2000lb or 3500lb axle for TTT

[madjack]

Brian, this comes from 20+ yrs and over 2million miles in the trucking industry and over 40 yrs of experience with private utility trailers...this is common practice in the industry for the best longevity and ride characteristics and holds doubly true for trailers that keep a constant load on them such as TD's have...ask Doug Hodder what 24 hrs hammering at 80MPH down the intestate can do the springs that are matched exactly to load...18wheelers, which have a 17k#'s per trailer axle load limit are typically spec'd at 25K#'s...I have also discussed this with engineers and find consensus with them on this issue...SUPPOSEDLY, this is not as critical with torsion type axles BUT I have done this as well on my builds and find that NOTHING moves in the camper because the ride is near perfect.........
madjack

[brian_bp]

Thanks, madjack. It seems that the key is the constant loading.
I'm more familiar with automotive ratings, where it would be very strange to find axle ratings 50% higher than the typical working load, especially on a front axle... and none of the suspension parts on most brands of vehicles are prone to failure.

Of course, with the average recreational trailer putting about the same road distance in a decade as a car does in a year, longevity shouldn't be a big issue.

Most of the moulded fiberglass travel trailers, which typically have rubber torsion suspensions using the lightest axle with sufficient capacity to just make the rated gross weight, seem to have no component reliability problems. It seems to take at least a couple of decades to kill the rubber in them, but it does eventually die, leaving them sagging and with little spring action left.

I suspect that one issue in the big rigs is that they typically have tandem axles (on both trailers and drive axles) which do not completely equalize, so each axle is subject to shock loads which are greater than a single axle trailer of half the weight and the same axle... this comes up each time a tandem teardrop or tiny travel trailer is discussed, and is specifically mentioned by the axle manufacturers.

[kennyrayandersen]

Kenny,

The more robust axle actually has no effect on shock loads as the rubber rating inside or the spring ratings determine shock absorption. Thus, an axle rated at 3500 lbs. won't make your ride any more stiff, by itself. The heavier axle, (which has the penalty of extra weight, as you note) is simply stronger, which is crucial. The weight of the wheels doesn't even pertain to axle loading (since it is unsprung weight) but is important to axle integrity. I don't want to snap an axle. In my case, you have to think of the overhang as the distance from a pivot point (where the axle is attached to the frame) or fulcrum. The entire wheel becomes a weight that is suspended from that fulcrum and acts like a lever, so that the increased force of that weight is actually the square of the increased distance from the pivot point. Since my wheel is about 40% farther away than recommended, the force on the end will be many times greater. At least that is the way I understand it.

I don't think that this is correct, and I'll try and explain why. The torsion axles vary two parameters (with regard to spring rate) in order to adjust for the trailer load weight. They are the length and diameter, or durometer rating of the rubber. By putting more material in or changing the diameter or hardness of the rubber the spring rate is changed; therefore, when a load is applied the deflection changes. The higher rated axle has a longer spring section which in fact increases the stiffness of the suspension. The g-loading the trailer sees in a function of stiffness; so, when you change the stiffness, the g-loading, or shock loading if you will, changes. Increase the stiffness [with a higher rated axle] and the shock loads do indeed go up. Though you are correct that the bending moment increases with an increased overhang, the axle manufacturer is not going to give you an undersized spindle IMO. The wheels and tires are unsprung, but the axle is not. Either, way you aren't getting any benefit to over-sizing the axle, but you are paying a weight penalty.

A 600 load axle is rated for 600 lb trailer (loaded) and that rating includes the spindles. The weight of the wheel is a force acting in the down direction, the reaction force on the wheel due to the weight of the trailer is up. Thus the weight of the wheel and tire is irrelevant, but the reaction at the wheel due to the trailer weight is what creates the bending moment (perhaps that is what you meant to say?). So, unless you PLAN on overloading your tear, I'm still not seeing the benefit of a higher-than-needed axle. The axle will be stronger, but is will 'tend' to beat the trailer up more.

Whether or not it is significant, I can't say, but I guess that since the loads that most of the tears are seeing (according to the finite element modeling that I've done so far) are not particularly significant, so perhaps not. Still, I guess I'm a weight freak as my day job is as a structural analyst in the aerospace field, so for me I'm trimming the fat. The uber ultra, so far, looks like I should be quite close to the 500 Lb fully-loaded weight including AC.

One other quick point that was ask about. The trailer weight is the maximum loaded weight and includes both cargo and trailer; therefore, a 2000 Lb axle would be used on a trailer, that when loaded, weighs 2000 Lb. Oversizing them a bit is probably not that big of a deal, and an OEM trailer manufacture that I've worked with in the past probably makes then a little stouter than needs be, but it's really not necessary and especially in the case of a little teardrop trailer.

