Hitch mounting / Leverage Question

Bobcat698

Bobcat698

Pure Diesel Power
Joined
Apr 24, 2006
Messages
2,229
Quick question regarding a pulling hitch mount location:

Does in matter WHERE the receiver mounts to the frame if the hook point is identical... IE if the receiver is mounted 6" further forward on the frame, but the hook point is in the same place, DOES it make any difference?

Probably a simple answer, thanks for the input.
 
Yes Imagine if it was behind the front tire. Would that make a difference? With hitch point being the same it's not as noticeable but does help.
 
Move the whole reciever as far forward as you can, then brace it to death. Moving it forward and extending the insert itself, will increase the stresses on it. If your hitch is flexing .5" at a 5" long reciever, it will flex an inch at 10" long. Which is an inch of hitch height. I am trying to envision the argument in my head why the new location is actually better, and it seems right, but I can't get the statics of it in my head right now to prove why. Someone with a few more mechanical engineering classes than I had(more importantly had them fewer than 10 years ago) will be more help.

I thought I had read some rules about hitches that limited how far the mount could be, something like not in front of the rear axle or something. It could have just been for a local pulling organization though.
 
mech2161 said:
Yes Imagine if it was behind the front tire. Would that make a difference? With hitch point being the same it's not as noticeable but does help.
Click to expand...

That was my thinking too, thanks for the input!
 
you can move the reese hitch about 7-10in on a dodge thats how far i moved mine depending on you hitch it will go around the spring hanger and with a stock receiver it will be right at the tailgait. if you move it farther than that and you do run a longer insert do not brace the insert to the frame you will not gain anything becauce you move your leverage point from the reese hitch mounting point to the inserts mounting point. unless you brace it back to where the reese hitch mounts to the frame. the farther forward everything mounts the more down force on the front tires/ less lift on the front end.
 
mech2161 said:
Yes Imagine if it was behind the front tire. Would that make a difference? With hitch point being the same it's not as noticeable but does help.
Click to expand...

I'm not convinced it's that simple.

If the hitch point is exactly the same, and assuming everything is fairly rigid, I think you'd be hard put to demonstrate a difference. Every time I sit down and draw out the forces and how they act on the truck, I can't see where it makes any difference where the reciever goes.

If someone has a better free body diagram that shows me otherwise, I'd love to see it, but I think Dan is right.

Pulling hitches that bear on the axle are another matter altogether.
 
I was hoping you would chime in Mat. I haven't heard free body diagram in years. If the frame flexes then I could see the advanatge to moving it closer to the axle, but the advantage there is just keeping hitch height. If the frame flexed tons the front wheels really wouldn't be lifted off the ground.

I have seen/heard several pulling setups that I thought were flawed. The variable that I think everyone forgets is flex. One of the ones that I have heard is running a bar(not putting the hook point on) from the reese reciever to say a point right around the back of the cab. If your frame had zero flex this would be a good plan, but if your frame/hitch flexes a little, it will actually be pushing up on the point just behind the cab. Which is totally opposite of the thought behind the bar. For the bar to have to work their would have to be zero flex in the frame/hitch, you would have to put a ton of preload on the bar, or make your hitch slide just a touch to actually let the bar pull down on the point behind the cab. I have a mechanical engineer that I bounce things off of at work that has a very good understanding of mechanics to help persuade me on these issues.
 
LOL thanks Dan!

I'm sitting here with my e-mech book re-learning what I used to know like the back of my hand.

I think where people go wrong in their thinking is that when you move the forces forward and backward on the frame, you get moment arms that people forget to include in the sum total of the forces. You cannot just move an attachment point and make everything go away....those forces are still there and have to be accounted for.

My gut feeling is that you need to ignore frame flex for now and understand the free body diagram first, then go back and add flex as a special condition....unless of course you know for a fact that your frame is flexing a boatload.

I also have this gut feeling that many people aren't looking at traction bar forces the right way either. When you think about the sum of the moments created by the axle housing, some of the "theories" out there can't be right.
 
How about you make a free body diagram taking into all the accounts of ladder bars, hitch mounting points, etc. Develop the equations and just let me fill in the variables! :) I don't think the GM flex much, but the 2nd gen dodges do, especially the 94-95's.
 
Oh boy! This sounds like a mechanics of materials problem mixed with a little dynamics. might be better to model the whole shooting match in SOLIDworks/COSMOSworks.

What must be remembered is what point you want everything to be relative to (the point of interest).

All forces and moments will be relative to that point.
 
