Trailers gone wild (fishtailing)

[MushCreek]

Hi! I haven't been here in a LONG time, what with life getting in the way. I was going to build a camper on a little trailer I had, and did, sort of. I built a box, then found a cool retro pop-up, which got me going quicker. The secondary purpose of the box was to haul stuff to our new place in SC. That's where we are now. I finally took the trailer on it's maiden voyage, which was a disaster! It would not safely go more than 50-55 without fishtailing all over the place. It took us 15 hours to go 600 miles on back roads- white knuckles the entire time.

Here's some data- The box is 6' high, 7' wide, and 10' long, plus a 2' Vee in the front. I also sloped the top for the front couple feet in an attempt at reducing drag. I had it full of stuff with a fairly low center of gravity- maybe 3000 lbs for the trailer plus load. Tongue weight seemed to be plenty. The trailer has a new axle and 15" tires, properly inflated. The tow vehicle is a big 3/4 ton van, weighing about 7,000 lbs. Again- new tires and proper inflation. There are only two possibilities I can come up with. 1) the hitch on the van is higher than the trailer, so it slopes back a good 4" or more. I think that's the culprit, and I'm going to get an adjustable height hitch to level it out. The only other possibility I can think of is some weird aerodynamic thing going on between trailer and van. The tongue on the trailer is very short, so it is quite close to the back of the van.

I once had a Bronco II that I put a bug deflector and a car-top carrier on. It drove fine with either add-on, but became dangerously unstable with both. Obviously an aerodynamic problem! Any thoughts on what my trailer problem could be? Did I miss anything? I'm headed back to SC in April, at which time I'll do some (careful) experiments to see if I can straighten it out.

[Gage]

If you can lift the tongue with the trailer loaded, you haven't got enough tongue weight. And I’ll bet you have a single axle with bias ply tires.

[madjack]

...yep, #1 cause of fishtailing is too light a tongue weight in relationship to total weight...tongue weight should be around 10% of total weight...mis-matched tires(including air pressure) especailly if bias ply, can, add to this greatly..............
madjack

[48Rob]

quote]I was going to build a camper on a little trailer I had, and did, sort of[/quote]

Jay,

The others already answered the big one, but your statement above is cause for further investigation.

While improper tongue weight is the most common cause for sway, there are several other things to consider that can cause sway, or greatly enhance sway caused by too little tongue weight.

Overall balance of the trailer, both front to back (tongue weight) and side to side are critical.
Sizing the axle, springs, and tires to the load/weight is pretty important too.
You mentioned you "had it full of stuff".
It would be a good idea to weigh the trailer, both empty, and full of stuff to be sure you are within the safe range of your axle/springs/tires rating.

Too much weight on one side can cause, or aggravate sway, especially when a truck is passing you.
Wind, along with passing trucks and an unbalanced load will give you white knuckles too!

That your connection point is not level is another issue.

You can probably solve all your sway issues very easily be rearranging cargo, but you really need to spend a little time to investigate and determine just what you have first.

A properly balanced trailer/tow vehicle will have no trouble running 75+mph without sway.
At worst, you'll feel a very slight pull/push.

Here is a basic guide that may help you;
http://www.timberman.com/RIG/sway.htm

Rob

[MushCreek]

I would say the loaded trailer is around 2500-3000 lbs. The axle, springs, hubs, and tires are rated for 3500. I can pick up 200 lbs. without too much trouble, and I couldn't budge the tongue- I can barely drag it sideways. The load was pretty well balanced side-to-side. As I said originally, light tongue weight doesn't seem to be the problem. The trailer is still fully loaded at my place in SC. I'll be back there in April, at which time I'll do some more investigating. Yes, the tires are bias play-as are all of the other trailers I tow successfully.

[madjack]

I would say the loaded trailer is around 2500-3000 lbs. The axle, springs, hubs, and tires are rated for 3500. I can pick up 200 lbs. without too much trouble, and I couldn't budge the tongue- I can barely drag it sideways. The load was pretty well balanced side-to-side. As I said originally, light tongue weight doesn't seem to be the problem. The trailer is still fully loaded at my place in SC. I'll be back there in April, at which time I'll do some more investigating. Yes, the tires are bias play-as are all of the other trailers I tow successfully.

...wellll, all that being said, the tongue weight should fall in the 2-300# range and sounds like it just might...balance of the load(side to side), matched air pressure of tires and alignment of axle would be my next checks.................
madjack

[eamarquardt]

I had a 67 VW Bug. It had a hydraulic steering damper. The damper failed on a regular basis. W/O the damper at about 40 mph the front end would start shimmying and it was quite violent. Just like trailer sway, something to be avoided at all costs.

