Do you really need ATC?
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If speed ismeasured simply by how long it takes to travel from points A to B. Then of course the caravan can not travel faster than the car because as you say they are attached.
However if the caravan is swaying from side to side and the car remains traveling in a straight line i.e. Before the snake starts. Then the actual distance traveled by the caravan between points A and B is greater therefore its velocity must be greater.
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I can't get my mind round the concept how with " the caravan is swaying from side to side " can be "Before the snake starts" ?
If the caravan is swaying from side to side the snake is already there.
The actual critical issue is whether the swaying is at one of the units natural frequencies or not.
The "speed" of the caravan between "A & B" will be greater but not its "velocity". The van will have cyclic velocities in both the longitudinal & lateral planes, that the car has not.
The ATC is proactive in that it detects and addresses minor amplitude sways BEFORE they can build up to critical levels.
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Our's came already fitted from new on a BPW chassis, it has only come on a couple of times in three years due to pot holes on dual carriageways, but it's like some of the other things we carry with us like first aide boxes and fire extinguishers that we may never use, but it's good to have them with you. Now day's HGV trucks have anti jack knife devices fitted from new, so I'm all for any safety device.
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Hello Lutz,
When a reaction such as a side force is put on the side of the van it pushes the van out of line, however, since the tires resist the force they act against the natural pendulum effect, now since the force has been induced into the tires the extra energy has to go somewhere and this is what accelerates the van towards the tow. Some of this is absorbed by the damper and ultimately pushes the brake on but at an angle to the tow vehicle, if it is only minor then the van returns to C of E naturally, if it is severe then the velocity of the sway creates a build up of potential energy for the return as it nears the limits of the swing and at the same instance it pushes the tow vehicle sideways - this explains why braking especially on a downhill makes the situation worse since the potential energy is increased even more. Take a look at some of the dash cam footage's and you can see this quite clearly. In severe cases it only takes three good swings to end in roll over because other resonance forces multiply the swings on each side.
If you need further proof watch the water skiers as they follow the tow boat then swing out to increase the velocity, when they reach the limit the rope goes slack if they fail to switch direction in time.
Another point raised is why then does the van brakes not apply to stop this - the answer is they lock up with the intense pressure and you can see the skid marks from this together with the side skid marks as the van goes broadside.
As I mentioned earlier you can work this all out using calculus derivatives and this proves the physics. You cannot find a solution until you know the cause. ATC is an active system not Pro active since it needs to detect the sways after they have started a Pro active system prevents a sway from starting in the first place and this cannot be achieved by braking both wheels simultaneously - all this can do is slow the combination to change the period rate of the sway. You ask what is? The answer is you need to apply an equal and opposite force at the center of equilibrium where the velocity would be at the highest - this stops the sway when the van lines up with the tow instantly under full control and it is done before sway is even noticed by the driver and does not slow the combination down.
John Wilson
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Hi guys
I see this is taking a lot of thinking but you can be assured I have spent hundreds of hours on this and come up with the same answers,
To get your head around the problem of the van accelerating on swings do this:- Draw a picture of the trailer and van inline then with the van at an angle you will see that the length of the combination has reduced when you scribe a line in an arc for the van, so this distance must be taken up somewhere or else. If the van had no tires it would swing like a pendulum and there would be no problem with the distance take up, however, the tires will not allow a full pendulum swing so the physics have changed and the tires will convert this energy towards the tow.
John Wilson
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Hi guys again,
In some posts they think sway is always when the van moves sideways - this is normal and you will never stop this because you cannot make the combination rigid it has to flex but under control. Now we are referring to a high velocity push on the side ( when we talk about certain velocities you have to work out what reference this is to) Swing velocities are what is refereed to here not combination speed/velocity, this seems to be where the confusion is. So just revisit what I said and think carefully of what it implies. When a severe push occurs it is stopped as it happens then if that did not do enough and the van returns back at a high velocity it is stopped again but at the CoE as it happens this is how it can never build up to dangerous situation - I found that 3-5 degrees of sway is ample before correction occurs but only on half the cycle not a full side to side swing as in ATC/iDC
Oh and by the way I do have a working prototype of the solution and I mentioned it on this site earlier - that's another matter.
