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Old 04-04-2010, 03:34 PM   #1
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A $24 Experiment in Weight Distribution

I am new to the forums, relatively new to RV’ing in general, and over the past couple of years of traveling have not thought much about weight distribution or sway control of the TT, but that has changed. We got into our ’01 25’ Excella several years back and inherited its Draw Tight without any damper or sway control. Have not had an issue with the hitch (probably because of the size and weight of the TV) but recently wanted to travel a bit more and needed that extra insurance to help with the “getting older and not as quick” syndrome ….bought and installed the Pro Pride.

So, now I have no worries about sway control (great built in feature) but have had little understanding about weight distribution, the mechanics of it, and any real numbers to put behind it. All I did before was to hook it up the way the previous owner showed me how to and blissfully down the road we went. Well, after reading many posts and threads on the subject of weight distribution and feeling a little uncomfortable about relying on a ruler and level, I made a couple of visits to my local CAT Scales and after the shock of the costs (a real bargain, two visits and several re-weighs) and intimidation of the process while having some on looking trucker (probably getting a good laugh at me, too), I now have some data to share and some thoughts on the results. I would like some feedback but due to dial-up, I can’t respond frequently.

The Characters involved:
2001 25’ Excella modified from twins to a single full
2002 GMC K2500, Crew Cab, Short Box, Duramax/Allison
Pro Pride Hitch---Set up using my old Draw Tight ball height-receiver as reference points---it provided for a very level TT-TV combination before when hitched using the 3rd link, A full moon rising, the tide going out, dog in the truck….no data, it just seem to work….probably a bad assumption but just didn’t seem to worry about it.

The Drama unfolds:
I took a couple of trips to the scales, and probably would like to take several more over time to increase the number of data points. The first trip was as the RV was while winterized. One empty propane tank, empty water, black and grey tanks, and drained lines. No food, and light on the misc. items. This would represent my lightest towing configuration as if I were to ever loose my mind and sell. This will never happen but I thought I needed a baseline. The second trip was with the TT filled with water, grey and black 2/3 full and a bunch of other things I thought I could throw in just for the experiment. In both weighings, no dogs, wife, generator, kayaks, fishing gear. Just the truck and a full tank of diesel to keep the results repeatable.

A note about the process. A really nice feature of the PP is that the WD bars are infinitely adjustable between min and max. I have mine set where at “0” the bars have just the slightest bit of tension when hitched and the jack retracted. The settings that I used started at relaxed position and at the jacks raised 4 ¾” with the dry configuration. This is where I got the TT/TV dead level (ok, plus or minus 0.5 degrees with my digital level.) Sorry I don’t have more data, but for now it’s a start. It would be nice to have a tensiometer or strain gauge attached to the WD bars to get feedback as to if they were being tightened evenly, for now I just use a torque wrench and monitor the torque at a setting to see if they are close to equal…couldn’t do that with the chains!

The numbers and a graph (See the pdf attachment)


Here is my analysis. While I do not believe that the spring rate on the WD bars are linear, I went ahead and put a linear overlay on the graph….seemed to work though, (HMMM?). The curious thing that I find in the data is that with the trailer loaded and the bars tensioned to 4 ¾” and beyond, it appears that no additional weight transfer go rearward of the hitch. Most all of the transfer is in the “re-distribution” of weight between the axles of the TV. Any explanation there? I am assuming that the WD hitch assembly becomes so “rigid” that at some point all you have is the flexibility to change the drivability of the TV. A variance in weight transfer of 60 pounds on the front axle of the TV is not a lot and maybe this is where there is “room” to soften the ride for the TV and TT….got to keep those dishes from rattleing.

To draw many more conclusions (and to see if the spring rate/distribution is linear) I believe I will require more data (dollars in the pocket for reweighs). In any event, the 4 ¾” setting seems to pull REALLY well either full or empty and getting the front axle back to within 97.5% of its original value is in my estimation, pretty close. And, not a bad experiment for $24.

