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Snaking

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#21 AWanderingLancastrian

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Posted 12 September 2017 - 11:16 am

My Elddis Typhoon single axle was susceptable to snaking and not only downhill!
It was especially prone to winds on either rear quarter.
Tyre pressures-noseweight and loading helped-especially keeping heavy stuff out of the back end.
Towing up and down the A9 it was subjected to some pretty high winds but of course crosswinds are not restricted to up here eg crossing the border on the west coast M6 A74M.

 

Hi ancell. Single axle instability and unpredictable behaviour often but not always caused by other vehicles,that was my main motive for only towing twin-axle caravans and large boxed trailers for the last 30+yrs.

 

Twin axle trailers/caravans have four points of friction down on the ground and that alone contributes to far greater stability than with just the two of a single axle.

This is never more evident than when reversing and maneuvering a twin axle on tarmacadam surfaces,the effort to turn is seen in the distortion that takes place at the walls of the tyres and if the rims are suffering any corrosion and the tyres are running under their correct/appropriate pressure they can/will spill the air from the tyres making for an even more precarious situation.



#22 ancell

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Posted 12 September 2017 - 11:27 am

I agree twin axles for towing are more stable in my experience of five decades towing 4 wheel and 2 trailers and single axle caravans.
My only reasons for not buying twin axle caravans in the past has been cost-4 tyres to replace 4 brakes to check-their lack of maneuvarability on site without a mover-levelling and securing them.

#23 Lutz

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Posted 12 September 2017 - 11:32 am

Instability is always dependent on speed. No outfit will always be instable regardless of how slow it is going. Every outfit has a threshold speed below which it will recover from a 'wobble' by itself without the need for any intervention. If the setup, such as the noseweight, etc. is correct, for a caravan this is usually somewhere between 50 and 60mph. Slowing down below the threshold speed will therefore always cure an unstable condition.

Slowing down will also deploy the overrun brakes on the caravan and this will also help to pull the outfit straight again. This is basically the same principle as what ATC does. It applies the caravan's brakes in order to straighten the outfit.


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#24 Stevan

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Posted 12 September 2017 - 11:54 am

AWL, despite having previously read these, and other, documents I cannot accept your fatalistic view. If an outfit is inherently stable and travelling below its critical, or threshold, speed (separate debate!), any sideways disturbance of the order likely to be encountered in normal driving will be followed by a smaller disturbance in the opposite direction, and back, each time getting less until it is straight again. Only if there is some inherent instability or approaching the critical/threshold speed, will each rebound disturbance be bigger than the one before leading to a snake. 


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#25 Richard080561

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Posted 12 September 2017 - 12:59 pm

Interesting. We are just back from a two site stay, no problems on the motorways on the first two legs, but on the final trip to home the van was very nervous over 50mph. Half way home I stopped to check tyre pressures and all was OK.  When home I noted that my nose weight was about 80 kg instead of the usual 100kg, I was also running with less weight over the caravan axle (we had eaten it!).  I'm surprised that 20kg would make so much difference.
I have always carried my 2 aquarolls + waste container in the end wash room,  I have always wondered if this exaggerates the "dog bone" effect.
For the questioner, I have occasionally forgotten to drain the hot water heater which is front right, and this is quiet noticeable on stability. 

Good point but not easy to measure.  Anyone any ideas?

One of these? https://www.obelink....pMaAmKtEALw_wcB
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#26 ChrisUK

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Posted 12 September 2017 - 04:27 pm

Nice, but as I would expect not cheap



#27 alano

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Posted 12 September 2017 - 06:09 pm

Check all the things mentioned - tyre pressures for car and caravan, loading of both car and caravan, noseweight.

 

Yesterday, there were weather warnings about high winds in the South-West reaching up to the Midlands, it may be that which triggered it.

 

I've got a similar journey today, down the M5/A30 to Cornwall, with another weather warning from 5pm.

Thanx Stay safe and enjoy your holiday



#28 alano

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Posted 12 September 2017 - 06:17 pm

Thanx everyone really helpful tips looking at my van the tyre pressure looks low so will re inflate  to correct pressures I will also check my nose weight as this may be a little heavy for my 3008 as they are quite low spec on nose weight,



#29 thebriars

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Posted 13 September 2017 - 04:53 pm

 

Hi ancell. Single axle instability and unpredictable behaviour often but not always caused by other vehicles,that was my main motive for only towing twin-axle caravans and large boxed trailers for the last 30+yrs.

