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Old 04-05-2006, 08:47 PM   #1
rickst29
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Default Tongue Weight vs. WDH sizing... don't forget the TV loading!

We have lots of posts which emphasize, rightly, that TM's 'tongue weight' figures are for totally empty trailers without air conditioners... or water in the tanks, or clothes, or food, or anything.

But, I don't see any posts which cover the other factor in sizing your WDH bars: the additional luggage which you put in the TV, behind the rear axle. This counts too! The WDH needs to bend/re-distribute that load away from the rear axle and back towards the front axle of the TV, just like it needs to redistribute the Tongue weight.

Here's an example, based on my stuff:

(Factor 1) 2619 unloaded tongue weight = 351 lbs (TM web site). -->plus<--

(Factor 2) Trailer load: 1400 lbs Gross Weight for water, battery, A/C, clothing, and etc. present on the Trailer but not counted in the TM "Dry Weight". The toungue weight should be increased by about 13% * 1400 lbs gross = 182 lbs. -->plus<--

(Factor 3) All the heavy tools and luggage I've loaded into the back of the 4Runner. Roughly speaking, count everything placed behind the center of the rear axle at 100%. For me, typically about 200 lbs more. This weight doesn't appear on the hitch tongue, it's all on the rear axle... which ain't where you want it, that's why you're using a WDH.

Total: 733 lbs with the 4Runner stuff considered, not the 533 lbs which you get by counting only the tongue weight after loading up the trailer. See the problem? In my case, 750 lb bars would be a very marginal choice, possibly subject to $$$ replacement if I hit a few bad potholes.
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Old 04-06-2006, 09:45 AM   #2
pbuck1
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Hey Rick, What a great subject for debate: should we count the TV load for WDH? Here's my 2 cents.
I'm not sure its valid to count the TV trunk contents. That weight is not taken via the hitch components, or more importantly, by the Class III receiver. It's the receiver which is "twisted" by the WDH bars against the TV frame to redistribute the tongue weight. It's probably too much to ask of the WDH to try to compensate for other weights which are not at the tongue.
And if we count TV contents, where do we stop? What about the people in the car? A 200lb friend or two in the back seat just forward of the rear axle but far from the front axle, would really change things, too.

I think you will be in more trouble if you get 1000lb bars and try to adjust them to offset all 700+lbs (just think of that twisting force on your receiver/frame) than if you just get the 750lb bars and offset the tongue weight by a few hundred pounds less which is all that's required to get weight and traction back on the TV's front wheels.

Paul
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Old 04-06-2006, 11:21 AM   #3
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The weight on the hitch isn't the same as the weight put on the back of the TV. Remember that any weight you transfer from the hitch is also loading the suspension on the trailer. The bearings on the TM are pretty small in comparison to those on the rear axle of the TV. Also, think about the trailer tires, which IMHO, are barely adequate for the load they are expected carry. If you're worried about the weight you add to the rear of the TV, maybe air shocks are called for - or maybe helper springs. I have seen trailer tires shredded because too much weight was put on them from a miss-adjusted WDH. I think I remember someone one this site mentioning ruining tires when the dealer adjusted the WDH too tightly.
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Old 04-06-2006, 06:25 PM   #4
rickst29
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Default Where do we stop?

Counting everything behind the rear axle is a good 'roughly speaking' first approximation, and gets you a heckuva lot closer to the right WDH setting than ignoring the out-of-balance TV loading you get when you pile all that stuff in the back. And so, this is what Draw-Tite recommends to Dealers in the 2006 catalog, "matching towing products to customer needs" starting on page 168. (I started this thread after reading Draw-Tite's instructions, and realizing that no one had ever brought them up in the forum before.)

If we were to be really finicky, then the type of vehicle would be a consideration. Pickups, especially short-bed models, don't have a lot of body structure out back... they're naturally heavier on the front axle, and need far less re-balancing help from a WDH. The first couple hundred lbs of Tongue Weight, UNDISTRIBUTED, actually helps balance these pickup trucks! But in contrast, an SUV like mine has a lot more stuff out back (big door instead of liftgate, goes all the way up to the roofline, etc.) and needs more help.

