That boat on the A9 ...

[Tumshie]

Never mind. ?

Edited November 25, 2018 by Tumshie

[MtB]

6 minutes ago, Tumshie said:

Never mind. ?

 

Oh no I missed wot u rote.

 

I was watching a video of Ringo Starr, most talented drummer EVAR....

[Tumshie]

Just now, Mike the Boilerman said:

 

Oh no I missed wot u rote.

 

I was watching a video of Ringo Starr, most talented drummer EVAR....

Phaaa....

[MtB]

1 minute ago, Tumshie said:

Phaaa....

 

Oh I'd forgotten about Animal!!!

 

 

[Guest]

55 minutes ago, Mike the Boilerman said:

 

What an excellent demo of the effect I was suggesting happened. 

 

Thing is, WHY does it happen when the weight distribution is wrong? 

 

You will note from the video that despite the weight distribution being wrong it wasn't until the trailer was pushed off line that it became unstable. In some cases you will get away with it, but if anything pushes the trailer out of line it will set up a pendulum motion caused by the excess weight at the rear which causes the swing to escalate.

 

The forward motion of the tow vehicle will tend to pull the trailer back in line but this only results in it swinging past the centre line and going the other way, and it repeats back again the other way and so on escalating with each swing.

 

The cause can be a simple gust of wind, a slight change of direction by the driver, a passing high sided vehicle (as discussed above), a binding brake on one side of the trailer, a rapidly deflating tyre or blow out on the trailer, the list is quite long.

 

Combined with the oscillating movement of the rise and fall of the nose of the trailer (as mentioned by CT above) this will soon convert into increased sideways movement, the result being an uncontrollable snake. Especially if the trailer is close to or exceeds the weight of the vehicle towing it.

 

If you could guarantee towing on a perfectly flat surface in a straight line, with no other traffic and in zero wind you could probably tow at quite high speeds even with a badly loaded outfit and not encounter an issue.

 

The reality of course is you cannot and you will readily encounter something that will trigger that initial movement.

Edited November 25, 2018 by MJG

[Tumshie]

8 minutes ago, MJG said:

If you could guarantee towing on a perfectly flat surface in a straight line, with no other traffic and in zero wind you could probably tow at quite high speeds even with a badly loaded outfit and not encounter an issue.

And there in lies the problem - when people look at some thing that's not quite right and say to them selves "it'll be fine" they're only picturing the perfect conditions not the things that might set that chaos theory into motion. ?

[Guest]

9 minutes ago, Tumshie said:

And there in lies the problem - when people look at some thing that's not quite right and say to them selves "it'll be fine" they're only picturing the perfect conditions not the things that might set that chaos theory into motion. ?

One of the issues is that there are lots of vehicles that can legally tow a trailer that is heavier than the vehicle towing it.

 

People often assume you don't have to take additional care if you do, 'coz it's legal innit'.

 

In reality of course you do.

[bizzard]

Request by Ditch crawler.    Tail wag, yaw.

[Sir Nibble]

Charity worker. WTF has that got to do with anything? Maybe they could have said cat owner, or horse lover, or east enders fan. Surely the only relevant description is "driver".

[Alan de Enfield]

1 minute ago, Sir Nibble said:

Charity worker. WTF has that got to do with anything? Maybe they could have said cat owner, or horse lover, or east enders fan. Surely the only relevant description is "driver".

But if he wasn't a charity worker he would have got 12 points and been banned.

 

Bit like the forum T&Cs, the judge decided to apply the rules 'favourably' in the circumstances.

[Iain_S]

13 hours ago, Mike the Boilerman said:

 

I note you fail to explain WHY... 

 

If you did, I would get it  

When a vehicle is cornering,  a sideways force is applied to the tyres, ands the treads will deform slightly, so that the wheels no longer travel in a direction perpendicular to their axes. The difference between the normal direcion of travel and the direction with a sideways force appled is the slip angle. If the slip angle is greater on the front wheels of a vehicle than the rear wheels,the result is undesteer: if the slip angle of the rear wheels is greater, the result is oversteer. Understeer results in a stable condition, as the vehicle takes a straighter line, and the sideways forces are reduced. Oversteer, on the other hand, is unstable, as the vehicle rotates, increasing the sideways forces, although in terminal cases the vehicle will follow a similar track on the road, Terminal understeer means that you go through the hedge on the outside of the corner. Terminal oversteer means that you go through the same hole backwards.

 

If the trailer is tail heavy, the load on the rear wheels of the towing vehicle is reduced, which increases the slip angle of the towing vehicle's rear wheels. This makes it unstable, as it is an oversteering condition. The driver will have to make constant steering corrections to maintain a sraight line, and the whole thing will be very twitchy.

