7k tandem axle vs 8k triple axle GVWR differences not adding up

[El_Supremo]

Hi, researching the 8k axle upgrade for the new fiver I plan to order soon, and am confused on the GVWR numbers. My Momentum 350G has 2-7k axles, which add up to 14k, and a GVWR of 16.8k, and I have read that the assumption is the other 2,800 lbs is on the pin, which makes sense to me. However, looking at a Momentum 397TH with 3-8k axles, the GVWR is 21k, which is 3k lower than the axles are rated for, and apparently nothing is assumed to be on the pin. Why wouldn’t the 8k axles get you a GVWR over 24k, assuming something is on the pin? Is this a paper de-rate?

[huntr70]

Just because the axles are rated for 24k lbs doesn't mean the rest of the trailer is.....

[alkbch]

Hi, researching the 8k axle upgrade for the new fiver I plan to order soon, and am confused on the GVWR numbers. My Momentum 350G has 2-7k axles, which add up to 14k, and a GVWR of 16.8k, and I have read that the assumption is the other 2,800 lbs is on the pin, which makes sense to me. However, looking at a Momentum 397TH with 3-8k axles, the GVWR is 21k, which is 3k lower than the axles are rated for, and apparently nothing is assumed to be on the pin. Why wouldn’t the 8k axles get you a GVWR over 24k, assuming something is on the pin? Is this a paper de-rate?

The pin box becomes the limiting factor at 21k.

[El_Supremo]

The pin box becomes the limiting factor at 21k.

I believe the top-end Momentum’s use the MORryde pin box, and I was looking at the MORryde pin box fit chart. The ones they list as replacements for Lippert that have enough capacity for a 20-21k GVWR rig are rated for 23k & 24k.

[El_Supremo]

Just because the axles are rated for 24k lbs doesn't mean the rest of the trailer is.....

As in the frame?

[Hoopy Frood]

Our previous Fuzion was a triple axle with 7,000 lb. axles and the GVWR was 18,000 lbs., so not too unusual.

[xrated]

I have a triple axle Momentum with three 7000 lb axles under it, and the trailer has a GVWR of 20,000 lbs. Even if the trailer is loaded up to 20K lb, there is still only going to be approx. 16,000 lb on the three axles combined. That gives me a cushion of roughly 5000 lb on the axles, and spread that out over all three of them is a 1666 lb cushion for each. Then there are the "G" rated tires that are rated at 4080 lbs each when inflated to the 110 psi cold pressure. The math on that says 4080 x 6 = 24,480. The tires are seeing what the axle is seeing....approx. 16,000 lbs, so the cushion there is 8480 lbs. That works out to 1413 lb per tire cushion.

Of course there are no trailers that are perfectly balanced and there is a bit more weight on one of the axles than the other one or two, and the same goes with the affected tires for that axle. But the point is that at least with the Momentum triple axle trailers, there is a pretty high degree of "over capacity" built into the trailer.

[huntindog]

Hi, researching the 8k axle upgrade for the new fiver I plan to order soon, and am confused on the GVWR numbers. My Momentum 350G has 2-7k axles, which add up to 14k, and a GVWR of 16.8k, and I have read that the assumption is the other 2,800 lbs is on the pin, which makes sense to me. However, looking at a Momentum 397TH with 3-8k axles, the GVWR is 21k, which is 3k lower than the axles are rated for, and apparently nothing is assumed to be on the pin. Why wouldn’t the 8k axles get you a GVWR over 24k, assuming something is on the pin? Is this a paper de-rate?

The pin box becomes the limiting factor. I "think" the frame should be good to 24K as the Luxe uses the same size I beam frame, and it is 24K.
Regardless, the upgrade is about more than carrying capacity. The 8Ks also have better brakes.

[geotex1]

The pin box becomes the limiting factor. I "think" the frame should be good to 24K as the Luxe uses the same size I beam frame, and it is 24K.
Regardless, the upgrade is about more than carrying capacity. The 8Ks also have better brakes.

Don't assume that. Lippert builds Luxe's frams designs and there's a lot of differences compared to the standard frames used by the mass manufacturers.

[huntindog]

Don't assume that. Lippert builds Luxe's frams designs and there's a lot of differences compared to the standard frames used by the mass manufacturers.

I never "assume" anything. I have however done a lot of looking under the hood of different manufacturers before plunking down my cash...While it is true that Lippert makes the Momentum frames, they are noticeabley stronger than the other mass produced units. Case in point, GD states that the entire carrying capacity payload can be loaded in the garage.... The others all state a lower garage weight limit that is less than the total payload CC. That means a stronger frame for GD.
But as I alluded the pin box is the limiting factor anyway, so I am not advocating exceeding that.... And the important thing in my eyes was the better brakes the 8K axles have