Tongue Weight Spreadsheet

[Shrug53]

I am considering an 11' tear built on a 9' bed. This is because the front and rear foot will curve up and not touch the frame. Do I need to figure my axle placement from the rear of the actual metal trailer, or from the rear of the body?

Many thanks!

[mikeschn]

I think you need to consider the entire teardrop AND trailer to determine the location of the axle. If you are a high powered mathematician you can calculate the mass of all the objects, derive their combined center of gravity, and then calculate moments of inerita so that you get a final tongue weight of 10%.

Or you can use a simple cut and dried formula that would place the axle at 54.5" from the back of the trailer. I used the cut and dried formula, and it didn't work. I have a tongue that is closer to 20% of the weight of the trailer.

I did take the liberty of doing a quick sketch of an 11' trailer with the wheel at 54.5" from the rear of the trailer (not the frame). Here's how it looks.



It looks okay, but if it works out like mine, you would have an incredibly heavy tongue weight.

The answer...
Build your trailer, and mock up your body on it. The play with axle locations until your tongue weight is where you want it. Then weld on your axle brackets and build the real body.

Look at it this way. You'll almost be in the second teardrop club with your first teardrop!

Mike...

[Shrug53]

I wish I could do that, but since I am having it all welded up for me at a shop, I need to figure it out ahead of time.
This may be a silly question but is there a reason the axles are not pleace in the middle? Is it just for looks? I have been working on a couple of new cabin car concepts including putting the kitchen inside. I will post those later. If I do it up like my most recent one, the bed will be all the way back and have batteries and water tanks underneath it, behind the axle.
Either way I am planning on putting all of the heavy stuff either over or behind the rear axle.

[mikeschn]

I'm no expert on trailers, but here's what I think.

If the wheel was exactly in the center, the tongue would weigh almost nothing.

And the potential for weight behind the axle to cause the trailer to sway from side to side increases.

And from a teardrop aspect, especially smaller teardrops, if you have your wheel in the center, there's no room left for the door.

Experts usually recommend that the tongue weigh about 10% of the trailer. To achieve that the axle is moved back some distance.

On smaller teardrops the axle is moved back even futher to get room for the door. Because these tiny teardrops are so light, their tongues weigh more, without causing any trouble to the tow vehicle.

On a larger teardrop like you are designing, it sounds like the most important criteria is to get your tongue weight to 10% of the trailer.

Can you have everything except the axle bracket welded? Bolt your axle bracket to the frame with a couple self tapping bolts, and put your mockup on top, and adjust the location? I would not do any towing with just the bolts though. Remember, keep it safe.

Mike...

[Shrug53]

Interesting. I have just reworked my interior to be able to shorten the trailer by a foot leaving me with a nice reasonable 6.5'x10' trailer.
With the heavy stuff I am going to have in the back (batteries, water tanks, A/C, Inverter) there should be enough of the trailer weight shifted back to allow me to put the axle 46.5" from the rear and I should be ok.

[mikeschn]

46.5 is a good starting point for a 10' long tear. What's the plan if your tongue is too heavy?

Mike...

[Ken A Hood]

I'm going to jump in here too............

Don't forget to make sure that on your side profile that the door will clear the axle(tires and fenders).

Yes, I'm still around, and going to go get some steel for my frame within the next couple weeks.........and might get a new axle also. I have a set of 2000lb split torsion axles, but am thinking of going with a solid axle(w/ brakes) since it will be easied to mount(perpendicular to the frame) and I won't need another crossmember at(near) the axle location.

And I'm hoping when I get my computer fixed(darn bugs!) that my AutoCad dwgs are still there, should have saved them to a CD

[angib]

Calculating the centre of gravity of a trailer doesn't involve any hard maths, just lots and lots looking up weights and estimating.

I did just that for my Superleggera design and the centre of gravity spreadsheet is here.

Anyone who wants more detail on how to do it is welcome to contact me and I'll do a write-up.

Andrew

[Shrug53]

Do not worry. I have a backup plan... a load balancing hitch and a few prayers?
Actually I was considering what you said. Bolting the axle on, and then once it is in the right spot, taking it back to the shop to have it welded in place. A bolted on axle should have no problems making it a few miles to the welding shop.

p.s. angib, I think you just gave me a brain tumor!

[Rebel]

Here is a link to axle info talking about stability...dig the picture in the article with the mule...

Kuffel Creek plans for their 10' comet suggests 40 inches on center for the axel. Of course, that is one option. Some say 1/3 distance from rear which that is. I have also heard a 60/40 division.

Best plan is as Mike suggests: Check the weight distribution before the final welding.

[Rebel]

http://www.synthx.com/articles/trailer-stability.html (now check out the mule...

