How to stop Jackknifing

Hello everyone. What causes Jack knife when towing? Do stabilisers help? What is the difference between stabilisers and Level riders. Thanks johno
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Reply By: Ross M - Monday, Apr 07, 2014 at 12:24

Monday, Apr 07, 2014 at 12:24
When the tow vehicle is traveling slower than the trailer ie, brakes faster than the trailer, the trailer can try and overtake the tow vehicle. It will side shift the coupling area of both and the trailer will try and pull alongside, usually not a good situation to be in.

Unless the trailer loses almost all of it's grip on the road ie skidding suitably applied braking force should see the trailer, trail, behind the vehicle.
Trailer brakes should be slightly more active than the vehicle to ensure it stays behind without wheel lockup. ie loss of grip.

A bad sway will make the rig get close to the jack knife point if towing equilibrium isn't maintained. If the tow vehicle is forced sideways as a result of sway, physics makes it slow down and the trailer catches up almost straight away, = jack knife.

Vehicle braking, instead of not just trailer braking, will add the the sway and increase the possibility of jack knife.
No brake pedal pushing if equilibrium has been lost. Electric controller activation may remedy the effect. Not a sure thing though.

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Reply By: HKB Electronics - Monday, Apr 07, 2014 at 12:29

Monday, Apr 07, 2014 at 12:29
Jackknifing is caused by the trailer pushing the car tail around. This happens because the brakes in the vehicle are more effective than the trailer.

If both the trailer and the car are stopping at the same rate, or the off center push from the trailer is not great enough to break the traction of the rear of the vehicle then the jackknife won't occur.

I don't believe stabiliser etc will help prevent this happening, others may disagree, stabilisers can make a van more of a pleasure to tow by leveling it and dampening
sway somewhat.

What to do to prevent jackknifing, don't pull to be a van for your vehicle, try not do do crash stops especially when car and van aren't in a straight line if possible, easier siad than done of course. Don't set the van brakes to aggressively, as once the wheels lock up it will loose traction and push the car even more.

Sure others will have different opinions and I will also be interested to hear these?

Cheers
Leigh

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Reply By: Dennis Ellery - Monday, Apr 07, 2014 at 13:10

Monday, Apr 07, 2014 at 13:10
If it is a large van keep your speed below 90 kph.
Keep your tow ball weight within recomended limits.
I have my caravan brakes set so that a light touch on the pedal brings on the van brakes without applying the 4WD brakes.
Practice this until it becomes an automatic reaction, as soon as you feel a sway starting.
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Reply By: The Bantam - Monday, Apr 07, 2014 at 13:44

Monday, Apr 07, 2014 at 13:44
Jacknifing can happen at any speed where the tow vehicle due to instability, loss of traction or otherwise failure of the vehicle mass to dominate the trailer mass...and thus control it.

The single easiest to controll factor is vehicle mass compared to trailer mass.
The heavier the vehicle is in comparison to the trailer the better chance the vehicle has to control the trailer.

While all types of trailer....semi-trailer/fifth wheel, dog trailers/quad trailers are all to some extent prone to jacknifing....by far the worst is the pig trailer (single axle group in the middle)..that is most caravans......that is why they are frowned upon in heavy transport....there is a very good reason they are called "pig" trailers.

But in recreational caravans we persist with this findamentally unstable format.....this is why there are these load distributing hitches and sway prevention devices......devices that are completly absent in heavy trasport.

A factor that compounds this whole problem is that people want to tow the biggest heaviest caravan the vehicle can legally...combine this with the very generous towing capacitities..that some of us view as rediculous....people should be concerned.

A vehicle that is towing well below..like 2/3 of its towing capacity on the highway will be far far safer than one that is maxed out....AND it will be far nicer to drive.

Stabilisation devices, load distribution, suspension modification, driving method and choice of speed all have a role to play.


The jacknife its self realy is not the issue...it is the inevitable result when any sort of vehicle & trailer combination gets out of control.

The issue is stability....of course reducing speed will reduce risk...but if your combination has a stability issue, speed will not solve the problem....the problem remains and it may come to bite you some day when a low speed is still too fast for the stability and traction that prevail.

If you combination is not stable and safe to tow at 100KMH on good, straight flat highway, under good conditions..it should not be on the road.

cheers
AnswerID: 530012

Reply By: Peter_n_Margaret - Monday, Apr 07, 2014 at 13:58

Monday, Apr 07, 2014 at 13:58
A vehicle with an overhung hitch (behind the axle) towing a "pig" trailer (as opposed to a "dog" trailer) is inherently unstable.
The extent of that instability is determined by the yaw inertia.
To minimise instability, keep length to a minimum and centralise mass as close to the axles as possible.

http://www.youtube.com/watch?v=PFzrWHTG5e8

The same instability does NOT apply to 5ers or "dog" trailer configurations.

