5th wheel hitch placement

[Harleyrider-TC]

I was searching the web for some answers on hitch placements on our RV toter and ran across this web site that might be able to answer my questions. I have a 2003 Pete 330 with 185in. wheel base that tows a 38 ft 5th wheel that weighs 18k lbs. and when I'm towing it on rutted roads it's a struggle to keep it on the road when I get above 55mph. It feels like I'm riding on marbles. The weight on the front axle empty is 8k lbs. and with the trailer on its 7500 lbs. The 5th wheel hitch is about 3in. back from center of axle. The truck is a 4 dr with a schwable conversion with a custom bed. Any ideas how to fix this problem?

 

[hot rod]

Welcome to the forum.

Did I read that correctly, your hitch is three INCHES behind the axle center? And that is to the center of the pin on the fifth wheel? It does not make sense that 3" could make 500# difference in the front axle weight. It's been a long time since high school physics, but my math comes up you should only have 60# change on the front axle at 3" vs. 185" wheelbase assuming a 3600# pin weight (somebody correct my math if I'm wrong). Even so, 500# should not make it handle as bad as you are experiencing, think about it, 500# is the equivalent of 80 gallons of fuel and I'm sure you don't notice any different handling on empty vs. full fuel tanks. Did you weigh it on the same day with the same fuel/passenger load? What are your rear axle weights with and without the trailer? The difference would be your pin weight of the trailer. Your truck is way bigger than my GMC 6500 Topkick that I tow with, and we are both much bigger than the 3500 dually the average idiot is towing that same fiver with. It should tow your trailer like you literally don't even know it is back there, regardless of where you place the hitch. I'm currently towing a 38' 15k fifth wheel camper, but I spent many miles towing a 40' 22k race car trailer with the same truck. My hitch is an inch forward of axle centerline and my my front axle weight goes up by less than 100# with a 5000# pin weight on the trailer. There are a bunch of guys towing fifth wheel campers with hdt's like yours, and most of those guys have their hitch 3 to 4 FEET behind the axle either to make room on the deck for a car or golf cart, or just to make it easier to back up, without any handling problems. I have to suspect your handling problem lies elsewhere.

Do you know the total weight of your camper and the pin weight? The rule of thumb is 20% of the total weight on the hitch, so if your total trailer weight is 18k, the pin weight should be around 3600#. Water in the camper can make a big difference there, my 100 gallon fresh water tank is well ahead of the axles and makes a big difference in pin weight full vs. empty. There is quite a bit of room for variation there and still handle fine, so anything in that range should handle just fine. My 22k trailer had 5000# pin weight and you literally didn't know it was back there at 75 mph (other than the weight on a grade of course), hills, curves, stopping, no problem at all. Handles like a sport car compared to towing with a dually. And your truck is heavier and has a higher capacity than mine.

Just brainstorming. Is your truck air ride or spring? If is is air ride, does your ride height stay the same loaded/empty, or does the ride angle on the truck (can't think of the term I want) change significantly? If it does change a lot, is there an auto levelling valve that could be bad? Have you gone through the front suspension/steering? King pins, tie rods, steering box play etc. could all give that same symptom which may not be as noticeable empty. Tire pressure? Most tire mfgs. have a load/inflation chart to give you proper inflation for the exact weight on each axle. Too high/low for the weight can make it feel weird, I know my Michelins call for only 75# of pressure on a 110# max tire for my weight, and it does make a big difference in ride. I'm also assuming your tires are in good shape to begin with. What about the angle of the trailer? A triaxle fifth wheel trailer is very sensitive to ride angle. If it is not very close to level, it will definitely handle all squirely, you need to be within an inch or two of level on the hitch height to handle well, and too low will handle worse than too high. Trailer axles bent or out of alignment? Broken springs/hangers on truck or trailer? Just tossing out ideas.

Good luck!

 

[Harleyrider-TC]

5thwheel hitch placement

Thank you for responding to my question Hot Rod. I've had the front end checked out and realigned and have put new mitchlin tires all around with 85lbs in the front tires and 78lbs in the rear tires. Our 5th wheel has tadem duals and was towing a little high in the front so as of yesterday I lowered the front end. I will be taking it on the road again next week for a trip and will see if this cures/helps the problem. If not we will take the next step.

 

[jasong-TC]

I'm wondering, could the wheels (being wider than the pickup trucks that made the ruts) be wandering into the ruts.
I've experienced this on dirt roads where the track width was wider or smaller than what I was driving.
Now if you're talking wash boarded roads, that's different.
It could be your shocks.
How much weight it's on the rear axle?
The air bags/springs may be too stiff.

 

[hot rod]

If his truck is like mine it is way oversprung empty, but that just makes the ride rough, it doesn't handle funny. And the ride is better with the trailer attached and weight on the springs. His handling gets worse with the trailer. My thought was that some problem with the truck (like king pins for example) may have not been real noticeable when empty, and just feels worse with the trailer. Lots of things it could be.

Tandem duals is an odd set up for a 5th wheel. Must be a high end rig, you usually don't see the nice expensive 10k axles on the mass produced units. While being level is still important, two axles trailers don't usually get as squirrelly as triaxles when they are off a bit. What kind of suspension is on the trailer? Torsion? Air ride? MorRyde independent? Do you have any sort of an air hitch on the truck or a 5th airborne type setup on the trailer end of the hitch? I've got a Trailer Saver air ride hitch on mine, and while the air pressure range is pretty broad, if you are way off it can allow the trailer to porpoise some on a rough road.

 

[hot rod]

One other weird thought. I once had a smaller tandem tag trailer that was always a miserable bitch to tow. It would be squirrelly and push the truck around way out of proportion to the size of the trailer. Never gave it real big thought, just figured it was geometry because we had the trailer built kind of short for the width. Towed it for many years, came time to replace the worn out torsion axles, and when installing the new ones and taking some measurements, it turned out the axles had been welded in at the factory slightly out of square with the trailer and with each other. Not enough to look odd to the Mark IV eyeball measuring tool all those years or to tow crooked behind the truck, but enough to see it on the tape measure. Welded in new squared up mounts and the trailer towed like a dream from then on. And it was a name brand cargo trailer. Go figure. Strange things happen out there.

 

[hot rod]

another thought just occurred to me on hitch placement. my truck has a fifth wheel with the pin an inch ahead of the axle that I normally tow with. But when I built the truck I also welded in a gooseneck ball on the deck behind the fifth wheel assembly because I had room, and what the heck then I could tow anything. That ball is about two FEET behind my axle, and I towed my buddies gooseneck trailer on a few occasions. his trailer is about 16000 with maybe 3000 over the hitch, and it towed fine, I noticed absolutely no difference from towing with my fifth wheel hitch. I didn't weigh that setup to see how much it changed the front axle weight.