Unless moving the receiver forward results in less flex(ie. hitch height loss) I can not see it being a big help if the distance from the hook point to the centerline of the axle is not changed. In my way of thinking you would be better off beefing the frame and receiver to stop flex then moving it forward if it does not result in a shorter overall hitch lenght. I have a hard beleiving that a 2" or even 2.5" that is say 6" longer insert will flex less then a beefed frame will with the receiver 6" further back. Boxing the frame should gain you more rigidity IMHO A 6" boxed frame rail should be far more rigid than a 2.5" solid bar if understand the Statics lesson that Mat gave me. If have wrong feel free to:kick: me Mat!:hehe:
 
All is true John, but if you can must move your hitch forward, get rid of a lot of the flex versus boxing in the whole back end of the frame. Which solution weighs less? Which solution puts more weight behind the rear axle instead of infront of it? I know I moved my hitch forward to get it closer to the axle and I think I used about 2 lbs of steel to reinforce the hitch for the extra leverage I put on my moving my slide in hitch out further.
 
By moving the hitch forward along the frame and keeping the "hook point" at the same place less force is applied to the front of the truck. Think teeter totter.
 
CSM diesel said:
By moving the hitch forward along the frame and keeping the "hook point" at the same place less force is applied to the front of the truck. Think teeter totter.
Click to expand...

The teeter totter effect is what is happening, but your still pivoting on the rear axle, and you still have the same length lever arm, so you really havent changed your leverage. There maybe other advantages to moving the hitch forward if there is excessive frame flex, etc, more of the little assumptions that are usually deemed negligable in doing calculations.
 
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What kind of calculations are we talking about here? To accurately model the average sled pull would take horrendous amounts of non-linear mathematics. Moving the "hook point" would provide more benefit than moving the hitch attachment point as far as moving the forces and moments around the rear axle.
 
zstroken said:
The teeter totter effect is what is happening, but your still pivoting on the rear axle, and you still have the same length lever arm, so you really havent changed your leverage. There maybe other advantages to moving the hitch forward if there is excessive frame flex, etc, more of the little assumptions that are usually deemed negligable in doing calculations.
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This is correct, and the principle in mechanics is called superposition. If the parts are rigid, then if the hitch point is the same, by the time you do all the math, you find out that they're equivalent.

It should make some sense, in that if you push the hitch point down 1", how much does the frame depress? ....about an inch. It's because the whole thing is a rigid assembly (and thus doesn't matter where it's attached). You end up making the hitch longer....so the loading point remains the same, and the forces superpose.

Like I said, if you have a pivoting puller-style hitch, then everything changes. But if we're talking rigid assemblies (which we all build) then I think you're stuck.

....but I'm still working on the FBD.


If you want an absurd example to think of, if you made a (theoretically weightless) weight bracket for the front that was attached to the bed (imagine something looking like a dumpster garbage truck), guess what, if this bracket put the weight in the same exact location, it would load the front end identically - because it's all about the center of gravity of the weights (and their location relative to the frame), not where the attachment point of the bracket is.
 
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Do you think this would be a good way to see if your thoery is true and i really want to try this, find some DOT scales and a force gauge and a ratcheting mechinism. Set the truck on the scales hook the ratcheting mechinism to the force gauge and apply certain amount of force to the hook point, then move the hitch forward recording how much weight difference there is on the all 4 tires between the two positions hence weight transfer do to the hook point.
 
DISTURBED said:
Do you think this would be a good way to see if your thoery is true and i really want to try this, find some DOT scales and a force gauge and a ratcheting mechinism. Set the truck on the scales hook the ratcheting mechinism to the force gauge and apply certain amount of force to the hook point, then move the hitch forward recording how much weight difference there is on the all 4 tires between the two positions hence weight transfer do to the hook point.
Click to expand...

Absolutely! If you want to go nuts, do what the draggers do and get 4 scales, one for each wheel. I've been hatching an idea like this for a while....it's neat to see others are thinking similarly (or worst case, you're just as goofy as I am).

The only thing unknown is the force in the sled chain. I would love to put a strain gauge in one and see what it says during a pass down the track. I think I could come up with some educated guesses, but I have never heard anyone say it's been measured directly.

Also you'd need the length of the chain on a particular sled, and how high the hook point is on the pan, to have the right geometry when simulating the loading. Remember, forces are vector quantities, so you must know force and direction.

I think some of you smart guys could come up with ways of estimating it :D
 
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:pop:

It's a physics lesson in here!!! :hehe:

Thanks for all your input guys!!
 
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