Trailer Example

[LeRepeteur]

Hello,

I am currently working on a design for a Project. I have been tasked with building a trailer, I know the COG and the lengths to the tire/axle from hitch and rear. My problem is finding my axle placement, I don't have a tongue weight and am unable to figure out how to find the proper placement. Can someone point me in the right direction? I've tried statics to figure this out but ultimately I'm getting nowhere in thought process of how to pick the placement.

Is there a way to calculate axle placement with just the COG? Can I figure out axle placement another way that is safer and more effective than the 60/40 rule. Please help!

Cheers,
LR

 

[rb1957]

so the axle should be close to the CG. I would have the CG ahead of the axle so there is some small down load on the hitch. I think you need to avoid up load on the hitch (and also excessive down load). I would guess at maybe 10% down load on the hitch, so that places the axles 1/9th of the hitch-to-CG distance behind the CG.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[3DDave]

If the CG is not ahead of the axle then the trailer is unstable. The 60/40 rule sets the stability. This safe - there isn't any "safer" though there can be more stability margin - that will depend on the spring rates of the suspension, the tires, the geometry of the trailer, damping, and more. If you don't know any of that, then 60/40 is a good start.

 

[zeusfaber]

It would pay to find out what range of preponderance is acceptable to the sort of vehicles that would have to tow your project if it was built. If your design doesn't respect that, you've lost.

A.

 

[BridgeSmith]

The tongue weight should be 10% of the total loaded weight. You can most likely ignore the small difference between the CG of the empty trailer and the fully loaded trailer, and model it as a beam with 2 supports - at the hitch and the axle. Load it with uniform load over the length of the bed or box, with a total equaling the trailer capacity, and move the support representing the axle until the weight on the tongue = 0.1* the total load. 1/10 of the bed or box length behind the center of the box will be a good place to begin iteration.

 

[waross]

I don't know where the 60/40 rule came from.
I have always seen the 10% rule.

1/10 of the bed or box length behind the center of the box will be a good place to begin iteration.

This is a starting place for a trial and error iteration.
When the tongue length is considered, the final placement of the axle will be further behind the CoG.
I have seen more. I pulled a very large trailer from Indiana to western Canada, with almost 13% on the hitch.
With 1800 lbs on the hitch, even with a very heavy duty equalizer hitch I had trouble keeping my headlights out of the tree tops.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[BridgeSmith]

With 1800 lbs on the hitch, even with a very heavy duty equalizer hitch I had trouble keeping my headlights out of the tree tops.

Yeah, with that much tongue weight, it should be on a 5th wheel or gooseneck hitch. That much load, that far behind the rear axle, is pushing the limit, even on a 1 ton truck.

 

[3DDave]

60/40 works out to about 10% tongue weight. 40% to the rear balances 40% to the front; this leaves 20% divided between the axle and the tongue => 10% on the tounge.

 

[rb1957]

@3DD ? Are you thinking of a trailer with two axles ? I'm thinking of a trailer with one axle, so the axle is behind the CG 1/9 of the distance between the hitch and the CG. no ??

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[3DDave]

No - I mean 20% + 40% in front of the axle and 40% behind the axle.

The only imbalance is the 20%, which is nominally 50/50 between the axle and the tongue => 10% each.

 

[BridgeSmith]

I'm thinking of a trailer with one axle, so the axle is behind the CG 1/9 of the distance between the hitch and the CG. no ??

It depends on the length of the tongue, relative to the length of the bed or box. The 20% of the load that's unbalanced, is the 20% at the front of the box, the CG of which is at the first tenth point of the box or bed from the front. If that CG is the same distance from the axle as it is from hitch, then you'd have exactly 10% of the load on the hitch, assuming a uniformly loaded bed or box.

Of course, it's going to be a rare case where the tongue length makes it work out exactly that way, but an even rarer case that the trailer is always loaded uniformly, so an approximation is generally the best you're going to do, anyway.

 

[rb1957]

no, if we're told a CG it doesn't matter the length of the bed, or the distribution of the load. I know it's an analytical ideal, but that's how we do calcs. We can say "let's consider 10% variation in CG position". We can say the axle is in the middle (or maybe 60/40 ?) of the bed and the tongue extends so far beyond the bed ... but that not how the problem was presented.

Maybe that's what "60/40" means ... and so you say 40% of the load is "balanced" on both sides of the axle, so 20% is "unbalanced". Maybe this is common terminology for guys who work with trailers and such ? Then the 20% is reacted by the hitch and the axle ... the 20% (of the load) is 50% (of the trailer bed) ahead of the axle, so the hitch is 50% of the trailer bed ahead of the trailer bed. Clear as mud ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[waross]

no, if we're told a CG it doesn't matter the length of the bed, or the distribution of the load.

Ok. You load the bed uniformly and place the axle where you will. Then I will select and place the tongue.
The longer the tongue, the less weight on the hitch.
It's called the law of the lever.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

Hi BridgeSmith:
It was what they call a destination trailer. Move it once out to the lake or campsite rather than building a cabin.
I was over 60 feet overall length.
My 1-ton was a dually that weighed in at close to 12,000 lbs with the fuel tanks full.
I made sure my tanks were not full crossing the scales.
You could call it a three generation trailer. Over the kitchen was a shallow loft for the grand-kids.
Over the master bedroom there was a deeper loft for the middle generation.
I generally hooked up late in the day and travelled about 80 miles the first day.
The second day, I used the bottle jack to put more load on the equalizer hitch.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[rb1957]

OP seems to have given up.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.