Need To Move Ball Mount Aft - Options?

Marceline

Dalehelman said:

IMHO The safest speed limit is what ever the traffic flow is. Interstate 84 through Idaho.

If you post 80mph people will drive 80mph (or faster), even if they’re in a vehicle that shouldn’t be going that fast.  No one should be surprised to learn that the states with the highest speed limits have the highest per capita traffic fatality rates. 

https://www.iihs.org/topics/fatality-statistics/detail/state-by-state

GrandmasKid

Having worked in hospitals for 41 years I can tell you that dying from an accident isn't the only thing we should be concerned about. Slow down, keep your distance and otherwise be respectful of others. If we can't do that we shouldn't be allowed to drive.

Denny16

That is a better solution by far.
cheers

AnOldUR

Early in this thread the discussion talked about the distance from the pin hole to the ball as being the relevant number. This had me scratching my head. My non-engineer brain pictures the important distance would be the distance from the rear axle (fulcrum) to the ball. Later posts agreed with this. The problem is that auto manufactures don't seem to be taking this into consideration when setting tongue limits. The 10-15% rule is applied instead. It's crazy that legal implications are based on generalizations when the math is not that difficult. For legal purposes some kind of ratio between tongue weight and axle to ball distance would be more appropriate. Here's a look at a Jeep Wrangler and a Ford F150. There's been talk here saying that a 12" extension will have a big impact. The distance between the ball and the rear axle is considerably different on these two TV's, yet they follow the same rule for determining tongue weight limits. None of this is as scientific as it should be. Just a lot of generalizations by companies trying to cover their butts. What's a person trying to use common sense suppose to do? Shouldn't a short extension on the Jeep give the same result as the standard F150 ball location?

gulfarea

AnOldUR Good point Art

Horigan

The issue and strength concern is the distance from the ball to where the hitch receiver mounts to the vehicle frame.  When a vertical load is applied to the ball, it imposes a twisting force (torque or moment) downward on the hitch receiver and the attachment bolts on the frame, and the nut inserts in the frame.  It's not just a vertical load imposed on these parts.  The receiver and frame attachments are designed for a particular vertical load and torque, with presumably some broad assumptions on percentage of tongue weight, dynamic loads, braking, etc. 

If you double the distance from the ball to the receiver attach points you double the torque (twisting force).  In my free body diagram way above to illustrate an example torque increase I measured from the pin since that's easier to measure and visualize.  Some receiver extenders wisely tell you to halve the tongue load.  

Another way to think of it is, if you put a 10 foot square bar in your hitch receiver and stood on it, you'd likely bend the hitch receiver and/or attachment to the frame.

Hope this helps.

Denny16

Sounds like a math issue coming...

cheers

AnOldUR

Horigan said:

Another way to think of it is, if you put a 10 foot square bar in your hitch receiver and stood on it, you'd likely bend the hitch receiver and/or attachment to the frame.

If that's a bet, I'll take it. I would think that the front wheels would lift off the ground before there would be any hitch damage. My uneducated guess says that with the ball out, say 5' from the TV rear axle and then adding another 10' for your hypothetical bar, a 200lb person standing on the end would put about 600lbs on the ball. Is that about right? Your illustration earlier used the end of the receiver as the fulcrum. I don't think that accurately represents the system we're dealing with here. I don't see the end of the receiver as a static pivot point. It will move about the rear axle. Any engineers care to chime in?

edit to add:
I'm a retired machinist. What do I know?

Horigan

Well actually..., I'm a retired Boeing Chief Engineer of Flight Controls and Mechanical Hydraulics, and I've only been retired for a little over a year, so I still remember some of this stuff. 

Let's consider that the hitch receiver is only 5" deep, per my diagram.  Let's assume it's half a foot.  When you stand on a ten foot bar at 200 lbs you're imposing a 2000 ft-lb moment about the aft end of the receiver.  To balance that moment on the front end of the receiver, 6" away, it has to react 4000 lbs to counter the 200 lbs ten feet away.  Now you're starting to yield parts that are intended to react 500 lbs tongue loads on a regular hitch that's less than a foot long.

Another way to think about it is with a teeter totter.  Think of one that has 10' on one side and 6" on the other.  Put 200 lbs on the 10' side and you need to put 4000 lbs on the other side to get it to balance. 