[Shrug53]

Poor mule. Actually I have read that when Mike brought up the topic of load distributing hitches.
I will know approximate weights of everything before I even start, so I am betting I can get pretty close. I used to just guestimate when I was drag racing and did pretty darn well. So, here's hoping!

[R Keller]

Andrew: ...and I thought I did obsessive spreadsheets! Wow. Good work!

Shrug: don't give up on trying to pre-calculate the tongue weight. The tongue weight on the Road Toad turned out to be pretty much what I had calculated (~15% unloaded, ~10% loaded), which is good considering the axle brackets were welded on before powdercoating and it'd be a real pain to have to move them

For what it's worth, here's how I did it. It's a little more complicated to explain than to actually do.

As an example, assume a 10-foot long trailer body that is a rectangular box (some adjustments would have to be made to calculations for teardrop shape). Now assume said trailer weighs 1,000 lbs and the weight is distributed fairly uniformly because of the box shape (chassis and body thrown together here for simplicity). Weight is 100 lbs. per foot of body length.

I used a starting point of the spindle center (not axle) at 3.75' (45") from the rear of the trailer body and 6.25' (75") from the front of the body. I have an additional 4.4' (53") length from the front of the trailer body to the center of the hitch ball.

Then I calculated the force moments by taking the total weight over the behind-spindle center and front-of-spindle center sections as acting as a point load 1/2 way from their respective distances from the spindle center. The tongue weight is a point load at the distance from the end of the tongue to the spindle center.

So:

Behind spindle-center moment: (3.75 ft. x 1/2) x (100 lbs./ft. x 3.75 ft.) = 703 ft .lbs.
Front of spindle-center moment: (6.25 ft. x 1/2) x (100 lbs./ft. x 6.25 ft.) = 1,953 ft. lbs.
Moment acting at end of tongue = 1,953 - 703 = 1,250 ft. lbs.
Tongue distance from spindle center: 6.25 + 4.4 = 10.65 ft.
Tongue weight = 1,250 ft. lbs. / 10.65 ft. = 117 lbs. = 11.7% of total trailer weight of 1,000 lbs.

Of course, we haven't considered the weight of the tongue itself and some other items in this simplification. We could refine our calculation by adding some other point loads for the bulkhead, galley cabinets, water tank, battery, propane tank. etc., by calculating their distance behind or in front of the spindle center. Set everything up in a simple spreadsheet and then play around with your initial assumed axle/ spindle center placement until you get the tongue weight/percentage that you want.

For example, assume an additional 175 lb. point load on the rear end located 2.5' (30") from the spindle center (437.5 ft. lbs.) and an additional 70 lb. point load for the tongue box and contents located 9" in front of the main body or 7' (84") from the spindle center (490 ft. lbs.). This is a net increase of 52.5 ft. lbs. (490-437.5) acting on the tongue, or an increase in tongue weight of 5 lbs. (52.5 ft. lbs./ 10.65 ft.).

This will all probably become clearer if you sketch it out. Sounds more complicated than it is.

The same concepts can be used to estimate the tongue weight by measuring the weight at the front jack. Or to figure out the strength of gas supports you need for the hatch.

For example, if the measured weight (using a scale) at the tongue jack is 150 lbs., and the distance from the tongue jack to the spindle center is 109", and distance from the center of hitch to the spindle center is 127" (tongue jack is 18" behind center of hitch), the tongue weight can be calculated at ((109*150)/127) = 129 lbs.

Or, if you have a 4-foot long hatch that weighs 50 lbs., you could calculate it as a 50 lb. point load acting 2 feet from the hinge for a 100 ft. lb. moment. If two hatch supports support the hatch 5" from the hinge, they would need to support a total of 240 lbs. (100 ft. lbs. /(5/12) ft.). Add 10% for temperature fluctuations and you get 264 lbs., or 132 lbs. for each gas support. Therefore, 150 lb. supports would work fine in this case and 120 lb. supports would be too weak.

Hope this helps.

Rik

[angib]

Rik said:

Andrew: ...and I thought I did obsessive spreadsheets!

Thank you - 'obsessive' is a much more polite word than several you could have used....

One suggestion I would make is to use a datum (measurement point) which is at one end or even beyond the trailer, so that all length measurements are positive.

I know it's intuitive to use, say, the axle but that requires forwards and backwards measurements and you are much cleverer than me if you can always get the direction right - I don't have any problem in believing I will always get them right, it's just experience that has taught me that it doesn't seem to come out that way.

Andrew

[Chip]

one method that has not been discussed and I think it deserves its due,,Many of us mathmatically chalanged tend to use this method,,,its the S.W.A.G. method,, for those not familiar with swag,,,(scientific wild a#$ guess),,,,two tool boxes and girlfriend standing on tail of galley I can lift tongue,with little trouble,,,yep its bout 90 lbs,,,works for me,,,,I did luck out though but it was an educated guess

chip