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I’m sorry everyone, but I am having a thick day. I just can’t seem to get my head around
Static Roll Thresholds.
Static adj. Stationary or inactive (of a force) acting but producing no movement
Roll n act of rolling over or from side to side
Thresholds n entrance, starting point; point at which a stimulus produces a response
The LTSA fact sheet #13 clearly states in paragraph three that, there are a number of new provisions, to improve the stability of heavy vehicles and to therefore reduce the risk of crashes due to poor stability.
I have read all the Fact Sheets on the subject (13 a,b,c,d,e,) I have attended meetings where obviously intelligent engineers have discussed the subject for hours.
I am told that a vehicle’s static roll threshold is a measure of the likelihood of that vehicle rolling over sideways. Vehicles with a low SRT are more likely to roll over when going around sharp bends and in sudden emergency manoeuvres.
Now with three million plus kay’s under my sizable belt and also my share of sudden emergency manoeuvres etched into my memory, I am a fan of anything that makes the truckers lot a safer one.
After thirty odd years in the game, I fully appreciate that the stability of goods service vehicles can be greatly affected by the way they’re loaded. The load needs to be arranged so that its mass and height are appropriate for the design of the vehicle and type of load.
I am reliably told that research into heavy vehicle roll-overs in New Zealand has shown that goods service vehicles with a GVM over 12 tonnes (class NC) and trailers with a GVM over 10 tonnes (class TD) need to have a SRT value of at least 0.35g (g is the acceleration due to gravity, and it’s constant).
If the vehicle doesn’t meet or exceed the required stability limit, then its stability can be improved by operating at a reduced load mass and/or reduced load height, or the vehicle can be modified so it offers sufficient roll stiffness to operate safely at maximum carrying capacity.
Now this last sentence is where I start to become concerned and more than a little confused, we are doing this to increase road safety and reduce rollovers, but we are allowed to modify the spring assembly so that that the magic figure of 0.35g can be attained.
So how is this wonderful modification being performed?
In Canterbury and I assume throughout NZ, by inserting a rubber block in the rear spring hanger to reduce the amount of travel by half of the spring and so provide the required roll stiffness.
Now I am just an old trucker and correct me if Im wrong, but wont this lack of travel in the spring assembly load up the suspension fairly quickly to the point were the inside wheels lift up, place all of the weight on the already compressed outside springs and tyres, while negotiating these tight corners and tip the vehicle over?
And what about metal fatigue, I’ll bet we see some horrendous chassis cracks in the not to distant future.
If this is “Best Practice, to improve the stability of heavy vehicles and to therefore reduce the risk of crashes due to poor stability.” then shouldn’t we think again?
But then, this is “just an old trucker’s point of view”
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