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Old 12-04-2012, 02:21 PM   #1
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weight distribution question

Any physics or math whizzes out there. My question is if you add 100 pounds to a cargo carrier on the back of a 5th wheel how would it affect the pin weight? I assume that the axles would act somewhat like a fulcrum and it would take 100 pounds off of the pin weight of the trailer since the added weight would fall opposite the front of the trailer. Am I on the right track?
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Old 12-04-2012, 02:52 PM   #2
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Looking at the very rough illustration below, it comes down to the weights and distances they are placed from the effective axle centerline. In this case, if W2 represents the weight placed on the carrier at the rear of the 5th wheel, then the effect on W1 (the pin weight) will be:

W1 = W2 x (D2 / D1)

If W2 = 100 lbs, D2 = 15 ft and D1 = 20 ft, then

W1 = 100 x (15/20)

W1 = 100 x 0.75

W1 = 75 lbs that will be removed from the pin weight.

The entire 100 lbs + 75 lbs = 175 lbs will show up as increased axle loading.

Rusty
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Old 12-04-2012, 03:55 PM   #3
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That makes sense to me. Thanks
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Old 12-04-2012, 04:07 PM   #4
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Rusty - reply is mostly to get this in my folder - but had to say Excellent.

Currently MH guy but can't have too much info. Curious - if you had unlimited axle capacity would there be an advantage to loading the rear end to = ~ 0 weight at pin. I know there are limits to weight at both positions and rearranging load can be necessary. Is there an optimal weight or % distribution at either or just stay under max for both? Again - curious and if dumb question - ignore - and I can cross bridge if I come to it.
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Old 12-04-2012, 04:19 PM   #5
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Quote:
Originally Posted by RustyJC View Post
The entire 100 lbs + 75 lbs = 175 lbs will show up as increased axle loading.
I was with you until this line, but now I'm lost. How can a 100 lb load add 175 lbs to the axle load? I understand that the weight is transferred because of the lever and fulcrum effect, but I'm not seeing how the total load can increase.
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Old 12-04-2012, 05:33 PM   #6
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The 100 lbs added to the carrier is obvious, but the 75 lbs that comes off the pin has to go somewhere, right? There are only 2 points where the weight of the 5th wheel is supported - the axles and the pinbox load plate. If the 75 lbs comes off the pinbox load plate, it is transferred to the axles.

Let's use a ridiculous example to illustrate this. Assume that a 5th wheel weighs 20,000 lbs total with 16,000 lbs on the axles and 4,000 lbs (20%) on the pin. If I could move enough "stuff" to the rear of the trailer to totally unload the pin, the 5th wheel would still weigh 20,000 lbs total, but with 0 lbs pin weight, all 20,000 lbs would now be carried on the axles.

To answer an earlier question, 5th wheels are generally designed to carry 20% (plus or minus 5% or so) of their total loaded weight as pin weight via placement of the axles. Although not as "touchy" as a TT, if the pin weight is too light, it can affect towing stability.

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Old 12-04-2012, 10:58 PM   #7
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Ah, now I get it. Thanks for the explanation. I have somewhat the same effect with my scooter on the receiver rack, only instead of reducing pin weight I'm reducing front axle load. Since I have a greater CCC margin on the rear axle the reduction in front axle load was beneficial.
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Old 12-05-2012, 07:48 AM   #8
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Quote:
Originally Posted by Moonshine94 View Post
I have somewhat the same effect with my scooter on the receiver rack, only instead of reducing pin weight I'm reducing front axle load.
Yep, the very same principle. The rear axle of your MH is the fulcrum point, so whatever load that comes off the front axle due to the see-saw or teeter-totter effect of the scooter on the rear must be added to the fulcrum point, the rear axle in your case. That's the problem with motorcycle carriers (and I wish I could use one for my BMW ); the 800+ lbs that I add to the rear of my 5th wheel for the bike and carrier will wind up unloading the pin by around 600 lbs and increasing my axle loading by 800 + 600 = 1,400 lbs.

To use my situation to illustrate:

5th wheel loading without bike:
Total weight = 15,500 lbs
Axle weight = 12,400 lbs
Pin weight = 3,100 lbs (15,500 lbs - 12,400 lbs)

5th wheel loading with bike
Total weight = 16,300 lbs (15,500 lbs + 800 lbs for bike and carrier)
Pin weight = 2,500 lbs (3,100 lbs - 600 lbs)
Therefore, the axle weight must be 16,300 lbs - 2,500 lbs = 13,800 lbs, or 1,400 lbs more than the axle weight without the bike and carrier, even though the bike and carrier only weigh 800 lbs.

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Old 12-05-2012, 03:38 PM   #9
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I'm glad you convinced them, but you would not pass my physics class with it. Sorry, a 100 lbs added to either side of the fulcrum adds 100 lbs to the total weight. You are trying to add a distributed weight (the 75 lbs.) to a real weight (the total weight of the trailer) and that's not how physics works. SRI, you get an F.
(Retired physics professor). But nice diagram, just go back to a college level course and get the fundamentals down.
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Old 12-05-2012, 03:46 PM   #10
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Sorry, a 100 lbs added to either side of the fulcrum adds 100 lbs to the total weight. You are trying to add a distributed weight (the 75 lbs.) to a real weight (the total weight of the trailer) and that's not how physics works.
In my post #2, I'm NOT adding 75 lbs to the total weight; I'm saying that, in addition to the added actual weight of 100 lbs, the pin weight is reduced by 75 lbs. That 75 lbs has to move to the axle weight in addition to the 100 lbs of actual weight added, so the axle weight (NOT the total weight) will increase by 175 lbs; the total weight increases by 100 lbs and the pin weight decreases by 75 lbs. The math is in another example I provided in post #8. The total weight (as I illustrated in post #8) has to be carried either on the axles or on the pin; whatever portion of the total weight that isn't on the pin must be on the axles (the fulcrum).

Prof, I had to pass Applied Mechanics in Engineering school. It's a basic forces and moments problem.

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Old 12-05-2012, 04:12 PM   #11
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I'm with Rusty, with all due respect to the Prof.

This is well known in the RV/auto industry.

Chevy used to publish a document for their pickups listing every option and it's effect on front and rear axle weights + or -. It was intended to assist slide in camper owners with axle loading.
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Old 12-05-2012, 06:56 PM   #12
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Sorry Rusty, misread your answer thought you said total weight. Changed your grade to an A.
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Old 12-05-2012, 07:03 PM   #13
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Changed your grade to an A.
My GPA and I thank you.

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Old 12-05-2012, 08:38 PM   #14
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Rusty,
Thanks. Every so often I find a 'light-bulb' moment and this was one of them. I have often thought about moving weight to the rear to reduce the pin weight, but never thought about the axle weights.
Joe
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