Weight distribution question

[Trackstriper]

This is a question about weight distribution of loads within a trailer, not tongue weight per se. I have a 6x12 cargo trailer about ready for conversion. There have been some discussions about dynamic stability issues when the mass is distributed toward the ends (front and rear) of the trailer rather than keeping it more centrally located. Moment of inertia stuff. I can't keep up with too technical a discussion but want to get some real-world feedback if anyone has it.

Here's the issue. Using the standard side door for entry/egress will be the easiest. I can build a shallow galley at the rear using the two barn doors. But with this arrangement, the longitudinal berths will be pretty much over the axle centerline so the weightier loads (fridge, water heater, shower assembly) will be somewhat forward or actually toward the nose (50# genny in tongue box, 5K AC unit, light cabinetry). I think I can get the tongue load balanced out within normal limits by placing batteries in the galley area along with some other items, but this places much of the "cargo" mass toward the poles, rather than centrally.

Another option is to use a rear entrance and modify the barn doors to a central 24" door and have a layout like the "compact" design from Andrew, with weightier items over the axle and the berths up front...kind of a shotgun arrangement. The side door could be used as an emergency exit if needed. This would require a fair amount of modification at the aft end, and I would prefer to retain a more stealth appearance to the trailer with stock doors in place.

The trailer is about 1100# empty and I hope to work as light as possible, maybe adding 500-600# if possible, but it has to be a mini RV for my purposes...full service, so I need certain creature comforts that aren't light (I'll be spending several weeks at a time in it...with the spouse).

It seems to me that with a higher moment of inertia the trailer would be both: a) more difficult to damp regarding sway from an erratic maneuver, and at the same time b) more to difficult to induce into a swaying motion. It should have a greater resistance to change whether from a stable condition or an unstable condition. Am I wrong headed in this thinking?

Do y'all think this is an issue to be concerned about given the size and weight of this trailer? The tow vehicle will be a full-sized van at about 6000#, maybe more.

Bruce

[Trackstriper]

Anybody?

[teardrop_focus]

You'll see some replies to your question during the week... when peepz are back from the long weekend and back in front of a 'puter...



But, since you ask, here are some random thoughts on the matter...

It seems to me that with a higher moment of inertia the trailer would be both: a) more difficult to damp regarding sway from an erratic maneuver, and at the same time b) more to difficult to induce into a swaying motion. It should have a greater resistance to change whether from a stable condition or an unstable condition. Am I wrong headed in this thinking?

Yes and no.

A higher polar moment of intertia will be more difficult to damp when swaying, yes... but I can't see how loading that way intentionally won't create greater resistance to sway.

What will help your trailer stabiity will be to:

1. Keep heavy items as low as possible. High-mounted items will not only act as a lever causing trailer chassis roll, but also act as levers that will load and unload the trailer's tongue weight... especially with a single-axle trailer... which affects the tow vehicle's rear axle load and tow vehicle traction, including handling traction.

2. If the trailer is equipped with radial tires, use a radial tire with a lower aspect ratio... in other words, a lower profile. I've seen some awefully-thin sidewalled radials on some camp trailers and with a too-tall a sidewall. A lower-profile tire such as a 65 or even a 60-series, properly inflated, will aid trailer stability against side-to-side swaying.

What type of springs will this trailer use? Leaf? or rubber torsion? Although very rarely seen, a leaf spring trailer with shock absorbers will be far more stable than one without.

Do y'all think this is an issue to be concerned about given the size and weight of this trailer? The tow vehicle will be a full-sized van at about 6000#, maybe more.

A 6000 lb full size van should tow a 2000 lb gross-weight trailer easily, but, again, mind the tongue weight. (I realize your post wasn't asking about tongue weight, but, hey...) :-)~ Keep the tongue weight to between 10% and 15%. With a 2000 lb trailer, that's between 200 and 250 lbs. You should be fine.

[angib]

It seems to me that a 6000lb van towing a 1600lb trailer isn't likely to be near the point where sway is a problem, if the trailer's sensibly designed. Weights near the middle of the trailer are best, but you don't seem to be adding enough weight to create a problem, so I don't think you need to be specially careful.

