Truck axle loading, predicting sag - Ford Truck Enthusiasts Forums



Truck axle loading, predicting sag

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Old 07-21-2017, 04:14 PM
backtrack2015 backtrack2015 is offline
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Question Truck axle loading, predicting sag

Hi. I have a diesel 2017 F-350 CCSB 4x4 on order. I've done some calculations based on the certification stickers from similar vehicles on the dealer lots and determined that my front axle will be at 5168 lbs. (GAWR 5200) and the rear at 3620 lbs. (GAWR ~7200) given my truck configuration and 720 lbs of people/dog/soda/snacks in the cab. As such, I'm very close to the front axle rating (springs) before I add anything more to the truck.

We are considering a fifth-wheel. The king-pin seems like it is typically mounted either over the rear axle or very slightly forward of the rear axle during towing (slider in forward position). Thus, I believe most of the king-pin weight is on the rear axle. Assuming a 2000 lb. king-pin is loaded 2" forward of the rear axle, the added load on the front axle (160" wheelbase) is about 25 lbs. My question to those who study these things: does this align with your real-world measurements/ experience? I feel like this might be true for a simple rigid static system but not true for one in which deflection of springs is part of the equation.

Second question: Is there a way I could predict the actual loaded height of the bed rails based on the unloaded height (front and rear) coupled with my assumed vehicle loading (options, people, trailer king-pin, and hitch)? This would help me greatly in shopping for a fifth-wheel. We know the factory-reported spring constants, but I'm not quite sure how to apply them based on the sequential loading etc.

Thanks for your help. -Steven
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Old 07-21-2017, 08:30 PM
backtrack2015 backtrack2015 is offline
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In case anyone is interested, I actually found someone who reported the loaded/unloaded heights for a 1500# pin weight. It sounds like the rear of a 2017 F-350 CCSB 4x4 drops just under 2-inches under that load. That's close enough for me. (Thanks Johndeerefarmer).

I'm still curious what folks who have used a CAT scale have seen in terms of front axle load due to the fifth-wheel. Thanks.
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Old 07-21-2017, 08:46 PM
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Putting almost 1900 on the rear axle from my pin added 140 pounds on my front axle. It doesn't add much. That's on my F350 LB extended cab. Squat is maybe 2".
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Old 07-21-2017, 09:23 PM
backtrack2015 backtrack2015 is offline
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Originally Posted by RV_Tech View Post
Putting almost 1900 on the rear axle from my pin added 140 pounds on my front axle. It doesn't add much. That's on my F350 LB extended cab. Squat is maybe 2".
Thank you. That is great information. The squat is entirely acceptable. As to the axle load, it looks like a modest fifth-wheel will put me over my front GAWR (spring limited at 5200) while being well under the rear axle rating. I'll have to contemplate that. I really wanted to be completely legal and was originally planning on a travel trailer (which would normally unload the front a bit). Hmmm.
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Old 07-22-2017, 06:23 AM
'65Ford '65Ford is offline
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From those numbers, a cab full of people would make the front axle overweight. Maybe the door sticker has a typo.
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Old 07-22-2017, 06:42 AM
backtrack2015 backtrack2015 is offline
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That is with 720 lbs worth of people etc in the cab. Nothing else. I'll run it over a scale and see how it actually is once I have the truck.
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Old 07-22-2017, 06:45 AM
Chuck's First Ford Chuck's First Ford is offline
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get truck to a cat scale.. get real numbers EMPTY..

if necessary have dealer upgrade to stronger front springs.. Ford offers 4 different ones.

my opinion. I think the snow-plow front springs are 7,000 pounds...
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Old 07-22-2017, 07:28 AM
Flyct Flyct is offline
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Steven,

Here's my sticker. Same truck you ordered but with HD front spring option. Front GAWR of 5600 lbs

I get a little over 2" sag with a pin weight of about 2600 lbs



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Old 07-22-2017, 07:39 AM
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I'd go back and order the snow plow prep (it is like 85 bucks) and that will give you 6K springs up front.
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Old 07-22-2017, 07:40 AM
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I've ran ours over scales at our local grain elevator many times with different combinations and the most weight transfer to the front was 100 lbs and that's with 3400 lbs of pin.

What you have to be concerned about is dynamic loading when you are going down the road, when you hit dips or bumps the trailer will transfer weight to the rear axle not the front because as the rear springs compress to take the load the rear end goes down and as the rear end goes down weight will transfer from the front to rear until the springs rebound back to normal ride height. It's like a sinking ship that's going down by the stern.

Denny
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Old 07-22-2017, 07:52 AM
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Thanks everyone for the added weight information etc. I should have ordered the heavy service front suspension (5600#) or snow plow package (6000#). That would have solved it. I'm not going to swap the order now.

I'll resist the urge to put a deposit on the fifth-wheel and simply wait to see the actual axle loads for the truck itself (CAT scale). I guess this is just a spring issue, so I can always get the 5600# springs and air-up the front tires a bit. I just have to decide whether or not I'm okay with being a smidge over the front GAWR as listed on the sticker. I'm happy with the rear 2" drop, so that is good news. Thanks again.

edit to add: My order is locked and the truck is being built next week otherwise I would modify my order.

Last edited by backtrack2015; 07-22-2017 at 08:12 AM. Reason: More info
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Old 07-22-2017, 08:38 AM
acadianbob acadianbob is offline
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I wouldn't panic until I saw the actual scale weight of your front axle. I'm having a hard time believing that you will be that close to capacity.
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Old 07-22-2017, 11:09 AM
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Quote:
Originally Posted by acadianbob View Post
I wouldn't panic until I saw the actual scale weight of your front axle. I'm having a hard time believing that you will be that close to capacity.
Amen. I've got the plow springs on my 2012. Didn't believe anything could ride this rough running empty. Absolutely hate them. Weigh it first! Then decide if there is a problem would be my advice.
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Old 07-23-2017, 04:15 PM
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My truck came with a 5600 lb front axle because of the camper package, I think. With a 2460 lb pin weight, my front axle is 4920 lbs, with about 350 lbs in the front seats and an 80 lb dog in the back seat.

My hitch sit directly over or slightly forward of the rear axle.
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Old 07-23-2017, 04:25 PM
Garyl53 Garyl53 is offline
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I thought the base springs for all 6.7L were 5600 for the front GAWR. I have 6000# and you don't want those unless you want to carry a plow! Way too rough ride.
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