 Add a wobble on the tailer, and the rear of the towing vehicle moves. If uncorrected, the drive from the towing vehicle may pull the trailer straight again, but when it is straight, it still has rotational momentum, and goes the other way, with the oversteering instability of the towing vehicle increasing the swing. After a few oscillations, the correcting force is insufficent to halt the swing, and the rig jack knifes. If the driver corrects, the problem is getting the timing right: the initial steering correction has to be reversed before the trailer is straight, or the oscillation will continue to increase.

Braking will make things worse, and the accepted advice used to be gentle acceleration. Snag with that, of course, is that you end up going faster, and more unstable than when you started.

[MtB]

Hmmm thanks for the detailed explanation, I'm beginning to see the mechanism by which this happens. 

 

16 minutes ago, Iain_S said:

If the trailer is tail heavy, the load on the rear wheels of the towing vehicle is reduced, which increases the slip angle of the towing vehicle's rear wheels. This makes it unstable, as it is an oversteering condition.

 

You seem to be saying here that a negative weight on the towing ball is necessary. I'm inclined to suspect this happens very rarely as it is bloody obvious when trying to hitch up and pushing the trailer hitch downwards to get it onto the ball would surely ring alarm bells for most drivers, so I doubt that happens very often. I suspect the unstable snaking is still perfectly capable of happening when the trailer is 'nose heavy', but just not enough. The thing I found unsatisfactory about the otherwise excellent demo video was they didn't actually tell us the values of the nose weight on the model trailer and car. I also suspect the 'slip' effect you describe on the rear tyres of the towing car is negligible in the case of the small model car in the demo yet the snaking effect is still demonstrated, and varies between stable and unstable with trailer weight distribution. I think there must be another mechanism at play additionally in the model in the video, but I'm still not sure what. 

[Paul C]

That's a good explanation of a situation where a trailer is so badly loaded it ends up with a negative tongue weight. In reality, you always want a positive tongue weight, BUT on a hitch trailer (ie not a 5th wheel or swan neck) its a bit of a balancing act, because too much tongue weight imposes a leverage and results in more than the tongue weight applied to the rear wheels - and the axle load of the front axle of the towing vehicle reduced - so less steering control. A long rear overhang exacerbates this.

 

Its one of the reasons why twin axle trailers are more stable than single axle - they can be loaded in various ways, and it doesn't result in much variance of the tongue weight. BUT the towing car's hitch needs to be at the right height - there's a standard, but its a range not an exact figure; and some 4x4s have trouble achieving it because the towbar ends up being a plough when the 4x4 is used for off roading.

 

Another thing worth mentioning about 4x4s - they are often better at towing because their wheels are bigger. Let me explain - if you can get the hitch below the centreline of the wheels, then during pulling/accelerating the trailer, the leverage imparts an angular momentum onto the front wheels of the tow vehicle; and during braking the opposite occurs. The trailer isn't trying to lift up the back of the towbar, instead its planing the towcar's rear wheels into the ground a bit more.

 

This is why (partly) tractors have big rear wheels, the hitch is well below the centreline. And its also why a typical tractor has almost no overhang and the hitch is only a few inches away from the centreline of the rear wheels - the closer the better here, obviously you need to have the physical space (you couldn't really have the hitch below the diff, etc it needs to be behind it) and you need enough to allow for turning.

[TheBiscuits]

I once had this happen to me on the M6.

 

I'd driven several hundred miles with a laden twin-axle vehicle trailer, and dropped off a load with no issues.  On the way back, the unladen trailer decided it wanted to sit with the rear wheels only just touching the road, making a rear-heavy trailer.

 

Ended up going backwards down the motorway, and the only thing I could do about it was put my hazard lights on when it started twitching going downhill in heavy traffic.  Nobody hurt and no damage, but a lot of explanations to the motorway police as they had to put in a rolling roadblock so I could turn round.

 

[Paul C]

5 minutes ago, Mike the Boilerman said:

 

You seem to be saying here that a negative weight on the towing ball is necessary.. 

Negative tongue weight is very easy to inadvertently occur with twin axle trailers when the hitch height is mismatched. (But we don't know the details here). I suspect in addition to just looking at overall weights, the accident investigation team in the A9 crash might have reloaded the trailer with the boat in the same position, presented it to the towcar and measured things like tongue weight too, since its a weight that needs to be in limit (tongue weight needs adding to the MAM of the tow vehicle too.....)

[tree monkey]

Happened to me once, many many many moons ago when I was a whippersnapper. 

Working for a landscaping firm doing a garden clearance, simple job clear all the shrubs and run the mulcher mower over the lot, remove shrubs from site.

 

110 Landy and long trailer all the lightweight shrub brash loaded and as the mowing was the last job that went on the back.

 

Driving over the Flint bridge and the trailer took what appeared to be complete control of the Landy, one second I was heading to the river Dee, the next about to crash into the central res.

 

To this day I have no idea how I stopped it and it was one of the scariest things that ever happened to me.