Cheers,
Peter
OKA196 Motorhome.
AnswerID: 530013

Reply By: Member - KeithB - Monday, Apr 07, 2014 at 16:09

Monday, Apr 07, 2014 at 16:09
Every trailer and caravan combination will have its own critical speed at which the rig becomes unstable and want to jack-knife.
That speed depends in a whole load of things from how the van is packed, suspension design, overhangs, wind conditions, bumps in the road - the list is almost endless.
I know this link has been seen on the forum before, but it's always worth another look.
University of Bath stability simulator
AnswerID: 530029

Follow Up By: Dennis Ellery - Monday, Apr 07, 2014 at 17:02

Monday, Apr 07, 2014 at 17:02
I agree speed is a big factor for large vans - mine is 7 metres with centrally located wheels (not the ideal location).

From Vehicle Dynamics by Collyn Rivers 2010
“Having researched this issue in depth for the past 10 or more years I would not personally tow any conventional caravan longer than five metres at over 90 km/h”
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Reply By: pop2jocem - Monday, Apr 07, 2014 at 20:27

Monday, Apr 07, 2014 at 20:27
The longer and heavier the van, the greater the towed speed and most importantly the distribution of heavy items to the rear can set up a potential jack knife situation.
Vehicle manufacturers seem to be in a race to see who can give their product the greatest legally allowable tow rating regardless of tow vehicle to towed trailer weight ratio.
There has been research done that appears to suggest that once a certain weight of van or trailer get past a certain degree of yaw there is no correcting it unless the towing vehicle has sufficient weight and tyre to road surface adhesion.
As has been mentioned a fella by the name of Collyn Rivers has a few publications that are well worth a read.

Cheers
Pop
AnswerID: 530064

Reply By: The Original JohnR (Vic) - Monday, Apr 07, 2014 at 21:13

Monday, Apr 07, 2014 at 21:13
Johno, one of the things that most seem to overlook is the way in which tyre and car design has changed over the years.

I have found that when towing something as small as a Kimberley Karavan a few years ago, tyre compatibility front to rear on the tow vehicle was important. Larger profile tyres allow more swing side to side on the tyres, particularly with lower pressures in tyres. Lower percentage tyres aren't quite as susceptible. That being said, on Australian roads, you don't want low profile tyres.

WD Hitches allow more pressure on the front, steering tyres and tend to stop the whip there, that when exaggerated, becomes enough to jack knife the car and caravan. I know with our Bushtracker behind the 100 Series, one link tighter on the WD hitch chain makes more relaxed steering. You don't need that in caravan park placements however, as you can strain the bars and bend them.

On a properly set up car and caravan WITH weight distribution hitch properly adjusted, I discount Colyn Rivers' commentary. Don't dispense with the hitch, though unless you are just pottering along or on very uneven surfaces
Cheers,
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John

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Reply By: Member - KeithB - Monday, Apr 07, 2014 at 21:34

Monday, Apr 07, 2014 at 21:34
I think another aspect of modern caravans is the use of independent trailing arm suspensions, which have very low roll centres. This makes the van tend to "roll" into a corner rather than "lean" into it.
I think that this, combined with a tendency to put spare wheels and tyres on the rear bumper of big vans, could be a major factor in stability. Colyn Rivers has suggested as much.
A number of manufacturers (Bolwell and Kimberly for example) are now offering conventional anti-roll bars with their trailing arm suspensions in an attempt to offset the low roll centres.
AnswerID: 530076

Follow Up By: awill4x4 - Monday, Apr 07, 2014 at 23:13

Monday, Apr 07, 2014 at 23:13
I've never seen any photo's or videos of vans on their sides with independent suspensions but I've seen lots pics of standard leaf sprung vans that have fallen over.
Regards Andrew.
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Follow Up By: Member - KeithB - Tuesday, Apr 08, 2014 at 08:33

Tuesday, Apr 08, 2014 at 08:33
Good point Andrew. Maybe the addition of double and single shockers on the trailing arms is saving the day - plus the fact that trailing arms eliminate bump steer.
But I can't help but think that asymetric link and five kink designs are inherently more stable.
Keith
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Reply By: Batt's - Tuesday, Apr 08, 2014 at 11:43