John Wilson
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Hi guys and yet again,
If you have ever looked at an engine and seen how the crankshaft turns with a connecting rod to a piston this will show you clearly how the piston accelerates as the crank turns from say the bottom of the stroke that is exactly what happens with your caravan as it sways. That is why it is so important to understand what is going on. Of course if you have never studied engineering it may take some time for it to sink in. When it does you will know more about snaking than you did before. You will be in a better position to decide what you need to do to avoid it an what solutions actually work.
John Wilson
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I it has have a Bailey Unicorn Cadiz series 3 ATC fitted as standard I have noticed it kick in a couple of times once on a very uneven road, I would certainly recommend the system.
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To clarify. My personal definition of a snake is the point at which the swaying of the caravan makes the towing vehicle to loose control. It is possible for a caravan to sway without this happening you can call it a sway or pre-snake condition. I am not saying it is desirable in anyway or safe.
With regards to ATC it must better have it than not to have it. In my opinion one refinement I would like to see is it not to activate below a certain speed. 2 Reasons firstly there is a speed hump at our storage where the ATC always deploys. I am always going at less than 5 mph at this point so it is irritating. The second reason happened yesterday. I was pulling off a site which had a roughish road onto an A road. The visibility on the A road is restricted because of a bend. So I want to pull out and accelerate. The ATC applied and it wasn't a fast get away and I felt vulnerable.
Still think the positives out weigh the negatives though.
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Thanks for your clarification of what you consider is a difference between a sway and snake.
I agree on the point that the ATC could be refined but In am very glad they chose not to. Doing so brings in further complications that must affect reliability and the cost to build which often reflects in the selling price and can affect the installation cost.
A present there is no speed sensor in the ATC or the caravan though there is in the car. So there would need to be either a speed sensor say via GPS computation added to the ATC or a sensor added to the caravan and telemetry added to get the new sensor on the van or the existing info from the car to the ATC.
All very doable but not at no extra build or installation cost, that I have no doubt would reflect on the selling price.
I said earlier In am glad AL-Ko resisted that option as keeping to the price point they chose to I am sure more have opted to have it than would be the case if they had complicated further, and it is more reliable even if you have to live with its “quirks”.0 -
My comments about a minimum speed below which wouldn't opperate was an aspiration more than a demand. I fully accept that it would add both cost and complexity to the system, whose major attraction is its simplicity. It does one thing it applies the brakes when it detects apparent in stability which takes energy out of the situation.
Somewhere in the depths of You Tube there is a german video of a caravan on a test track with and without ATC operating it is an impressive demonstration.
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With or without ATC, one has left it darned late to intervene if the caravan is already swinging so much that it already affects the stability of the towcar. The amplitude will never be that great at the first swing so that at that stage even without ATC it will still be possible for the driver to respond manually in an appropriate manner. However, without some sort of active intervention, the amplitude of the swing will increase with each oscillation and it becomes more and more difficult to bring the caravan into a straight line again. When the sway has become so violent that it already affects the stability of the towcar an accident is almost inevitable.
I was able to regain control of caravan already swaying so much that it was using up two lanes of motorway by applying the brakes. (The towcar in that case was so heavy relative to the weight of the caravan there was little feedback to the driver that something untoward was happening at the back and hence it was already getting a bit late to respond by the time I noticed - an indication that a low weight ratio is not necessarily always favourable).
Contrary to statements made above, my experience is that the caravan's overrun brake will not actuate when the caravan is swaying. Locking of its brakes resulting in skid marks from the caravan's tyres will only occur if the driver in panic does an emergency brake. Skid marks from the caravan can also be the result of the its tyres losing lateral grip as the caravan is skidding sideways when the swing is very large.
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I have only had one snake and there was no warning, it was instant immediately after a high speed coach passed very close.
Luckily it eventually recovered but it was an experience I don't want to repeat.
So I say fit ATC, shockers, and balance the wheels, and keep the speed down. Pay attention to nose weight and caravan loading.
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A number of people are sceptical about being able to accelerate out of a snake. I agree that most outfits wouldn't have the power to do this but you would think that accelerating the car would have the same affect as braking the trailer?
