1st trailer question.

[Desolation]

After years of looking I have decided to built what it now called an Expedition trailer. My "Idea" folder was started long enough ago that I labeled it "Off Road Trailer".
I've been reading and stealing ideas for a few evenings; I'm going to have a few more questions as I finalize the CAD drawings here is the first question I have not found an answer to.
The tongue of this trailer will telescope allowing about a 30" reduction in the over all length. The question that this brings up is would towing on pavement at speed improve with the increase in tongue weight (full short)?

2nd. Question.
Because this is "my" trailer I decided to locate the coupling as high as I can for better angles off road, and I have designed the frame of the trailer accordingly. However I got to wondering if there is an optimal geometry... for example with a 33" OD tire is there a relationship to the height of the coupling?

Thanks in advance! This is a great site!

 

[R_Lefebvre]

1) That's difficult to question to answer as there are a lot of variables, many depending on the tow vehicle in question. Generally speaking, you want 10-15% tongue weight. I don't think more really helps with stability beyond that. Some heavy duty trucks may ride better with more weight on them.

2) I don't think their is a relationship to the height of the tire per se. Having it at the height of the frame of the truck can reduce forces in the coupler. However, some people are building trailers with really high ground clearance, which results in a really high CG and possible tippiness.

It's all about compromises, you have to decide what you want to do with the trailer. Personally, I think there's only so far you can go with a trailer in any case, due to traction on the tow vehicle, etc. So I went with a lower frame height and lower cg on mine, giving me more cargo space, etc.

 

[Desolation]

Thanks Rob!
I agree on all counts and when the trail gets bad obviously I'll unhook, but I decided to compromise in those places in particular, and find I have once again exceeded the limited scope of my experience.
In addressing the tippy I have decided to do an exo-cage, making the trailer as Fool(me)proof as possible

 

[slimtwo]

Well you definately chose the right place to get you information from, and Rob, I feel is one of the many great sources of information on trailer builds. I, like you have been compiling information for my future trailer build, and picking the brains of those who are either in the process, or have completed their trailer. Well good luck with your trailer, I hope to see some posts from you with some pic's. Who knows, I might be able to "borrow" some ideas from you.

Happy Trails!

 

[jcbrandon]

...
The tongue of this trailer will telescope allowing about a 30" reduction in the over all length. The question that this brings up is would towing on pavement at speed improve with the increase in tongue weight (full short)? ...

Welcome aboard.

I don't think changing the length of the tongue by 30" will significantly change the tongue weight, unless you have a lot of cargo on the adjustable portion of the tongue that also gets moved closer to the axle.

Let's look at an example:
Say your trailer weighs 1,150 pounds with the tires carrying 1,000 pounds (500 pounds each) and the remaining 150 pounds on the hitch for about a 15% tongue weight.

If you shorten the tongue length by 30", how much total weight have you moved to the axle? Really, just the weight of the structure that moved. So that's what, 20 pounds of steel? So the tongue weight decreases by 10 pounds and the weight on the axle increases by 10 pounds. That won't significantly affect the tongue weight.

What you have done is significantly changed the length of the lever formed by the hitch point and the tire contact patch. And increasing the length of this lever, rather than shortening it, will improve towing at highway speeds -- by reducing sway -- up to a point.

Don't confuse an adjustable tongue length with changing the position of the axle in relation to the center of gravity of the trailer. That's the only way to significantly change tongue weight with a given total trailer weight.

 

[R_Lefebvre]

Sorry no, there's a little more to the math than that, but I'll start with your numbers. Let's say 1150lbs, with an axle to short tongue distance of 60". To end up with your tongue weight of 150lbs, the CG is 7.82" in front of the axle. If the axle is extended 30", to 90" total, the tongue weight becomes 99.92 pounds. So a 50% reduction.

You can't figure it the way you did, you must use a Free Body Diagram.

 

[Desolation]

Welcome aboard.

Thanks! This place is a wealth of info!

