E350/450 Cab-Chassis w/ Composite Box Body

[gator70]

There are several posts above, including one with a weight distribution spreadsheet. So yes, I am paying attention to axle weights.

What is the source for the specific 82% value (18% de-rate)?

12ply is a leftover from the equivalent ply ratings for bias-play tires, it isn't very useful. Load index is by far the best choice for tire comparisons in LT tire ranges.

Typical real-world percentages​

Use case	% of tire max load typically used
Passenger vehicles (P-metric tires)	70–80%
Light trucks / vans (LT tires)	75–85%
SUVs at GVWR	80–85%
Trailers (ST tires)	85–90% (less margin by design)

If a tire were routinely loaded at 100%, it would overheat and fail under real driving conditions.

 

[luthj]

Typical real-world percentages​

Use case	% of tire max load typically used
Passenger vehicles (P-metric tires)	70–80%
Light trucks / vans (LT tires)	75–85%
SUVs at GVWR	80–85%
Trailers (ST tires)	85–90% (less margin by design)

But where did you source those tables from? (I sincerely hope it wasn't from an AI summary).

I am not arguing against safety margins, but 18% is a very specific and fairly high margin.

The wheel and tire manufacturers load tables for commercial and LT tires already have some safety margin baked in.

 

[gator70]

These are numbers I'm trying to reach.

My rear axle is rated at 11,000lbs, truck weight is 6900 lbs, dry.

My tires are rated at 3970lbs per tire.

Water and fuel is heavy. As I engineer AUX fuel tanks and freshwater tanks, I try to move them forward when possible.

 

[luthj]

So you don't have a source, and those are your personal targets. It is totally fine to have personal design goals, but please don't go around telling folks to derate their tires 18% without that caveat. If you got that data from an AI summary, I strongly advise to find some better first sources, ideally industry specs, regulations, etc.

I would also note that "moving weight forward" blindly can cause just as much of a problem. Overloading the front axle worst case. Though less of an issue on a medium duty truck. Much better is to actually measure curb weights, and the CG of the box. Then calculate the adjusted CG for the added weight (engineering vs guessing).