When you put stuff in the very back of a typical long-bed pickup, it's much more unbalanced than putting the same stuff at the back of a regular-bed pickup... because most of 'em are built from the same chassis, with the 'long-bed' differing due to a much longer overhang. Stuff loaded behind the rear axle unloads the front axle, just like a trailer tongue... it doesn't matter that the TV "body" extends into this overhang area. Yes, passengers in the back seat will be loading the rear axle somewhat more than the front axle. But they won't be UNLOADING the front, and there's no need to balance your TV axle loading to the last 1/10 ounce. As a rough cut, do what Draw-Tite says: simply add on just the total weight of luggage stored at/behind behind the rear axle. To the extent that you DON'T count this stuff, your TV front axle is more likely underloaded, with the resulting steering/braking issues.

- - - - -
I must strongly agree with Jim's point, though... the *STANDARD* 2619 and 2720 axles/tires are IMO underconfigured, and even though you want to bring your weight back to the TV front axle (where you need it for steering and brakes), owners of these models must be sure not to push the WDH too far, overloading the Trailer axle.

You can see by, my sig, what *I* think to be the proper solution for the 2619 and 2720.
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TM='06 2619 w/5K axle, 15" Maxxis "E" tires. Plumbing protector. 630 watts solar. 450AH LiFePO4 batteries, 3500 watt inverter. CR-1110 E-F/S fridge (compressor).
TV = 2007 4runner sport, with a 36 volt "power boost".
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Old 04-07-2006, 04:10 PM   #5
Denny_A
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Default Here's what I.......

.....did, then never paid attention again.

With my 4Runner sitting level and UNLOADED and filled with fuel, I measured the front/rear height at selected, easily identifiable points - my baseline reference.

Plugged my WDH hitch into the receiver, then measured the new F/R heights. After that I started piling weights on the hitch and measuring new height F/R at each 50 lb interval. After having added 600 lbs (free weight discs) I had a table of the effect of load on the hitch.

I then plotted an an x-y graph of Front height change vs. weight and Rear height change to weight.

When actually loading the vehicle with the trailer tongue (no cargo or pax), I then had a handle on tongue weight without actually weighing the tongue.
We usually loaded our trailer the same way for travel, so I also had a good idea of the effect of cargo and pax as measured at the hitch. I.e., If the cargo is loded in the TV for travel, and the trailer tongue is on the hitch as dead weight, the change if F/R heights indicated the combined effect on the hitch. Add a couple of pax up front (using free weight equivalents), I could use the combo-graph to come up with a number.

Typically one shoots for the front measurement CHANGE to be equal to the rear height change or less than (by up to 1/2 inch) the rear height change. Within that range the cargo plus pax effect was nil. But, I wasn't loading 200+ lbs of cargo behind the axle.

So, from then on I dropped x-links and measured F/R height change within 1/2 inch whilst ensuring the front NEVER dropped more than the rear!

For my 2720SL, 750# spring bars were just right!
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Old 04-08-2006, 01:32 AM   #6
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Default Yep, no surpise there!

Quote:
Originally Posted by Denny_A
..... But, I wasn't loading 200+ lbs of cargo behind the axle.
I think we're in blustery, finger waving, 100% total agreement! If you don't abuse your 4R the way I do, then the 750 lb bars are gonna be fine . But if you load as I will, then they'll be lookin' a bit marginal (or, maybe even WORSE than "marginal").

I think you've illustrated ONE point of this thread exactly... since you haven't got all my #$%$ loaded back there, you've got nothing to add (beyond your tongue weight) in Draw-Tite's recommended forumula. But folks other folks like me, who DO pile lots of stuff behind the rear axle of their TVs should do the math for their loading, definitely add some lbs and check the result.
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TM='06 2619 w/5K axle, 15" Maxxis "E" tires. Plumbing protector. 630 watts solar. 450AH LiFePO4 batteries, 3500 watt inverter. CR-1110 E-F/S fridge (compressor).
TV = 2007 4runner sport, with a 36 volt "power boost".
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Old 04-13-2006, 09:31 AM   #7
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I find this an intriguing subject, so I decided to go back to first principles and work out what happens with a WDH. So, I've produced two spreadsheets which I've attached (hopefully successfully). I'm an electrical engineer not a mechanical one, so I had to try to recall my high school applied math (levers, moments, etc.) which is pretty rusty, so I cannnot guarantee the accuracy. It would be great if someone expert could check the math, assumptions, etc. and I've included the equations I used for that purpose.

