Me Again wrote:
TomG2 wrote:
The grey area is the difference between the sum of the axle ratings and the total vehicle rating(GVWR). Using the axle ratings alone, all the builder would have to do is put a heavier rated rear axle under their vehicles to raise their GVWR? I don't think it works that way. Otherwise, they could put a 7,000 pound rear axle under a F-150 and raise the rating to over 10,000 pounds? More goes into the gross vehicle rating than simply axle ratings.
The rear tires are what sets the RGAWR. AAM units on the Dodge and GM are rate to over 10K(10800 to 11500 depending on where you look).
That was true even back in 2001 on my Dodge with a Dana 70 axle rated to 8580 pounds, those with Dana 80 have a rating of 10100.
My Rear GAWR is listed at 6084 or two times the standard LT245/75R16E tires rated to 3042 lbs. However the truck came with the factory optional LT265/75R16E rated to 3415 each!
Those with 3042 rated tires can push that limit with a mid size 5th wheel.
Only on RV forums do people get so rapped around the axle on these issues!
Chris
The weakest link of the whole rear frame/chasiss/suspension/axle/wheel/tire unit is what sets the RGAWR number. The tires are not the weakest link on all trucks. On most 1/2-tons and below, the tires are rated greater than the axle and springs. On most 3/4 and 1-tons, it is the opposite. There are exceptions all over.
If one is going to play hometown engineer and load beyond the manufacturer stated RGAWR, then one must fully educate oneself as to the details of ALL of the parts that make up the rear end of a truck. This includes not only the tires, axle housing and springs, but also the wheels, brakes, hubs, lugs, spring hangers/shackles, shocks, sway bar and even the frame of the vehicle, as well as the cargo bed that is mounted to that frame.
Just because a Dana 80, for example, is rated by Dana as an 11,000 lb axle, does not, in any way, mean that the rear chassis assembly of a pickup using the Dana 80 axle, is also rated to 11,000 lbs, nor does it even mean that the whole axle assembly, as equipped by the vehicle manufacturer (which includes brakes, hubs, bearings and lugs), is rated to 11,000 lbs either.
Each vehicle manufacturer that uses that Dana 80 axle bare housing, specs their own brakes/hubs, calipers/wheel cylinders, pads/shoes, bearings, lugs, differential, axle shafts, etc, that are used to make up the completed axle assembly. These parts can be, and often are, rated less than the Dana bare housing!
This is equally true for other axle manufacturers, such as AAM, that provide axles for different vehicle manufacturers. The same is true for manufacturers such as Ford/Sterling, Nissan, Toyota, etc, that make axles in-house and use similar models in different vehicles, within their own product lines, or cross-sell to other manufacturers, that base their vehicles on common chassis (such as Ford and Mazda).
Even Dana's 11,000 lb rating for the housing, is nominal. A longer axle with the same housing tube diameter and thickness, will have lower carrying capacity than a shorter one, all other things being equal. Hub and bearing design and position of the load center over the spindle section of the housing, as well as the position of the springs along the housing tube, also create different stresses on the housing, and thus different max load capacities for different specific applications of the axle model.
The manufacturer takes into account the capacity of the springs and spring buckets (for coil springs) or hangers/shackles (for leaf springs), shocks and shock brackets, sway bar and brackets, and other suspension members (trailing arms, panhard arm, etc), if equipped, as well as the the frame of the vehicle that the axle is attached to.
Tires have to ride on wheels. Wheels have weight and pressure ratings, just as tires do. Many folks forget this and only consider the tires and just ASSUME the wheels are up to the ratings of bigger/stronger tires than the truck was equipped with by the manufacturer. Not so. One must investigate the actual ratings of the wheels.
ALL of these things make up the rear chassis/suspension system of a truck and ALL are considered in the manufacturer's GAWR number.
Can we change and upgrade weak links and make a stronger system? Sure we can :). BUT, we must do so in an educated fashion, not just assuming everything else is good to go, when in fact it may not be, if we expect to be sucessful for the long term.
Be wise, my friends, be wise, and educated.
