How is actual weight over each axle figured

[Scramblin_Jim]

I did a search and came up with several different answers to weights. There are curb weights, Gross Vehicle Weights, Axle weights. But nothing really made me say "ah ha"!

So for my 1983 Scrambler the owners manuals says the GVW is 4150 lbs. Then that means a Scrambler SHOULD carry no more than the total of all cargo, people, fluids, and options which equals 4150 lbs.

So to determine my particular vehicles capacities then, I must weigh it and this will tell me what my vehicles curb weight is. This weight is the weight of a full tank of gas, no driver, no passengers, no cargo, just a vehicle with all its options.

If the above two assumptions are correct on my part, then the difference between the GVW and the curb weight should tell me how much weight in cargo, passeners, extra options is available to fully load the Scrambler to its maximum safe limit of the GVW.

Am I correct so far?

I have not measured my 8 yet but for argument sake lets say it has a curb weight of 2800 lbs. (others on this forum said their 8 came in about 2777 lbs.) That would leave 1350 lbs of cargo, extra options, and people to carry.

The owners manual also states that the gross axle weight rating is 2200 lbs for the front and 2700 lbs for the rear. So how is the curb weight then distributed over each axle? Looks to me without any rear cargo most of the curb weight is seen as a downward force at the front axle due to the engine. I am assuming than that the extra 500 lb capacity of the rear axle will support the additional 1350 of cargo. How does that work?

If I install an Aero fuel tank (33-gals or about 214 lbs of weight (assuming gasoline weights 6.5 lbs/gal.) when full), then that adds 117 lbs more weight in the rear over a 15 gallon tank and reduces my cargo capacity down to 1233 lbs. Some have suggest I would need to boost my rear spring capacities. In the specifications section of the "Factory" tab of this site, it is stated that the rear springs are 4-leaf rated at 185 lbs/in. std. Ya buddy - whats that mean in English? Will they work for the extra 1350 of cargo?

Why do some suggest beefing up the springs just because I am adding 117 lbs of fuel?

I need this one explained to me.

Thanks,

 

[kohldad]

The Weight issue

Yes, you are correct on the GVWR and curb weight.

When adding cargo to the vehicle, placement of the cargo determines how much weight is added to which axle.

If you add 100 lbs of cargo over an axle, then all 100lbs is added to that axle.

If you add 100 lbs of cargo in the middle between each axle, then the weight is divided evenly between the two axles so 50 lbs to each axle.

If your wheel base is 100" and you add 100lbs of cargo 20 inches in front of the rear axle, then you will add 20 lbs (100 lbs times 20"/100") to the front axle and the remaining 80 lbs to the rear axle.

So far, so good and fairly straight forward when you think about it.

Now for what some people can't understand..

Assuming the same 100" wheel base, let's add that 100 lbs of cargo in the bed that is 20" behind the rear axle. All of the cargo weight is carried by the rear axle PLUS additional weight transfered from the front axle to the rear axle due to the leverage of the cargo. To calculate this, let's use the rear axle as the center point. Since the cargo has 2000 in-lbs of torque (100" times 20"), tries to lift the 20 lbs from the front axle (2000 in-lbs divided by 100"). This 20 lbs of lift is seen by the rear axle as 20 lbs of extra weight it must carry. Therefor, in this example, the front axle carries 20 lbs less and the rear axle carrie 120 lbs more wieght.

If we use your extra fuel capacity of 177 lbs, the wheel base of a stock scrambler at 103.4", and figure the center of gravity of the extra fuel at 14.5" behind the rear axle. The rear axle is actually carrying an extra (177 lbs plus 24.8 lbs (177 lbs times 14.5" divide by 103.4") for a total of 201.8 lbs.

Of course, this doesn't account for the extra weight of the tank.

 

[kohldad]

the Spring Issue

So know we know the extra fuel is forcing an extra 201.8 lbs on the rear axle.

So why do some folks now say upgrade the springs.

Springs are rated by lbs per inch. Surprisingly, the factory specs actually give spring rates for the scrambler at 185 lbs/inch. Of course, this is for new stock springs and would be actually less for worn springs (hence spring sag on older springs).

Since the 201.8 lbs is carried on two springs, each spring is only carrying 100.9 lbs. At the factory rating, this means the vehicle will settle 0.56" (100.9 lbs divide by 185 lbs/inch).

Still with me or did you go glassy eyed like my daughters?

 

[bigwalton]

Eric put on his engineer's hat this morning

Perfect explanation!

Remember, your Scrambler may be rated to carry that weight (and that's when it was stock, like Eric mentioned) but it doesn't say HOW it carrys that weight... i.e. on the bumpstops in the rear.

This is similar to the look I'm referring to. This was leaving Anchorage with the back of the Jeep packed top to bottom and side to side with stuff:

 

[1x1_Speed_Craig]

You guys are smart.

 

[Scramblin_Jim]

EXECELLENT!

Thanks that really helps.

 

[mulescj8]

Redneck explanation

If it's sittin on the bumpstops then it's overloaded lol

 

[bigwalton]

If it's sittin on the bumpstops then it's overloaded lol

It can't be overloaded, it could still move!