Bill
Attached Files
File Type: pdf cat scale readings.pdf (25.7 KB, 81 views)
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Old 04-04-2010, 05:37 PM   #2
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Playing with numbers can be fun but it is a good idea not to forget why the measures are being made.

re: "Most all of the transfer is in the “re-distribution” of weight between the axles of the TV." -- can you envision the situation where the TT was completely off the ground, cantilevered behind the TV? -- I think the lesson, though, is about the fact that your precision greatly exceeds the accuracy of measure at this point in the experiment.

A good focus for the experimentation would be to start with the primary reason for weight distributing hitches. There are two major reasons so there might be some debate about which is primary but I think the choice should be rather clear.
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Old 04-04-2010, 06:45 PM   #3
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Agreed, Seems a little silly to talk about about a 60 pound weight shift when your system is over 7 tons. i like the data and have wondered about those that worry about over-hitching or overly stiff suspensions that "pop" rivets out of their AS. My interest is only in looking for the answer to a question that I also wonder about, what is the real purpose of WD and is it to restore weight to the front axle, level the towing assembly, move the weight of the tongue to the TV? I just really don't know and have assumed that it is the former from many of the threads that I have read over the last several years.

The concern that I am coming to is with overly tensioned WD bars, do you put too much stress on the TT frame/monocock(sp?) to where you just do more damage than good? I look at my data and wonder, should I relieve the bar tension to soften the ride and keep the redistribution of weight to the front axle to something around 95% or even less.

Bill
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Old 04-05-2010, 09:47 AM   #4
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Sorry if I get a bit vague. This topic brings out some folks who seem to be rather possessive of their views and beliefs and are not open to thinking about anything that challenges them. That can create a painful discussion.

Quote:
The concern that I am coming to is with overly tensioned WD bars, do you put too much stress on the TT frame/monocock(sp?) to where you just do more damage than good?
You have a good measure for the damage (rivets coming loose). That raises the question of whether you have a measure for the beneficial side of the problem to balance against the adverse effects.

A friend was having problems with the cushions bouncing off his trailer couch while going down the road. He dumped the load leveling and now the cushions stay put. As an additional benefit, his ride is much more pleasant, especially when on rough roads like I80 over Donner Pass. He has a measure for the 'beneficial' but it is qualitative and that displeases some.

Quote:
what is the real purpose of WD and is it to restore weight to the front axle, level the towing assembly, move the weight of the tongue to the TV?
These raise the questions: About why you might need weight on the front axle; About what is a reasonable drag or droop in the TV for the loads it is designed to carry; -- The last one isn't quite right as tongue weight is carried by the TV anyway, a load leveling system moves it forward to the steering axle and aft to the trailer axles.

InlandRV Andy is quite adamant about what shock loads and vibration do to an Airstream in discussions with those who figure more is better when it comes to spring bar sizing. Some use devices like the Centramics balancers to help reduce vibration but that doesn't help shock from oversized springs.

The real question is about the purpose of weight distribution. Perhaps one way to consider that is to extrapolate it to an extreme. What is the effect of a lot of weight on the extreme rear other than just being off level? What would it do to the driving experience? What would a reasonable driver do to compensate? How could you measure this effect directly?

(note, been there done that with a friend I helped to pick up a lot of surplus wharehouse shelving. The drive back with an overloaded pickup across Connecticut was 'interestiing').

The spring bars in modern hitches are only one way to address the issues raised by load distribution. There are others, mostly involving choice of tow vehicle and its suspension. Improvements in automotive design have opened up options in the latter that many have not grasped (accepted?) as of yet. Load leveling with spring bars is being superseded (in part) by technology and many find it hard to part with the past and explore new ideas and new options.

The key is that your hitch must do something for your driving experience. Can you notice what this is? Can you measure it in some way? How much is in your head and how much is in your hands or the seat of your pants? How much is illusion and how much is a real contribution to your driving?
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