Towing Weights

[swall]

Over the years, I have read the sections in my vehicle manuals governing max weight for a trailer to be towed. The governing parameter seems to be the combination of trailer weight and tow vehicle weight. I had always assumed that this related mainly to braking capacity of the tow vehicle. Is there something else?

 

[Compositepro]

The main factor governing max trailer weight is the weight of the tow vehicle. The traction of the tow vehicle in acceleration and braking is proportional to its weight. Of course it will also have heavier brakes. Trailers will be designed to also add some weight to the tow vehicle.

 

[rmw]

I have towed loads with PU trucks that grossly (pardon the pun) exceeded the Combined Vehicle Weight Rating of the towed combination and probably violated several laws concerning GCW's with respect to licensing and permitting (although I did have a CDL when I did it). I even had to double the capacity of the trailer axles from 3.5K teach to 7K to carry the loads - destroyed those 3.5K axles. In all cases I used the trailer brakes and wouldn't attempt something like that without functioning trailer brakes. The CGW was somewhere north of 26K lbs. Tractin wasn't a problem since I typically had 5-6K lbs in the bed of the PU truck. I drove this load coast to coast several times. After I changed operation methods, the PU truck gave me excellent service for several more years and was in excellent shape when I sold it (to my mechanic who knew exactly what shape I kept it in.)

Scariest moment was once in Colorado when the truck brakes got good and hot and boiled the water in the brake fluid (hygroscropic fluid) and vaporized in the brake lines taking my truck brakes out of the picture. Between the trailer brakes and some serious downshifting I am still here to relate this, thankfully.

rmw

 

[IceStationZebra]

There are several factors that come into play. A few big ones that I can think of that are mainly combined GVW related:
- engine cooling and longevity
- trans torque capacity, cooling and longevity
- axle cooling and longevity
- brakes (depending if trailer has brakes, could assume no for worst case scenario)
(and don't forget OEM bragging rights in the case of PUs)

I can't find the article now, but I read one a couple months ago that discussed how SAE came out with a new differential oil viscosity mostly for 1 ton trucks. The thicker oil was eating into the fuel efficiency and the thinner oil wasn't passing durability tests.

ISZ