Guide to Fitting Spring Brakes on Trailers
It has been agreed with the transport industry representative bodies that the fitting of additional spring brakes to the third axle of a tri-axle trailer, (or a second axle of a tandem) where they already exist on two axles (one on a tandem), will be treated as an alteration generally not requiring inspection.
This is subject to consideration being given to the advice in the attached annex and to the trailer being less than 8 years old at the time of notification of the change.
Subject to consideration of the changes, VOSA reserve the right to inspect all trailers.
Modification of a trailer more than 8 years old will require an inspection as it is a legal requirement to bring the trailer up to a later standard as described in the attached annex.
Any modification to a braking system is required to be notified to VOSA on a form VTG 10.
Forms are available from a VOSA testing station, from Welcombe House 91/92 the Strand, Swansea, SA1 2DH, or from the VOSA website.
The completed form, detailing all the changes made should be sent together with the appropriate fee to the above address.
If an inspection is not required you will be sent an acknowledgement and a sticker to attach to the plating certificate to amend the DtP number.
If an inspection is required you will be contacted by the test station that you nominated on the notification form regarding an appointment.
Annex
Advice to operators modifying a Trailer Spring Brake Parking system
Operators fitting additional spring brakes need to consider the potential delay in parking brake operation, due to the increased volumes of air added to the park brake circuit needing to be released to apply the brake.
On most trailers applying the park brake control does not immediately apply the trailer park brakes. This is caused by the relay emergency valve in the circuit applying the service brakes first, when the red line is disconnected.
Instances have occurred where parked trailers have moved unexpectedly some time after the truck has left the trailer. Usually the red line has been disconnected but the park brake has not been applied. However, it is possible where the red line has been disconnected and the park brake is applied to get both service and park brake circuits to “balance” causing neither circuit to fully apply a brake. Movement of the trailer can occur, causing serious injury.
A similar situation can also exist during unconventional tractor/trailer coupling procedures. If the modifications are not carried out correctly, there can be a significant delay introduced into the depletion of air from the parking brake circuit allowing the combination and/or trailer to move.
Where additional spring brakes are fitted VOSA would recommend consideration of the following points before modification of the parking brake system.
All Trailers
Changing existing spring brake units with piston type units is likely to increase system efficiency without further modification. This is an easy “fix” since no additional piping or changes are necessary.
A quick release valve can assist with park brake response.
Pipe diameters should not be less than 10mm in the park brake circuit to maximize air flow.
Pipes should be routed to reduce their lengths to a minimum.
Manifold blocks, elbows, bends and joiners should be avoided, since all these fittings increase the restrictions in the pipe.
Utilisation of a relay valve (instead of a quick release valve) will help in increasing speed of park brake response. Signal lines in this case should be of small diameter to keep response times to a minimum.
Care should be taken to ensure there is sufficient clearance to accommodate additional spring brake units, particularly where space is limited eg. on a self-tracking/steering axle.
Operators should be advised to contact the trailer/brake system manufacturer or their agents, for advice on proposed alterations.
Trailers more than 8 years old (additional requirements)
If the trailer is more than eight years old before the date of the VTG10 application the braking system must be brought up to the current standard. This is a legal requirement and is irrespective of the alteration proposed, eg this may only be a tyre size change.
The main requirements are:-
An Anti-Lock system permanently powered through an ISO7638 connection and preferably with an alternative backup power supply. (To allow the ABS to operate when coupled to a tractor without an ISO7638 connector). This applies to all trailers of 3500kg and above (total axle weight in the case of a semi trailer or centre axle drawbar, and GVW in respect of a full drawbar).
Automatic Slack Adjusters.
A two line system meeting the requirements of C&U Regulation 15.
This can be a system operated by a Service and an Emergency line only or by Service, Secondary and Emergency lines where the Service /Secondary are a two into one system).
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