Fixed Fifth Wheel.

yes but once its set so that your at max on the steers with max on the drives, the weight transfer between the drives and the steers will be the same. the only real difference would be if you were going to run at less than half tanks of fuel to move a hundred pounds of weight off the drives to the steers running the 5th further forward than normal in order to scale. they will normaly let you slide threw at 100 lbs or so over on a axle anyway. i just wouldn't be so concerned about it to not buy the truck or to swap the 5th wheel for a slider, id just get a near gross load on and make sure its in the right place to start with and run it till the 5th wheel needed to be replaced then consider putting a slider on. but at the cost if the o/p really wants a slider he can put one on, im just of the opinion that id spend the 600 bucks going threw the new truck and doing some pm work before putting it to work before id worry about the 5th wheel being fixed.

 

Not true. Reworking the amount of weight in the nose of the trailer (in front of the kingpin), also adjusts the distribution between drives and steers. So if you have a fixed 5th wheel and some newbie warehouse worker stacks a bunch of heavy stuff on the nose, you can be over on the steers, under on the drives, OK on the tandems, and OK on gross. If they swap out the heavy pallet on the nose with a lighter one from the middle of the trailer, you will then be OK on weight and still maintain the same gross.

A sliding 5th wheel could just move a notch, but the fixed would have to be reworked.


For what its worth, I had a fixed 5th wheel on my Prostar and never had an issue. I had a sliding 5th wheel on a Cascadia and had to run in a different notch once every 6 months or so. The Maxxforce prostar was 1500lb lighter than the comparable DD15 Cascadia so it could handle 48,000lb loads (rare) while the Cascadia could only carry 46,500 (more common).

 

where is the weight ahead of the kingpin pushing on the truck? Ahead of the kingpin, or right on the kingpin/5th. It makes no difference, the trailers weight is still in the same spot. If it makes the steers heavier, it makes the drives heavier

 

If there was no kingpin and the trailer plate was simply resting on the 5th wheel, you would be correct. However, there is a kingpin and it can apply a small twisting force -- moment -- through the 5th wheel.

Normally that moment is inconsequential, but if you have a load of mixed freight and someone puts dense items on the nose, you may find that that you scale at 12,300 33600 34000 when you normally are 11,900, 34,000 34,000


Additionally, you may pull different trailers. A 36" kingpin setback on a Wabash might be an inch or two different from a 36" kindpin setback on a Hyundai

 

BS

A small twisting force? What way? Are you suggesting the additional weight in the front causes a structural failure in the trailer and allows the king to tilt forward thus tilting the whole truck forward? Most 5ths are lose enough to allow some kinkpin play. If the play is so much it makes a difference in weight, you might want to repair the kingpin because it shouldn't tilt at all. It could just be to much weight in one spot for the trailer spec. Most can handle 40 over 8 feet. If you load 40k with in the first 4 feet of the trailer i could see this happening. But the weight distribution for the truck is the least of your problems. Your trailer nose is gonna break off because the trailer is loaded beyond it manufactured spec. if the load is with in spec, the kink shouldn't tilt forward. Seeing as the king pin plate goes behind the pin as well as in front, any weight in the 4 feet behind the king should counter balance it. If the trailer really tilts though,, something is broken.

In theory anyway, the trailer is supposed to rest on the 5th and the kingpin only protects against horizontal motion. The king shouldn't really support the trailer. The 5th can tilt. Even if the king was tilted forward, the spring tilt of the 5th would accommodate just like it would when the tractor starts a hill but trailer is still flat. So, even if the trailer bends like its not supposed to, even if the king is tilted, the 5th will tilt without tilting the truck forward.

(Unless you have a fixed, non tilting, 5th, but then wouldn't your trailer break loose every time you hit a hill?)

 

so, who cares, the weight still hits the 5th in the same way. You could have a sliding kingpin. it would make no difference to distribution on the truck. it would effect total weight on truck, but not distribution.

If you get a trailer loaded perfectly, move you tandems so there is 46k on the truck. Then move your 5th so there is 12 on steers and 34 on drives it will always be distributed perfect aside from fuel.

Get antoher trailer with different king or sliding king. Adjust tandems so 46k is ont he truck and it will be 34 on drive, 12 on steer every time. UNLESS you change fuel weight.

difference in king pin set back is the same as adjusting your tandems. The box sits on 2 spots, the trailer and the 5th. Its as simple as that. Adusting the connection point to the tandems, or the connection point to the kinpin will adjust how much weight goes on those 2 points. Adjusting the 5th changes where that point sits on the truck.

Now with flatbed I ride with the 5th back to give room for headache rack and me room to work. Also some loads stick off front. I can still gross 80K even though i am never up to 12 on my steers since my trailer can haul 40k with the split axle. I dont have sliding tandoms or sliding kinkgpin, just a sliding 5th. if im to heavy on the drives, i can slide 5th forward to put some weight on steers, but shifting the load forward will just make the problem worse. (and yes i have pulled vans to)

 

one trailer is prob just heavier and it has nothing to do with kingpin. to clarify, distribution should be the same full loaded with 46k on the truck Empty it will vary. its not necessarily a liner equation as you get lighter. Also as you load a spring steer axle it sags while the air doesnt, thus shifting weight forward. So, if you test empty, the distribution will be off since the steers are taller. With air ride steers some of the variables should be eliminated.

 

Not all loads are evenly distributed. This is one example from Tractor Supply Company: 4 pallets of dog food, stacked 2 high, on the nose followed by 30 rolls of insulation, then 4 riding lawn mowers, and capped off with 10 bundles of fence T-posts.

In shear yes, but moments can add to (or subtract from) shear forces. Which is why engineers used to painstaking draw shear-moment diagrams before CAD programs. Here is an example relevant to trucking: https://ecourses.ou.edu/cgi-bin/ebook.cgi?doc=&topic=me&chap_sec=03.2&page=case_intro

 

Im not arguing the point of if you can break a trailer by over loading the front. My point is, even if some how you managed to bent the kingpin, the 5th leveling would make up for it and thus 0 effect on load distribution.

Although, if bent to far, either the locking jaws wont lock right or the 5th plate wont sit touch the trailer. In the first case, it might not sit forward enough and put a tiny bit more weight on drives, but I'd worry about it coming out before i worry about weight. besides, were taking half an insh which is less then one click on a sliding 5th. The 5th plat would take up any tortion even if not latched right.

If the bent king is strait in jaws, then the front of the 5th wont be touching the trailer. Int his case the weight is still on the center of the 5th, but the chance of pop out is pretty high.


Point is, even with bent kingpin...if it manages to latch and remain flat there will be no affect on distribution. If it doent latch right or remain flat, distribution probably still wont be affected, but i wouldn't roll with it like that.


But this is mylast comment....The mods here like to get angre at me when i argue to much and i think im approching that point. Ifyou would like to discuss further pm me.


In answer to OP, as long as 5th is in right spot, sliding isnt required. In flatbed sliding is nice so you can run further back to give yourself more space most of the time, but slide up when required to for weight. In van/reefer, leave it in the sweet spot for weight all the time. If its not set to your liking a non sliding can be moved to the sweet spot, but this is best done when you are loded to make sure you have the spot correct.

 

To the OP:save your $,just check if the 5°wheel is in the right spot (meaning the right distribution off weight between steer and drives).

To others:Richter is right.
Sliding the 5°wheel will NOT put more or less weight on the truck.
It will only change the ratio between steer and drives.
Sliding the trailertandems is the only way to put more or less weight on the truck.
Tandems forward...............less on truck.
Tandems backward..........more on truck.