of the "rubber" axle is similar to the sealing gaskets used on the windows and doors of an Airstream trailer. It is obvious that these seals take a "set" over time regardless of whether or not the doors or windows are used periodically or not. Rubber elastomeric life is a function of time, compression, temperature, and the elastomeric ("rubber") formula. Of these, consider the time the trailer axle remains under the maximum compression - the only two variables of the above four that we can actually control.

Below is a shot of a cut apart of my old axle. This axle is from a 1978 axle - 31 years old when this picture was shot. It is apparent that there is little life left in the "compression-relaxation-rejuvenate" cycle. Hardly any "rejunavation" here after about six months of total relaxation.



Take a look at the maximum "guarantee" of any rubber torsion axle manufacturer - not many over a year or so. I talked to design engineers from no less than three manufacturers and all agreed that any axle more than 15 or so years old is running on borrowed time.

The pic below is my solution to easily unload the compression on the torsion axles. The bar between the two axles is welded to the mounting bracket, so no welding was done on the axle tube, nor is there any compressive load directly on (or under) the axle tube.



Below is a view of the jacking beam actually in use.



The jacks between the wheels take up all or part of the load on the tires, and also act as an additional stabilization point when in use.

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Old 09-23-2009, 10:35 PM   #12
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My trailer is pretty heavy and the axles are nearly nine years old now. Each time I lift it up to work on the tires I think I might risk damaging something or someone. Im very careful with a 40 ton bottle jack but anything can happen. I plan on replacing the axles in a few years which is probably much earlier than the sales guy told the original owner...
I personally wouldnt worry about the axles too much as they are doing what they were designed to do.. hold up the trailer. Sometimes I think we gearheads worry too much about stuff. I love the idea of suspending the axles while you park but I think I wouldnt bother. If it makes you feel better my axles will have four thousand pounds on each all winter. I think they will still love me in the spring time when we are ready to go..

Cheers Vinnie
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