Tuesday, Apr 08, 2014 at 11:43
Don't rely on stabilisers etc quite a few people who use them or who are advised to use them don't know how to set up the tow vehicle, trailer or van correctly they are taking a short cut or hiding an incorrectly set up tow vehicle trailer or van in the first place. Make sure your suspension is in good condition fit heavy duty springs to the rear of the vehicle which will cope with the extra weight talk to a suspension specialist about how much extra weight you plan on putting on the rear of your vehicle and don't skimp on you estimate lots of people refuse to do that because they don't want a harsh ride that's rubbish it will improve the ride the people who drive around with a sagged rear end are asking for trouble and should be pulled up by the cops for endangering themselves and other road users. One of the most important thing is to make sure your trailer or van is " SITTING LEVEL" with the tow vehicle adjust your trailer brakes according to the instructions take time to learn how they operate and how to use the manual override . Loading the trailer or van correctly is critical have more weight forward of the axel putting weight on the tow ball but not exceeding the max tow ball weight I find somewhere around 60 to 80% is ideal which you can quite often measure with a sturdy board placed on bathroom scales this will bring you back to why you really need to upgrade your rear springs and fitting load assisting air bags will also help to keep you set up level if you require them. Don't take short cuts set it up correctly the first time and you will have years of trouble free safe towing. Remember your towing don't swerve radically around hazards pot holes etc look down the road plan ahead remember the amount you turn your steering wheel will be exaggerated by the time it reaches the vans axel drive safely.
AnswerID: 530104

Follow Up By: Batt's - Tuesday, Apr 08, 2014 at 12:06

Tuesday, Apr 08, 2014 at 12:06
Also if you have it set up correctly there is no reason why you can't safely and confidently drive at the posted speed limits if you want to be it 100 kph or 110 kph because your not relying on stabilisers etc to compensate for a poor set up. As you can guise my pet hate is these type of devices that give people false confidence in their set up people I'm driving on the road with who just want to hook it up and drive and have no interest in learning or understanding what their driving. Take a look at a road train with say 7 pivot points they weigh a lot more than a van and don't require these towing aids.
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Reply By: Nomadic Navara - Tuesday, Apr 08, 2014 at 16:56

Tuesday, Apr 08, 2014 at 16:56
I don't think it has much to do with the relative balance between the van and tug brakes. There is a lot of cases where the brakes were not used. Have you not seen the number of reports in the news where vans have jack-knifed and flipped? Most of these have a comment from the driver in the form of "we don't know what happen, we were travelling along nicely and then things happened without warning." If the jack-knifing problem was a problem with brakes then these accidents would not have happened as brakes were not used.

The problem is all tied up with the distribution of weight in the van and the relative size of the van to the tug. The distribution of weight in the van includes the way the van is built as well as the way it is loaded. For best stability vans should be built with the centre of gravity well forward of the axle. They also should have the weight concentrated towards the middle of the van. The more the weight is concentrated in the middle the lighter the ball weight can be.

Critical speed: this subject was broached earlier. The critical speed of a van is the speed that when that speed is exceed the van has the potential to become unstable and uncontrollable. I use the term "potential to become unstable" as things feel fine as things will seem fine until some external force upsets the travel of the van (and then things go ape bleep .) People often get the the impression their long heavy van is stable as when they try to make it snake they find it hard to make it do so. Collyn Rivers sums up the effect well in his article "Vehicle Dynamics:"

"A long end-heavy caravan commonly
feels more stable than a short one with
centralised mass (short twin-axled ‘vans are
usually very stable but often feel ‘twitchy’
due to their fast but minor movements),
but such impressions are illusory.
Inertia is not the same thing at all as
inherent stability: a giant container ships
seems ultra-stable - until a rogue wave rolls
it too far - and its inertia keeps it rolling).
“My mega-van always seemed rock stable
until the day it jack knived and rolled over”
is a very common post-accident reflection."

One of the problems we have is those who unwittingly reduce the critical speed (and thus the stability of their van) by adding boxes and liquid containers to the ends of their vans. In an effort to increase the stability of our vans we should be building them as the Europeans do. You will note they don't have thing on the ends of them like bumper bars and spare wheels. the front boots are a lot smaller, they don't have front or rear kitchens.

When people add things to the ends of the van they consider that if they add equal amounts of weight to each end to maintain the same ball weight then all is well. It is not. Remember back when we started to balance our wheels. All the tyre retailers got those devices that you sat your wheels on, you then added weights so the wheel became dead horizontal (a bullseye level was provided to assist in the process.) This was referred to as static balance. In short time this was found to be fairly useless, particularly with the wider tyres. The next step was dynamic balancing and that is still in use today. It's the same with caravans. Static balancing does not save your bacon, you need to consider the dynamics of the moving rig. When you add weight to your van tha dynamics of the van are not changed in a linear manner. The effect of the added weight works in a square law manner. That is, the yawing effects of the added weight increase in in a square law as the distance from the centre of gravity is increased.

See this link Vehicle Dynamics
PeterD
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