Lots of posts here about the conventional analysis which is limited by the payload rating. Some as low as 1090 lb.
In another thread, I tore into the need for a 3.92 axle ratio when the rest of the truck is the same and the 8-speed transmission with 3.21 gears provides more torque multiplication than the old 6-speed with a 4.10 ratio (15.1 vs 12.3).
In browsing the Internet I found two themes: the sum of the front and rear GAWR is more important than the GVW and the capacity of the rear axle is both under-rated and limited by the tire's capacity, which is easily, and on this forum often, changed.
For this discussion, I will use numbers from a 5.7 Hemi eTorque 4x4 Crew Cab with 5-7 box and sticker payload of 1150 lb, mid-range for a Limited. Some people are nearly 100lb less, Some 300 lb more. GVW is 7100, front GAWR is 3900, rear GAWR is 4100. and as indicated in the sticker, curb weight is 5950. Further, that weight is 3450 front, 2500 rear. I will use tires the OE 20" size of 275/55-20 SL 113T which have an OE pressure of 36psi. At that pressure, they have a rated load capacity of 2535. However, metric loads don't have the LT safety margin and need to be divided by 1.1 resulting in 2305.
Further, we will assume the truck has either the OE 4-corner air suspension, an Air Lift spring added to the back, or has changed the rear steel spring to one capable of handling the load.
Add the two GAWR and the axles have a capacity of 8000. With a weight distributing hitch there is a fair amount of latitude in balancing the weight front to back. 7100-5950 is 1150. 8000-5950 is 2050.
There is some significant margin built into the ratings for the axles. At 2305 lb each, the two rear tires have a combined capacity of 4605. Allow for some production variation, inaccuracies in tire pressure gauges, and normal variation from temperature changes and leakage and it is clear that is the limiting factor. An LT275/55-20 Load Range E is 3085 at 80psi. That is 33% increase. Larger sizes that fit: LT275/60 (3415), 295/55(3415), 305/55 (3640, but really should have a +12 instead of +19 wheel) up to 295/65 (4080, but at 35" diameter should be in a steel spring car with a 1.5+" level kit or Bilstein 5100 shocks adjusted for 1.5" or greater).
Can the axle safely handle a 6000 load? Not on the OEM Springs! However, with the correct springs (as mentioned in the assumptions) and LT tires, could 4800 or 5000 be a more realistic rear GAWR?
That payload would be equivalent to an F-150 with Max Payload package. With that package, the F-150 gets LT tires (275/55-20 or equivalent 18" size), stiffer springs, 3,73 electrically locking rear, and a larger diameter ring gear. The Ram Max Towing only on a 2wd quad cab Tradesman/Big Horn also has a larger diameter ring gear. The axle housing (that carries the load) and the brakes are unchanged.
Do both and the assumed usable GVW is 8900 and the payload is 2950.
The GCWR if the Ram is 17,000. A 10k trailer with 15% tongue weight and 1000lb crew and cargo would have a vehicle weight of 8450 and a total weight of 16,950. Pretty much what buyers were expecting.
Remember, this analysis is predicated on using an LT Load Range E tire and having either an OE air suspension or Air Lift supplemental air spring in the rear. I strongly recommend a Hensley or ProPride hitch. These are 4-bar linkages that create a virtual fifth wheel dramatically increasing stability. Taking the truck out to the GCVW limit should demand it. Another option is an actual fifth-wheel trailer since the higher payload makes that possible. Ford lists F150 specs for a fifth-wheel trailer.
Now please tear my analysis apart!
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Updates:
1) There is an SAE standard for GCVW: J2807. First used on Toyotas, it is now widely used. It is the reason the 3.21 axle ratio is marked down on GCVW. However, I haven't found an equivalent for payload and many manufacturer's don't publish their GVWR with their trailer specs.
2) The 6x5.5 wheels have load capacities typically of 2200 lb to 2500 lb. 8-lug wheels have load capacities over 3000 lb. Net, this means 4400 lb to 5000 lb will be the max rear GAWR. It came up in a Ford Forum where the Heavy Duty Payload package has a 4800 lb GAWR but the wheels had 2200 lb load rating (special Heavy Duty Package wheel) https://www.f150forum.com/f82/heavy-duty-payload-oem-wheels-397577/
3) The factory air suspension has an overload state. Owners have found it to be somewhere around 2000 lb payload, although those tests put the load entirely in the bed.
4) There are several designs for air springs. Some go inside the coil spring, at least one replaces the coil spring, the others replace the jounce bumper inside of the coil spring. Of the last, some require cutting the jounce bumper mount and others don't.
5) In the original post, I indicated the Ram Max Tow package changed the differential ring gear size. I haven't found any reference to that when looking. Ford does change its ring gear size from 8.8" to 9.75" with an HD 9.75". Corrected below
Update 2:
Item 5 in update 5 was wrong. It does include a 10" differential ($2000 to buy the parts). Don't know if the housing or axles are different. Is there a website that has a complete and accurate parts list? The sites I've found always seem incomplete.
Update 2a:
It is a 9.75" Dana 60 with an open rear, and it is only the center section. In heavy-duty towing getting the load moving from rest is important. The standard has 7 starts on a 12% grade in 5 minutes. It is max combined vehicle weight rather than a payload issue. For me and I presume for most here, the GCVW is more than adequate. The issue is the tongue weight, cabin passengers, and bed cargo.
