Here’s a question we get asked, and even some of our own team have scratched their heads over it: if you have two safety chains rated at 2 tonnes each and you cross them over, does that give you 4 tonnes? No. Each chain has to be strong enough to hold the whole trailer on its own. Here’s what the rules say.
The short answer: chain ratings don’t add up
The national design rule for trailer connections, Australian Design Rule 62/02, says that trailers over 2.5 tonnes ATM must have two safety chains, and “each safety chain must meet separately the strength requirements” (clause 14.3.1.1). For trailers up to 3.5 tonnes, that requirement is a chain complying with AS 4177.4.
So the second chain is a backup, not extra capacity. Your trailer’s ATM (Aggregate Trailer Mass, the most it can weigh fully loaded, shown on its plate) must be covered by each chain’s rating, not by the two ratings added together.
How many chains, and what rating?
Vehicle Standards Bulletin 1 (VSB1), section 9.2, sets it out by ATM:
- Up to 2,500 kg ATM: at least one safety chain complying with AS 4177.4, or a safety cable with the same certified load capacity.
- Over 2,500 kg up to 3,500 kg ATM: two safety chains complying with AS 4177.4 (or cables with the same certified rating).
- Over 3,500 kg ATM: two chains of Grade T steel to AS 2321, and each chain’s minimum breaking load must be greater than the trailer’s ATM.
Reading the stamp on the chain
AS 4177.4 chain is marked with the maker’s or importer’s identification, “4177”, a hyphen and two digits, repeated at least every fourth link. “4177-35” means a 3,500 kg rating. VSB1’s selection table matches the chain to the trailer:
- 1,000 kg chain: trailers up to 1,000 kg ATM
- 1,600 kg chain: up to 1,600 kg ATM
- 2,500 kg chain: up to 2,500 kg ATM
- 3,500 kg chain: up to 3,500 kg ATM
Grade T chain for heavier trailers is marked T, 8, 80 or 800. VSB1 also notes T-rated safety chains shouldn’t be welded.
Worked examples staff can use
- Two 2-tonne chains: the trailer’s ATM can’t be more than 2 tonnes. Crossing them doesn’t make it 4 tonnes.
- 2,000 kg box trailer: one chain stamped 4177-25 meets the rule. A second chain is fine as extra security, but isn’t required at that ATM.
- 3,000 kg caravan: needs two chains, and each one must be rated for 3,000 kg or more, so in practice two 4177-35 chains. Two 4177-25 chains don’t qualify, even though 2,500 + 2,500 is 5,000.
- 4,000 kg trailer: over 3,500 kg, so AS 4177.4 light trailer chain is out. It needs two Grade T chains, each with a breaking load above 4,000 kg. VSB1 gives the minimum as 6 mm chain with a 5,100 kg breaking load.
Shackles count too
A chain is only as good as what joins it to the car. Transport Victoria recommends load-rated D-shackles complying with AS 2741, with a break load limit at least 1.5 times the trailer’s ATM. Its examples: a 750 kg trailer needs a shackle rated to at least 1,125 kg, and a 1,000 kg trailer at least 1,500 kg. By the same rule, a 3,000 kg caravan needs shackles rated to at least 4,500 kg each.
Transport Victoria also says it doesn’t recommend lifting hooks (clevis, Mawby, sling or eye hooks) as a towing connection, because those lifting-style hooks aren’t designed for towing. That is different from manufacturer-rated towing safety-chain hook kits (for example Austlift’s G80 and G100 sets, which Austlift states to AS3776-2015, AS4177.4 and ADR 62/02, with published ATM tow capacities and breaking forces). VSB1 and Circular 0-1-3 still describe connection devices mainly in shackle terms and are largely silent on those kits — the written standard lags industry practice. Silence is not the same as “hooks are illegal.” Austlift publishes ATM tow capacity with a stated 4 to 1 break factor; that ATM figure already includes their safety factor.
Our shop default remains a rated Grade S shackle matched to the chain and ATM. If you’re looking at a Mawby-style or Austlift hook kit, check the maker’s towing/ATM rating, the markings, and that it suits your chain size, then ask us in store for an estimate against your trailer plate.
Why cross the chains?
If you use two chains, Transport Victoria says to cross them under the drawbar before attaching them to the tow vehicle. That helps the trailer keep its direction and makes the drawbar less likely to hit the road if the coupling comes off. Crossing doesn’t change their rating.
Chain length and attachment points
Length
The documents we checked don’t set a length in millimetres. As a practical guide, the chains should have enough slack to turn tight corners without pulling tight. They should be short enough that they don’t drag on the road, and that the drawbar can’t drop to the ground if the coupling lets go.
Where they attach
Under VSB1 and ADR 62/02, the chain attachment points on the trailer must be on a substantial structural member, as near as practical to the coupling. With two chains, there must be one each side of the drawbar’s centreline. Each point must take a pull equal to the full ATM along the drawbar and half the ATM upwards. That’s tested for each chain separately, which is the same “each chain on its own” idea.
FAQ
Can I use a safety cable instead of chain?
Up to 3,500 kg ATM, yes, if the cable has a certified load capacity at least that of a compliant AS 4177.4 chain (VSB1).
My chain isn’t stamped. Is it OK?
Check with us before you tow. VSB1 says you need evidence that a chain meets the standard, and an unmarked chain gives you no way to show its rating.
Does my trailer need chains if it has breakaway brakes?
Trailers with a rigid drawbar need safety chains whether or not they have breakaway brakes (VSB1).
Sources
- Vehicle Standards Bulletin 1, Revision 6 (Department of Infrastructure), section 9.2
- Australian Design Rule 62/02, Mechanical Connections Between Vehicles, clauses 14.3 and 14.4
- Transport Victoria: Caravans
- AS 4177.4 (safety chains) and AS 2741 (shackles) are paid Australian Standards. The requirements above are as quoted in VSB1, ADR 62/02 and Transport Victoria.
This is general information only. Always check your trailer’s plate, the markings on your chains and shackles, and the current standards.
Want your chains and shackles checked against your trailer’s plate? Call us on 1800 486 922 or visit Ballarat Trailers & Towbars, 4 Ballarat-Carngham Rd, Alfredton VIC 3350.
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