Scale/Weight/Axle Question

There should be minimal change in steer weight based on load where that 5th wheel is set, as it's almost on centerline of the tandem. I'm wondering if you have suspension height issues. If the airbags on the rear axle are overextended a bit, they generate slightly less force than they do within the design range. (page 25 in this link has a chart which illustrates this: http://www.firestoneip.com/site-resources/fsip/literature/pdf/AirideDG.pdf) This would result in the front axle carrying more load, and since the 5th wheel is behind the front axle, it would lever some of the weight off the steer. The same issue can happen if the suspension is too low, as the front bags would then generate more force than they should, with the same result.

The load curve of the bags is fairly linear over a range of several inches in the center of the suspension travel, meaning all the bags are carrying equal amounts of the load. But outside that range, the effective spring rate of the bags varies and can cause these kinds of issues. I'm not guaranteeing this is your issue, but the symptoms are all there. Good Luck!

 

Its not hard. I slide tandems forward on our trailer to very front. I scale. Then adjust accordingly at 350 pounds per hole. Not that difficult or worth fighting over

 

I was refering to post 37 when i said you wouldn't make it to 12k if your fifth wheel was all the way back.

i know i haven't been doing this trucking thing very long, but i'm pretty sure that if your trailer gets loaded heavy in the front, and your kingpin setting is far back (lets say 36" instead of 18") then you'll be heavy on your steers.. take that same load and shift it backwards a few feet in the trailer and it'll solve that issue.

Adjusting your axle, and fifth wheel positions are the alternatives to adjusting the load. If you can't adjust the load you adjust the axles and or fifth wheel, if you can't adjust your axles, and or fifth wheel you adjust the load..

That slide rail looks like it was positioned for a 18" or 24" kingpin setting.. it's all the way back and still ahead of center.. but if all of this just recently started happening, and nothing on the truck changed, and the kingpin settings on the trailers didn't changed, then the only viable option left is that the trailers are being loaded different

 

No matter where the load is in the trailer, all the transfer takes place at the 5th wheel. Moving the load will change how much of the load the truck is bearing, but it will not change the steer/drive ratio, only adjusting the 5th wheel does that. If the 5th wheel had no pivot, then where the load is in wagon matters, but the pivot pins remove the ability of the trailer to apply torque to the truck frame, and all the force is applied vertically to the 5th wheel.

 

if that were true, then there would be no need for a sliding fifthwheel.. you'd set it so that you'd have 12k lbs while you were 80k gross and it'll never have to be moved again because the only way you could become over weight on your steer is if you were to move your fifthwheel right? but no, that's not how it works.. sliding fifthweels are options because different king pin settings will cause a different steer axle weight because cause of the position of the load in relation to the bridge of the truck/trailer combo. the other way to change the position of the load in relation to the bridge without moving the fifthweel or tandems is to move the load within the trailer.

i understand what you're saying about the transfer of weight happening at the kingpin.. but you have to keep in mind also that where the weight is position in relation to the kingpin/fifthwheel will also determine how much is actually transferred..

 

but it only effects how much goes to the fifth wheel! It won't change the balance between the drives and steers. If various loads puts 34K on the drives, it doesn't matter where in the trailer the load is, the steers will be the same weight with each load.


The reason for sliding 5th wheels is it allows you to transfer weight forward when you are trading cargo weight for fuel weight.

 

ok, so if i had a trailer, and the tandem were slide all the way forward, and it had a 36in kingpin setting, and the fifthweel was centered.

Then i took a 22k lbs pallet and set it on the tail of the trailer, all of that added weight would go onto the tandems?
I say yes, i say 100%

now if i move that pallet half way between the drive axles and the tandems?
I say 50% drive 50% tandems

Now over the tandems, but not at the front of the trailer?
I say 90% drive 10% tandems

Up against the bulk head of the trailer?
I say 10% steer 90% drives..

I have a fixed fifthwheel.. the company i leased to loaded me right at 80k lbs of gravel and did it so that i was 12/34/34 with half a tank of fuel.. we had to adjust the fifthweel forward to get it like that, but that's how it is now and it's how it's been.. on all of my loads i have to watch how they load me so i can make sure i get it all in the right spot.. some drivers do two piles, one in the front and one in the back.. other do three front, middle and end.. i try to do one unless it's too much product then i just do two in the center.

 

Actually, it's way more complex than that, but I'll get to that at the end.

Close. It depends on exactly where the 5th wheel is and on your wheelbase, but just so we have numbers, 5th wheel 13" in front of centerline on 260" wb (that's 5%,) would give 2.5% steer 47.5% drive 50% trailer.

That one would be closer to 4.5% Steer 85.5% drive 10% tandem

What changed between those last two examples that would suddenly cause a change in the steer? Nothing changed as far as the truck is concerned, and it wouldn't change the ratio of steer to drive weight added either. All the truck knows is a given amount of weight was added to the 5th wheel, but it will continue to distribute that weight based only on the 5th wheel position. It would increase the steer weight, moving the weight to the nose, but it would also increase the drive weight.

Here's the complicated part. Because the weight is beyond the kingpin, it adds more than it's own weight to the 5th wheel, and takes that same amount off the tandem. In this case that amounts to an extra 82#. So the numbers would be roughly 5.2% steer 98.6% drive and -3.75% tandem. The situation in the first example of the load being behind the tandem would have a similar effect, with more than the pallet's weight added to the tandem and a small percentage taken off the drives. This is a significant effect on a carhauler with the back two cars completely behind the tandem, each of those cars add about 125% of their weight to the tandem while taking 25% of their weight off of the 5th wheel.

I'll gladly run through the math if you'd like, it's all calculated using torque arms based on the distances involved.

The point I'm trying to get you to see is that there is no way to change the ratio of weight added between steer and drive by changing the weight in the trailer. If they load you so that you are heavy on the steer, you have to be heavy on the drives as well! All of the force is vertical on the 5th wheel, the truck has no way of knowing whether the force came from a single piece directly over the kingpin or a piece twice as heavy in the center of the trailer.