Are Fifth-wheel Trailers Ever Unstable ? ? ?
 

I am experienced on towing with hitches at the back of the tow vehicle, including a catastrophic fishtail accident. My question is:

Are fifth-wheel trailers pretty much immune to fishtailing and other forms of instability?

I will be building a fifth-wheel trailer from scratch and could use some advice from those with fifth-wheel experience. My F-350 had a lot of fifth-wheel experience before I got it, but it isn't talking. I have a new Binkley (Holland) 12" fifth-wheel (inverted) to match the Binkley telescoping kingpin in my flat-bed. The fifth-wheel is an "oscillating" type, but I do not need to use it that way.

Archie, most people that pull 5ers will say that they are immune to the sway of a bumper pull trailer.

I had a 26' Gulfstream 5er (8K lbs.)pulled by a 91 F450 Super Duty diesel, that got a little sqirrelly one day. We were on the interstate running 65-70 MPH, roads were dry, no wind, no trucks within a mile, when for no reason the trailer swayed back and forth twice and then straightened out on its own as I was reaching for the brake controller. I stopped and checked everything I could think of, tires, brakes, warm hubs, and found nothing. We pulled this trailer with the same truck for 25K miles and never had it do this again. Sort of like a "pay attention" wake up call.
My current rig is a 99 37' 5er pulled with a R-Model Mack, so I don't think I'll have to worry about any instability with this rig.

That is the main reason for a fiver plus you can tow more in fiver form than TT

Jeff,

I am just wondering about the extremes. Of course, there are cases when horrible rear-loading will make even a properly designed tow vehicle & trailer unstable. I am thinking of making my rig a "slider" so I can move the rear axle back for stability and forward for low-speed maneuvering and loading (my trailer will be designed to tilt).

In general, you are correct, but when I "run the numbers" on my planned system, it is surprising how small the extra capacity is. I'll see if I can do a short version:

My '68 F-350 weighs 6,500 unladen, with a 10,000 GVW => 3,500 net.
Tandem-axle trailer, 2,000 unladen, with a 7,000 GVW => 5,000 net.
Just add the two net weights, because they are independent.
This gives 8,500 unladen, with a 17,000 GCVW => 8,500 net.

My '68 F-350 weighs 6,500 unladen, with a 10,000 GVW => 3,500 net.
Fifth-wheel trailer, 3,500 unladen, with a 13,500 GVW => 10,000 net.
Because 3,500 of the trailer weight is on the truck, the truck's net is already included in the trailer's net, so do not count it twice.
This gives 10,000 unladen, with a 20,000 GCVW => 10,000 net.

I have done this calculation a lot of times and would love to have somebody point out an error. The numbers are in part due to choices already made. I am using a single-axle, dual-wheel configuration the same as my F-350 by my choice. I KNOW that if I went to tandem-axle dual-wheels, the story would be much different. Another part of my loss is the very heavy (2,000#) flatbed I have - I am considering making an abreviated fifth-wheel deck.

There are several reasons why I am still going to build the fifth-wheel:
o I already have acquired almost all the parts.
o I want the stability of a fifth-wheel.
o Some loads cannot be divided between the truck & trailer decks.

Archie, the plan of making it a slider for ease of weight distribution sounds like a good one. The only thing I question is, why the single axle on the trailer? Tandem axles are more stable by design,(they want to keep going straight) and will also carry more of the load, instead of having it on the truck.

Just curious, what are you hauling? Parts? Heavy equipment?

Jeff,

The heaviest thing I hauled was 7,500 pounds of Caterpillar D-2, which is no longer mine. I most often haul machine tools - lathes & mills. I am trying to standardize tires & wheels between my F-350 and its trailer. I have owned two tandem axle units and have a lot of respect for their capabilities.

I do not like having a tandem rig fully loaded and seeing (in the mirror) the tires trying to come off the rims during a sharp turn - I actually did get one (tubeless) to come off its rim with a bang while spinning a trailer with a 5,000 pound sailboat on it with my forklift. (Scared the stuffing out of my wife who was standing right by the trailer!) Another thing I did not like was when pulling on a very large radius turn (again, fully loaded), the trailer did indeed want to go straight, so I would get a straight stretch until the tow vehicle put enough side pressure on the tongue, a correction, then another straight, & so on. Not really horrible, but a little uncomfortable in heavy urban traffic on an elevated cloverleaf.

My main goal is to have a trailer deck the same height & width as my flatbed. Then I can load it with anything on pallets with my forklift. With a tandem-axle trailer with the bed between the wheels, everything has to be loaded at the rear and winched forward. I have done this enough to be tired of it. I am pursuing the unattainable goal of having one trailer for all purposes.

I know I could do a bed over tires tandem which used my same wheels, but then my slider could not slide as far. The design I am considering should be able to serve as a drive-on loading ramp for itself and for the truck deck. I do not want to go tandem-dual because the cost of wheels & tires would be more that what I can justify for the amount of hauling I do.

Thanks for you thoughts - I appreciate the feedback. Answering your questions helps me de-bug my ideas.

Does anyone have any experience with oscillating-type fifth-wheels?
 

The Holland/Binkley fifth-wheel I have is normally used in inverted mode on the trailer, but is sometimes used in "normal" orientation on the tow vehicle. My F-350 was already fitted with a Binkley telescoping king-pin so I bought a Binkley fifth-wheel to match. I did a lot of reading on the internet and learned what options I had.

Big truck fifth-wheels mostly have a single pivot axis that allows the trailer and tractor combination to bend when negotiating rises and dips. When negotiating a road-side dip while turning, the springs try to compensate, the frames twist, and the tires of the trailer even lift off the ground if it is unloaded. I have seen this, so I assume many of you have as well.

