The Physics of Towed Vehicles

Flat Towing A Vehicle Behind Your RV

Many motorhome owners flat tow a vehicle (all four wheels on the ground) on their RV camping trips. We here at flat tow our modified Jeep Wrangers on our adventures, and have for over 120,000 miles. We appreciate the freedom and convenience of having a small vehicle instead of using our RV to explore new places.

Towing anything is a pretty big responsibility in our opinion. State laws vary, but trailers over a certain weight require trailer brakes… typically 1500 lbs. While trailers are usually required to have braking systems, many state towing laws are vague at best as to a requirement for towed vehicle brakes. Some states require a stopping ability within a specified distance from a stated speed, and many require a "break-away" brake for situations where the towed vehicle becomes unhitched, but as far as we know, there is no national standard for supplemental braking systems in the USA.

Logically speaking, if state laws require a trailer of 1500 pounds to have brakes, it seems to us that towing something that can easily weigh twice as much (our Jeep weighs over three times as much at nearly 5000 lbs.) should have a braking system too. However not everyone agrees with the above logic, and there are a large number of folks out there towing a vehicle without supplemental brakes.


While the debate on whether you need to have a supplemental brake system on a towed vehicle goes on among many in the RV community, we hope by presenting the following information, we may convince you that having a supplemental braking system in a vehicle you tow is necessary. We have no financial interest in any supplemental brake manufacturing company.

The Physics of Towed Vehicles

I will start out with one of the illustrations used to introduce the topic. A man and his wife were traveling though Texas in a Diesel pusher pulling a Jeep Grand Cherokee. He was traveling at the speed limit and was maintaining a safe distance when traffic suddenly shut down. He immediately jumped on the brakes at full bore. As he began to rapidly decelerate, he could feel the Jeep begin to push harder and harder on the back of the coach. At the last second before impact, he swerved to the left to avoid a collision with the tractor trailer stopped in front of him; but only half of the motorhome cleared the trailer. The trailer cut through the passenger side of the coach like butter, taking his wife's life on impact. Since there had been numerous accidents of this type in a relatively short period of time, Michigan State University's accident reconstruction team was dispatched to see the cause of the accident, as well as to see if there were any measures that could have been taken to prevent this tragedy. The head of the project concluded that if the towed vehicle had been using a supplemental braking system, the coach would have stopped at least one foot before impact rather than four feet after impact.

I am sure that the first question that comes to mind is "Why couldn't a forty-footer with air brakes and an exhaust brake handle the weight of a 4000 lb. towed? It is still well under my GCWR." I believe the definitions of the weight ratings will help clear up some of the confusion.

Dry Weight- The basic weight of the coach. No fuel, water, passengers, cargo, etc.– just the "nuts and bolts"

Curb Weight- The "ready-to-roll" weight of the coach. Includes all fluids and a full tank of fuel. Does not take into account passengers or cargo.

Gross-Vehicle Weight Rating (GVWR)- The maximum amount of weight the coach's chassis can handle (air bags/springs, shocks, brakes, etc.). This weight includes fuel, water, passengers, cargo, trailer tongue weight, food, and everything else. Most of the time your engine can comfortably pull more than you GVWR.

Gross Trailer Weight Rating (GTWR)- The total loaded-down weight of a trailer. The GTWR includes the weight of the trailer as well as all cargo and fluids on board. The static tongue load must be 10-15% of the total GTWR. This is the number used to determine which class of hitch and tow bar (Class III, Class IV, etc.) is necessary.

Gross-Combined Weight Rating (GCWR)- The combination of the GVWR and the GTWR.

*To find the maximum towable weight rating subtract the GVWR from the GCWR.

Here are some common misconceptions

1) While it is true GCWR minus the GVWR is the maximum amount of weight a vehicle can pull, it is not the amount of weight the vehicle's chassis can stop. A SAE Class IV hitch has a weight rating of 10,000 lbs., but most chassis manufactures specify that any trailer over 1,000-1,500 lbs. (depending on the manufacturer) must be equipped with a braking system.

2) Supplemental brakes are necessary even if the total combined weight does not exceed the GVWR.

