The Physics of Towed-Vehicle Braking

[Nortons RV]

Originally Posted by 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

thanks brent for the info this is as important today i remember a article on kinetic energy where two trucks crash head on at 50 mph the energy at 50 mph at the point of impact being one # and by adding 10 mph faster it would be quadruple the force thanks again

[melvonnar]

Originally Posted by Nortons RV

thanks brent for the info this is as important today i remember a article on kinetic energy where two trucks crash head on at 50 mph the energy at 50 mph at the point of impact being one # and by adding 10 mph faster it would be quadruple the force thanks again

Question ?
Do you honestly think most people will read this long explanation; you are correct in your explanation, but the average person will stop reading after the first paragraph.
I've been motor homing for 30 years; no accidents wore out 4 motor homes, traveled nearly 500,000 miles. so I'm not a novice, for most, the breaking system is a great idea and I would encourage it, I personally don't have one. I now drive a 34 foot motorhome pulling a Toyota PU. And O yes I'm 85 years young.
have a nice day

[KanzKran]

Stopping distance after brake application on flat ground between a coach with no toad and the same coach with toad (but with no brakes) increases in proportion to the ratio of the combo weight (coach plus toad) to the coach (alone) weight. Adding brakes to the toad with the same capability as the coach reduces the braking distance of the combo back down to that of the coach alone, speaking in general terms and in an ideal world.

IF the braking force the MH can apply to the tires is limited by the friction between tires and road surface, or by the anti-lock brakes preventing wheel lock-up (skidding) by monitoring for onset of skid and modulating the brakes, which is still friction limited, or even by the brakes themselves if they're not capable of locking the wheels (weak brakes, in other words), then adding mass to the system via a tow bar will increase stopping distance in direct proportion to the increase in total mass, all other things being equal.

What follows is just an expansion of the above, so no need to read if the details don't interest.

A 30,000 lb coach with a stopping distance from 60 mph on flat ground of 428 ft* will decelerate at -9.05 ft/s^2. The horizontal force at the tire/roadway interface will be about 8425 lbf.

If you add a 4000 lb vehicle, towed with a horizontal tow bar (so no vertical force component), and without brakes of its own, then the mass increases to 34,000 lbm. Friction force at the tire/pavement interface is still 8425 lbf, but acceleration decreases to -7.98 ft/s^2. Force in the tow bar would be 990 lb as the toad is pushing against the coach while the coach is doing all the braking.

Stopping distance increases from 428 ft to 485 ft, an increase of 57 ft. You can run through each scenario using Newton's second law and the equations of uniformly accelerated motion, but it still works out to 428 ft x 34,000/30,000 = 485 ft. Or a 13.3% increase in stopping distance no matter what the actual tire friction value is, assuming tire friction or weak brake force governs, and any weight combo can be used to estimate stopping distance increases after brake application.

If tire friction governs and you put the extra 4000 lb inside the coach, the coefficient of friction (friction force divided by weight force, or .281 in this example) remains the same of course, so the braking force increases and the stopping distance remains the same.** If the brakes themselves are weak, then moving the extra toad weight inside the coach doesn't make any difference, as tire friction is not limiting, and the longer stopping distance will still result. Same is true with brake fade, or down grades, but that's next level stuff.

If you activate the brakes in the toad such that it too decelerates at exactly the same rate as the coach without the toad, then the force in the tow bar is zero, acceleration remains at -9.05 ft/s^2, and stopping distance remains 428 ft. This was mentioned in one of Demco's posts, and it's not surprising.

Of course, you'll never get the toad brakes to exactly match the coach braking, but the trend remains - toad braking in addition to coach braking reduces stopping distance to some degree, and in fact, if the toad has better tires with a higher coefficient of friction to the road, it may actually shorten the stopping distance a little below what the coach alone (no toad) can do.

I have no opinion on whether a toad is a trailer, or what states require brakes at what threshold, or if it's consistent with any manufacturer's specs or requirements, but toad braking certainly will reduce stopping distance in almost all scenarios, so it's a personal judgement as to whether or not someone wants to add brakes even if they're not legally required to. But if you need 485 ft to stop and you only have 450 ft until you run into something, it won't end as well as if you only need 428 ft to stop.


