Monday, Jul 13, 2015 at 13:26
Batt's
Further to my follow-up above, the speed limit is not the figure at which EVERYONE should travel. It is the maximum speed in favourable conditions that you're allowed to drive at.
Now, before anyone has a hairy fit, I'm not condoning the Mr Magoos of this world who drive unreasonably slowly, totally unaware or uncaring of the queue of traffic behind them.
But I am saying that "favourable conditions" embrace more than just the weather and road surface. Since this thread is about towing caravans, perhaps the most important aspect of "favourable conditions" is stability of the rig. Stability is affected by load distribution, speed and the geometry of your rig, among other things.
You can't do much about the geometry other than make an informed choice of tow vehicle and caravan and perhaps offset any ill effects with the addition of a weight distributing hitch, yaw damper,
suspension modification, etc.
Load distribution is affected by whereabouts in the van you put your heavy stuff - if you're conscientious you can control that. It is also affected by how the van was designed and where the manufacturer has placed built-in heavy stuff - something you cannot control except by knowledgeable choice of van.
So given that you've loaded your van as best you can to retain stability, there remains the issue of speed and its effects on stability.
This video clip shows two things:
One is that a van loaded optimally with weight close to the axle is inherently more stable.
The other is that a van that has less than optimal load distribution is still stable at lower speeds, but quickly becomes unstable at higher speeds.
Batt's, perhaps your van is inherently stable at the speed limit and will settle down quickly after a minor input like a gust from a passing truck. Or perhaps it is unstable and just waiting for something to set it off, as demonstrated in the last part of the video clip where the unstable model was trailing perfectly until it received a single small input that then made it uncontrollable.
This clip recently posted on this site demonstrates my point in real life. On that road it is reasonable to assume the truck was probably doing about 100, but it doesn't really matter. Whatever the speed of the truck, the car and van would have been going a bit faster, all nice and stable, and caught up with the truck. No doubt the car and van accelerated during the overtaking manoeuvre, perhaps not. Regardless, it can be seen that the van had already started to swing before it passed the truck. It was not upset by a quick return to the left lane. More likely it was upset by a change in pressure (read apparent gust) as it moved out of the truck's wake.
That just shows how precarious this stability thing is at speed. There may be absolutely nothing in reserve even for a planned manoeuver like a simple overtake, let alone an unplanned one like a swerve to miss a pothole or an animal or a person, or a gust from willy-willy or a passing truck in the opposite direction.
I think it is totally unreasonable to advocate that all van drivers should tow at the speed limit. (I'm talking highway limits here, 100 and over) It may be ok for some rigs, but I'd be willing to bet the farm that for most it is not, and is probably unsafe - even for you.
Cheers
FollowupID:
843249