ARTSA 18th April 2008

The ARTSA Braking and Stability College recently considered a proposal to revise the upper compatibility limits in ADRs 35/02 and 38/03 which apply to unladen vehicles that have load-proportioning brakes. 

This matter is also of interest to the ITC and both ITC and ARTSA are co-operating in this consideration.

Australian Design Rules 35/02 and 38/03 were released in 2007. There were various significant changes, including the introduction of unladen  compatibility limits for vehicles that have load-proportioning brakes. The purpose of the limits is twofold:

The unladen compatibility limits apparently apply to any vehicle that has a mechanical load proportioning brake system.  They do not apply to an electronic brake distribution system unless it includes a mechanical proportioning valve.  

The compatibility limits are the same for both ADRs 35/02 and 38/03. They are shown below. 

The rules allow the unladen performance to be predicted by computation based on the validated laden compatibility performance. Usual computational practice is to not allow for load transfers so the computed performance could be out by ~10%.

A sensible adjustment approach is to set the load-proportioning valves so that the friction utilization of each axle group of the combination vehicle is about the same. This requires knowledge of the brake system on the mating vehicle. 

The ADR’s deal only with type-approval of one vehicle model and they cannot be applied to individual combinations. This is the fundamental difficulty. Whilst the state road safety regulations and the Australian Vehicle Standards Rules (AVSRs) do deal with combinations, the practice has been to swap trailers between trucks routinely which disturbs matching of brake set-ups.

Sample computations show that the upper limit in ADR Figure 2 prevents the valves being set to achieve reasonable friction balance on semi-trailers. This is potentially dangerous because a vehicle having a trailer with low braking capability could have a dangerous risk of prime-mover jack-knife. In fact there is not acceptable alternative to a computation of friction utilization levels being done on a case-by-case basis.

An accompanying document was provided by Bisiteknics.  It illustrates the issue for trailers. In the example a sensible load-proportioning setting is about 60%.  However, the rule limits the setting to about 40%. 

For semi-trailers, there are four situations that need to be considered, as shown in Table 1.

 

Table 1 is simplistic because it does not consider other destabilizing effects such as mixing of drum and disk brakes or threshold pressure imbalance. Furthermore, multiple trailer combinations could have a mixing of load-proportioned and non load-proportioned brakes on the one vehicle.

The case 2 in Table 1 is the most concerning because jack-knife is the most dramatic, sudden and uncontrollable form of poor dynamics. Case 3 is also a concern.  However, it is beneficial to have the trailer slightly over-braked so that it applies a stabilizing ‘brake from the rear’ force.   Hence case 3 is a more tolerable situation.

The proposed change to the brake rules (ADRs 35/02 and 38/03) is to amend the upper limit as shown in Figure 2. That is, to allow setting of load-proportioning valves at the 60% level. 

(a)

(b)

Figure 2. Proposed changes to ADR upper limit line (marked Proposal).

(a) Comparison of ADR, ECE and proposed limits for a semi-trailer.

(b) Comparison of ADR, ECE and proposed limits for a prime-mover.

The ECE semi-trailer limits were supplied by Mr Greg Burns (Airbrake Corporation) for the following trailer dimensions. The ECE regulation 13 has vehicle-specific limits 

The upper limit puts a restriction on the braking level for load-proportioned vehicles only. Non load-proportioned vehicles will have higher levels. Is an upper limit needed at all? If the setting is based on sensible computations then an upper limit is not required.  However, the rule does not give guidance about how to do this and furthermore, cannot enforce sensible settings on particular combinations. So respondents argued that the proposal line is too low.

On balance the upper limits in the unladen compatibility limits should be amended to limit the setting as shown in Figure 2. Without an upper limit there is no need to justify the setting that is used.

Additionally a normative guide should be included in each ADR to illustrate a sensible setting scheme. Sample computations should be provided for both  semi-trailer and tipper and dog combinations.  The purpose of this normative information is to guide manufacturers and modifiers as to a suitable rationale for setting load-proportioning valves. 

Some practitioners were concerned that the Vehicle Safety Services group requires computations for intermediate load levels and that this is onerous and unnecessary.  Intermediate computations might be justifiable if the models used for the computations are precise so that fine tuning of the setting is possible.  In practice, the valve will be set to achieve acceptable performance in the unladen condition.  The point of doing intermediate computations is apparently to ensure that the load-proportioning brakes do indeed produce a graduated adjustment as load is varied. If operators unload from the rear on a ‘milk-run’ delivery round then the setting may be inappropriate because of increasing brake imbalance as load is reduced.  

It is proposed that the following proposals be put to Vehicle Safety Services within Dept of Infrastructure:

That ADRs 35/02 and 38/03 be amended as follows: