Braking Systems & VSB1 Compliance
The extra tare weight of an enclosed body means these trailers reach braking thresholds sooner than an open box trailer of similar cargo capacity.
The Governing Standard
Trailers with an Aggregate Trailer Mass (ATM) of 4.5 tonnes or less are covered by Vehicle Standards Bulletin 1 (VSB1), currently Revision 6. Braking requirements are set by GTM (Gross Trailer Mass) — the load on the trailer's own wheels once coupled — not ATM.
Braking Requirements by GTM
| Loaded GTM | Requirement |
|---|---|
| 0 – 750kg | No brakes required |
| 751 – 2,000kg | Braking on both wheels of at least one axle |
| 2,001 – 4,500kg | Braking on all wheels, plus an automatic breakaway system |
Because an enclosed trailer's own body (roof, walls, doors) adds tare weight a box trailer doesn't carry, it's worth checking the empty/tare GTM of a specific enclosed model before assuming cargo alone determines which bracket you'll land in.
Overrun (Mechanical) vs Electric Brakes
Mechanical overrun braking is fully compliant up to 3,500kg with no electric brake controller required. Electric or electric-over-hydraulic brakes are common on heavier enclosed trailers, particularly those set up as mobile workshops carrying substantial fixed equipment in addition to cargo.
Questions & Answers
Do enclosed trailers need brakes more often than box trailers?
In practice, often yes — the added tare weight of a roof, walls and doors means an enclosed trailer can cross the 750kg or 2,000kg GTM brake thresholds with less cargo on board than an equivalent box trailer would need.
What's GTM vs ATM for braking purposes?
GTM is the weight sitting on the trailer's own wheels once coupled — this is what VSB1 uses to set braking requirements. ATM is the total loaded weight including what's transferred to the tow vehicle at the coupling.