Thread: Towbar for D2 + Jayco 25ft caravan

Thats a nicely written article,,

but just a couple of things spring to mind after reading it,,,

Pitching just doesnt happen with top of the line shockies on the tow car,,

If "there can be up to 500kg of up and down force when pitching"
and you've just added a WDH,, havent you just added extra upwards force??

wont a "bullett of sideways wind" hit both the front AND the back of the van?



I'm happy that you have a "nice" van to tow,,

I'm going for a quick search,, there was a vid posted not so long back that demonstated just what we are talking about,, now,, where was it,,,

[nomedia="http://www.youtube.com/watch?v=5gyc4Dmgbew"]YouTube- Land Rover's "Trailer Stability Assist" for Range Rover, Range Rover Sport and Discovery 4/ LR4[/nomedia]

and this tech does exactly what the article says,,

it absorbs the motion.

not only that ,, it does it on both axis,,

Originally Posted by Pedro_The_Swift

Thats a nicely written article,,

but just a couple of things spring to mind after reading it,,,

Pitching just doesnt happen with top of the line shockies on the tow car,,

If "there can be up to 500kg of up and down force when pitching"
and you've just added a WDH,, havent you just added extra upwards force??

wont a "bullett of sideways wind" hit both the front AND the back of the van?

I'm happy that you have a "nice" van to tow,,

I'm going for a quick search,, there was a vid posted not so long back that demonstated just what we are talking about,, now,, where was it,,,

Pedro,

Thanks. I am not an engineer so I can't argue for or against WDH except from personal experience, but a couple of quick reactions to your points:

Pitching just doesn't happen???? Sure it does, regardless of whether there is a trailer attached or not, unless you have incompressible tyres and a completely rigid suspension, and even then it will happen after hitting a bump when the wheels leave the ground. (Pitching being any movement about the lateral axis). All that shockers do is damp that pitching so it doesn't continue too long after the initial upset. Perhaps that is what you meant, but it only takes one quick pitch down on the towbar, and that is what you get when the van brakes come on, to reduce the weight on the front wheels and therefore the efficiency of your brakes when you need them most. Mine are Bilsteins, which are not top of the line, but are at least reasonable quality.

The WDH will indeed increase the upward force, but it will also substantially reduce any downward force, such as with heavy braking, which generates the upward reaction.

A natural cross wind which is not blowing through obstacles and is therefore a steady wind, will indeed affect the entire side of the van, and therefore it will always tend to yaw the van UNLESS the effective keel area forward of the pivot point is identical to the effective keel area aft of it. However that rarely occurs because of trees, buildings etc on the side of the road, so in practice the cross wind is usually acting first on the front and then on the rear. In addition, one of the unnatural wind effects which is particularly common and potentially dangerous is the bow wave from a heavy transport which, when overtaking, affects the rear first, tending to move the nose of the tug one way, then affects the forward section, tending to move the nose the other way. The end result is a drift towards the overtaking vehicle. And I can tell you that it is quite "interesting" when a heavy transport overtakes when you have not noticed its approach! This is one situation where I have no doubt that trailer stability assist would be very very helpful.

I am all for getting rid of the weight and complexity of the WDH IF trailer stability assist can fully replace it, but so far that does not seem to be the case. Trailer stability assist can't do anything to affect the fundamental problem which is weight distribution. "All" it can do is help to detect and then try to counter incipient sway before it gets too much (and that is a great help if the vehicle can stop the sway before it gets out of hand). But it is only one of the issues that WDH appear to help solve. In my one personal experience of dangerous sway, its onset was so sudden that I seriously doubt the ability of trailer stability assist to help before the forces exceeded the ability of the tyres to retain grip. We were probably a microsecond away from jack knifing or worse after probably one to three seconds since the upset was intiated. A quick hit of the van brakes alone solved the problem instantly. Any way I have invited LR to give their views on their decision, and as a back up I have foreshadowed my desire to include the van on the D4 test drive if I decide to go further with replacing the D2.

I guess we have hijacked the thread, but caravan handling can be a real safety issue, not just for those towing but also for any unfortunate soul who has a van jack knife in front of them. I have enjoyed the discussion.

This site is also interesting, but doesn't prove anything apart from the effect of weight distribution: Bailey of Bristol - Caravan Stability Studies

Check out the simulator.

Whilst I agree with Pedro that the article is quite good at explaining the principles, there are two matters that are not quite right, one practical and one pedantic. I also think there are a few practical summary conclusions which would help to be made, too.

Collyn Rivers is right that in the absence of any other influence, a body will rotate around its centre of mass. However this is only a minor effect in terms of the impact of the trailer on the towing vehicle. The major vertical and horizontal influences are the rotation of the trailer around its axle and wheel pairing. There is a lesser horizontal influence relating to the rotation around the hitch point. The discussion of rotational dynamics relating to the distribution of the mass of the trailer and energy absorbion are all valid.

The practical summary points I would make are:
1. the centre of mass of the trailer should be just forward of the trailer axle and as low down as possible.
2. the heaviest parts of the trailer should be as close to and as low down to the trailer axle as possible
3. all other things being equal a longer draw bar will result in lower vertical and horizontal loads on the hitch point.

The pedantic point I would make is that under our SI (metric) system of units, mass is measured in kilograms and weight, which is a force, should be measured in newtons. If you wish to talk of weight in kilograms you should use kilograms force (kgf) for which the unit of mass is the metric slug. For you older types who pine over the imperial system, you either use mass of ‘pounds mass’ (lbm) with force in poundals, or you use force of ‘pounds force’ (lbf) and mass in slugs.

Mundy

Originally Posted by Mundy

Whilst I agree with Pedro that the article is quite good at explaining the principles, there are two matters that are not quite right, one practical and one pedantic. I also think there are a few practical summary conclusions which would help to be made, too.

Collyn Rivers is right that in the absence of any other influence, a body will rotate around its centre of mass. However this is only a minor effect in terms of the impact of the trailer on the towing vehicle. The major vertical and horizontal influences are the rotation of the trailer around its axle and wheel pairing. There is a lesser horizontal influence relating to the rotation around the hitch point. The discussion of rotational dynamics relating to the distribution of the mass of the trailer and energy absorbion are all valid.

The practical summary points I would make are:
1. the centre of mass of the trailer should be just forward of the trailer axle and as low down as possible.
2. the heaviest parts of the trailer should be as close to and as low down to the trailer axle as possible
3. all other things being equal a longer draw bar will result in lower vertical and horizontal loads on the hitch point.

The pedantic point I would make is that under our SI (metric) system of units, mass is measured in kilograms and weight, which is a force, should be measured in newtons. If you wish to talk of weight in kilograms you should use kilograms force (kgf) for which the unit of mass is the metric slug. For you older types who pine over the imperial system, you either use mass of ‘pounds mass’ (lbm) with force in poundals, or you use force of ‘pounds force’ (lbf) and mass in slugs.

Mundy

Can't agrue with that!