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Old 03-09-2016, 11:59 AM   #1
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Tongue Weight

I've been calculating weight and balance for aircraft for 40 years but that always begins with known weights on each gear, an empty or Basic Operating Weight and CG, and looks for a loaded CG, not a new weight on each gear. I have a trailer and want to be able to establish a tongue weight for different loading through calculations, not re-weighing every time I change/re-position a load.

OK.....here we go. My trailer (stock by reputable manufacturer) weighs 1560 lbs empty with a tongue weight of 250 lbs. That's too much on the tongue, and the single axle is located quite rearward (Front of tongue is my reference datum line at 0", front jack wheel at 11", enclosure front at 29", axle at 146.25" and back of trailer/enclosure at 193.5").

I mounted a spare tire, and secured a jack inside the trailer in the front, bringing the Basic Operating tongue weight to 309 lbs. I can't put one motorcycle in it, as far aft as possible, without the bike's CG being forward of the axle (further increasing the tongue weight), let alone two bikes. I believe I need to move the axle forward and am trying to figure out how far.

I may be suffering from some cranial flatulence, but it seems to me there should be a way to calculate how moving the load fore or aft affects tongue weight.

Another question on something I may have simply forgotten, having been retired for eight years. I know the CGs of the bikes so is it sufficient to just use the CGs in tongue weight/trailerCG calculations, or do I need to use the front and rear axle weights and arms (in inches) which span the trailer axle? In other words, the front axle (384 lbs.) of the bike is in front of the trailer axle, and the rear axle of the bike (422 lbs.) is aft of the trailer axle.

Any help would be appreciated, or a link to some magical online calculator would also be helpful.
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Old 03-09-2016, 03:52 PM   #2
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General rule of thumb for axle placement is 60/40.

60 percent of the deck length in front of the axle and 40 behind.

Thats just a starting point. I built a trailer with 55/45 but it was a purpose built trailer.
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Old 03-10-2016, 08:08 AM   #3
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While it is an interesting observation and calculation, a trailer is not an airplane, and weight distribution is not nearly as critical. What is your tow vehicle and how much tongue weight is it rated for? 500 lb tongue weight is no problem with most tow vehicles. Most trailers pull best with a good proportion of the weight on the hitch, and I don't think 20% or even 25% is too much on a small trailer. Have you tried to see how it pulls? Usually utility trailer manufacturers design their unit for with lots of room up front because they would rather see it front heavy, than not enough weight up there .....
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Old 03-10-2016, 08:38 AM   #4
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Here is a simple online Beam Calculator.

http://bendingmomentdiagram.com/free-calculator/

There are simpler ones but this one has distributed and point loads.

Brian
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Old 03-10-2016, 01:42 PM   #5
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"General rule of thumb for axle placement is 60/40."

That sounds more reasonable. My axle is at 75.6%. My tongue weight at Basic Operating Weight is at 19.2% before I load anything. Loading even one bike as far aft as possible places it's CG 13" forward of the axle giving me a tongue weight of 18.6%.
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Old 03-10-2016, 01:55 PM   #6
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Originally Posted by WiliWali View Post
While it is an interesting observation and calculation, a trailer is not an airplane, and weight distribution is not nearly as critical. What is your tow vehicle and how much tongue weight is it rated for? 500 lb tongue weight is no problem with most tow vehicles. Most trailers pull best with a good proportion of the weight on the hitch, and I don't think 20% or even 25% is too much on a small trailer. Have you tried to see how it pulls? Usually utility trailer manufacturers design their unit for with lots of room up front because they would rather see it front heavy, than not enough weight up there .....
Tongue weight capacity isn't the issue for me. I can have 1110#. I'm more interested in the manageability of of a disconnected trailer balanced against a safe towing tongue weight. This isn't a big trailer. The most it will ever weigh is around 3500#, so I'd like to re-position the axle to give me a tongue weight between 10 - 15% with either one or two bikes in it.

In the UK the minimum allowable TW is 4% with a desired range between 5 - 7%. However, they are limited to 60 mph. At those TWs, instability can begin to affect things at around 65 -70 mph. That being the case, I would prefer to keep my TW closer to 10%.

Thanks for your input.
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Old 03-10-2016, 02:10 PM   #7
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Originally Posted by BrianGlenn View Post
Here is a simple online Beam Calculator.

Free Online Bending Moment Diagram Calculator and Shear Force Diagram Calculator | Bending Moment and Shear Force Diagram Calculator

There are simpler ones but this one has distributed and point loads.

Brian
Thanks, Brian. Playing around with it this morning, I think I can make this work for me.....if only by coincidence. You see, it doesn't allow for a beam (empty trailer) weight entry so I couldn't establish an accurate TW with the trailer and two bikes. However, I noticed that the CG for the forward-most bike was within .02" of the Basic Operating CG of the unloaded trailer. That was close enough to just combine the weights into a single load point and add the other bike's weight and CG to the mix. The result was a TW of 16.6% which I'd still like to get closer to 10%.

Thanks again for the link. I think I can use it to establish how far I can move the axle forward. I just find it hard to believe that in the wide world of the internet, there isn't a calculator like this out there for trailering.
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Old 03-10-2016, 05:31 PM   #8
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Breezyman


You might try Towing Calculator | Husky Towing


PM me I have one in Excel that you can save.
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Old 03-11-2016, 08:51 AM   #9
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Breezyman


You might try Towing Calculator | Husky Towing


PM me I have one in Excel that you can save.
Thanks for the link but the Husky calculator contains the following:

"The calculator assumes that cargo added to the trailer will be evenly distributed using the same pin/hitch weight percentage as the empty trailer. You can adjust the percentage of hitch weight by manually entering a new figure."

It is, therefore, not useful to me. I need to be able to assign an arm to the weights which is necessary to calculate where to move the axle or load in order to reach a desired tongue weight.

Thanks again.
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Old 03-11-2016, 09:07 PM   #10
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if you are going to move the axle forward, and you are capable of the work, why not make it adjustable at the same time--similar to large semis where they can move the axles to adjust for loading?
With this light of a trailer, ought to be some off-the-shelf stuff to accomplish a movable system.
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Old 03-12-2016, 05:52 PM   #11
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Dont count the tongue in the percentage as to axle placement, only the loadable area. Also why not mount the spare at the rear ?
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Old 03-15-2016, 08:29 AM   #12
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Originally Posted by wingnut60 View Post
if you are going to move the axle forward, and you are capable of the work, why not make it adjustable at the same time--similar to large semis where they can move the axles to adjust for loading?
With this light of a trailer, ought to be some off-the-shelf stuff to accomplish a movable system.
Joe
I'm considering this but it will mean somehow making the fenders (attached to the box and not the axle assy). and the suspension movable also so I don't believe it will be practical.
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Old 03-15-2016, 08:36 AM   #13
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Dont count the tongue in the percentage as to axle placement, only the loadable area. Also why not mount the spare at the rear ?
If I mount the spare tire in the rear, I don't have room to get two rather large bikes in and out.

I'm curious as to your recommendation about not counting the tongue in the percentage. Why is that? My whole purpose in these calculations is to establish an axle position which gives me an acceptable tongue weight, as a percentage of loaded trailer weight, under the loaded conditions I am likely to incur.
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Old 03-16-2016, 09:28 AM   #14
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The tongue is not a loaded area of the trailer in general. Yes some people mount stuff up there but it wasnt designed that way, so you only count the loadable deck area as the toungue is a minor amount of weight in front of the loadable area.
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