*a random value I plucked off the internet for illustration purposes, and which does not include recognition, reaction, and brake application times, which doesn't change with the addition or deletion of toad brakes

**lots of assumptions made, including the coefficient of friction remaining unchanged with tire force, which it's probably not, but those things are refinements and do not change the physics on a gross, first-cut level.

[tom3855wales]

A great read, thank you, but i kept getting hit with [Moderator edit] ads all through the section which I dont appreciate not.one bit. Not your deal i know, just wanted to let u know if you havent gone back and read your posts.

[Jagob]

Great discussion. I was looking seriously at a surge brake type system that mechanically apples the toad brake. I took my Jeep Wrangler down the drive and at about 20 mph, killed the ignition, and started applying the brakes. The first application was pretty effective but after about the 3rd it was just about zero braking. I’m wondering how effective any of these braking products really are if you aren’t using some type of vacuum booster. Based on my simple test, these type systems (with no vacuum boost) are doing very little other than giving you a false sense of security. DISCLAIMER- totally new to MH and toad pulling and just trying to educate myself!!!

[Sweetbriar]

When I've been asked why I have supplemental braking (Ready Brute tow bar when I was flat towing and now an Acme dolly with brakes) for my tow vehicle I reduce my answer to the simplest of terms -

If I can add 3500 to 4500 lbs of weight to my coach and not increase stopping distance that would be a good thing, right?

[Jagob]

Sweetbriar,
I totally agree, that’s why I’m asking my question. After the boost is gone from the braking system, it doesn’t seem like you’d be getting much help from it. I’m hoping some folks who use them will chime in here and correct me, because for cost and simplicity, that’s the kind of system I’m looking at. After my test drive that I detailed above, I’m doubting myself.

[Sweetbriar]

Originally Posted by Jagob

Sweetbriar,
I totally agree, that’s why I’m asking my question. After the boost is gone from the braking system, it doesn’t seem like you’d be getting much help from it. I’m hoping some folks who use them will chime in here and correct me, because for cost and simplicity, that’s the kind of system I’m looking at. After my test drive that I detailed above, I’m doubting myself.

The mechanical connection between brake pedal and the piston in the master cylinder is still there you just don't have the assistance of the vacuum. You, as the operator, will need to work more to stop the vehicle when boost is lost but the system capabilities are still there. By design braking assist devices are fail safe.

The surge system on the Ready Brute bar I was using could pull the brake pedal in my car a lot harder than I could push with my leg. I did break the cable that was connected to the brake pedal once and I read of another owner that bent the brake pedal during a panic stop. Plus you also have to consider tow break away systems still function without assistance.

[Skipper Lee]

Originally Posted by DriVer

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.

Thank you. We just bought a 38 ft Holiday Rambler endeavor and are trying to decide on a tow dolly or 4 wheels on the grond system. I have had numerous people say they have towed for years with no brakes on the dolly when I would ask if it has drum or electric brakes. Your article sealed this one up for us. I previously towed a 30 ft pull be hind with a dodge Ram and had electric breaks and no incident and am looking for advice as to purchase a tow dolly with electric brakes or a 4 wheels on the ground tow bar type and would like to hear opinions on which is better and safer. Thanks again for this post, it is very timely for us.

[NXR]

Originally Posted by Skipper Lee

I previously towed a 30 ft pull be hind with a dodge Ram and had electric breaks and no incident and am looking for advice as to purchase a tow dolly with electric brakes or a 4 wheels on the ground tow bar type and would like to hear opinions on which is better and safer. Thanks again for this post, it is very timely for us.

There's lots of posts in this forum on that subject. Both can be equally safe if properly equipped. "Better" usually starts a limited Holy War.

A lot depends on whether you already own a vehicle that can be "flat towed" or "dolly towed" and if not, would you consider buying one. Not all vehicles can be towed four-down or on a dolly. Some must be towed up on a trailer.

If you choose flat-towing, you need to assure that a vehicle you already own is capable of being flat towed. Some are and many are not. You can start here if you do not know but the Owners Manual is always authoritative: https://www.motorhome.com/download-dinghy-guides/

If you're limited on towing capacity then the weight of a dolly plus the weight of the car may be a problem.