In another thread, I tore into the need for a 3.92 axle ratio when the rest of the truck is the same and the 8-speed transmission with 3.21 gears provides more torque multiplication than the old 6-speed with a 4.10 ratio (15.1 vs 12.3).
In browsing the Internet I found two themes: the sum of the front and rear GAWR is more important than the GVW and the capacity of the rear axle is both under-rated and limited by the tire's capacity, which is easily, and on this forum often, changed.
For this discussion, I will use numbers from a 5.7 Hemi eTorque 4x4 Crew Cab with 5-7 box and sticker payload of 1150 lb, mid-range for a Limited. Some people are nearly 100lb less, Some 300 lb more. GVW is 7100, front GAWR is 3900, rear GAWR is 4100. and as indicated in the sticker, curb weight is 5950. Further, that weight is 3450 front, 2500 rear. I will use tires the OE 20" size of 275/55-20 SL 113T which have an OE pressure of 36psi. At that pressure, they have a rated load capacity of 2535. However, metric loads don't have the LT safety margin and need to be divided by 1.1 resulting in 2305.
Further, we will assume the truck has either the OE 4-corner air suspension, an Air Lift spring added to the back, or has changed the rear steel spring to one capable of handling the load.
Add the two GAWR and the axles have a capacity of 8000. With a weight distributing hitch there is a fair amount of latitude in balancing the weight front to back. 7100-5950 is 1150. 8000-5950 is 2050.
There is some significant margin built into the ratings for the axles. At 2305 lb each, the two rear tires have a combined capacity of 4605. Allow for some production variation, inaccuracies in tire pressure gauges, and normal variation from temperature changes and leakage and it is clear that is the limiting factor. An LT275/55-20 Load Range E is 3085 at 80psi. That is 33% increase. Larger sizes that fit: LT275/60 (3415), 295/55(3415), 305/55 (3640, but really should have a +12 instead of +19 wheel) up to 295/65 (4080, but at 35" diameter should be in a steel spring car with a 1.5+" level kit or Bilstein 5100 shocks adjusted for 1.5" or greater).
Can the axle safely handle a 6000 load? Not on the OEM Springs! However, with the correct springs (as mentioned in the assumptions) and LT tires, could 4800 or 5000 be a more realistic rear GAWR?
That payload would be equivalent to an F-150 with Max Payload package. With that package, the F-150 gets LT tires (275/55-20 or equivalent 18" size), stiffer springs, 3,73 electrically locking rear, and a larger diameter ring gear. The Ram Max Towing only on a 2wd quad cab Tradesman/Big Horn also has a larger diameter ring gear. The axle housing (that carries the load) and the brakes are unchanged.
Do both and the assumed usable GVW is 8900 and the payload is 2950.
The GCWR if the Ram is 17,000. A 10k trailer with 15% tongue weight and 1000lb crew and cargo would have a vehicle weight of 8450 and a total weight of 16,950. Pretty much what buyers were expecting.
Remember, this analysis is predicated on using an LT Load Range E tire and having either an OE air suspension or Air Lift supplemental air spring in the rear. I strongly recommend a Hensley or ProPride hitch. These are 4-bar linkages that create a virtual fifth wheel dramatically increasing stability. Taking the truck out to the GCVW limit should demand it. Another option is an actual fifth-wheel trailer since the higher payload makes that possible. Ford lists F150 specs for a fifth-wheel trailer.
Now please tear my analysis apart!
////////////////////////////////////////////////////////////
///////////////////////////////////////////////////////////
Updates:
1) There is an SAE standard for GCVW: J2807. First used on Toyotas, it is now widely used. It is the reason the 3.21 axle ratio is marked down on GCVW. However, I haven't found an equivalent for payload and many manufacturer's don't publish their GVWR with their trailer specs.
2) The 6x5.5 wheels have load capacities typically of 2200 lb to 2500 lb. 8-lug wheels have load capacities over 3000 lb. Net, this means 4400 lb to 5000 lb will be the max rear GAWR. It came up in a Ford Forum where the Heavy Duty Payload package has a 4800 lb GAWR but the wheels had 2200 lb load rating (special Heavy Duty Package wheel) https://www.f150forum.com/f82/heavy-duty-payload-oem-wheels-397577/
3) The factory air suspension has an overload state. Owners have found it to be somewhere around 2000 lb payload, although those tests put the load entirely in the bed.
4) There are several designs for air springs. Some go inside the coil spring, at least one replaces the coil spring, the others replace the jounce bumper inside of the coil spring. Of the last, some require cutting the jounce bumper mount and others don't.
5) In the original post, I indicated the Ram Max Tow package changed the differential ring gear size. I haven't found any reference to that when looking. Ford does change its ring gear size from 8.8" to 9.75" with an HD 9.75". Corrected below
Update 2:
Item 5 in update 5 was wrong. It does include a 10" differential ($2000 to buy the parts). Don't know if the housing or axles are different. Is there a website that has a complete and accurate parts list? The sites I've found always seem incomplete.
Update 2a:
It is a 9.75" Dana 60 with an open rear, and it is only the center section. In heavy-duty towing getting the load moving from rest is important. The standard has 7 starts on a 12% grade in 5 minutes. It is max combined vehicle weight rather than a payload issue. For me and I presume for most here, the GCVW is more than adequate. The issue is the tongue weight, cabin passengers, and bed cargo.
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