When towing the goal is to remain lane-centered without driver inputs (beyond what was normal for the solo vehicle). Cummins and Kenworth both use the number of steering inputs per X-miles when high as deleterious to fuel mileage.
A weight distributing hitch [WDH] is required even on 1T pickup trucks when trailer tongue weight [TW] exceeds 400-lbs. A trailer with a TW of 400-lbs may weigh as little as 2,500-lbs. The bar is low. The simplest understanding is that a WDH "returns" the Front Axle [FA] to unhitched height/weight values, and that this ensures "normal" steering and braking by so doing. The remainder of the TW is "distributed" over the Rear Axle [RA] and Travel Trailer [TT] axles. When a large force plays against both the TV and TT at slightly different times,
the leverage action of the trailer tongue is spread out across all axles of both vehicles effectively dampening "problems".
Now, payload of a truck is, in a manner of speaking,
irrelevant to the discussion, overall, as
TW is a static measurement of a dynamic problem. That is to say, an 800-lb TW can be 12,000-lbs under the wrong circumstances. The "arm" of the trailer tongue, extending back to the center of the axle is a lever that can upset the TV, drastically. The tail wags the dog. I once watched a portable generator that I know weighs around 1,800-lbs takes a dually pickemup clean off the road. A dry road.
A download of a spreadsheet made by
Nickcrowhurst on AIRforums.net showing forces acting upon the rig is
here; the thread is a discussion of these factors. One can play with different trailers and tow vehicles given a handful of measurements. This, and the below, are ways of understanding how SAE looks at tow rig dynamics, the forces acting upon the tow vehicle different from solo, and how to re-establish solo handling to the best extent.
The following is chart made up by
Ron Gratz a contributor on RV.net, handy for it's brevity and checks. By using a three-pad certified scale (see CAT Scale Locator, online) one can fine-tune a WDH beyond the usual measurements of before/after heights given in Towing Guide instructions by vehicle and hitch manufacturers. Specifics to vehicle ought to be followed.
For a truck, the usual result of proper hitching, today, is that
once the FA has returned to the unhitched weight value that the balance of the TW will be distributed from 75-25 to 60-40 on the RA and TT axles respectively. (Copy and print a few):
Weighing #1 -- TT attached and Weight Distribution Activated
Let Front Axle Load be "FA1"
Let Rear Axle Load be "RA1"
Let TT Axles Load be "TT1"
Then, while in same position on scales, take
Weighing #2 -- TT attached and Weight Distribution Not Activated
Let Front Axle Load be "FA2"
Let Rear Axle Load be "RA2"
Let TT Axles Load be "TT2"
Then, drive off scales and drop TT. Return to scales and take
Weighing #3 -- TV only -- TT Not Attached
Let Front Axle Load be "FA3"
Let Rear Axle Load be "RA3"
From the above values, you can calculate:
TV weight = FA3 + RA3
Gross Combined Weight = (FA1 + RA1 + TT1)
- should also be equal to (FA2 + RA2 + TT2) if scale weights are correct
TT Weight = Gross Combined Weight - TV Weight
Tongue Weight = (FA2 + RA2) - (FA3 + RA3), or
Load Transferred to TT Axles
when WD System in Activated = TT1 - TT2
Once these values are established, and some subsequent ones done the same way to account for different loadings, the range of WDH adjustments are known for a given rig.
Now is when tire pressures can be zeroed in as highest pressures are not always to the point in braking/handling questions. Same for alignment problems on the TV, slack in the steering gear, etc. In short, any TV shortcomings are magnified
and we do not want the actions of the TT to have any farther effect. IOW, if the rigging is ideal, then the diagnosis of handling, braking and FE problems can be isolated and treated.
Now, as to type of WDH; next post.
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