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Old 10-28-2009, 03:14 PM   #99
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Just a BTW, use of a spell-checker would making reading some of these posts easier and more understandable.
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Old 10-29-2009, 03:01 AM   #100
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Originally Posted by DriVer View Post
wa8yxm, In some jurisdictions a car being towed on a dolly is a triple.

I agree with you in that anything towed behind a powered vehicle is a trailer.
what jurisdictions?
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Old 10-29-2009, 05:29 AM   #101
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Webster's definition:
"One that trails".
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Old 10-29-2009, 05:17 PM   #102
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I saw this thread and thought I would add it.

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.


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Old 10-29-2009, 05:24 PM   #103
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Brent,

Thank you for taking the time to publish this piece.

Mike
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Old 10-29-2009, 05:30 PM   #104
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Supplemental brakes are similar to insurance and fire extinguishers.

A waste of money until you need it, then it's priceless.
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Old 10-29-2009, 06:28 PM   #105
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Jake Brake

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Supplemental brakes are similar to insurance and fire extinguishers.

A waste of money until you need it, then it's priceless.
You might say I'm over-doing it but I use my Jake brake all the time. If I see that the Green Light has been on for some time at the next intersection, I take my foot off the gas and apply the Jake Brake. I leave it on until I come to a complete stop. I use very little brake until the last 100 feet. It has gotten to be routine. The Jake Brake does turn my brake lights on. In bumper to bumper, stop and go traffic I put it on and leave it, most of the time, just turning it off while stopped, until the situation changes. I am also in 4Th gear on a 6 speed Allison during heavy traffic. I would really miss it if I didn't have it. The tricky part (for me) is learning when a green light will change before I get to the intersection. I didn't learn this technique on my own. I have googled sites where RV instructors give detailed instructions on exhaust brakes. There is also a lot of info on the PacBrake site. I know they are not exactly the same but the end result is what counts in my book. I also bought the video "How to drive an RV like a pro." Every little bit of info has helped me. I have only had my MH for about three weeks and I'm in "learning mode" right now. This is not my first MH; I did have a MH before this one but it was gas and the diesel is a whole nother animal! This forum and others are full of info for the new diesel operators. Anyway...that's my two cents.
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Old 10-29-2009, 06:42 PM   #106
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Hindsight is wonderful--if I had hindsight, my motorhome would not be sitting in the repair shop getting put back together after someone broadsided me.
I could have prevented that accident by starting my trip 5 minutes later.
I'm not making light of the motorhome accident' I'm just saying things happen.
Perhaps in the motorhome accident you are refering to, imagine if this happened: The toad had brakes, instead of hitting the tractor trailer in front of him, it had the brakes locked up and jackknifed into another car and killed the entire occupents. that didn't happen, but it could have. I'm just a realist
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Old 10-29-2009, 10:12 PM   #107
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Eye Opener

Let me give a quick observation here. I came up on an accident a number of years ago where a car that evidently did not use any braking ( no skid marks), (medium size car, approx. 3200 lbs.) ran a stop light at approximately 60-65 mph (according to witnesses) and hit a semi truck lowboy loaded with a D8 Cat complete with blade, broadside in the intersection. That D8 Cat and blade weighed somewhere around 70,000 Lbs. (I owned the same year model D7 Cat and it weighed precisely 58,000 lb.)

That 3200 # car knocked that Cat off the low boy and it landed on top of the car. 6 young boys lost their lives in that accident. I am not a scientist but sure believe I do not want 3200 or even 2000 # behind me without brakes. If a car that size can knock a lowboy out from under a 70,000 lb. Cat. (that is literally what it did) or the Cat would not have landed on top of the car.

As Paul Harvey said, now for the rest of the story. It required a 100 ton crane to be called in to lift that Cat. off that car before ANY of those DEAD bodies could be extracted.

Just an observation. Put that in your pipe and smoke it. When the smoke clears you should see how much force a medium size car generates when traveling at highway speed. Also how much energy it takes to stop it.

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Old 10-30-2009, 10:19 AM   #108
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As it happens.. I had an event Monday which told me sometihng about the need for aux brakes.

The cross bar that holds the tow hitch on the motor home is held to the frame by two L brackets each with 3 bolts.

BOTH, yes BOTH brackets broke.. the result of this is that the hitch was no longer connected to the motor home. Safety cables also hook to this bar. so the only connection was the electrical wires.

Well. someone got my attention, I pulled over and stopped, the car, which has a US-Gear brake system..... Was right where it belonged. It stopped right along with the motor home.. Only "Secondary" damage was the rear tire on my bicycle.. it rides on that same hitch and when the hitch fell the rear wheel hit the pavement and was fairly well torn up..... I'll get a new one soon, 150 to fabricate and weld on new, stronger brackets, about 50 for a new rear wheel on the bike, and ZERO in damage to the car or motor home from the car crashing into the coach due to no hitch... Now... I'd say that system just paid for itself.
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Old 10-30-2009, 12:08 PM   #109
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OK folks.

Our rules are quite clear about discussing the topic and not each other.

Let's get back on topic and off of each other.

Thanks!
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Old 10-30-2009, 12:10 PM   #110
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OK folks.

Our rules are quite clear about discussing the topic and not the poster.

Let's get back on topic and off of each other.

Thanks!
Amen
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Old 10-30-2009, 09:17 PM   #111
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I just drive 40 mph and everyone is behind me.
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