The Stovebolt Site Forums The Shop Area General Truck Talk 5th Wheel Mount Location?

I know diddely about 5th wheels, but I am hoping that I will need to soon.

I've seen them mounted behind the rear axle, on top of the rear axle and in front of the rear axle. I'm also handicapped by an engineering degree.

It seems to me that the mount should be 1/3 of the wheelbase forward of the rear axle, since there are two wheels on the front and four in back.

But that doesn't take into account the static center of gravity of the truck. I suspect that, because of the weight of the cab and drive train, an unloaded truck has about 60% of it's weight on the front wheels.

I would also think that, ideally, you would want the center of gravity of the loaded truck where you would have 33% of the weight on the front, and 66% on the rear. (Duallies on the rear. 50/50 otherwise.) My reasoning is that this puts a reasonable amount of weight on the front wheels for steering, and a reasonable weight on the rear wheels for load carrying and braking.

I've also seen some sliding 5th wheel mounts. These would seem to be great for fine tuning.

My next question is about goosenecks. I asked a bed manufacturer about fifth wheel placement and he said it didn't really matter because you can get trailers with goosenecks any length you want. Is that true?

So what do you think?

My F-350 has the ball right over the rear axle. Works just fine. By the way it's a long wheel base crew cab dually.

Are you planning to install a full-size 5th. wheel for semi-trailer type loads (80,000 lb. GVW) or a smaller one for towing something like a horse trailer, etc.? The purpose for carrying the trailer kingpin weight slightly ahead of the rear axle is to allow some transfer of trailer weight to the front axle. Max allowable 18-wheeler axle weights are:

front: 12,000 lb.
drive axles: (dual drive) 34,000 lb.
trailer axles: (dual axle) 34,000 lb.
total gross weight: 80,000 lb.

We would slide the 5th. wheel forward to put the max weight on the steer axle to get legal over scales, then slide the 5th. wheel back for easier steering between scale houses. Sometimes a trooper with a set of portable scales in a rest area would rain on our parade, and we'd get a ticket for overweight on the drive axles.

If you're setting up a gooseneck ball hitch or a small 5th. wheel for a travel trailer, put the pivot point just slightly ahead of the rear axle centerline for the best trailer handling. Going behind the axle CL will take weight off the steer axle, and make the handling a little squirrely at times. It also does funny things to trailer tracking while backing up.
Jerry

Just a few inches froward of rear axle centerline is the preferred placement for a goose neck ball.

Actual placement may be influenced by the length from ball to either rear corner of bed, and the trailer tung "reach".

On a heavy truck with a not so heavy trailer it is acceptable to put the ball behind the rear axle if not possible to put it forward as a first choice.

Grigg

You won't find as many adjustable gooseneck ball setups as fifthwheel setups, especially in pickups. This is because the gooseneck ball must be braced differently and generally requires a hole in the bed. Mine is not adjustable, but only takes the pulling of one pin under the bed to remove the fifthwheel and install a goosneneck ball in the bed.

Hotrod Lincoln, thanks for you input. I had a feeling that an OTR driver would know a lot more about this than I do.

Grigg, as always, thanks. "You da man!" You can call me "Grasshopper" any time.

Baldy . . . hunh? I didn't know there was a difference between a 5th wheel mount and a Gooseneck mount. So you're saying that a Gooseneck connects to a ball in the bed and a 5th wheel attaches to a socket? I honestly didn't know there was a difference.

Yep! Goosenecks are generally stock trailers, as in cattle and horses, pulled by pickups or flat beds with a well for the ball. Fifthwheels can be the smaller size used in pickups that pull RV trailers or the larger type used on the tractor of a semi-truck.

goosenecks generally work better for rigs that have to go offroading (horse trailers etc.) because they can swivel in any direction,where RV type 5th wheels are more "locked" together and can't articulate over rough ground as well. As a result,5th wheels aren't as likely to sway suddenly and empty all the cupboards as an RV with a gooseneck might,since the truck will have a lot of influence over the sideways motion of the trailer. I set the 5th wheel plate on my '57 about 4 inches forward of the rear axle centerline,and it worked great.
Speed

I just bought a 5th wheel camper and had the 5th wheel hitch mounted as part of the deal. The company mounted it 6 inches in front of the rear axle. As for the sliding hitch, they are used on short box trucks so that you can slide the unit away from the cab for tight turns when turning or backing into a tight place. My truck is an extended cab short box '07. I did not get the slider hitch because it was about $400 more. I do have to watch myself when backing into a camp site so that I don't hit the back of the cab. Don

Glad I'm hearing this conversation now. I'm fixing my flatbed and I'm installing some bracing under it to accept a gooseneck ball and / or a fifth wheel mount.

When my truck is complete, I was planning on taking it to the local scales and measuring the weight on the front and the weight on the rear. I know my maximum front and rear axle weight. Based upon that I can determine the total load weight I can put on the hitch. I can then take this max hitch weight and use it to determine where the hitch should sit to distribute this weight between the two axles. I've been told by many people it'll be about 4 - 6 inches in front of the rear axle.

I have a question about the 5'th wheel hitches. How do they connect to the truck bed? I'm assuming some bolts, but what are the dimensions of the attachment surface?

Thanks.

They are not universal, each manufacturer is different. Mine mounts to the bed by way of the receiver of a gooseneck assembly and has well distributed pads that contact the bed of the truck, but aren't actually attached. That is why I can change from fifthwheel to goosenech just by pulling one pin ubder the bed. The receiver assembly is mounted to the frame with bolts through the frame an u-bolts around the frame,IIRC.

4" forward of the drive axle is a good generic set. Too far forward will make for a rougher ride, and under a heavy load will cause your frame to flex more than necessary. Too far back is not to be considered. What Baldy said about mounting is dead on.