So, apparently you have all the stars in "correct alignment" for your trailer to sway. I'd suspect that your suspension, tires, balance, etc. are well aligned to hit the same resonant frequency and sway at that frequency. Changing any one of these might get you out of the "frequency band". I'd, though, change as many items as you can to make sure that you completely mis-align the stars. If you're load is 3000# and your tires are rated for 3500# you are nearing the edge of the envelope, IMHO as I always like a significant safety margin. One could argue that the manufacturer has already factored that into the equation but you are having difficult so maybe some more margin for error would be a good thing. If they aren't properly inflated you can't expect them to perform properly. Stiffening up the suspension will also help mis-align the stars. Adding shocks to the trailer's suspension would be a plus. Utilizing a weight-distributing hitch would also probably help also.

I'm aware that none of these changes are cheap but they are a heck of a lot cheaper than an accident.

As SC says: "Just my too cents".

Hope this helps.

Cheers,

Gus

[angib]

Another cause of sway apart from low tongue weight is big inertia - if you have heavy weights at the front and back of the trailer you can still achieve a reasonable tongue weight but the 'dumb-bell' weight distribution increases the likelihood of sway - or, more accurately, reduces the speed at which sway will start.

If you have big weights at the front and back, is there anything you can do to move them towards the axle (preferably just in front of the axle)?

[mikeschn]

Where is your axle in relation to the to back of the trailer?

Mike...

[48Rob]

Another cause of sway apart from low tongue weight is big inertia - if you have heavy weights at the front and back of the trailer you can still achieve a reasonable tongue weight but the 'dumb-bell' weight distribution increases the likelihood of sway - or, more accurately, reduces the speed at which sway will start.

Hi Andrew,

If a trailer weighs say, 3000 pounds, and you have 12% (360 pounds) tongue weight, but you have 500 pounds behind the axle and 2500 pounds forward of the axle, does this constitute "dumb-bell" weight distribution?
(Since the axle placement is being kept rearward to allow for pleasingly aesthetic door location, weight is added to the rear to keep tongue weight from becoming too great)
The axle placement would also dictate to a large degree the overall balance, no?
At what point would the design start to suffer, when the weights fore and aft are similar, even if the axle is well back and if so, wouldn't you be forewarned by the loss of appropriate tongue weight?

I'm not arguing, but I am trying to learn, as I am in the midst of a design...

Rob

[eamarquardt]

If a trailer weighs say, 3000 pounds, and you have 12% (360 pounds) tongue weight, but you have 500 pounds behind the axle and 2500 pounds forward of the axle, does this constitute "dumb-bell" weight distribution?
(Since the axle placement is being kept rearward to allow for pleasingly aesthetic door location, weight is added to the rear to keep tongue weight from becoming too great)
The axle placement would also dictate to a large degree the overall balance, no?
At what point would the design start to suffer, when the weights fore and aft are similar, even if the axle is well back and if so, wouldn't you be forewarned by the loss of appropriate tongue weight?

I'm not arguing, but I am trying to learn, as I am in the midst of a design...

Rob

I don't know exactly how you are planning to distribute the weight in your trailer in its "ready to hit the road state" but the criteria you reference above (trailer 3000#, 360# tongue weight, 500# behind the axle, and 2500# in front of the axle) seems, intuitively, like it would difficult/impossible achieve. I'd like to see details on your "weight distribution plan" and perhaps some calculations. PPPPPPP. PM me if you don't know what PPPPPPP means.

Cheers,

Gus

[MushCreek]

The heaviest object in the trailer is a large garden tractor, close to 1000 lbs., and it is pretty much centered over the axle. In case it isn't apparent, I'm currently using this rig as a cargo trailer, so I can easily re-distribute the weight. I've had poorly loaded trailers before, but never had one behave this badly, esp. with a big, heavy tow vehicle. I'm REAL curious to see how it acts empty. Things I'll check will be tongue height, and whether the axle is square to the trailer frame. If it were some freak of aerodynamics, I'll either lengthen the tongue or change the shape of the box.

[mikeschn]

Don't forget to measure and tell us the distance from the back of the trailer to the center of the axle.

Mike...

[MushCreek]

If I recall, it's about 55", and that would make it 65" to the front of the box.

[mikeschn]

5" behind the center of the trailer. That's not nearly enough. Even with your weight properly distributed, that layout lends itself to fishtailing.

Can you have the axle moved rearward?

Mike...