Edit:  Regarding my focus on the receiver, that's where the loads are most elevated.  Yes they are transmitted through the structure to the rear axle and yes if the receiver and supporting structure were strong enough you could lift the front wheels off the ground with a sufficiently long hitch bar, but the loads in the receiver would be very high given the short moment arm it has to react the long hitch bar loads.

AnOldUR

@Horigan ...  I believe your edited statement is correct. The stresses within the receiver tube is a small independent system with its fulcrum being the end of the tube. That tube has a collar at the end to reinforce it. The risk of failure at the end of the receiver tube is very small. The forces in your previous diagram are independent from whats applied to other locations and connection points.  Baring a collar failure, they have no impact on the overall system where the fulcrum is the rear axle. The numbers on your illustration are misleading and do not represent how an extended hitch impacts tongue weight and sway. It isolates the stresses at the end of the receiver tube and ignores everything but failure at this single point.

mods:
If you feel this conversation is getting too argumentative and has no benefit to readers please let me know. I'll delete and move on.

Sharon_is_SAM

@AnOldUR - if the weight impacting the receiver does not impact the rear axle, how is it that when looking at TV payload you must subtract the trailer tongue weight?  

AnOldUR

Sharon_is_SAM said:

@AnOldUR - if the weight impacting the receiver does not impact the rear axle, how is it that when looking at TV payload you must subtract the trailer tongue weight?  

I'm not sure what comment of mine you're referring to. I don't recall saying that the weight on the receiver doesn't transfer to the rear axle. It clearly does. From what I understand, the TV payload is defined as the weight of passengers and cargo and that part of the payload capacity is tongue weight. I believe it's any additional weight that rests on the vehicle’s wheels.

Horigan

AnOldUR said:

@Horigan ... 
mods:

If you feel this conversation is getting too argumentative and has no benefit to readers please let me know. I'll delete and move on.

@AnOldUR No worries.  I'm good.  I didn't feel we were getting argumentative.  I'm sorry if I came across that way.

The bottom line is that I personally would not use a hitch extender without reducing the tongue load, regardless of what the hitch extender documentation says.  I was just trying to explain why some extender documentation say to reduce tongue load.  Everyone is free to use these as they wish.

AnOldUR

Horigan said:
The bottom line is that I personally would not use a hitch extender without reducing the tongue load, regardless of what the hitch extender documentation says.  I was just trying to explain why some extender documentation say to reduce tongue load.  Everyone is free to use these as they wish.

We are in complete agreement here. I came into this discussion looking for information because I thought I'd need a few extra inches of extension to gain access to the back of my Jeep. Turns out that extending the ball mount was not necessary. My replies were mainly just me trying to justify the math and get it all straight in my head.

Sharon_is_SAM

@AnOldUR - “ Baring a collar failure, they have no impact on the overall system where the fulcrum is the rear axle.”   Sounded like you discounted the impact on the rear axle.  Glad you don’t need to extend your hitch after all.  

rh5555

I think we all agree that making the trailer longer will not negatively affect the handling of the TV, and may even improve it.  So...

This is the modification I'm thinking about for our TAB400:

and shown with the tub in place (for reference):

It will require minimal modification to the frame (at the front, where the 3"x4" box tube passes through) and can easily revert to the original design at a future date.  The jack gets moved off-center so that the box tube doesn't get a big hole in it.

Anyone want to try this out for me?

AnOldUR

rh5555 said:

I think we all agree that making the trailer longer will not negatively affect the handling of the TV, and may even improve it.

You won't get that agreement from me. Too many variables. It will decrease your tongue weight. That being and improvement or not depends on your starting point. Furthermore, "A longer trailer can lead to the increase chance and severity of trailer sway." (quote from etrailer.com)

Bayliss

@rh5555, I tend to agree with @AnOldUR, but that is just based on what I have read over time.  You should contact the frame manufacturer to get their input and advice before making an alteration like that.

rh5555

@AnOldUR: I think your quote is a little misleading.  They are talking about getting a longer (overall) trailer (like an Avia), not talking about extending the trailer tongue.  I found this write-up quite interesting.  Also his section on Strength.  For a full on analysis, see the attached pdf.

gophergrad

I had to get a stinger with a much greater drop when I changed to the RAM 1500 as a TV.  The one I got from etrailer had a longer horizontal distance than did the previous one.  I was worried about the tailgate, but it just clears.