What limits tow ratings for unibody vehicles?

[JDgreen227]

My '04 Lesabre (3.8 liter V6, automatic) is rated for a 1,000 pound towing load from the factory. The hitch I installed has a 3,500 pound rating. My utility trailer weighs 800 pounds with no cargo, meaning I have a 200 pound carry capacity if I go by the factory ratings. Yesterday I helped someone pick up a new garden tractor and mower from the dealership, and the tractor/mower combination weighed close to 800 pounds. The car acted like the trailer was still empty, there was ample power and I barely felt the additional weight when braking (no trailer brakes) and the temperature gauge stayed within half scale even though I was running the A/C in near 100 degree weather.

So, exactly why is the car rated for only a half ton? I am thinking cooling capacity is the limiting factor as the entire cooling system only holds 8 quarts. Thanks for your input.

 

[Dmace]

Did someone cut you off so you had to make an emergency stop?
If you had, you would quickly see why they're limited. Brakes and tire size/load capacity are pretty limiting as well.

1600 lbs is still nothing even for a car but if it was rated at 2500 lbs, people would load them to 3500 lbs...

 

[JDgreen227]

Did someone cut you off so you had to make an emergency stop?
If you had, you would quickly see why they're limited. Brakes and tire size/load capacity are pretty limiting as well.

1600 lbs is still nothing even for a car but if it was rated at 2500 lbs, people would load them to 3500 lbs...

Good points, car is rated for a maximum load of 4780 pounds GVWR of vehicle, passengers and luggage and tires are rated 1610 pounds each @ 35 psi for a total capacity of 6440 pounds. Obviously tires are not a factor. Curb weight with full tank of fuel and driver is just a shade over 3750 pounds.

 

[jeffsw6]

There probably aren't many people deciding between the Buick sedan and the Lincoln sedan based on the towing capacity.

With that out of the way, I bet the limit is how much trailer you can put on it before the entire car begins to warp. As you know, the load-bearing system of a LeSabre is nothing like a ladder-frame truck. You might think you have plenty of brakes to stop that thing in a pinch, and maybe you do; but the car might not be straight afterward!

 

[Bob77064]

Years ago people pulled travel trailers with trailer brakes with body on frame cars. Just like today's trucks the trailer hitch was frame mounted.

 

[bcp]

What limits tow ratings for unibody vehicles?

The country you are in.
-------------------------------------
Example-same car:

UK 2315 lbs
Canada 700 lbs
USA 0 lbs
------------------------------------
From:
The Great American Anti-Towing Conspiracy

"On Toyota’s UK website, the Yaris is credited with a towing capacity of 1050kg/2315lbs. That’s right in line with the old rule of thumb that a car can safely tow an amount equal to its own weight.

But here in the land of the (not so) free, the Yaris’ owner’s manual admonishes: “Toyota does not recommend towing a trailer with your vehicle.” The unnamed author goes on to give a partial pass to our northern neighbors: “In Canada only, total weight of cargo and trailer not to exceed 700lbs.” Please leave your trailers at the border? Perhaps this partial exemption reflects Canada’s status as being somewhere between English and American. But the logic is lost on me."

 

[nikdfish]

One of the reasons I bought my Dodge Magnum RT AWD in 2005 was the potential for a 3500 lb towing & 350 lb tongue weight capacity (w/weight distribution), of course the 5.7 Hemi was part of the attraction. As an AWD RT it already had the upgraded cooling & brakes and I later swapped out shocks and springs for the rear nivomat self leveling shocks (the only component I was missing to duplicate the tow package). I didn't go with the Dodge hitch, so I ended up with 3500 lb tow & 300 lb tongue weight capacity (w/out weight distribution). We then picked up a 3500 lb capacity box trailer & later got a 5x8 open bed tilt trailer to use with it.

We gave the old '97 Ram 1500 4x4 to our boy & used the Magnum/trailer combo to take its place. 7 years & 150,000 miles later it still does, though the wife did end up getting a Ford 150 for hauling gardening supplies (since she refuses to pull a trailer...).
The Magnum actually towed better in terms of handling and acceleration than the V8 Ram did with similar towed weights.

On occasion we have pulled the box trailer loaded to a 3400 lb total (per the truck stop scales) up & down the hills between WV & NC in summer heat with no issues and running a steady 60+ up the grades. I think the wind resistance of the tall box has as much or more impact than the weight. My take on the main difference in ladder frame vs. unibody is the nature of the attachment points between vehicle and tow hitch, and the potential affect of designed impact absorbing crush zones on the strength of those attachments.

Nick

 

[aczlan]

My 1997 Volvo V90 has a unibody frame (like the 1993 940 and 1989 740 that proceeded) and it (like its predecessors) is rated to tow 3500 pounds with 350# of tongue weight and brakes on the trailer. My wife's 2002 Caravan (also unibody) has about the same ratings, but worse brakes (no ABS, drums in the rear vs 4 wheel discs and ABS on the Volvo).

Aaron Z

 

[s219]

I am sure it has to do with the strength of the frame, the engine/transmission cooling, the brakes, and then legal concerns.

My last unibody SUV was only rated to tow 2000 lbs with 200 lb tongue weight. My new one is 5000 lbs and 500 lb.

I agree the biggest limiter, which you don't want to mess with, is braking. Second would be transmission. You probably don't have a temp gauge for that, and a LeSabre surely doesn't have a transmission cooler. It wouldn't take much to cook a stock car transmission when towing a lot of weight.

 

[triptester]

On unibody construction the 1/4" or thicker hitch material attaches to and is supported by only 14 guage sheet metal.