It certainly saved my brother from a serious accident when towing a boat and it worked for me on one single occasion when towing a light van with a very powerful car at modest speed. Of course, there is no way of knowing whether the snake would have self corrected but at the time it seemed very clear that accelerating had stopped it.
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I currently have a t/a with ATC towed by a Discovery which is very stable and I never had a hint of a sway even from passing buses etc.
However going back 30 years with an old caravan towed by a family saloon I did suffer movement when being overtaken by fast vans and buses. A trick commonly used by caravaners when seeing an overtaker approaching was to ease across more to the right of your own lane (not across the white lines) making the overtaker also to move to their right, then ease yourself back over to the left just before the other vehicle reached you. It helped give a wider gap between the two of you so more space for the displaced air to disperse. Just a gentle easing across with no sudden change in steering.
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Hi guys
If you are still not convinced that is your problem, however, here is another thing I discovered during the development of my system - When the overrun brakes are applied during the extreme snake the friction hitch lock caused the overrun shaft to jam the brakes in the on position from the side force on the shaft, this was worse if there was just the slightest wear on the shaft or there was insufficient grease on the shaft, even so I would not recommend not applying the device unless you have another damping device installed or a proper control system in place. Just to illustrate this to the nay sayers, this concept is used in rotary power steering systems to enable the steering shaft to lock the hydraulic valve that permits power to flow to the rams and this device actually runs in hydraulic oil such is the high friction involved when a side force is applied to a shaft. I must add that the brakes are hard on from the inertia and the situation is irreversible anyway so don't worry to much about this. Just remember don't let a sway develop in the first place.
And luttz I agree the best thing to do is - nothing, just allow it to settle down slowly if possible since accelerating seems the obvious thing to do, however, in practice when the brakes are held on the van will resist realignment because both brakes are on so it will be hard to pull it out to align it - just try this in your yard with the hand brake on - you will not be able to pull it around.
John Wilson
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I get the impression that a lot of the responses here are based on theory or experience in one-off situations. In the course of my work during my working career (before the days of ATC) I was involved in many series of tests requiring deliberate inducement of a snake under widely varying conditions and not once did the caravan's brakes actuate unless the car's brakes were actively applied. (Maybe the caravan brakes would have come on in the extreme, but we never let it get that far). Moderate braking, just enough to actuate the overrun, was always the remedy. Sometimes just releasing one's foot from the accelerator pedal would already be sufficient. Early intervention as soon as instability is detected but before the amplitude of the sway increases to unmanageable proportions was the key. With experience that can be done by the driver himself, but electronic aids are usually more reliable and quicker to react in an appropriate manner. I am not aware of instances where ATC has failed in this respect.
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Hi Lutz
Yes I agree with what you say, however, there are situations that need to be considered and I base all my comments on fact not theory alone. As a qualified transport engineer with 40 years experience I know better than to base my comments on loose hypotheses. Just to add to my previous comments:- just as the brakes lock on the wheels will lock up (caravan brakes are very powerful) and at that instant the tires loose all grip and enter the laws of friction when the whole van will slide just as the guys who use handbrake turns to slide around corners. The rule here is that a locked wheel travels faster than a rotating one- fact. this is the finale to a complete rollover. I fitted switch and a warning light to the overrun so that I could see through my rear mirror how often the brake applied, with 1.5 tonne pushing on the overrun even decelerating downhill with the engine even and with wind pressure on the van above 50 mph.
My take on this subject was to find out the cause and effect without any doubt so that I could build a solution that covered all events. The bases of my conclusions were that you have to prevent sway from starting because nothing can stop the oscillations beyond a specific amount. Even my system cannot correct if sway gets over 40 degrees. So i make sure that the oscillation are killed on half a cycle of the sine wave ie at the center of equilibrium. That means it can accept a sway to one side but can never return and go over center. For those who are unsure when a sway may turn into a snake put a line on you rear window to line up with one on the van, if the sway returns past the center take extreme care, if it stops at the center your ok. To all reading this let me know if you understand this.