On a very small percentage of big rigs, the fifth-wheel is an "oscillating" type. This means that it has a second pivot axis that allows the trailer to sway independently of the truck. The Holland site explained that the application for this was on tankers where the tank is the frame and too much torsion leads to problems. Maybe some of you have also seen the evidence of this problem - diagonal wrinkles in the tank. Most tankers are highway only and I assume tanker drivers (& many others) are careful not to take too sharp a dip or rise while turning. I have yet to see a full-size oscillating-type fifth-wheel other than in pictures.

The previous owner of my F-350 had done a lot of fifth-wheel towing and felt strongly about the benefits of having the trailer & truck locked together, with regard to sway. This makes a lot of sense because the whole set of springs on both vehicles are resisting sway and you do not have two independent sway frequencies that could cause problems.

I initially ordered the "plain" Binkley, but the dealer said I could have the oscillating model cheaper if I did not mind doing a little modification. It turned out that there was a minor (< 1/64") mis-alignment between a hole and its reinforcing plate from poor jigging during welding. Literally a few licks with a file was all that was needed. Anyway I have an oscillating-type fifth-wheel. On my 15 acres I do not have a dead flat turn-around area, so I think that the extra axis will ease my nerves during maneuvering on my property. I do plan on making a system to easily lock out the extra axis when I am going down the interstate, especially with a tall load and high cross-winds.

One of my questions about fifth-wheel stability is to find out what everyone else knows about this. You goose-neck guys that have a ball & socket type coupler have the equivalent of an oscillating-type fifth-wheel - do you have more sway problems than a "rigid" fifth-wheel?

Sounds like the rocker type hitch I have is pretty much what you are calling "oscillating". Is that what they call it for the big rigs ? I know some rocker type hitches are designed to lock out that feature; but on mine it's designed to be flexible all the time. I tried carrying a huge cooler on a platform I added to the back of my camper, but a test drive showed this brought it so close to neutrally loaded (almost no pin weight) that it was clunking as it rose and dropped in the hitch. Despite this, it still went down the road stable, with no sway - and that's using a smaller short bed pick up as the tow vehicle. Trailer about 5500 lbs. Needless to say I didn't continue to tow it that way; just an example of how stable a fifth wheel can be.

95van,

Thanks for your input. The term "oscillating" is a bit strange, I admit. I think that is is used by the technical people that design big rig components. I am confident no marketing person would use that term as it sounds bad, especially in a discussion of trailer stability.

Now that you mention it, I remember the word "rocker" (not the best term either!). I did not spend too much time on non-Holland sites, because that brand was already pre-selected for me.

I guess the question for people with rockers that can be locked out, is:

When to you lock it out and when to you let it "rock"?

In my experience this is not much of an issue unless you are using under rated tires and axles. I have had triple axle single as well as dually trailers and never had a tire come off the rim. And believe me I have had them in some crazy places and watch the rims and tires flex, but if it is rated correctly and you have 80psi in the tires you should be fine.

BTW a friend of mine has a tandem dually tilt with sliding axles that works great. The slide and tilt work off a hydraulic setup.

mudmaker, Thanks for the slider info.

Regarding the tires, I did not mean to say that tires were going to be coming of the rims during towing. I was originally amused by watching the tires fight themselves during tight turns necessitated by some of the poorly designed gas stations, including people that wait until you are gassing up and then park in the red zone right in front of your exit path, while their partner pulls in behind you. The long turns issue was as I said - uncomfortable. It made for "white knucke" steering in urban traffic, which is a VERY small percentage of my towing.

My trailer was not overloaded when I did pop a bead off the rim, although the tires were the original bias-ply (35 psi) tires. I was much happier all around when I put on 65 psi radials. All the same, I like would to be able to jack-knife my trailer, and a fifth-wheel setup makes it easy to take it clear to 90 degrees where the trailer is being spun around.

Considering all the compromises, I have come down on the side of a single-axle dual rig. To me the tandem axle single-tire has one big advantage that would recommend it to me if I did all my driving in the mountains - it could be fitted with four disk brakes and shed heat much better. My F-350 has huge drums all the way around and my trailer will match. These drums stop quite well, but I know their limitations - they will not do a series of panic stops nor will they last long without fading if they are used continuously. On the other hand I will have the gearing I need to hold back with engine braking. (I also have considered putting a retarder in the center of the trailer axle - anyone have any ideas?)

if you are going to be loading with a forklift you might want to put a set of landing gear on the back of the trailer, like on a single axle straight truck. otherwise you might overload the trailer axles.

Thanks, 04superduty,

Good tip & worth repeating. On my tandem axle trailer when I occasionally had to load up to 2,000 pounds on the back of the deck and then winch it forward, I used a couple of axle-stands under the frame.

I also had a fun ride once when I forgot to put the wood blocking I normally used under my fork-lift trailer's rear. My little Clark forklift is small, but it weighs 4,000 pounds. As I drove it onto the back of the trailer there was enough leverage to lift the tow vehicle's rear end enough to break traction and everything started to roll/slide down the slight slope. I realized that there was not a lot of choice in this situation and just kept driving the forklift past the axle and then things slid to a halt.

Now you know why I do not fail to think about this tip when loading a trailer.

P.S.: This is another reason why the pro's chock all their wheels. I have seen signs on loading docks reminding the drivers that they get on service until the trailer wheels are chocked.

I guess if you already purchased the hitch then use it, but if it were me with small wieght capacity you are dealing with I would use a gooseneck if your worried about binding the hitch and twisting frames etc. The only disadvantage I have found on a gooseneck is on high profile applications. In a flatbed or say less than 9' tall they are great towing and you have less to deal with in the bed.