Weight on a bar calculations?

Weight on a bar calculations?

Renegade_Azzy
Renegade_Azzy

April 13th, 2012, 1:57 pm #1

Working on a project for a hitch mounted tire carrier. It will fold down and retract. Being my intent is for offroad applications, and tires get big and heavy, I dont want to overload a class3 hitch. How can I figure out my max load when extended?
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sniper1rfa
sniper1rfa

April 13th, 2012, 4:02 pm #2

that is presented to the hitch using a standard ball hitch at the max rated tongue weight, measuring from the ball to the mounting point of the hitch. Then you have a figure to check the tire carrier against.

For example, if the normal ball is 12" from the mounting point and accepts a load of 500 lbs, you're applying 500ft-lbs at max tongue weight. If your tires will stick out 3 feet than you can run a third of that, or 166lbs, before you're overloading the hitch.

Keep in mind, however, that hitches are not designed to handle torsion - the ball will not transmit that force so they don't need to handle it. Use straps to support the left and right sides of the tire carrier, beef up the hitch, or be very cautious with how much weight you put on there.
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Maker of Toys
Maker of Toys

April 13th, 2012, 4:24 pm #3

Working on a project for a hitch mounted tire carrier. It will fold down and retract. Being my intent is for offroad applications, and tires get big and heavy, I dont want to overload a class3 hitch. How can I figure out my max load when extended?
As Sniper1fa points out, it's a simple moment arm calculation:

Make yourself a nice dimensioned sketch of what you propose to build AND the existing hitch, complete with the hole for the locking pin(s.) Getting the distance between holes and the centers of the major masses correct is enough; you don't have to mike the holes. A view from the side of the vehicle will suffice.

Then scale, measure or calculate (as a right triangle) the distance from the geometric center of the load (tire and wheel plus any other mass (like brackets, fuel cans, etc.) to the center of the locking pin of the hitch reciever. (which serves as an easy datum point. . .)

The mass of the tire-wheel-mount combo times that lever arm times whatever fudge factor you want to give for the dynamics of going down the road (I'd suggest at least 3.0, but YMMV) equals the 'moment arm' of the load. You can stop here if you like; that number equates to a torque that you can compare with the hitch's allowable TONGUE WEIGHT for a rough answer. Don't forget to add the mass of the tube that runs between the tire, etc and the reciever portion; there's a whole integration you can do if you want, but I'd just lump it at the center of the tire to build in a little extra safety factor.

If you feel like a better approximation: (but still an approximation), then measure, scale, calculate (or whatever) the distance between the geometric center of the hitch ball and the locking pin hole. Multiply that by the permissible TONGUE WEIGHT (not trailer mass), and that's the allowable moment arm for a trailer vis-a-vis bending something. The sheer load for towing is going to be much higher, but it's going to be more-or-less linear, and so not as applicable.

Compare the moment arms, with fudge factors to suit your comfort level, and you're there.

you can do a similar calculation to guess at the loads you might impose on bolts close to the reciever.

I'd post a sketch, but I'm at work. . .
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Renegade_Azzy
Renegade_Azzy

April 13th, 2012, 5:36 pm #4

Except for the part about the ball. No ball hitch on this thing, just the bar that goes into the receiver, like those basket carriers.

Take a mix of a yakima folding bike rack and one of these:


http://www.amazon.com/Yakima-DoubleDown ... 983&sr=1-2
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Maker of Toys
Maker of Toys

April 13th, 2012, 7:10 pm #5

so the reason we're mentioning the ball is a compare and contrast thing: you don't need the ball, you just need to know where your load is relative to where a *ball would be if you installed it.*

so, to reiterate:

1) calculate the load on a 'virtual ball' for the maximum trailer allowable for your receiver. for that you will need to measure/calculate the distance that the ball would project from some datum (say, the locking pin, or alternately, the rear set of attachment bolts)

2) calculate the torque that 'max load' virtual trailer places on the mounting points to the vehicle, using the distance of the ball from the datum as the lever arm. Note this as the permissible torque for your hitch/vehicle combination.

3) calculate or measure the mass of your new rack and its contents.

4) calculate or measure the distance from the SAME POINT on the hitch receiver (as your virtual ball) to the center of mass for your new rack and its contents.

5) calculate the torque generated by your rack at its lever arm, as seen by the datum.

6) apply a 'just in case' multipler to account for the vibration and shock of rough travel.

7) compare the permissible torque recorded in step 2 to the final number for the new rack generated in step 6.

8) if the number on line 6 is less than the number on line 2, this is your margin of safety, and you may proceed with due caution.

9) if the number on line 6 is greater than the number on line 2, you will need to redesign.



Welcome to the wonderful world of educated guesses and compromise that makes up the engineering discipline.




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Renegade_Azzy
Renegade_Azzy

April 13th, 2012, 11:13 pm #6

nt
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Maker of Toys
Maker of Toys

April 14th, 2012, 1:47 am #7

. . and it might not be obvious that the input value you want in step 2 is the 'virtual tongue weight,' not the total weight of the trailer. Depending on who you ask, that might be as much as 10% of the total mass of the trailer, but certainly won't be the ENTIRE mass. . .

Hope that helps!
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