The Physics of Towed-Vehicle Braking

[Demco Manufacturing, Inc.]

At request of DriVer, this is an elaboration of a portion of the seminar I presented at the 2007 iRV2 National Rally in Branson, MO. This is technical information and I've tried to present as simply as I can, but what makes sense to me may not make sense to you. So, feel free to ask for any clarification.

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 trail 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 (who was in contact with SMI) 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.

Brent Schuck
Research and Development
Graphic Design

[Mike Finnerty]

This explanation should be a constantly resident topic easily available and hard to miss for anyone towing or considering towing a vehicle.

[ISLAPP]

Thank you, Brent. That is very informative and I have made it a featured topic so it stays at the top of the forum.
Mike

[TXiceman]

Brent, thank you for the article. As a mechanical engineer, I try to explain the physics to people, and some listen....some do not.

The arguments I hate to see are the ones that claim the state law specifically says TRAILERS over XXX # require brakes and this does not apply to towed vehicles. Whether the coach is pulling a utility trailer loaded with 3500# of bricks or feathers, 3500# is 3500# trying to push you when you are trying to stop. The coach does not know if you have a 3500# load of bricks or a 3500# dinghy.

We towed a dinghy when we had a motorhome and we had brakes on it as well as a break away system. We had to make a couple of emergency stops and I am 100% convinced that the brakes on the dinghy made the difference.

You can not put a price on safety and a persons life.

Ken

[DriVer]

Originally posted by SMI Manufacturing, Inc.:
This means the afore mentioned 3,500 lbs. would equal about 9,470 lbs. of force on the back of the coach in a panic 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.

Brent, Thank you so very much for this more than insightful article. I hope that everyone will read this article and see for themselves how critical auxiliary braking systems must be engineered.

Buy the best system that you can at a price you can afford. Your life may depend on it.

[rebelsbeach]

Great post Brent...

I certainly am loving my Air Force One!!

[Toterman (David)]

Thanks Brent for taking the time to post this information. I have taken the liberty of posting your thread with credit over on the Escapees forum . This information is not just confined to towing a car but any form of trailer or vehicle.

David

[Tom and Patty]

I would further think that having a vehicle pushing from behind would tend to force the MH to continue traveling in a straight line. Making the MH slower to change direction in an evasive maneuver, like the Michelin video that shows how to control a sudden loss of air pressure in a tire....by accelerating first, braking second.

We never gave a thought of towing without supplemental brakes. Pulling a PT Cruiser with Silent Partner.

Thank you for the info Brent.

[RustyJC]

Originally posted by Tom and Patty:
I would further think that having a vehicle pushing from behind would tend to force the MH to continue traveling in a straight line.

Well, except for the hinge in the middle (the hitch coupling), you might be right. Think in terms of a tractor/trailer rig. If the tractor has more braking force than the trailer, the trailer will push the tractor into a jack-knife situation because of the yaw flexibility of the hitch. Granted, the relative weights are reversed with a heavy MH towing a light toad, but the physics still apply.

Rusty

[JandM1947]

Greetings Brent & forum readers,

I had my SMI Air Force One installed today by Jesse & John at SMI HQ in Evansville, IN. The installation was very professional, in spite of the fact that Jesse's regular installation partner was at a rally out west and it was the hottest day of the year (102). What dedication - thanks guys, you're great!

I am very impressed with the AFO unit. It's easy to hook up & almost invisible. The safety benefits are unquestionable & I'm really glad I decided to use the SMI system for my unit.

[Vegascpl]

I've been one of those hold outs who thought there was no way my light weight hybrid would ever stop my motor home!

I looked at the Air Force One and I liked it. However, I'd rather pay someone for professional installation than spend 6 hours crawling around under my coach. I also want to know it's working correctly and that if there is a problem I have someone to go back to. The closest place to Vegas that has even done an installation was in Lake Havasu.

I'm now thinking about the Roadmaster which is similar and installed for about the same price as the purchase price of the SMI. http://www.roadmasterinc.com/product...r/brake_m.html

Anyone have any experiece with this system?

Thanks for the information on the thread. It convinced me I should add a breaking system.

Michael

2005 Dutch Star 40' 4 slide
2005 BMW K1200LT motorcycle on lift
2006 Ford Escape Hybrid

[Tom and Patty]

I believe the Roadmaster will be applying pressure to a dead pedal, no power boost. The SMI charges the power booster with vacuum, therefore the in car cylinder doesn't have to apply high forces to brake the toad.

Our PT weighs in at about 3500 lbs which I don't consider a lightweight. The Ford Escape Hybrid weighs in at close to 4K.

I cannot imagine anyone towing a car without a supplemental braking system. At the worst, you probably have no breakaway system .

[Demco Manufacturing, Inc.]

Michael,

In an effort to keep this thread devoted to generic information about tow braking, I am starting a new thread to specifically answer your question. Ford Escape Hybrid Braking

Brent Schuck
Research and Development
Graphic Design

[BryanL]

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.

I am not happy having problems with the 'lecture' after so many positive reviews. But it does grate on my understanding of physics and my distaste for hyperbolic fear mongering.

The weight does not change no matter how fast you try to change the speed of the rig. Weight is defined as the force due to gravity, not any change in inertia.

The mass also does not change.

The whole argument is much simpler than presented. Stopping quicker means you need more force than stopping slower. Stopping more mass, which has a greater weight, requires more force than stopping less mass. That's all.

As far as I know, any towed vehicle over 1500 to 3000 lb, depending upon state of registration, is required to have braking. This makes me wonder about the scare story used in this presentation and whether it is really appropriate to an honest message.

The limiting factor on stopping force is almost always the tire friction on the road surface. If you slam on the brakes, both your towing vehicle and the towed vehicle should be laying down rubber.

Without the FUD, the lessons I see are these:

1) if you are towing any significant weight without proper brakes, you are probably towing illegally.

2) you must compensate in your driving for the size and weight of the rig you are driving.

3) your RV does not handle like a sports car and should not be expected to.

4) rig your RV and maintain it properly so that it is dependable and is safe if driven properly.

5) safe driving depends upon a confident attitude based on experience and training. Trepidation and fear are indications that this experience and training is insufficient for safe driving.

I note that RV's are generally the safest vehicles on the road and that the primary cause of accidents (after DUI) is not paying attention. Stay awake and alert, take care, and drive safe.