Tow Capacity Calculations

[hdrehder]

We finally weighed our Jeep Grand Cherokee/14DBS rig on Saturday, and I put together this chart below (hopefully, it will enlarge like a photo when you click on it).  Looks like I'm over in TV weight and under in trailer/tongue weight, with about 500 more pounds to my tolerable max.  If I split that between tongue and trailer axles, then I'd be as heavy as I want to be.  If I'm off in my assumptions, would be glad to hear it.  Also happy to share my spreadsheet if any interest.

When the dealer mounted my E2 WDH, they covered the sticker that states GVWR and GAWR figures.  I'm wondering what those say.  We have the beefy 14" wheels and offroad axles.  Wonder if that increases the overall GVWR from 5,000 to 6,000 for the trailer....  Looks like the GAWR is 3,000 per axle, but can't tell for sure.

[attachment id="763" thumbnail="1"]

[daplumbr]

The chart enlarged fine. It took a minute to absorb it all; it's pretty comprehensive. My interpretation is that you are correct in your assumptions. All in all, it looks pretty good, except for those TV axle weight numbers. When I visited the factory, I noted that the axles used (at least in 2014) for the standard package and the off-road package where the same axles; they simply mounted the off-road package axles on a wedge of aluminum that raised them a bit for the larger wheel/tire clearance. So, if the axles are the limiting factor in weight capacity, then the same 5000 lbs applies. If the tires are the limiting factor, you would have to go by the info on the sidewalls of the regular ST and the off-road package LT tires. I really doubt that the off-road package would increase the capacity from 5000 to 6000 lbs, in any event. 

[charliem]

[font size="3"]CMac,

First let me complement you on the detailed collection of data. Then let me make a few observations:
 [ol type="decimal"][li]I agree your tongue weight, sans WD, is 6160-5860=300.
[/li][li]The total trailer weight, sans WD, is 3840+300=4140.
[/li][li]Tongue weight is only 300*100/4140 = 7.2%. This is too light and is totally a function of trailer loading. Anything less than 10-15% is asking for sway problems. This is the first thing to work on. Move weight forward and shoot for around 500#.[/li][li]Removing and reattaching the E2 bracket to read the GVWR is only a few bolts. No problem unless the sticker is damaged.[/li][li]The trailer GVWR is primarily a function of axle and tire rating; secondarily a function of frame design and axle attachment. Look for a sticker on the axles themselves. They might be the standard 3500# axles LL uses on other trailers. Then check the tire load capability at max inflation. This from tire published data. Take the lesser of the two values as GVWR. The frame issues, if any, should be negligible. LL has been very conservative in their advertised GVWR in the past.
[/li][li]The off road axles are just the standard axles mounted at an angle, as has been noted by Merlin. On my 21RBS the BFGoodrich LT tires are rated slightly lower than the standard ST tires and lower than the combined axle ratings. Therefore my GVWR is limited by the tire ratings.
[/li][/ol]

[/font]

[hdrehder]

Thanks Merlin and charliem for your comments.  Charlie, I thought that I might hear that the tongue was too light.  Thank you for confirming. We've already got two propane tanks and two batteries.  We're storing bikes in the trailer, so may look for a way to put them on the tongue. There is a product called a bike bunk, but it's seems overly expensive to me, as it is only a mount that lifts over the tanks and requires that you provide your own rack.

[daplumbr]

But, wouldn't adding tongue weight aggravate the overage on the rear axle of the TV? I may be reading the chart wrong, but that's what it appears? Perhaps you could consider lightening the trailer overall, if possible, to get closer to the 10-15% factor for tongue weight. Like Charlie, I applaud you for really looking into weight distribution. You'll be safer for it. 

[charliem]

[font size="3"]Yes, increasing tongue weight will increase the load on the TV rear axle, but that's what the WDH is for. The WDH shifts rear axle load onto the TV front axle and the trailer axles. The TV rear axle is within factory limits now so it should be OK. I'm also curious as to what is being carried in the trailer. The 4140 seems a lot compared to LL's advertised 2650 advertised dry weight, even given the two propane tanks and two batteries. Are those two bikes Harleys?   8-)
[/font]

[hdrehder]

[quote timestamp="1429615585" author="@charliem" source="/post/9688/thread"][font size="3"]I'm also curious as to what is being carried in the trailer. The 4140 seems a lot compared to LL's advertised 2650 advertised dry weight, even given the two propane tanks and two batteries. Are those two bikes Harleys?   8-)
[/font][/quote]To quell your curiosity, we carry (almost) all the comforts of home.  But I too wonder what puts us so far over the advertised dry weight.  The Harley wannabies are Trek and Rock Hopper mountain bikes, so that doesn't add too much.  Extra propane, extra battery, 5 - 14" tires, 1/4 fresh water, couple bags of ice in black tank. No dumbells or other weird weighty things. ... Just dug into the purchase paperwork and the TX Dept of Transportation forms state a dry weight of 2,980 and a carrying capacity of 3,020.  So it appears that the offroad package (14" tires and shifted axle) increases the GVWR by 1,000 to a total of 6,000 lbs.   

[tincan]

I have to say I do not understand. You are not over weight at all. If your GCWR is 13,000 pounds you, at the most, weight 10,000, then you are golden.  24% under max GCWR

Tongue weight cannot be calculated that way.  GCW minus GVW(TV) minus GVW(Trailer) should equal zero every time.

10,000 - 6160 = 3840 which is your exact GVW(trailer)

You cannot use GVW(TV) only, you have added a hitch with bars to the TV GVW in your GCW calc.

Tongue weight is about balance over the pivot of the axles not about GVW(Trailer).  I could could stack a 1000 pound of lead on a trailer right over the tongue or right over the rear bumper and get completely different results for tongue weight, but the total trailer weight remains unchanged.  

