Weight Formulas

[Spyke]

Okay

I did a search and I don't know exactly what to call the formulas, so I am asking for your help. What I want to do is figure out how adding an extension / weight on my receiver hitch is going to change my weights on the rear and front axles. Thanks in advance

Steve

 

[DuaneOlson]

good question--if to big of an extension or trailer you should get an equilizing hitch to put the weight where you want to some extent

 

[DPelletier]

Spyke,

I haven't found any good information about the effect of an extension with the stock class IV hitch, but if you go to www.torklift.com they have a chart showing the effect of various length extensions with thier "class V" hitch (superhitch). Keep in mind the stock ratings of 500/5000 and 1000/10000 without and with weight distributing bars respectively and then you can "do the math" from there.

Hope this helps,

Dave

 

[Spyke]

Thanks

Guys for the info I have Torlifts superhitch with a 36" extension now. I am trying to figure out how much 700lbs on this extension is going to increase my rear axle load since my common sense tells me it should be more than the 700lbs I just want some science to tell me how much.

 

[DPelletier]

Sorry Spyke,

I misread your original post. You're right, the force increases in a direct linear relationship with the extension length. Damn, I'ts been way too long since I've been out of university! I'll see if I can come up with the formula you're looking for. Its a simple one really.



Dave

 

[Spyke]

Dave

Thanks, thats one of the reasons I posted here. I know formulas are out there but it has been along time since I left the public system.

Steve

 

[SRadke]

I tried to find you a formula but the more I tried the more complicated it got. The simple formula is force x distance x 2. (Toung weight x distance from the rear axle x 2 because you've transfered weight from the front. ) The problem with this formula is that the truck is on springs so it doesn't necessarily "pivot" around the rear axle.



Look at it this way. If you put 1000 pounds on your trailer hitch the truck goes down at the rear axle and up at the front axle. At some point in the middle there it remains the same height. That is the "pivot" or fulcrum. Using that point for the fulcrum the physics formulas work but the problem is that point moves depending on the spring rates and load you've applied. If you apply enough load to get the rear suspension bottomed the fulcrum now becomes the rear axle. I guess I didn't help much did I.



-Scott

 

[DPelletier]

Scott,

LOL

I looked at the same stuff and came to largely the same conclusions. I'll keep trying. Everything I learned dealt mostly with kinematics (stuff thats moving with V velocity and A acceleration). I'll keep trying.



Dave

 

[SRadke]

If it was me I'd go down to the truck scale on the farm with a piece of 2x2 to stick in the reciever and weigh the truck axles at diffrent hitch configurations. It would take some time but you can't mess up those calculations. Of course you could do the same thing on your local backroad truck scale.



-Scott

 

[Spyke]

Scott

That is what I was looking for something along that line. I figure that the springs would have to be taken into effect, but for simplicity I would use the axle as the pivot measure from the center to the end of the extension and figure 100lbs at 7 feet from the axle puts 700lbs on the axle. The problem is with this scenario 700lbs on the extension is now putting 4900lbs on the axle not much room for weight on the truck. I do have acces to portable scales but I figured if I could use a tape measure, pencil and paper I would do this at my desk. The scales I would use are a bit heavy and combersome for one person.



Thanks

Steve

 

[SRadke]

I rethought this whole thing. The formula I mentioned above is a torque formula. Placing a 700 pound load on a 7' lever arm out the back of your truck will apply the same torque to the truck as placing a 4900 (7" x 700lbs) pound load 1 foot behind the pivot point (which we're calling the axle for argument's sake). This does not necessarily mean you've added 4900 pounds to your axle. I still can't come up with a formula for you but that one will not work.



Here is the simplest I can come up with based on the knowns (enter your own numbers because I'm guessing)



Truck front axle: 4500 lbs

Truck rear axle: 3000 lbs

Trailer toung weight: 700 lbs

Trailer axle weight: 7000 pounds.



The weight on the trailer axles will not change unless you use a load leveling device so basically you will have no more than 4500+3000+700= 8200lbs on your pickup. How it is distributed I don't know but assume you've added the entire 700 to the rear and a wild guess say 400 from the front transfers to the back. Now you have a truck and trailer that weigh in at front axle 4100, rear 4100, trailer 7000 for a grand total of 15,200 GVW. That doesn't seem so unreasonable does it? I think so much more clearly at night.



-Scott

 

[DPelletier]

Scott,

Your right it is a torque formula: Torque = Force x Distance. Makes sense because a Force in lbs x Distance in Feet = Torque in Ft Lbs. Bottom line is that it won't tell Steve what he wants to know. In you example above, assuming no WD bars, the longer the extension, the more weight is transferred from the front axle to the rear axle. In fact, if you made the extension long enough it would ovecome the weight of the truck on the front axle and lift the nose off the ground. At that point ALL the weight of the truck and the tongue would be on the rear axle.

This is bugging me!!!!!



Dave

 

[klenger]

I'm kind of curious for the reason for a 36" extension. If it's because you have a big slide-in camper on the truck, then between the weight of the camper and any load on the hitch, you are putting some serious weight behind the rear axle of the truck.