Unstable twisting of Chevy LUV pickup trailer (1 Viewer)

[marcfj60]

I did a little searching but couldn't find an answer. So I'll get my questions posted up.

My father recently acquired a trailer that used to be a Chevy LUV Mikado pickup. The frame in front of the bed sticks out about 4' so the total trailer length is about 10' long. It has the original axle and springs under it.

We installed a light kit from Harbor Freight on it, took it to the MVD and got a new VIN and permanent tags on it.

It tows fine when it's empty or has a couple hundred pounds in it. I needed to haul some dirt off one day and that's when I ran into a problem.

I loaded it until the bump stops were about an inch from the top of the axle tubes. They are normally about 8 inches above them unloaded. When towing the trailer things are fine below 15 mph. Once I exceed that speed, the trailer will begin to oscillate in a twisting motion, compressing the spring on one side and unloading on the other, then shifting this behavior to the wheel on the other side.

I know I had a load on the tonge of the towing vehicle. I loaded the trialer with weight bias toward the front, but not significantly. I don't know exactly what the load was but my '99 UZJ100 didn't seem excessively loaded on the rear suspension. I bought and installed some new shocks, thinking I just needed to dampen out the oscillation. But that didn't fix it.

My theory is that the springs are just too light duty for more than a couple hundred pounds. Once they are overloaded, they quickly get into a resonant natural frequency that you can only stop by slowing down.

I would like to be able to carry more than 300 lbs with this trailer. A good goal would be 800 - 1000 lbs of cargo.

One idea I was given was to replace the leaf pack with more beefy ones. I have a set from an AWD Chevy Astro Van but looking at dimensions, I'm going to have to install a new front mount, move one set of U-bolt holes over 1/2" on the axle bracket and fabricate and shim the rear shackles.

Another idea I had was to find some coil springs from the front of a 4-Runner or Tacoma and install them between the top of the axle and the bottom of the frame, replacing the bump stop bumpers. I was going to poke around the U-Pull-It yard and see what I could find. It'd be really nice to find something that doesn't require too much cutting/drilling to get the spring cups off.

One other thought I had was that the tires were not proper tires for the trailer. They were just standard load tires, not trailer or even LT tires. Do I need to run ST tires on this or can a decent set of LT tires with at least a Load C rating be ok as long as I put enough air pressure in them? I'm going to make a guess that the trailer weighs about 700 lbs. I never would exceed 1,000 lbs of cargo in it.

 

[titanpat57]

To me it seems that there is not enough tongue weight by the problem description, but of course there are so many big words in that post...I was just looking for pictures after the first sentence...

Try front loading to see if you can get 15-20% of the load to the tongue, as long as it's not straining the truck.

If that doesn't work, well, then you're therory is probably correct, and airbags may offer some safety insurance too.

 

[marcfj60]

As I was typing I was thinking that my post really needed some pictures to help complete the mental pictures. A video showing the weird twisting would be ideal but I'm too busy trying not to crap my pants when it's doing that to take a video. Sorry about the big words. Too many years in engineering school has made it hard to use other words.

Based on the 'squat' of the tow vehicle, I'm estimating there was about 300 lbs on the tounge. I had the sand loaded in the trailer such that the sand was up to the top of the bed at the front and then it tapered down to nothing at the tailgate, as an attempt to shift the weight forward.

Would airbag helpers be better (or easier) than trying to put some coil spring helpers?

 

[titanpat57]

I think so, there are some pretty adaptable models...maybe a search on forums related to the truck itself may provide some direction, or perphaps even some of the "repeat" problem....just thinking outloud.


oh..

and that was hilarious about trying not to crap your pants.....

 

[Brian in Oregon]

You need to get an accurate weight of the total trailer light empty and loaded, plus the weight on the tongue. You can span two bathroom scales with a board and put the landing gear on it to get th tongue weight. The empty and loaded weight will probably have to be done at a truck scale.

The trailer tongue weight should be between 10% to 15% of the TOTAL GROSS trailer weight. Never go under 10%. I'll bet dollars to donuts your trailer is less.

 

[Living in the Past]

A picture would help. Seems most trucks have more bed behind the axle and most trailers have more bed infront of the axle. On a truck the rest of the frame with a engine act as a counter balance. I thought about making a trailer out of a stout bed but planned on switching the front of the bed with tailgate just for this reason. So the question is how much the the bed is behind the axle as compared to in front of the axle?