If you make the assumption your trailer is perfectly balanced you would have 10% of trailer weight pushing down on the ball of your hitch. In your case 380 pounds.

The reason they make tongue weight scales is that you cannot calculate it easily.  You have to know the weight of everything, trailer structure included, and do a massive sum the moments about the pivot equation.  This can be short cut by starting in the middle, making an assumption the trailer is balanced at 10 % and only do the equation for stuff you add.  It is SOOOOOOOO much easier to just set the tongue on a scale.

You were at a scale, You drive on so trailer axles are off scale but tongue in on scale, park, unhook, level trailer, drive TV off, collect weight. Presto you have tongue weight.  No way to calculate it with the numbers you presented.

[pinstriper]

Yeah, my 14DBS with just about all the options and then loaded up with clothes, food, domestics, just 1 battery scales at 3600#. I dunno how I would add 500# to it without hauling silver ingots or something.

[charliem]

[quote source="/post/9738/thread" timestamp="1429669744" author="@tincan"]I have to say I do not understand. You are not over weight at all. If your GCWR is 13,000 pounds you, at the most, weight 10,000, then you are golden.  24% under max GCWR

Tongue weight cannot be calculated that way.  GCW minus GVW(TV) minus GVW(Trailer) should equal zero every time. [font color="e61967"][font size="3"]This tongue weight is correct if you use the "Actual Weight sans WD" column. [/font][font size="3"]The TV-GVW in this column actually includes the undistributed TW. The tongue weight in the "Actual Weight W/WD" column is not truly tongue weight.[/font][/font]

10,000 - 6160 = 3840 which is your exact GVW(trailer) [font color="e61967"][font size="3"]The [font color="e61967"]38[font color="e61967"]40 is [font color="e61967"]only the [/font][/font][/font]weight on the trailer axles. You must add TW[font color="e61967"]=300 to obtain total trailer weight.[/font][/font][/font]

You cannot use GVW(TV) only, you have added a hitch with bars to the TV GVW in your GCW calc. [font color="e61967"][font size="3"]True, but minor effect[/font][/font]

You were at a scale, You drive on so trailer axles are off scale but tongue in on scale, park, unhook, level trailer, drive TV off, collect weight. Presto you have tongue weight. [font color="e61967" size="3"]Correct. It's also easy to measure TW with a bathroom scale, a 4x4, and some diagonally  cut 2x2s[/font][font color="e61967"][font size="3"].[/font][/font]
[/quote][font color="1947e6" size="3"]
And it's way too early in the morning for me to be this deep in it !  Coffee time.   :-/
[/font]

[charliem]

[quote source="/post/9740/thread" timestamp="1429670998" author="@pinstriper"]Yeah, my 14DBS with just about all the options and then loaded up with clothes, food, domestics, just 1 battery scales at 3600#. I dunno how I would add 500# to it without hauling silver ingots or something.
[/quote][font size="3"]Please send large sample of silver ingots. I'd like to try it.   [/font]

[tincan]

[attachment id="769"]

I have made a basic sheet to explain what I mean with a sum of moments calculation.  Sorry CharlieM, I know it is early, but once you engage the brain of an engineer,... well you can guess.

This is a very basic and simplified example with a TON of assumptions. 

Tongue weight cannot be calculated by subtracting trailer and tow vehicle weight from gross combined vehicle weight. 

If you weight the TV(with hitch) and the trailer separately, it should equal GCVW(weight of truck and trailer when hitched).  If you are using a WDH you should see a shift in weight per axle, but total weight will remain unchanged.


I will correct myself and say that on a multi-plate scale like a CAT truck scale, if you put the unhitched trailer on 2 plates, and you level the trailer, you should get the tongue weight and the axle weight for the trailer all at one time. 

[charliem]

[quote source="/post/9757/thread" timestamp="1429707516" author="@tincan"]

I have made a basic sheet to explain what I mean with a sum of moments calculation.  Sorry CharlieM, I know it is early, but once you engage the brain of an engineer,... well you can guess. [font color="1979e6"][font size="3"]Yeah, I know. That's what wakes me[font color="1979e6"] at night. Just can't shut it off.[/font][/font][/font]

This is a very basic and simplified example with a TON of assumptions.  [font color="1979e6"][font size="3"]No way I'm going there even when I'm awake. Theory i[font color="1979e6"]s solid, but [font color="1979e6"]way[/font][font color="1979e6"] to many unknowns.[/font][/font][/font][/font]

Tongue weight cannot be calculated by subtracting trailer and tow vehicle weight from gross combined vehicle weight.  [font color="1979e6"][font size="3"]There are several ways to look at CMac's [font color="1979e6"]data. We can get all wrapped up in where the WDH and bar [font color="1979e6"]wei[/font][/font][/font][font size="3"]ghts are, but we end up [font color="1979e6"]within 20# of the TW. I [/font]think we can agree on two things: The TW is less than 10% and TW can be more accurately determined by direct measurement. [/font][/font]

If you weight the TV(with hitch) and the trailer separately, it should equal GCVW(weight of truck and trailer when hitched).  If you are using a WDH you should see a shift in weight per axle, but total weight will remain unchanged. [font color="1979e6" size="3"]Totally agree.[/font][font color="1979e6"]
[/font]

I will correct myself and say that on a multi-plate scale like a CAT truck scale, if you put the unhitched trailer on 2 plates, and you level the trailer, you should get the tongue weight and the axle weight for the trailer all at one time.  [font color="1979e6" size="3"]Totally agree here too, though I'm not sure of the accuracy of the CAT at 300# ??[/font]
[/quote]