I did the dolly route for a year and switched to flat tow. I find it more convenient even though the up-front expense is usually higher than a dolly. As I've aged I value long-term convenience over a one-time cost savings.

If you can do a new post in this forum and explain your goals you will get a lot of advice. The trick will be sorting through it all.

Good luck,

Ray

[powerboatr]

since we are snowed in, i have been doing alot of re-reading posts related to stuff that intrigues


this post is one of the top


the braking scenario really is good


i set my trailer brakes to be on the heavy side as in they apply a bit more aggressive to not only handle weight of trailer , but to also pull back slightly on the coach, why not use the two extra axles out there with big brakes on the axles.


now we tow an edge with tow bar and patriot braking system.

still use trailer, but not nearly as much.


but with the patriot i set it to be a bit more aggressive in its application so when i stop , i feel like the edge is not even there


last trip, we had an unexpected road bumps and stuff through some construction and i never thought it would dislodge or move the patriot.
but that has to be the only explanation as to why it was out of position when we arrived at the park. but a few miles before the park, we were moving along at about 60-62 and headed down a long hill , and a tanker truck about 1/2 -3/4 mile up, suddenly smoked his tires and stopped to turn short to the left, i mashed the stop pedal hard and felt the bus was not slowly fast enough and pressed harder, REAL hard. i also felt like i was being pushed from behind. I was rapidly running out of road surface and no where to go but to the right, into a sloped grassy area, no emergency lane was present, just grass then AIR, or hit him dead into the rear of the tanker trailer.

i hollered at my wife to cinch up as i felt we were going to hit the tanker and it was going to be abrupt. I was by now pressing with all my might on the brake pedal and we were slowing and i thought to myself this is a no win situation, rear end the taker truck or go off in the grass and flip and die slowly, vice going through the window and die a bit faster. mind you this is all happening in mere seconds, probably less than 10.

then just as all seemed to be WE are going to die, the tanker started moving to the left and into his road, i swerved to the edge of the lane and just missed the rear of his trailer as he rolled . i was shaken.


we had a mile or so to go to the park. when i got there i stopped, got out and the horrific smell of brakes was overwhelming .


we checked in and went to set up, this is where i found the patriot askew and was not pressing the brake pedal.


i am still not sure how or why other than the construction bounces and uneven pavement.
but when we go home i sat in the passenger seat and had my wife actuate the patriot over and over and ended up making a small backer board for the front of the seat and designed it in a way that the board and patriot now hit the seat track base vice the cushion. this was really weird and i had never had this happen before

[cphollis]

We are new to RVing (July 2020) and quickly realized we needed a toad to fully explore when travelling. Cue large research project for me. The "best flat tow" for us turned out to be a 2021 Jeep JL 2DR. Besides, I like jeeps.

When it came to braking systems, there are no shortage of people who will tell you they don't use one, and they've been just fine. I hope for everyone's sake that their luck continues to hold. Plenty of examples in any RV park.

Being a geek by nature, I thought "there's no way that pulling 4000+ lbs without a brake assist is going to be OK". Simple physics, as explained here. Yes, it was all expensive. Yes, it takes time to set the brake system up and uninstall it.

That's all part of the deal, just like paying for a reserve chute when skydiving. And you won't meet many people who regret not paying for that insurance.

[RAJORDAN53]

I'm a newbie to RV living. Just bought my first RV, a 2014 Tiffin Allegro Red 33AA. Anyone familiar with towing a Ford F150 4x4 (2014) behind the Allegro? I know the F150 4x4 is flat towable, but wondering about overall weight capacity and hooking up break assist. Any help/advice would be much appreciated.

[walt2137]

Originally Posted by RAJORDAN53

I'm a newbie to RV living. Just bought my first RV, a 2014 Tiffin Allegro Red 33AA. Anyone familiar with towing a Ford F150 4x4 (2014) behind the Allegro? I know the F150 4x4 is flat towable, but wondering about overall weight capacity and hooking up break assist. Any help/advice would be much appreciated.

If the truck is a crew cab you are talking about close to 6000# , if you do decide to tow the truck i would susgest the old style M&G brake system it is one of the simplest and in my openion best toad brake system out there the only problem with it is it wont fit all toads but the M&G 110 brake system will fit your 2014 150, i will post their web site.
https://m-gengineering.com/products/...-brake-system/