 

Twin axle trailers/caravans have four points of friction down on the ground and that alone contributes to far greater stability than with just the two of a single axle.

This is never more evident than when reversing and maneuvering a twin axle on tarmacadam surfaces,the effort to turn is seen in the distortion that takes place at the walls of the tyres and if the rims are suffering any corrosion and the tyres are running under their correct/appropriate pressure they can/will spill the air from the tyres making for an even more precarious situation.

I too would disagree with this. Its not just down to friction on the ground. Its down to how much lateral movement there is on each of the axles. In the case of a caravan, under-inflated tyres or tyres with weak side walls will allow this sideways movement which will enable a snake to rapidly start. A twin axle will be more stable because there will be less lateral movement on the tyres, and yes there will be increased friction when the van is turning.

 

The only snake I ever had was, I believe, caused by very weak tyre walls, and my advice to Alano would be to have a good look at the tyres themselves as many older caravans were fitted with tyres that were actually inadequate for the job. Fitting heavy duty (i.e. 8 ply)  trailer tyres will make a huge difference to stability in such cases (and are less prone to blow-outs and punctures). It could also be a problem with budget car tyres that allow a lot of lateral movement too. An easy way to check side wall strength is to make sure the caravan is hitched up, or the jockey wheel secured so it cannot move, and the try to push the back corner of the van sideways. If the back of the van will move sideways and the tyres flex when pushed, then think about a tyre change.

 

On my last caravan, the tyre pressures were incorrect in the handbook, and after a year the dealer agreed to pay for 3 new tyres. I was staggered just how thin the tyre side walls were; they were no thicker than a bicycle tyre.

 

One of the few positives about my current Bailey is that at least good quality tyres have been fitted, and thus the stability is excellent.


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#30 Steamdrivenandy

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Posted 13 September 2017 - 06:42 pm

Just another thought, you don't say what type of 3008 you run but lots have a 1300kg Towing Limit. From what I can see a Typhoon has an MTPLM of 1300kg so it's potentially right on the cusp weightwise.

 

Now some Peugeot's, Citroen's and others have a Gross Train Weight Limit that is less than the total of their Towing Limit plus Gross Vehicle Weight. In effect if you run the caravan at MTPLM (hard not to) then you have to reduce the maximum weight the car can take and vice versa. So you need to check your car's plated Gross Train Weight (GTW) and then add it's GVW and Towing Limit together if the two figures match, fine, if they don't and you've been travelling with a goodly load then you may be illegal and may have to think carefully about your actual weights. 



#31 FrankBullet

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Posted 14 September 2017 - 09:38 am

It's an interesting read this and all the links are very useful.

Generally, I find my outfit very stable; I can certainly feel a degree of 'pull' and 'push' when overtaking or being overtaken (typically hard-driven 7.5t vehicles cause the biggest issue) but nothing overly dramatic and as has been suggested I try to get as far away from a large vehicle as possible. I think the very short rear overhang of my car helps a great deal in this respect and I do put the awning over the axle, slightly forward.

So far I've only had one 'moment' and the only worrying thing for me is that I couldn't work out how it had been created - in a line of traffic on a 60mph dual carriageway I was in the outside lane overtaking slower cars on the inside lane; before I knew it my caravan was pitching left and right which, by the time I'd lifted off a little and firmed my grip on the wheel it was over - it's a piece of road I travel regularly with the van (it was on the way home from storage) and it was loaded in the same way as normal - no large vehicles or apparent crosswinds to be seen and all tyre pressures good; the experience didn't worry me but there was a nagging concern that I couldn't pinpoint what had happened. The only thing I could perhaps think of is that usually the bikes are on the roof of the car when I tow but it wasn't on this occasion and I wonder whether they help the air flow (if not my economy!) for some reason.

It's not done it since (touch wood) having travelled through France loaded up at an indicate 105km/h - I actually appreciated the experience as I've now 'felt' it which might sound odd!
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#32 Steamdrivenandy

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Posted 14 September 2017 - 09:51 am

What about the 3008's GTW?