John Wilson
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True, but the amplitude of the swing must be considerable for the caravan's tyres to lose lateral grip. A side-to-side sway of, let's say, 3° either way should be noticeable by the driver as it is the equivalent of about 40cm on an 8m long caravan. He should see that in his rear view mirror even if it's not sufficient to be detected in the behaviour of the towcar, and it would indicate the need for intervention. However, it would not result in the caravan's brakes coming on and certainly not to the extent that the tyres lose lateral grip. However, such a pendular movement should be more than enough to actuate the ATC.
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I concur with that Lutz
If you take this example the situation become clearer for those seeking knowledge:- When the overrun is presented with a force 10kg it will start the to move towards braking albeit slowly, which is what it is supposed to do and will do upon any deceleration (10kg against 1,500kg is easily overcome) so this implies that any sway will start to push against the overrun damper so that in most cases sway is under control and moves back to center under control. If the sway is pushed hard the damper then will take about 100kg to compress (you can test this yourself easily) This means that the resultant force is now absorbed by the tow car from an angle and it pushes the car rear end sideways ( This is what the driver feels immediately) This causes the potential energy to increase dramatically and results in the Kinetic energy ( The velocity of return) to increase also. If it is accompanied by additional resonance ( wind etc) it will gather velocity ready for the next oscillation and off you go again. However, if the brakes are applied at the limit it causes the potential energy to be snap released which gives it an extra push ( that's what happens when a jet takes off the pilot puts full power on then releases the brakes to give extra push) so you now begin to see what situations build up to a problem. Do not be afraid to question if you do not understand rather than criticize before you know.
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Hi The question for this post was "Is ATC worth it" Well I concede that until there is a workable, viable alternative to you I would say yes. That may surprise some of you after what I have posted. If you are trying to decide ask the manufacturer some questions. How many swings of the van does it take to activate. The last I heard it is 2 large swings over a specific g or 4 smaller swings at a lesser rate. Now the latter accounts for the majority of tugs people commonly feel but most are a waste of fuel and wear and tear on the outfit initiated by bumps, cornering, or wind effects but they cannot take the chance it may lead to something greater. The 2 large swings probably are something to be concerned about, however, If you get hit with two large swings it will be hit or miss if soft braking a 3.5 tonne combination will slow it down in time to stop the third critical swing - that is what you need to question. Then account for the inefficient concept of braking both wheel simultaneously that prevents the van alignment in that short space of time. Independent braking of the wheels (differential braking) will pull the van around inline instantly - without needing to slow down. so ask them why they do the former.
If you take ESP on cars they arrange independent braking to help the driver keep control (fantastic innovation but don't rely on it) it is electronic so full of problems. However, in conjunction with a van differential system it is amazing and if you can get TSP (trailer electronic program even better. these two car systems are not the same as ALKO make you believe for ATC they do not do the same. They are electronic but operate completely differently so beware.
John Wilson
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My personal experience of ATC is that it takes far less to activate than what you are suggesting Landlubber. Mine activated at very slow speeds (less than 15mph) on very lumpy roads entering and exiting my storage yard. It also activated on a B road with relatively sharp humps and dips, without any discernible sway whatsoever... I am not sure if ATC sensitivity is adjustable, but ours seemed quite sensitive.
That said, not as sensitive as the built in protection on our new car. We have had a Land Rover Discovery Sport for a little over a year, and last year, whilst driving down the M5 at 60mph, the cars own system kicked in before I even felt anything. Looked in my rear view mirror to see the caravan well off centre due to a gust of wind and no sign of ATC on that occasion, I am convinced the car saved the day.
Again to return to original post, for the sake of a couple of hundred quid, I would rather have it than not. Even the most careful driver can find themselves in a situation where ATC could make the difference between life and death.
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Whilst i concur with Lutz on part of his response I don't think he understood when braking occurs to lock up of the brakes. Of course if I am in front of a group in a lecture I can elaborate using a few stage tools so I understand - but beware many misunderstandings do lead to lack of clarity. I am contemplating a YouTube video to get the message across and also show the mathematics to back it up - I promise only an outline of the maths and how calculus is used to analyse the conclusions - I am not a mathematician just an engineer that knows how to use the maths - thanks to Newton etc who did all that work. Ps you can find Calculus on many YouTube sites if you have the time to study it and it will be enlightening for you.
John Wilson
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