[madjack]

Bryan, static load is the point to what I was saying about ratings...if you have a trailer with a 1k load rating and you have 1k worth of weight on that axle at all times (static load), the axle is at it's maximum recommended deflection at all times...this puts constant stresses on the springs and will thus weaken the springs over time(ask any metallurgist)...now, hammer down the Intestate for an extended trip with that spring which is fully loaded to begin and you end up with a situation in which the spring can literally beat it's self to death, because the "G"force loadings are ALWAYS over the max recommendations(mass and momentum...thank you Mr Wizard)...seen it happen WAYYYY too many times...this is the reason many older campers you see, seem to be "squatting" or are in need or spring replacement(read vintage trailer forums about this common problem)...I find it very disingenuous of you to at one time knock a manufacturers recommendations(ProMariner) and at the same time use manufacturers guidelines to post info...the info I have posted comes not from a book or reading tech manuals but from hard won experience in the trailering industry and not just the heavy 18wheelers but trailers of all types and sizes.........
madjack

[brian_bp]

... static load is the point to what I was saying about ratings...if you have a trailer with a 1k load rating and you have 1k worth of weight on that axle at all times (static load), the axle is at it's maximum recommended deflection at all times...

If the static load is equal to the axle rating, then the suspension will sit at its maximum static deflection, not at the maximum allowed deflection during travel (shock), right? The Dexter Axle manuals for Torflex show static and shock deflections nicely, to illustrate this (yes, Torflex use rubber springs but the idea is the same and the Torflex drawings are often shown in this forum).

...this puts constant stresses on the springs and will thus weaken the springs over time(ask any metallurgist)...

My understanding is that constant stress is not the killer of steel structures (including springs); the killers are deflection beyond elastic limits, and cyclic deflection approaching those limits. One great thing about steel is its elasticity, but there limits to it. If the axle's springs are forced past the designed shock deflection repeatedly ("hammering" down a road), I can see how that certainly would be bad.

[kennyrayandersen]

Bryan, static load is the point to what I was saying about ratings...if you have a trailer with a 1k load rating and you have 1k worth of weight on that axle at all times (static load), the axle is at it's maximum recommended deflection at all times...this puts constant stresses on the springs and will thus weaken the springs over time(ask any metallurgist)...now, hammer down the Intestate for an extended trip with that spring which is fully loaded to begin and you end up with a situation in which the spring can literally beat it's self to death, because the "G"force loadings are ALWAYS over the max recommendations(mass and momentum...thank you Mr Wizard)...seen it happen WAYYYY too many times...this is the reason many older campers you see, seem to be "squatting" or are in need or spring replacement(read vintage trailer forums about this common problem)...I find it very disingenuous of you to at one time knock a manufacturers recommendations(ProMariner) and at the same time use manufacturers guidelines to post info...the info I have posted comes not from a book or reading tech manuals but from hard won experience in the trailering industry and not just the heavy 18wheelers but trailers of all types and sizes.........
madjack

I believe that you guys are talking about two different things. I believe that they are both valid, but a little different.

Brian is right in that a 1000Lb trailer is NOT loaded to it's maximum dynamic loading -- just it's maximum static loading. Dynamic loading is another issue. Typically, according to a truck and trailer handbook I haven't seen for 20 years, over-the-road vehicles are designed to 3g on-road and 5g off-road. The 34,000 lb load typical to the rear axles in an 18-wheeler is a static number (I'm going by memory here). When a truck goes onto the scales -- this is the number that is checked (Not only does the gross weight, typically 80,000 lb total, need not to be exceeded, but the per axle weight on any of the axles can't be exceeded).

However, this is not the maximum load that the axle (or trailer for that matter) is designed for. The maximum is the static load times the intended use factor (3gs on-road, 5gs off-road); so, it possible that someone chooses to use a little over capacity on the axle hoping to extend the life, but for every lb you have in the rig, it's one less lb you can haul, so these days even the commercial folk are being more weight-conscious.

After many thousands of miles (I'm thinking teardrop trailers don't fall into this category unless they are consistently over loaded (which may actually be the case with a vintage TTT)) fatigue can set in and the suspension can sag etc. This is usually after MANY thousands of miles though -- the harder the use the fewer miles before this occurs.

I think a bigger issue for the torsion-type axles in that over time the rubber will break down and have to be replaced (even this should be 5-10 years down the road). The slight over-capacity won't mean much if the rubber has gone to pot. So, I think if you e-mail Dexter, tell them your gross weight, they are going to tell you to get the appropriately sized axle. I could be wrong.