The spreadsheets are in Microsoft Works format since a lot of people don't have Excel. They need unzipping.

In the first spreadsheet you enter your measurements and weights in the outlined boxes ( the inital values are for my TV/TM combination) and the spreadsheet calculates what the WDH tension must be to equalize the net load on the TV front and rear axles. Since this is the maximum tension you want to put on the WDH bars ( the general guidelines allow you to leave a bit more on the rear than the front), I've included a second spreadsheet.

The second spreadsheet allows you to enter your own WDH tension (per bar).
So with this you can mess around with different values and see what happens on the front, rear and TM axles.

One thing I found interesting using the first spreadsheet is that the WDH bars really have to work hard to compensate for trunk contents for only a little benefit at the front axle. For my vehicle combination, 200 lbs in the trunk instead of 50 lbs unweights the front axle by only 24 more pounds, not really significant overall, but the WDH tension has to increase by more than 200 lbs to compensate, taking the bars into a higher load category. I'm not convinced it's worth it in most cases.

Of course, all this is all moot if my spreadsheets are not good.

Paul
Attached Files
File Type: zip WDH spreadsheet 2.zip (2.0 KB, 122 views)
File Type: zip WDH spreadsheet 3.zip (1.9 KB, 87 views)
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Old 04-13-2006, 02:24 PM   #8
Denny_A
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Default

Quote:
Originally Posted by pbuck1
I find this an intriguing subject, so I decided to go back to first principles and work out what happens with a WDH. So, I've produced two spreadsheets which I've attached (hopefully successfully). I'm an electrical engineer not a mechanical one, so I had to try to recall my high school applied math (levers, moments, etc.) which is pretty rusty, so I cannnot guarantee the accuracy. ......snip.....

One thing I found interesting using the first spreadsheet is that the WDH bars really have to work hard to compensate for trunk contents for only a little benefit at the front axle. For my vehicle combination, 200 lbs in the trunk instead of 50 lbs unweights the front axle by only 24 more pounds, not really significant overall, but the WDH tension has to increase by more than 200 lbs to compensate, taking the bars into a higher load category. I'm not convinced it's worth it in most cases.

Of course, all this is all moot if my spreadsheets are not good.

Paul
Your calcs look fine to me. Numbers reveal that going from 50# in the trunk to 200# raised the spring bar force be a total of 160#. Not real smart use of a WDH. It would be better to move the CG of the weight as close to the rear axle as possible.

The disturbing effect is the additional down force on the trailer axle. If one were loaded to 3300# and had an axle limited to 3500#, the 305# down force (200# trunk wt.) on the trailer causes the axle limit to be exceeded. To me, that looms as the bigger problem when attempting to compensate for a heavily loaded trunk.

I tried 400 lbs in the trunk and 914 lbs on each spring bar so that the front and rear axles had the same additional load. The down force on the TM axle increased to 358# (compared to 305 at 200# trunk load), whilst the front axle was loaded an extra 54#. Doesn't seem a useful tradeoff to me.

Additionally, one would need to be knowledgable of the max static torque load which can be applied to the Class 3 hitch.

BTW - nice work.


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Old 04-13-2006, 04:55 PM   #9
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Default Please post the .xls versions

Paul, I'm really looking forward to your formulas!

However, those of us on Linux can't work with 'Works'. (Here would normally follow many sentences of grandstanding about "supporting criminal monopolists" and etc... Let me just say that I strongly recommend openoffice.org to my Windows customers too, and skip the rest.)

Neither openoffice.org nor koffice has bothered to spend their S/W Development time on "import" of .wks worksheets. They have good compatibility with xls. I don't know about other Office Packages, these are the two which I use and support with a little Q/A from time to time.

Thanks in advance
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TM='06 2619 w/5K axle, 15" Maxxis "E" tires. Plumbing protector. 630 watts solar. 450AH LiFePO4 batteries, 3500 watt inverter. CR-1110 E-F/S fridge (compressor).
TV = 2007 4runner sport, with a 36 volt "power boost".
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Old 04-13-2006, 05:39 PM   #10
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Here are the same spreadsheets in .xls format (both are in the same zip file):

Paul
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File Type: zip wdh spreadsheets.zip (6.6 KB, 115 views)
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