Do we know for certain what causes torsion axle sag? - Fiberglass RV

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Old 11-12-2014, 10:09 PM   #1
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Do we know for certain what causes torsion axle sag?

In addition to my membership here, for several years I've been a member of a yahoo group forum for A-frame trailers (Chalet, Aliner, etc.). A trailer repair person on that forum recently said, "The only reason that torque flex axles wear out is overloading the weight."

That got me to wondering. I know I've read here many times the belief that the rubber in a torsion axle stiffens and distorts with age and with non-use, resulting in a sagging trailer and a harsh riding trailer. However, I've never seen documentation. Do we know of any statements by torsion axle manufacturers, one way or the other? Is it at all possible that sag could always (or even usually) be caused by overloading, rather than by age and/or relative lack of use?

If the cause really was overloading, then jacking the trailer up for storage would really be a waste of time and effort.

To test the hypothesis somewhat: I know there are plenty of sagging Scamps, for example. Currently, Scamp 13s come with 2200 lb axles and 16s come with 3500 lb axles. It seems difficult and unlikely to exceed these ratings, but perhaps Scamp was using lower capacity axles 20-30 years ago? Anybody know?

Anyone have any documentation or hard evidence that would prove or disprove the statement he made?

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Old 11-12-2014, 10:18 PM   #2
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I have no hard proof but age and overloading would both be a contributing factor in my opinion.
For a comparison a rubber band sitting in a drawer will stay the same basically forever. put a 10 lb weight on it and let it sit like that for a week and it won't snap back to it's original position..... now overload it with a twenty lb weight and the results will be even worse. I may be way off base but it is my reasoning that memory of the rubber will come into effect.
That's why when i store for very long periods of time i jack the trailer up halfway to a neutral position like it would sit on a rack in the store when new.
I too would like to see some strong written proof of what's real

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Old 11-13-2014, 12:56 AM   #3
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Rather than wonder, I wonder(?), if the axle manufacture could give an answer to the question?
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Old 11-13-2014, 03:54 AM   #4
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Good question Frank but I doubt any company is going to give any hard line stats for life expectancy. I had the same thought as Joe after reading the first post question. A rubber band in a drawer, no sun light or uv and it falls apart when you stretch it a little bit. Tires on any trailer that sits most of the time outside are a good example of uv checking. Torsion axles don't get the uv or sun light either. The components used to make a rubber band or the rubber in a torsion axle I'm sure is different but they will all break down at some point. Even metal fatigues with age and becomes brittle and prone to easier breakage. I guess bottom line is it may depend on where you live and how you use your trailer makes a difference of how long the torsion axle holds up before needing to be replaced. A 25 to 30 year usefull life sounds pretty good to me even with over loading factored in.
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Old 11-13-2014, 04:09 AM   #5
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Old 11-13-2014, 04:42 AM   #6
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Perhaps this thread, with pictures of the interior of an axle will help: What a torsion axle should not look like
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Old 11-13-2014, 05:35 AM   #7
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Another very interesting article that basically says ALL rubber will deteriorate over time, just in different ways and at different rates.
Care of Objects Made from Rubber and Plastic - CCI Notes 15/1
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Old 11-13-2014, 07:04 AM   #8
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I am not a Chemist, nor did I stay at a Hoiliday Inn Express last night but IMHO rubber ages. I believe this to be true based on my own life experiences. I am sure it did not help my Scamp's axle's time to failure that the factory used an under-rated axle back in the day. I am 54, and expect my new 3500 pound axle to about do me for the duration. If I happen to out live it I will gladly install another one. After all, since it is now bolted on instead of welded it should be alot easier next time!
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Old 11-13-2014, 07:15 AM   #9
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You were hoping for a simple, one source only answer? There isn’t one. The answer is more likely all of the above.

Rubber will degrade over time due to chemical changes, fatigue due to flexing, heat and ozone. So long as rubber is asked to operate in its “elastic” range the effects will be minimized but not eliminated. The elastic range being where it returns to its original shape after deformation.

Additionally, heat is an issue because rubber will take a “heat set” such as what is sometimes seen in tires. Generally most tire flat-spotting will work out but after reheating/running the tire but there is always some deformation that doesn’t work out. Usually it reduces to something less than what the vehicle is sensitive to. Same thing for any other rubber part.

Overloading can risk asking the rubber to exceed its elastic range to operate in its “plastic” deformation range. Plastic deformation implies something does not return to its original position after deformation. Think of plucking a steel ruler over the edge of a desk. Pluck too hard and ruler bends permanently. That would be plastic deformation. I use the term “risks” because I don’t know whether other axle parts might fail due to overloading long before the rubber gets to its plastic deformation point.

And the elastic/plastic transition point moves. The transition to plastic deformation generally lowers as the material is (over) heated. And as I’ve often related of tires, rubber doesn’t heal. Abuse an axle (or tire) once and that damage is carried for the rest of its service.

And I’d be surprised if there weren’t polyester or nylon cords longitudinally imbedded in the rubber components to add tensile strength to the bands and they’re not as flexible as the rubber is and may in fact be the part that is plastically deforming.

All of this is interesting (to a geek like me) but most of the torque is likely coming from that square metal bar running through the center of the axle. Steel being less flexible than rubber, it’s probably plastic deformation of the bar that’s causes the late life sagging. Its properties change too over time.
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Old 11-13-2014, 07:58 AM   #10
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FWIW until the mid 80's Scamp was installing 1.2K lb axles on the S-13's and 2.2K lb. axles on the S-16's. Also people assume a lot of S-13 axels are bad because they sit low but in reality they sit lower because the swing arm angles usually start at an up angle where the S-16&19's start with downward angle swing arms.
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Old 11-13-2014, 08:42 AM   #11
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My 15' Trillium 4500 came with a 2000lb axle. Seemed kinda light, so I went with a 3500lb axle. I would go with a 2500lb axle next time, (on another trailer).
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Old 11-13-2014, 09:29 AM   #12
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Old 11-13-2014, 10:45 AM   #13
Name: will
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Our '79 Scamp 16' had a 2200# axle. I replaced it with a 3500# Dexter torflex derated to 2400#. I wanted the heavier bearings and larger brakes.
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Old 11-13-2014, 12:19 PM   #14
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Originally Posted by P. Raz View Post
Every picture tells a story.
I have to wonder exactly how something like that fails. I would not think all four “rubbers” give up simultaneously or even at the same rate. So one gets a little weak and puts more stress on the next weakest one and so on. They might even swap the order around a bit, whichever one is the weak link at the time. I have an old backhoe, and it is a constant source of amusement and frustration. I fix one leak in a hydraulic circuit and pretty soon something else in that circuit is leaking. It finally dawned on me that a leak is acting as a pressure limiting device. Once fixed the pressure finds the next weakest area and causes it to start leaking and relieving pressure, and so on.

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