Chris, I have no experience of US tyre types, but I've seen plenty of folk say that any passenger car tyre is more likely to cause sway problems because they are much less stiff, side to side, than dedicated ST trailer tyres.

Andrew

[stomperxj]

I agree with Andrew. If it has 14" or 15" wheels I would suggest an LT tire instead of a regular passenger car tire as they have softer sidewalls...

[teardrop_focus]

D'oh! You two are correct!

(My mistake; my comments were based only on my experience in that I only use performance tires on my car that are designed with stiffer sidewalls for handling and not "regular" or touring-class radials.)


YES! Use the radial with the stiffest sidewall as is practical for one's budget... in this case, both Andy's and stomper's recommendations for using an LT or light truck tire are spot on.

[Trackstriper]

Thanks for the input. I hitched up the trailer yesterday and loaded it with 500# of cargo, distributed front and rear only, to put the weight out at the poles. Pulled it 140 miles at varying speeds, no problems with sway, even after inducing some tail waggle at speed. It damped out pretty well. So I guess it's not a problem at this point with my towing combo.

The tires that I had on it were the stock bias ply specials that came with the trailer, Nanco brand (Chinese?). I picked up a set of aftermarket 15x6" alloy wheels a while back and put some 70 series BFG LT radial tires on them. I'll try that combo with the same test.

BTW, the fuel consumption sucked. Normally I get 15 mpg at cruise. I pulled 10 mpg with the 6x12 and wasn't keeping up with interstate traffic, maybe 65 mph. When I pull my 4x6 cargo trailer, loaded to a higher weight than the 6x12, I only drop to about 13-1/2 mpg at cruise. I only slightly know it's back there.

The tranny wanted to hunt for the right gear; with OD off the engine seemed to run pretty high rpm at 65mph, with OD engaged it was geared to tall and a bit iffy on the flats, really lugging on slight upgrades, then wanting to downshift. I need a gear between the two options, unless I drive 55. I may have to change my tire size down a bit to find a workable combo. I guess I'll put a tach in the van to see what I've got going on.

In my case the aero drag is killing me with the 6x12 and maybe I'll have to do some workarounds with that too.

Appreciate the help.

Bruce

[teardrop_focus]

Trackstriper

The tranny wanted to hunt for the right gear; with OD off the engine seemed to run pretty high rpm at 65mph, with OD engaged it was geared to tall and a bit iffy on the flats, really lugging on slight upgrades, then wanting to downshift. I need a gear between the two options, unless I drive 55. I may have to change my tire size down a bit to find a workable combo. I guess I'll put a tach in the van to see what I've got going on.

A tach would be a very good idea. If it is determined that your 3rd gear (Drive, assuming the trans is a four-speed auto) cruise at 60 is 3200 or below, you should be able to live w/ that comfortably. It's cruising above 3000 to 3200 where thing begin to seem busy. 3000 isn't as economical as 2200, but you have no choice unless you were to change the drive axle tire diameter at least an inch, more than likely two to gain the split in gearing you're searching for. And unless the speedo drive is recalibrated, you won't notice the difference according to MPH... something to consider. Now I'm confused... hahahaha...

Turn OD off, then use the button as a manual shifter to upshift for slight downhills or with a tailwind. You can downshift with the button as well, as when preparing for an ascent... (you can also simply use your foot). If the trans in electronically controlled, the vehicle's PCM (powertrain control module) won't allow any potentially-damaging shifts. Turning the OD off and therefore preventing this "hunting" you speak of can actually prolong transmisson life and have the trans run slightly cooler, albeit due to the slightly higher engine speed.

You're definately on the right track. Again, a good V8 will cruise happily all day long at 3000 revs... it's just not quite as economical.



Finally, here's a nice reminder from forum user Frog, from another thread:

Frog

Sometimes driving in direct drive instead of overdrive might actually increase mileage by letting the engine work in a more efficient part of its power band. But again, without testing, its just academic speculation. Slow down, save gas, have a safer ride and enjoy the scenery; or speed up, burn more gas, increase safe stopping distances and get there faster without seeing anything but the highway.