When a manufacturer "rates" an item, what criteria do they use?
For example, our rear axles are rated to handle "X" pounds. Is that rated for sitting parked, driving 55, making high "g" turns??? Under what conditions is this rating good for???
O.K. you brainy guys what's the "million dollar answer"

 

I would say it's determined by what the lawyers tell them to say.

I would also say, withtout certainty, that the ratings are for conditions that are within the operating and loading specifications and recommendations listed within the owners manual.

Let's say your truck is maxed at the GVWR of 8900# full of rock dust. it'll hold it while parked, at 55 mph but high-G turns probably won't be the order of the day...

 

I've read where people speculate that GCWR or "max trailer weights" are determined by the truck's ability to get the load up to a certain speed. Seems logical considering GCWR appears to be based on rear axle ratio. As for the other ratings, I suspect they are 5 to 10% less than the failure point for the rated item. But I guess you're asking where that failure point actually is in terms of speed or force. I'm not sure anyone really knows- not even the engineers who design the stuff. If you find out, let us know.

 

>I've read where people speculate that GCWR or "max trailer
>weights" are determined by the truck's ability to get the
>load up to a certain speed. Seems logical considering GCWR
>appears to be based on rear axle ratio.


Would it be safe to add "...up to a certain speed within a certain amount of time"?

The difference in max towing capacities between Excursions with 3.73's & 4.30's (4x4, V10) is 400lbs. (9600 vs. 10,000) I'm quite certain that I could pull 10K (and probably even 11K as the 02 specs state), but not within the same "time to speed frame" as a truck equipped with 4.30's.


Which brings up another thing that I've been pondering...

Is a 4.30 ring gear stronger than a 3.73 ring gear of the same manufacturer? Stronger meaning structurally stronger.

 

>Is a 4.30 ring gear stronger than a 3.73 ring gear of
>the same manufacturer? Stronger meaning structurally
>stronger.

Structurally stronger?, no! they both go through the same hardness
or anealing process.i used to know the level of hardness they were at
but have since forgot.

Things that I always thought Tow ratings were based on were braking power
size springs,size and sidewall ply of tires,gear ratios,hp vs torque curve of engine being used.

Springs are tested for lbs before breakage and decurve or deflection

Brakes are rated in brake horse power/vs size and area

These trucks are strong, but they aren't as strong as the older ones.

I remeber my dad had a f350 1ton dual wheel truck 20 to 30 yrs ago
that put these trucks to shame as far as plastic an steel in it.
and it had grease fittings everywhere cause I had to grease it.

Rich

Ford Trucks Built Tough!
not with rocks
Watch out "X" Mayor of Truckville

 

>Structurally stronger?, no! they both go through the same
>hardness
>or anealing process.

Cool. Thanks for the info Rich!

>Things that I always thought Tow ratings were based on were
>braking power size springs,size and sidewall ply of tires,gear ratios,hp vs torque curve of engine being used.
>
>Springs are tested for lbs before breakage and decurve or
>deflection


OK, I dig what you're sayin' BUT the only thing that would change in my example is the ring and pinion. Everything else stays the same...

 

>>I've read where people speculate that GCWR or "max trailer
>>weights" are determined by the truck's ability to get the
>>load up to a certain speed. Seems logical considering GCWR
>>appears to be based on rear axle ratio.
>
>
>Would it be safe to add "...up to a certain speed within
>a certain amount of time"?
>
>The difference in max towing capacities between Excursions
>with 3.73's & 4.30's (4x4, V10) is 400lbs. (9600 vs. 10,000)
>I'm quite certain that I could pull 10K (and probably even
>11K as the 02 specs state), but not within the same "time to
>speed frame" as a truck equipped with 4.30's.
>
>
>Which brings up another thing that I've been pondering...
>
>Is a 4.30 ring gear stronger than a 3.73 ring gear of
>the same manufacturer? Stronger meaning structurally
>stronger.