 

Snaking is one of those things where people will swear blind that their rig is stable and even though they're slightly over in some way or other it's nothing to worry about, 'cos it's never happened. And sometimes the conditions never occur to make it happen and then the vanner thinks their policy is totally justified. Equally, all the elements can suddenly come together and cause a 'snake' and all of a sudden the reason for the limits and advice suddenly crystallise. 



#33 JTQ

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Posted 14 September 2017 - 10:02 am

It's an interesting read this and all the links are very useful.

Generally, I find my outfit very stable; I can certainly feel a degree of 'pull' and 'push' when overtaking or being overtaken (typically hard-driven 7.5t vehicles cause the biggest issue) but nothing overly dramatic and as has been suggested I try to get as far away from a large vehicle as possible. I think the very short rear overhang of my car helps a great deal in this respect and I do put the awning over the axle, slightly forward.

So far I've only had one 'moment' and the only worrying thing for me is that I couldn't work out how it had been created - in a line of traffic on a 60mph dual carriageway I was in the outside lane overtaking slower cars on the inside lane; before I knew it my caravan was pitching left and right which, by the time I'd lifted off a little and firmed my grip on the wheel it was over - it's a piece of road I travel regularly with the van (it was on the way home from storage) and it was loaded in the same way as normal - no large vehicles or apparent crosswinds to be seen and all tyre pressures good; the experience didn't worry me but there was a nagging concern that I couldn't pinpoint what had happened. The only thing I could perhaps think of is that usually the bikes are on the roof of the car when I tow but it wasn't on this occasion and I wonder whether they help the air flow (if not my economy!) for some reason.

It's not done it since (touch wood) having travelled through France loaded up at an indicate 105km/h - I actually appreciated the experience as I've now 'felt' it which might sound odd!

 

The multiplicity of the bow-waves from cars being overtaken as they track along the van's side could possibly create the right frequency to "excite" the unit? This seems quite common with for example car transporters that generate a load of bow waves.



#34 FrankBullet

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Posted 14 September 2017 - 10:29 am

Good point, especially as all the vehicles were in the lane to pull off so quite close to each other - I'll keep my eye out in future

Regarding the 3008, if it's a 1.6HDi (especially an EGS) or the Hybrid4 then the caravan will definitely be over the maximum towing weight never mind GTW, I would imagine it's a 2.0 in which case it could still be over the GTW - ours drops from 1550kg to 1350kg as you use the payload up.

Edited by FrankBullet, 14 September 2017 - 10:30 am.


#35 Paul_B

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Posted 14 September 2017 - 07:04 pm

I too would disagree with this. Its not just down to friction on the ground. Its down to how much lateral movement there is on each of the axles. In the case of a caravan, under-inflated tyres or tyres with weak side walls will allow this sideways movement which will enable a snake to rapidly start. A twin axle will be more stable because there will be less lateral movement on the tyres, and yes there will be increased friction when the van is turning.

 

The only snake I ever had was, I believe, caused by very weak tyre walls, and my advice to Alano would be to have a good look at the tyres themselves as many older caravans were fitted with tyres that were actually inadequate for the job. Fitting heavy duty (i.e. 8 ply)  trailer tyres will make a huge difference to stability in such cases (and are less prone to blow-outs and punctures). It could also be a problem with budget car tyres that allow a lot of lateral movement too. An easy way to check side wall strength is to make sure the caravan is hitched up, or the jockey wheel secured so it cannot move, and the try to push the back corner of the van sideways. If the back of the van will move sideways and the tyres flex when pushed, then think about a tyre change.

 

On my last caravan, the tyre pressures were incorrect in the handbook, and after a year the dealer agreed to pay for 3 new tyres. I was staggered just how thin the tyre side walls were; they were no thicker than a bicycle tyre.

 

One of the few positives about my current Bailey is that at least good quality tyres have been fitted, and thus the stability is excellent.

 

Interesting post and I've found the same.

 

My caravan tyres are rated R95 but as I had difficulty in finding some this time round I had commercial tyres at R104/102 fitted, these required a bit more pressure but the improvement is noticeable.

 

I've never had a snake (touch wood) and now I get very little sway when buses etc pass me :)







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