Simple upgrades to keep your trailer brakes road legal

Year after year studies are published by organisations such as the Health and Safety Executive (HSE) and the British  Agricultural and Garden Machinery Association (Bagma) revealing that many farm trailers have brakes that fail to pull them up within the legal stopping limit.

Companies and individuals are responsible for ensuring that agricultural vehicles they drive are safe and in good working order – both on and off road – and that means ensuring brakes are working effectively and comply with the legal braking distances.

Any vehicle, trailers or trailed appliances stopped on public roads found to have defects would leave the driver and owner liable for prosecution by road traffic law enforcement agencies or the police.

See also: Video: Effective upgrades for top-class trailer brakes

The law – a few reminders

  • Maximum gross train weight 24.39t
  • Maximum speed 20mph (32kph) – only vehicles such as the JCB Fastrac and the Mercedes Benz Unimog with sprung axles front and rear may travel faster than this
  • Minimum braking efficiency – hydraulic brakes (up to 20mph): 25%
  • Minimum braking efficiency – air brakes (above 20mph ABS and dual-line failsafe system required – can also be hydraulic): 45%
  • Maximum gross weight of a conventional drawbar (unbalanced) trailer: 18.29t (even if the manufacturer’s plate says otherwise)
  • Maximum drawbar loading: 3t

But with farming margins tight as ever and machinery prices in a seemingly unending upwards spiral, a new trailer is probably going to be an expensive bet.

In an effort to improve things, Bagma has come up with a list of simple, (relatively) low-cost things you can do to upgrade your existing brakes.

Brake upgrade – Bagma suggestions

  • Larger (3/8in or 1/2in) diameter hydraulic brake hose £100-£150
  • Larger (25mm – 35mm) brake rams £120-£260
  • Replacement lever arms (including manual slack adjusters) £120-£150
  • Hydraulic breakaway failsafe system £300-£450
  • Parking brake improvements £100
  • Hydraulic load sensing £280-£450
  • Replacement commercial spec. axles £2,000-£2,500

We wanted to find out just what effect BAGMA’s recommendations would have, so we put them to the test.

We chose a 10-year-old 12t AS Marston Ace as our test candidate, as it’s been a perennial best-seller for decades and there’s a big fleet of them out there.

With 400mm diameter drums, 80mm wide shoes, 20mm rams and 1/4in pipework, it’s pretty standard ag spec.

1. Adjustment

Braking adjustment

Before touching anything we gave the trailer a run in its found-on-farm state. Unsurprisingly, it failed pretty convincingly, with just 14% braking efficiency.

The brakes on this standard spec 12t AS Marston Ace have a conventional lever arm with a split end to couple it to the drum’s splined cam shaft. This shaft twists to squeeze the shoes tight against the inside of the drum.

Thanks to regular maintenance the levers weren’t seized in place, so we were able to loosen them and knock them off, cranking the cam shaft right round with a set of Stilsons until the shoes made contact with the drum and then refitting the lever.

With that done we re-attached the ram using the right hole in the lever to get the angle between the two as close to 90˚ as possible, to allow the ram to exert maximum force.

Re-test result: 16.2% braking efficiency (+15%)

2. Bigger hoses

Larger hoses

Like many of its counterparts, new a decade ago, our test guinea pig had pretty lean 1/4in pipework. We decided to give it a serious upgrade, going straight for 1/2in hose and fittings.

If you get technical about it and work out your Pi R-squared sums, that equates to an increase in bore of almost 300% – a serious increase in oil flow.

Re-test result: 15.4% braking efficiency (-4%)

Not a good result – bigger hoses with smaller rams do not necessarily equate to better braking.

3. Bigger rams

Braking rams

Again pretty much standard fitment for a trailer of this vintage, 20mm rams look pretty puny when lined up alongside their 30mm equivalents.

We whipped off the old ones and welded these on in their place, being careful to maintain a 90˚ lever-to-ram angle.

Re-test result (1/4in hose): 33.6% braking efficiency (+107%)

A huge leap in performance, even with the old pipework.

Next attempt with the bigger hose:

Re-test result (1/2in hose) – 38.3% braking efficiency (+136%)

Breaking contacts

  • Totrax, Boston, Lincolnshire – 01205 280 578
  • TractAir, Brough, East Yorkshire – 01482 576 222
  • Erentek, Waddington, Lincoln – 01522 720 065
  • ADR Tyremart, Newark, Notts – 01400 283 820
  • BPW, Leicester – 0116 281 6100
  • GES, Peterborough – 01733 210 021
  • Granning Axles, (UK Sales), Warrington, Cheshire –  01925 817 689
  • Hunton Legg Saxmundham, Suffolk – 01728 663 010
  • J.H. Milnes Penistone, Sheffield – 07748 114100


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