Believe it or not, all things being equal, the lower numerical ring and pinion will actually be "structurally" stronger due to number and size of teeth(much the same as more splines in an axle= stronger)

The greater tow rating of the higher numerical gearing is based on the ease of getting the load moving and the less strain on driveline parts(namely clutch/convertor)

 

Components/systems are rated for static and dynamic loading. Testing takes into account steady state as well as transient conditions. Criteria to use for setting an initial target of "xxx loading" would be material properties, projected loading (as in "g" loading), and environment. In other words, when you are designing a system for a truck, as in payload, you do analysis of suspension components - steel in springs/tie rods/ball joints, etc., then the interaction between these components and durometer of damping components (shocks,tires,...). Static calculations can be done with these interactions. Dynamic forces can be calculated to determine limits to the system. Then confirmation of the system is done under varying environments (temperature, humidity, corrosive, ground type).

For towing, the big factor everyone left out is the cooling system capacity. If the powertrain can't be cooled for the ambient, RPM range of operation, it doesn't matter what gear ratio you have. The main reason that cars can't tow as much as they used to? Not enginepower, or rear end gearing, but electric fans do not flow as much air under load as engine driven fans. Cooling system can't reject heat at a greater rate than it aborbs.

 

>Components/systems are rated for static and dynamic loading.
>Testing takes into account steady state as well as transient
>conditions. Criteria to use for setting an initial target of
>"xxx loading" would be material properties, projected
>loading (as in "g" loading), and environment. In other
>words, when you are designing a system for a truck, as in
>payload, you do analysis of suspension components - steel in
>springs/tie rods/ball joints, etc., then the interaction
>between these components and durometer of damping components
>(shocks,tires,...). Static calculations can be done with
>these interactions. Dynamic forces can be calculated to
>determine limits to the system. Then confirmation of the
>system is done under varying environments (temperature,
>humidity, corrosive, ground type).
>
>For towing, the big factor everyone left out is the cooling
>system capacity. If the powertrain can't be cooled for the
>ambient, RPM range of operation, it doesn't matter what gear
>ratio you have. The main reason that cars can't tow as much
>as they used to? Not enginepower, or rear end gearing, but
>electric fans do not flow as much air under load as engine
>driven fans. Cooling system can't reject heat at a greater
>rate than it aborbs.



Of course all your info is correct. But we are comparing EXACT trucks(including cooling systems and all the other info posted above) with the only diff being the axle ratio.

 

Here is an engineer's view on the subject...

All machines are designed with some safety factor in mind. Common safety factors for vehicles range from 1.5 to 2. A safety factor of 1 would be the minimum strength needed to meet the maximum design stress. To limit failures, machines are always designed with a safety factor greater than 1. Without knowing exactly what safety factor that Ford designs their axles to, it is impossible to speculate as to what the maximum payload capacity is.

Now for gear ratios...
A higher gear ratio (more speed reduction) results in higher torque at the axles at a given power input. I do not know if Ford uses different diameter axles for different gear ratios. If they use the same axles, you have more safety factor cusion on the 3.73 gears than on the 4.10 gears.

IMO: Your truck is rated at a value that Ford is confident that you won't break it at. In order to be confident at that rating they overdesigned everything. If you occasionly moderately overload it you're most likely not going to hurt anything.

 

You get more thatn just a gear ratio change when you order trailer tow/non trailer tow options. The vehicle is outfitted as a system, not a component swap. Their is no EXACT comparison. Gear ratios yield different mechanical advantage, so your observation on getting up to speed quicker is correct for the 4.10 vs 3.73. Spring rates, wire harnesses, and cooling systems are factored into the equation for any particular vehicle ordered.
Some info for you: GCWR = Gross Combined Weight Rating = Weight of loaded vehicle PLUS loaded trailer weight.
Ford trailer weighting criteria is insane when compared to our competitors.

 

Just to reiterate what most of us have been talking about...

Differences in ratings between two trucks configured the same except for axle ratio. Obviously "packages" of equipment will cause variances in capabilities.

 

Actually NoMo, I was answering your ramblings in your first post. If you strictly compare ratios for axle, then you do not increase your GCWR rating. You increase your vehicle's ability to propel the load you are rated at. Nothing more.

 

 

>If you strictly compare ratios for axle, then you do
>not increase your GCWR rating. You increase your vehicle's
>ability to propel the load you are rated at. Nothing more.


2000 XLT Excursion 4x4, V10, 3.73's = 17,000lb. GCWR
2000 XLT Excursion 4x4, V10, 4.30's = 18,000lb. GCWR