As for the weight shift, based on my experiments with a scale and a trailer changing only the tongue length increased the weight indicated on the scale.

I should note that the weight shift is not my reason for doing it.

 

[jcbrandon]

Sorry no, there's a little more to the math than that, but I'll start with your numbers. Let's say 1150lbs, with an axle to short tongue distance of 60". To end up with your tongue weight of 150lbs, the CG is 7.82" in front of the axle. If the axle is extended 30", to 90" total, the tongue weight becomes 99.92 pounds. So a 50% reduction.

You can't figure it the way you did, you must use a Free Body Diagram.

Can you give me some tools so that I can understand this better? I don't see how moving just the hitch point forward or aft can have a significant affect on the location of the center of gravity relative to the center of the axle.

Is it because the trailer tongue is a lever, the CG is the load, and the axle is the fulcrum? Is so, do we assume that the trailer frame, the lever, is level? Would the results change if the trailer were not level?

And, just to clarify, are we talking about merely moving the hitch point in relation to the axle? Or are we discussing the possibility of moving the axle in relation to the rest of the trailer?

 

[Desolation]

Can you give me some tools so that I can understand this better? I don't see how moving just the hitch point forward or aft can have a significant affect on the location of the center of gravity relative to the center of the axle.

Is it because the trailer tongue is a lever, the CG is the load, and the axle is the fulcrum? Is so, do we assume that the trailer frame, the lever, is level? Would the results change if the trailer were not level?

And, just to clarify, are we talking about merely moving the hitch point in relation to the axle? Or are we discussing the possibility of moving the axle in relation to the rest of the trailer?

1. Moving the hitch coupling closer to the axle.
2. To have tongue weight the CG must be ahead of the axle some amount. If nothing else changes and the tongue is shortened the ratio of the distance between the axle and the CG to the CG and the coupler changes thus the load or tongue weight changes.
3. I could be wrong but that is what it looked like to me...

 

[R_Lefebvre]

Can you give me some tools so that I can understand this better? I don't see how moving just the hitch point forward or aft can have a significant affect on the location of the center of gravity relative to the center of the axle.

Is it because the trailer tongue is a lever, the CG is the load, and the axle is the fulcrum? Is so, do we assume that the trailer frame, the lever, is level? Would the results change if the trailer were not level?

And, just to clarify, are we talking about merely moving the hitch point in relation to the axle? Or are we discussing the possibility of moving the axle in relation to the rest of the trailer?

Moving the hitch point doesn't change the CG at all relative to the axle. But you've got it now, the trailer tongue is a lever, and the longer lever lessens the force. You have to do a moment calculation.

Remembering the number off the top of my head, the CG is 7.82" in front of the axle, resulting in a torque of 8993 in-lbs on the body of the trailer. I determined the 7.82 with a bit of work. So, 8993 in-lbs on a 60" lever results in 150 lbs of force. 8993 in-lbs on a 90" lever results in 100lbs of force.

If the height of the CG and the hitch is exactly at the same height as the pivot point (the axle, or possibly the contact patch of the tire, depending on how you approach this) it won't matter what the level of trailer is. However, that's not realistic. So, say the CG is 15" above the axle and 7.5" in front. As you lift the tongue, the CG migrates backwards relative the axle. By about 30° tilt (I'm winging it right now, could be off), the CG is directly above the axle, and the trailer is ready to tip backwards.

You can really feel this on my trailer now. The CG is just in front of the axle, not much tongue weight, but the CG is maybe 36" above the axle. It doesn't take much tilt to get the trailer to want to tip over backwards.

 

[ntsqd]

JC, the Free Body Diagram was developed just for this sort of thing, but I find that for just considering axle and tongue weights with a horizontal trailer frame that the set-up for a Shear-Moment Diagram paints a clearer picture. I spent some time last night looking for a page that was a simple explanation of how to do this and with the applicable relatively simple formula listed. Didn't have a lot of luck with that.

I'll try to have a second go at it after this weekend.