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Old 01-27-2017, 06:40 PM   #21
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Originally Posted by Brian B-P View Post
Not unless truck #2 has a longer cab and thus the same wheelbase as truck #1. It isn't just the distance from axle to ball, but the ratio of this distance to wheelbase which determines load transfer off of the front axle (and thus WD effect needed), and stability.
Yep, my thoughts too. The wheelbase is the main factor.
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Old 01-27-2017, 07:20 PM   #22
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Originally Posted by Brian B-P View Post
Not unless truck #2 has a longer cab and thus the same wheelbase as truck #1. It isn't just the distance from axle to ball, but the ratio of this distance to wheelbase which determines load transfer off of the front axle (and thus WD effect needed), and stability.
Okay, more info on wheelbases (and relative engine weight, for what that's worth):
Truck #1 = 158" (heavy 7.3L diesel)
Truck #2 = 145" (5.0L V8)
So, when the hitch ball is the same distance behind the rear axle, how much difference does a 13" shorter wheelbase make? I knew I should have paid more attention in those math classes....
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Old 01-27-2017, 08:03 PM   #23
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Okay, more info on wheelbases (and relative engine weight, for what that's worth):
Truck #1 = 158" (heavy 7.3L diesel)
Truck #2 = 145" (5.0L V8)
So, when the hitch ball is the same distance behind the rear axle, how much difference does a 13" shorter wheelbase make? I knew I should have paid more attention in those math classes....
In would think generally that the longer the wheelbase, the more leverage force would be required to distribute weight from back to front. But yeah, I didn't pay enough attention in math - or geometry for that matter.
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Old 01-27-2017, 11:17 PM   #24
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I'm already using an 11.25" ball mount (center of retaining pin hole to center of ball hole), and I think it's going to be close. If not, Hidden Hitch offers a 14" ball mount...
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Riddle me this (using real measurements):
Truck #1, 8' bed, 52" from center of rear axle to hitch receiver opening.
Truck #2, 5.5' bed 48" from center of rear axle to hitch receiver opening.

So, since there is a 4" difference in distance from center of rear axle to hitch receiver opening, it seems I would have equal towing "geometry" using a 4" longer ball mount with Truck #2 compared to Truck #1. Is that right?
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Originally Posted by War Eagle View Post
Okay, more info on wheelbases (and relative engine weight, for what that's worth):
Truck #1 = 158" (heavy 7.3L diesel)
Truck #2 = 145" (5.0L V8)
So, when the hitch ball is the same distance behind the rear axle, how much difference does a 13" shorter wheelbase make? I knew I should have paid more attention in those math classes....
So, guessing that the face of the receiver is about 2" beyond the pin hole (and hoping that I've caught all my typos and put all the numbers in the right places),

Truck #1 (8' bed, heavy 7.3L diesel)
158" wheelbase
52" from center of rear axle to hitch receiver opening
  • + 11.25" ball mount (short) - 2" = 61.25" from rear axle to ball
    and 61.25 / 158 = 38.8 % (this is the fraction of hitch weight which is transferred off of front axle)
  • + 14" ball mount (long) - 2" = 64" from rear axle to ball
    and 64 / 158 = 40.5 % (this is the fraction of hitch weight which is transferred off of front axle)
Truck #2 (5.5' bed, 5.0L V8)
145" wheelbase
48" from center of rear axle to hitch receiver opening
  • + 11.25" ball mount (short) - 2" = 57.25" from rear axle to ball
    and 57.25 / 145 = 39.5 % (this is the fraction of hitch weight which is transferred off of front axle)
  • + 14" ball mount (long) - 2" = 60" from rear axle to ball
    and 60 / 145 = 41.4 % (this is the fraction of hitch weight which is transferred off of front axle)
Fortunately, the truck with longer overhang also has more wheelbase, so they work out about the same... but the longer truck appears to have slightly better geometry for stable and controlled towing with the same mount. In this case, the geometry of the shorter truck even with the shorter mount ends up almost the same as the longer-overhang truck with the longer mount... and the heavier truck may (depending on suspension) may handle the trailer better, since the trailer weight isn't scaled up to match the bigger truck.

Even the 2.75" difference in mount length matters some, since it is roughly 5% of the distance to the rear axle. The extra mount length is proportionately more significant to the shorter truck.

I can't imagine wanting to use even an 11" ball mount, let alone 14", but I do understand the desire to clear the tailgate. Life is full of compromises.
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Old 01-27-2017, 11:20 PM   #25
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Not to over-think this, but it seems like a tow vehicle's suspension creates a hybrid combination of Class 1 and Class 3 levers depending on weight distribution of the tow vehicle - the heavier the font end (diesel engine, RanchHand bumper, snow plow, etc.), the more like a Class 3 lever. As a heavy front end Class 3 lever, the front axle ends up acting as the fulcrum, the rear springs act as the effort (pushing up), and the hitch ball bears the load. In the words of "My Cousin Vinny", does that hold water?
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Old 01-27-2017, 11:37 PM   #26
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Thanks for calculating that all out, Brian! Very interesting that they came out so similar. Just to muddy the water a bit more, Truck #1 is an F-250 SuperDuty with an additional heavy duty suspension option (rock solid ride, used mostly as a farm truck but handles interstate travel fine, too). Truck #2 is an F-150 standard suspension. Both are 4x4. Based on your calculations, I'm confident both can pretty much equally handle our new 21' on the end of a 14" ball mount if we need to go that way. I really want to be able to lower the tailgate, and I'd really rather not use the Jack-E-Up jack mount if we can avoid it. We'll see how things go once the 21' gets here. Thanks again for sharpening your pencil and going through all the math... Dale
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Old 01-27-2017, 11:37 PM   #27
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Originally Posted by War Eagle View Post
Not to over-think this, but it seems like a tow vehicle's suspension creates a hybrid combination of Class 1 and Class 3 levers depending on weight distribution of the tow vehicle - the heavier the font end, the more like a Class 3 lever. As a heavy front end Class 3 lever, the front axle ends up acting as the fulcrum, the rear springs act as the effort (pushing up), and the hitch ball bears the load. In the words of "My Cousin Vinny", does that hold water?
I had to look up the lever class definitions - haven't used those for decades! They are based on which point is the fulcrum, but there are no immovable points in this suspended system, so it doesn't matter which one you use. It would matter more if the calculation was of motion distances and one point was fixed in place.
  • I used a Class 1 lever approach... calling the hitch weight the "effort", the front axle the "load" or "resistance, and the rear axle as "fulcrum" between them. The overhang and wheelbase numbers fall neatly into this.
  • You could use a Class 2 lever approach (treating the front axle as fulcrum, so applying hitch weight to increase the rear axle load); this allows direct calculation of the increase in rear axle load first, but it is just the hitch weight plus load transferred from the front.
  • You could use other approaches if you want to consider the whole load on each axle, and that would "hold water" too, but I'm just calculating changes - it just seems easier to me.

Whatever method is used, the reality is the same so the same numbers come out. I chose my approach because the amount of front axle load reduction is what people seem to be concerned about (especially those using weight-distributing hitches), and I just find it easiest.
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