 

Older and wiser me takes great care when towing

 

[TheBiscuits]

 

4 minutes ago, Paul C said:

BUT the towing car's hitch needs to be at the right height

Exactly what was wrong in my post above.  Towbar too low, so (just) lifted the rear 60% of the trailer off the floor.

[Alan de Enfield]

6 minutes ago, Mike the Boilerman said:

I'm inclined to suspect this happens very rarely as it is bloody obvious when trying to hitch up and pushing the trailer hitch downwards to get it onto the ball would surely ring alarm bells for most drivers,

Yes - a trailer may have a 'low' nose weight (normally for each car the min & max 'nose weights' are specified and within that range it should be stable.

 

My 3-axle 'flat bed' trailer has a low nose-weight as the trailer is 'balanced' across all 3-axles. The nose weight is increased when it is loaded.

 

My 3 axle 'horse trailer' has an extremely heavy nose-weight (as the axles are further back beneath the 'horse' section of the trailer). The nose-weight is actually above the maximum for the car, when 'empty' but is very stable (note what I did there ?) and tows well both loaded and unloaded.

 

[Machpoint005]

1 minute ago, tree monkey said:

To this day I have no idea how I stopped it and it was one of the scariest things that ever happened to me.

 

Older and wiser me takes great care when towing

We ex-motorcyclists would call it a "tank-slapper" -- especially if we've had one as a result of a rear tyre blowout. 

[Paul C]

1 minute ago, TheBiscuits said:

 

Exactly what was wrong in my post above.  Towbar too low, so (just) lifted the rear 60% of the trailer off the floor.

Interestingly (a possible diversion though) the type approval regulations make it a bit worse now. A sensible fitment on some cars is a height-adjustable tow hitch but nowadays both the towbar would need to be type-approved to allow one, and the device itself would need to be.

 

Something like this:

 

[tree monkey]

4 minutes ago, Machpoint005 said:

We ex-motorcyclists would call it a "tank-slapper" -- especially if we've had one as a result of a rear tyre blowout. 

Never had a tank slapper on a bike, had the rear wheel lock up a few times, largely through poor conditions and my stupidity 

 

To add, which I suppose is a similar effect

Edited November 26, 2018 by tree monkey

[Alan de Enfield]

24 minutes ago, Paul C said:

Interestingly (a possible diversion though) the type approval regulations make it a bit worse now. A sensible fitment on some cars is a height-adjustable tow hitch but nowadays both the towbar would need to be type-approved to allow one, and the device itself would need to be.

 

Something like this:

 

That was always (??) the standard fitting on Land Rovers (well the Discoveries I had anyway)

[Paul C]

I have to admit I don't know the exact situation, its possible a LR is one of the few vehicles to have a towbar which accepts height adjustment attachments (and is type approved for one); also remember that up until very recently, commercial vehicles didn't need type-approved towbars where cars did. 

[Guest]

2 minutes ago, Paul C said:

I have to admit I don't know the exact situation, its possible a LR is one of the few vehicles to have a towbar which accepts height adjustment attachments (and is type approved for one); also remember that up until very recently, commercial vehicles didn't need type-approved towbars where cars did. 

My 2002 TD5 Discovery had a type approved height adjustable tow bar.

And very useful it was too.

 

 

 

 

[Mike55]

45 minutes ago, Paul C said:

That's a good explanation of a situation where a trailer is so badly loaded it ends up with a negative tongue weight. In reality, you always want a positive tongue weight, BUT on a hitch trailer (ie not a 5th wheel or swan neck) its a bit of a balancing act, because too much tongue weight imposes a leverage and results in more than the tongue weight applied to the rear wheels - and the axle load of the front axle of the towing vehicle reduced - so less steering control. A long rear overhang exacerbates this.

 

Its one of the reasons why twin axle trailers are more stable than single axle - they can be loaded in various ways, and it doesn't result in much variance of the tongue weight. BUT the towing car's hitch needs to be at the right height - there's a standard, but its a range not an exact figure; and some 4x4s have trouble achieving it because the towbar ends up being a plough when the 4x4 is used for off roading.

 

Another thing worth mentioning about 4x4s - they are often better at towing because their wheels are bigger. Let me explain - if you can get the hitch below the centreline of the wheels, then during pulling/accelerating the trailer, the leverage imparts an angular momentum onto the front wheels of the tow vehicle; and during braking the opposite occurs. The trailer isn't trying to lift up the back of the towbar, instead its planing the towcar's rear wheels into the ground a bit more.

 

This is why (partly) tractors have big rear wheels, the hitch is well below the centreline. And its also why a typical tractor has almost no overhang and the hitch is only a few inches away from the centreline of the rear wheels - the closer the better here, obviously you need to have the physical space (you couldn't really have the hitch below the diff, etc it needs to be behind it) and you need enough to allow for turning.

I learnt to drive on a 'Little Grey Fergie' many moons ago and it had a hydraulic 'automatic' hitch fitted which was virtually under the diff.  More modern tractors which are also much heavier do tend to have the hook further back.