A66 caravan smash

So, to which bit do you object?

High nose weights (which are indicative of a trailer C of G well forward of the trailer axle) result in a very stable combination provided that the towing vehicle can stand it.

I tow an automotive dyno not a caravan. And sadly for you I do have a clue. And the physics does not change whatever you call your trailer. Truck. caravan burgervan dynamometer or whatever.

He wouldnt know! Basic stability physics obviously eludes him!

John Wright ( snipped-for-privacy@pegasus.fs.com) gurgled happily, sounding much like they were saying :

And the glider?

Do you have any specific objection to the post you replied to? Or would you care to explain your version of physics?

As it stands "you really don't have a clue do you?" doesn't do much to refute anything.

It possibly didn't after that.

had glid.

OK then. A caravan, with one or more axles in the middle is designed to carry all its weight on its own axle(s), without transferring any significant load to the towing vehicle. Because the towbar is situated behind the rear axle of the towing vehicle, any vertical downwards load on the tow bar will result in increased load on the rear axle and decreased load on the front (steering ) axle, hardly favourable conditions for directional stability. This will happen no matter how strong strong the rear suspension on the towing vehicle. Any comparison between an articulated lorry and trailer and a caravan is bogus because they are completely different systems. Those are my specific objections.

It wasn't too badly damaged - the canopy was smashed but it was flying again inside a month or two.

Interesting!! Surely if you have an efficient braking system on your caravan, the de-acceleration will cause the mass of the loaded caravan to produce a force acting through it's CofG and pivoting around the contact point of the tyres on the road thereby producing a downward force on the tow ball of the towing vehicle.

To reduce the effect of potential lifting of the front wheels of the towing vehicle it is important to select a towing vehicle with a minimum distance between the rear wheel contact point on the road and the tow ball, I am not aware that this is a criterion which is normally used for selecting a towing vehicle. Don

Quite possibly, but I would have thought that the towing vehicle would produce an equal if not greater force in the opposite direction as its own C of G attempts to roll over the front axle under braking.

If the caravan/trailer is loaded properly and the tyres and brakes are in good condition with tyres correctly inflated then everything should be OK anyway. I'm sure vehicle manufacturers must take this into account when designing vehicles and setting the maximum towing weights.

Wrong. There HAS to be some weight on the tow bar to get the required stability. Ask a manufacturer if you cant understand basic physics!

Yes.

But because of the lack of leverage between the rear wheels and the tow bar the effect is very marginal. And some extra weight on the cars rear wheels aids stability anyway...

hardly

Actually it improves directional stability. It causes understeer if anything at all which means it wants to travel in a straight line. It may however increase the understeer when cornering on the limit Not something you would want to be trying while towing a caravan hopefully! In any case the lightening effect on the front is tiny. Do you realise you would need to add well over ten times the cars own weight to lift the front wheels!

This will happen no matter

Only in your mind. A trailer is a trailer. You can call it a truck or a caravan. The only difference in engineering terms or physics is that ther centre of gravity is much further in front of the centre of pressure in a trucks trailer. This leads to a) greater stability. b) greater weight on the tow point. Moving the c of g of any caravan forwards by for eg shoving all the heavy luggage food etc forwards will and does increase stability. If the caravan snakes at your given motorway speed then you need to increase the towbar weight. Simple physics.

Your "objections" are at odds with the physics, the facts and the engineering.

Not to any significant extent.

Remember cars are intended to be driven with or without up to 5x passengers of 200 lb each.

Totally wrong. The connection between an artic and it's trailer is supported directly underneath. No rotating moments in the vertical plane are imposed upon the tractor unit.

So all this is tosh.

Adding weight will lower the frequency and amplitude (for the same stored energy) if the system goes into oscillation. That is the simple bit.

I don't think it's simple. If you think it's simple show us your working out.

I have a degree in engineering that would qualify me to gain admission to the Institute of Physics, and I got it before the days when they gave them away like tram tickets.

That aside I don't see the analysis of an oscillating system comprisiing a car and caravan coupled together as simple at all.

There is the "restoring force" element which arises from the tendency of the caravan to fall in line behind the car pulling it. Ultimately limited of course if the caravan is big and ugly enough to influence the motion of the car.

Then there is the tendency of the caravan to fall into the lowest energy configuration in the face of the aerodynamic forces exerted upon it being dragged through the air. Being square on to the wind might not be the lowest energy configuration. Sailing ships don't sail like that. Maybe a caravan "snaking" is analogous to a sailing ship "tacking".

There are dynamic effects such as masses and moments of inertia to take into account.

I could go on for a long time.

If you say it's simple show us your working out.

DG

Small trailers are unbraked. Caravans are braked but as there's no annual inpsection, a great deal of them don't ever see a grease gun outside the warranty period.

We are talking about lateral not vertical stability so thats not even relevant.

Err no. Its entirely valid. Explain using physics and show your "working out" if you disagree. Tosh means nothing apart from you cant!

Actually thats COMPLETELY wrong! Adding weight (moving the C of G forward has only the effect of increasing the stability and makes the thing want to go straight. Adding weight to the rear or moving the c of g back decreases stability. Its obvious, provable, and as expected by any competent engineer.

Again?

I am 46 and do have a degree in physics.

Its not "simple" bit a forward c of g always helps stability.

Its lower every time you move the c of g further forwards in front of its wheels! Purely because it gets further out of line in the first place.

No sensible comparrison can be made from tacking and the natural instability of a box as seen from the side. ANY flat plate needs to be balanced at around 28 percent of its chord (like a helicopter rotor blade or flying wing) to be aerodynamically neutral or remotely "stable". A square box with the wheels in the middle does not come close which is why the stabiility goes down as speed goes up. Only a fin (aircraft style) would be able to truly restore this ballance. If this wasnt the case then a few pounds tow bar weight would be all thats required for neutral stability at any speed. But unfortunately while mass and c of g physics is taken care of by "some" tow bar weight the aerodynamic ones are not. As speed increases one overtakes the other and it starts to snake. IF you add a few extra pounds to the towbar weight the added stability helps compensate for the high speed aerodynamic centre of pressure problem caused by the caravans shape. The greater the nose weight the faster you can travel in stable configuration.

Which so far we didnt allow for. If we do then the nose weight needs increasing further still. In practice this can be found easily by loading the caravan more nose heavy until its stable at your chosen max required speed.

You did.

Aussies have developed anti snaking using electric brakes but it is illegal in Europe

I was stuck behind a caravan on Friday for 20 or so miles, it went past about 5 laybyes, only one car got past, it went fastest at 50mph down hill, up hill was down to 35mph in a 60.

I ended up just behind, I eventually flashed them - they though problem and slowed down - a van got past then the rest of us.

Next layby I paused to let the last few cars past, I was towing - but I got a friendly wave.

The one we followed was ignorant and had a scribbled on reg number L160KUJ I think

If you're going to tow anything, 200bhp or more is required in my book. No fun at all otherwise.

Tim..

My Disco Td5 only has 136bhp, but drags around my trailer & race car without undue effort. But it does develop 232lb/ft of torque.