I've been searching but have come up empty handed on the following question:

What are the differences between a 2800lb and 3750lb 9-inch rear axle.

For 1981, say Axle code 01 vs 13.

Are there differences in the housing, axle shafts, bearings?

I'm guessing as a general rule the 2800lb shafts had the small-bolt pattern, while the 3750lb shafts had the larger pattern.

Thanks,
Jason

 

Try this thread.

 

Yes, thank you, that is exactly what I was looking for.


If I understand the conclusion: the difference is bolt pattern and brake size.

 

Yep. Same bearings, so it has to be the wheels.

 

Snapped this of the stock wheels tonight. 1400lb x 2 =2800lb, so there you go.

I wonder if the limitation is purely in the rim. It's easy enough to find aftermarket wheels with a 1900lb load rating.

 

That explains it very well. Thanks.

 

I mean, from a practical standpoint, with all else being equal in the axle, would Ford would make an axleshaft/hub that was exactly 21% weaker? Especially given the measurements that were posted in the other thread.

Brakes of course are another consideration, but my trailer brakes are excellent. Basically, I'm looking for a little bit of extra overhead as with a 4500 lb trailer I'm really pushing the weight limit on the rear axle (and my GCVW is out the window). It's occasional use and I'm not trying to turn my light-duty 1/2 ton into a real 1/2 ton or anything.

If this truck wasn't a 3rd generation, in family since new "heirloom", or I was regularly pulling 5K lb loads, there are a lot of decent 3/4 ton trucks out there for not that much scratch.

 

I'm sure Ford didn't make it weaker. It had to have been the wheels that limited the weight carrying capacity of the truck, but the brakes weren't much better. So, changing out the wheels would certainly help.

 

You don't have much tongue weight, even with the 4500lb trailer correct? Not over 1000lbs I would think.

 

Oddly enough, a 5000# trailer with a 500# tongue weight is the max for a weight-bearing hitch on a new F150, so I can't imagine what it would be for a F100 in the early 80's.

You may be wondering how I know about the limits on a new F150. That came up recently when a fellow Sea Ray forum member mentioned that the owner's manual for his 2013 F150 states that you have to use a weight-distributing hitch after 5000/500#. So, I scoured the owner's manual for my 2015 F150 and it isn't in there. Not a word about when or even if a WD hitch is needed. However, there's a link in the manual to Ford's RV & Trailer Towing Guide. Sure enough, it has this table:

 

Correct, it should be about 400-600 lbs depending on how well I've loaded the trailer.

Which puts me right about there.

With 2 full tanks of gas, 2 grown men, plus the cap, the truck weighs in at about 4200 lbs, giving me an effective payload of 500 lbs -- which is kinda horse****, but it is what it is: the biggest 1/4 ton truck you've ever seen

Assuming a 50/50 Front/Rear weight bias (I figure this is worst case, which is a good conservative place to estimate from)

2100 lbs over the front axle.
2100 over the rear.

So, 500 lbs before I hit my GVWR of 4700lbs; 700lbs before I hit the RAWR of 2800lbs

Even with a perfectly loaded 4500lb trailer, with a 10% tongue weight. I'm pretty damn close. As I don't have a truck scale, it's not hard to see things being just a bit too far foward on the trailer, which could bring the tongue weight from 450 to 600 lbs pretty damn quick.

Throw a spare engine block and misc parts in the bed of the truck and I'm maxed out.

Note, with a 4200 lb truck and 4500lb trailer, I passed my GCWR 1025 lbs ago. This is also worse case as in summer I tend to run without the cap and may or may night not have a passenger, which gives me another 400 or so lbs to play with.


I'm not proud, if I really screwed up my calculations here, please tell me.

 

For reference, I have a class III hitch weight-bearing hitch. The hitch is rated at 600# tongue / 6000# trailer. I'm only assuming the frame of the truck can take it. It's not a swiss-cheese frame and it has a thousand or so miles of pulling various loads over the last couple years with no ill effects yet.

The hitch is rated for 1000# /10000# when used with a weight-distributing setup.

As my trailer is only rated for 6000# gross.....

 

Sounds like you have it scoped out. Should be fine with stronger wheels/tires and good brakes.