The handling of light trailers, boat trailers, caravans and even road trains has been the topic of great discussion and debate for years. There is little comfort to the owners and operators in having Gross Vehicle Mass (GVM) and/or Gross Combination Mass (GCM) and being technically legal, when the handling stability is degraded and unsafe.
Background
Revisiting history, the B-doubles of the mid 1980s were typically eight axle tri/tandem, and to achieve weight distribution often the laden centre of gravity (CoG) of the rear trailer was often near the geometric centre of the tandem axle group.
As a result there were a few seemingly unexplained roll-overs. For B-doubles the introduction of tandem/tri and then tri/tri configurations mitigated some of the sensitivity of rear trailer loading.
The problem was the relationship between the laden centre of gravity and the ‘S-dimension’ (or drawbar length). The typical laden centre of gravity of a semi-trailer is in the range of about 55 to 65 per cent of the S-dimension.
However, there has been an issue with loading requirements, especially refrigeration where the laden CoG often exceeded 75 per cent. This, in combination with the fifth wheel located behind centre of the towing trailer axle group in combination with some suspensions, exhibited significant roll steer, dynamic stability was often impacted.
Recently, I had reason to drive a truck to Melbourne. It’s been a while since I’d driven a truck on a significant trip and it provided an opportunity to revisit that environment and make some on highway observations.
The average Joe motorist really does not give trucks the respect they deserve. Leave a bit of a gap for safe stopping and some lunatic who is either totally ignorant or has a death wish dives into the gap.
It would seem that the driver licensing system fails to educate, or is it simply impatience and the style of aggressive driving is their new norm and then authorities wonder why there are some many accidents.
Another observation is that there are more than a few caravans showing less than desirable dynamics. Maybe the caravans were hastily loaded to avoid the bushfires in central Victoria.
European Regulation R55 – Uniform provisions concerning the approval of mechanical coupling components of combinations of vehicles contains some specifics that not referenced in the Australian Design Rules nor (for light trailers) Vehicle Standards Bulletin #1.
Importantly, R55 includes a specific reference for a centre axle trailer (pig trailer) in, ‘the vertical load imposed on the coupling of the towing vehicle shall not exceed 10 per cent of the maximum mass of the trailer, or 1,000kg, whichever is the lesser’.
This is important for pig trailer design and loading. The definition of a pig trailer: ‘a trailer having one ‘Axle Group’ near the middle of the length of the goods-carrying surface is vague’. Near the middle is very subject to interpretation.
Trailers and caravans
In the case of boat trailers, an owner, who approached me, had been to the sales dealer who indicated that the trailer really needs an additional axle (and they could sell him a kit for $3000).
However, after weighing the boat and trailer (single axle) the weights were confirmed within ATM and GTM, but there was a problem the coupling vertical load which substantially exceeded the ECE R55 requirement of less than 10 per cent of the ATM, being upwards of 15 per cent.
The trailer axle position was adjustable, with a sliding suspension subframe.
The suspension was moved forward, the coupling vertical loading now being nominally 10 per cent; and importantly the owner, an MC license holder, is now very comfortable with on road dynamics and still has $3,000 in the bank account.
With the case of a caravan, it was an expensive exercise for the owner, who purchased a new caravan and from day one was concerned about the towing stability as speed increased trailing fidelity decreased to the extent that he was he limited towing to around 80 km/h. The dealer advised that heavier tow vehicle was required, so he purchased a Ram 1500 – no difference to the towing.
The independent suspension was identified as having near zero roll steer and the caravan weights were confirmed to be within ATM and GTM, but there was a significant problem, the coupling vertical load which was very low and the weight distribution dependent on fresh and grey water status was very low, around 2-3 per cent of ATM.
With some ballast 140kg, increasing the coupling load to around 6.6 per cent of ATM. The caravan is very stable at 100 km/h. The challenge now is to find a resolve to move the water tanks for improved distribution.
For centre axle trailers (pig trailers), too much vertical loading on the tow coupling is not good! This results in tow vehicle pitching. Where there is too little vertical loading on the tow coupling is not good! Results in laden CoG being near the centre of the axle group where any towed dynamic upset impacts on the trailing stability.
Ideal loads
What is the ideal coupling load? The general principles apply to both light and heavy pig trailers.
With many examples, a design target for light pig trailers is a coupled vertical loading of 8-10 per cent of the ATM.
For heavy pig trailers drivers should aim for 10 per cent of the ATM and if the coupling vertical load is going to exceed 1000 kg, the application is likely a tag (semi) trailer, consult a heavy vehicle engineer.
On heavy fixed drawbar pig trailers, it is also important to ensure that the coupling has the appropriate V-rating.
It has recently been highlighted that some trailers are fitted with weld-on towing eyes; a review of major suppliers suggests that weld-on towing eyes do NOT have a V-rating, as an operator, if in doubt, check with your coupling supplier.
Ultimately, regardless of light or heavy vehicles, the dynamic principles do not change and is an issue that all owner/operators of these vehicles, whether truck and trailers, light trailers, or even caravans, need to be cognisant of. Safe travels.