 

[marcfj60]

When there's no cargo in the trailer, the tounge weighs nearly nothing. The axle seems nearly centered under the bed which is 6' long. Looking at some photos online, there appears to be a little more bed behind the axle than in front of it, maybe 60/40 split. There is about 4' of frame sticking out in front of the bed to the hitch. I had the tounge sitting on a cinderblock (I need to get a landing gear for it) such that it was about level with the ground. I set the spare tire in the back of the bed and it was enough weight to shift the CG to behind the axle, making the tounge lift up and the trailer pivot back until the tailgate hit the ground.

So a light tongue does sound like part or all of the problem. I've been thinking about adding a platform on top of the tounge to hold a cooler, Honda CT90, propane bottles, whatever. I could make it real beefy and that would help shift the CG forward. I also want to mount the spare tire up there, so that could help.

I'll look into airbags, landing gear, spare tire mounting and the platform and also take some photos the next time I'm at my father's house, where it's stored.

 

[fireflyr]

My M100 does the same thing when the tongue weight is too light. You have to put your loads towards the front to stabilize the trailer.

 

[Brian in Oregon]

Weld some thick steel plate over the A frame assembly (assuming you bent the frame rails to make one). That will add ballast.

 

[marcfj60]

I didn't build it, but the guy who did cut/bent/welded the frame into an A in front of the bed. Based on advice and experience posted here, I'm going to mount a trailer jack, a spare tire holder(and tire) and weld a platform with some steel to the top of the A and see how she does. I'll also try the board and 2 bathroom scales trick to attempt to get a reasonable tongue weight.

I can always add airbags, rubber springs, coil springs, etc to beef up the suspension later if I need to.

I'm also thinking about replacing the ball coupler with a lunette ring and using a pintle instead. Does a fixed ring provide enough room for trailer to truck twist for mild off-road use? Would it be better to fit it with some kind of swiveling lunette? I'm just concerned that I might be off-road, on a forest road somewhere and my Cruiser and my trailer will be twisting different directions. I know a lunette can stand up that kind of motion a lot better than a ball coupler can.

 

[Living in the Past]

I'm sure doing all that will help. The down side is you will always have a heavy tongue weight even when the trailer is empty. Depending on how you use the trailer most of the time you might be able just load front of the axle with the heavier stuff. If you carrying dirt, rocks or sand might think about having a way to add weight for those times. A spare mounted just on the front side of the bed would probably help to balance out the empty trailer. The front end shouldn't tip up in the air just because the spare is back there. Putting the spare on my M416 in the back just help take some of the tongue weight off when moving it around by hand empty. But it doesn't want to tip up. I'm just not sure I would want to add a lot of weight to the tongue all the time when it's really only needed in worst case. Because of the way the trailer was built (more bed behind the axle) the amount of weight needed changes depending on the load and where it is. I moved a bunch of used bricks in my M416. On one of the trips I loaded the back heavier thinking I would keep some ofthe weight off the lunette. Big mistake, even with my 100 series it was trying to push the back end around going down a hill.

 

[SteveH]

I have built about 5 pickup bed trailers from Japanese trucks. I think your tongue is too short, and the tongue weight too low. I did learn that adding weight (toolbox) to a light tongue will help, but it won't totally solve the wobbly-at-speeds issue. I have learned to make the tongue as long as possible (not always convenient, I realize) as it adds to the stability of the trailer. The only time my best trailer wobbled (a '77 Ford Courier longbed) was when it had about 2000# of rock in it, which is more than you likely should put on a pickup bed trailer.

To increase tongue weight, move the spare to the front of the bed panel, above the frame and outside the box. Consider adding a toolbox. Remove the gas tank (if still equipped) and any other hitches and junk on the rear of the bed of this trailer. The steel plate platform is a great idea. Add a 'landing leg' so that your hitch coupler doesn't lie in the dirt, and this leg will add weight, too.

I still think that 4' is just too short for the tongue, but you have to experiment and fiddle to get it right.

 

[marcfj60]

I'm not ambitious enough to stretch the tounge out so I'm thinking I'm just going to need to experiment with loading and tounge weights.