Common logic would say that 3500 lb. towed vehicle would put 3500 lbs. of force back of the coach. Remember, the 3,500 lbs. is measured vertically, not horizontally. At rest on flat ground, the 3,500 lb. vehicle is putting 0 lbs. of net force on the coach. So what is all the fuss about? A 3500 lbs. vehicle does not always put 3500 lb. of net force on the back of the coach. The amount of direct force is directly proportional to the rate of motion.

Newton's laws of motion explain this phenomenon. Here is how the laws are normally summed up:

1) The Law of Inertia – An object in motion will stay in motion until it is acted upon by a net force. Application- The towed vehicle will stay in motion until something stops it (e.g. friction, gravity, brakes, brick wall, etc.)

2) The Law of Acceleration – The force of an object is equal to the mass times the acceleration Application- The force of the towed vehicle on the back of the coach is the weight times the rate of deceleration.

3) The Law of Reciprocal Forces – For every action there is an equal and opposite reaction Application- The inertial mass generated by the towed vehicle will equally increase the braking effort of the coach.

Law number two is the key to understanding what happens to the coach in a panic stop. Newton stated it like this (translated from Latin): "The rate of change of momentum of a body is proportional to the resultant force acting on the body and is in the same direction." Think of it like this, would you rather get hit in the face by a baseball that I tossed at you, or a MLB fastball? According to the "I don't need a brake" mentality, they should feel the same. It is, after all, the same ball, isn't it? What changed? The rate of motion.

This law states that the force of an object is equal to its mass times its acceleration (F=ma). Another common misconception is the definition of acceleration. If a object is traveling at 60 mph at point A and 60 mph one second later at point B, its acceleration is not 60 mph, it is 0 mph per second. If an object is traveling at 60 mph at point A and 40 mph one second later at point C, its acceleration is 20 mph per second. Acceleration is defined as "the rate of change of velocity per unit of time." The faster you try to stop, the higher the value "a" (acceleration) is multiplied by the static mass "m," making the force "F" much higher. The ability to decelerate depends on the coach, the towed vehicle, and the weather conditions, but an average value would be 2.7 times the static weight of towed vehicle in a panic stop starting at 60 mph. This means the afore mentioned 3,500 lbs. towed in a panic stop towed has the same amount of force as a 9,470 lbs. towed in a medium stop.

Simply stated, it comes down to this: When you are trying to stop in a panic, you are not only trying to stop faster, but you are also trying to stop more weight.

The above presented by Brent Schuck
Research and Development
SMI Manufacturing – Used with permission

SMI Manufacturing is a supplemental brake system producer and we don't endorse any specific manufacturer of these devices, however the above information is the best scientific based explanation we've seen for the need for a supplemental braking system for use when towing with your RV.

Supplemental Brake Systems & Flat Towing – Final Thoughts

Supplemental brake systems are relatively expensive and that is probably the primary reason their use is rejected. The most logical argument we've seen for not using a supplemental brake is device malfunction causing substantial brake system damage of the towed vehicle. There are few examples of these problems documented we are aware of.

The sad example of the fatal class A motorhome accident above we hope will cause serious consideration of the potential consequences of opting to not use one these RV safety towing systems.

There is a lot more to flat towing a vehicle than just brakes. First and foremost is that not all vehicles can be towed with all 4 wheels down. You must check with the vehicle manufacturer to determine towing ability. The vehicle you are using to tow with must have an adaquite towing capacity. Many RVs extend the frame rails to fit a larger RV on, these units often have receivers that are rated at only 5000 lbs. It's easy to overload a tow vehicles equipment with a towed vehicle! You also need a properly aligned tow bar and properly installed towed vehicle base plate, safety cables, emergency break-away system, and stop, turn, and running lights. Your towed vehicles front wheels must be free to steer, so you may need to leave a key in the ignition to release the steering lock.

The big drawback to flat towing is you can't back up. We have found that if everything is perfectly straight when we stop, we can back up our towed vehicle a few feet at most, and only in a straight line.

You can spend a considerable amount of time and money preparing a vehicle to be towed. But once prepared for towing a vehicle, you will be rewarded with ease of vehiclular mobility when you arrive at your RV camping destinations.

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