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Drum or disc - that is the question

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Keeping vehicles on the road and not unnecessarily in the workshop is key to a successful operation. That&#;s why, when developing its braking systems, BPW has invested heavily. The result is high-performance, low-maintenance equipment designed to keep downtime to an absolute minimum and customers&#; vehicles on the road for longer. 

Trailer and tractor compatibility, as well as a vehicle&#;s working environment, are critical factors to performance and should be considered when selecting the brake for your operation. Here are some pointers to help you make the right decision. But remember, if you&#;re debating whether to go drum or disc, there is no clear, black-and-white answer. Everyone has his or her opinion. We&#;ve highlighted some key points so you are better informed when making that decision. 

COST: Cost is a major factor when buying and in most cases, it is one of the first considerations in deciding whether to fit drum or disc brakes. As well as the purchase price, you should consider the cost of replacement parts, maintenance costs, and labour time, i.e., long-term cost-effectiveness. 

While a disc brake has fewer replaceable components than a drum brake, the initial cost of purchasing a disc brake set-up is on average higher. The long-term advantage is that with modular components, there is less to replace, therefore servicing time is reduced. The cost of replacing modular parts for a disc brake is much higher when compared with a drum equivalent, with the calliper and disc accounting for the large majority of the cost. 

When comparing the BPW standard times for replacing equivalent components, the difference is negligible, and, of course, the standard times are just a guide. Experience would be the deciding factor when comparing the labour time to change components. 

As a result of the higher cost of disc brake components, the drum brake residual value is deemed higher, particularly when fitted with BPW axles. At the end of the trailer&#;s useful life, the owner may wish to sell it, and with the market favouring drum brakes, this will be easier. Another purchasing consideration when buying used trailers is the cost to renew components that have had a long service life, and, as mentioned previously, drum brake components cost less to replace.

OPERATIONALLY: Choosing the right brake is dependent on the day-to-day operation of the trailer. The disc brake has better braking efficiency at higher temperatures and is less likely to be subjected to brake fade issues. This is because as the disc increases in temperature, it expands toward the pads. When the drum becomes excessively hot, it expands away from the shoes and on rare occasions, can expand to the point where, no matter how far you press the brake pedal, the shoe will not make good enough contact with the drum to stop the vehicle in time. However, with the fitment of retarders/exhaust brakes, this issue is very rare in the UK. 

The disc brake set-up is fairly exposed to the elements, and dirt, grime, and water ingress from the road surface will have a bearing on braking efficiency and the maintenance intervals required. If a disc brake is used on muddy roads or tracks, the vent holes on the disc rotor can become blocked, causing the disc to become hotter much quicker and for longer periods of time. Drum brakes, on the other hand, are better protected from outside elements, blocking the majority of moisture and dirt, which is why they are a preferred choice when it comes to certain applications such as forestry and site work. 

If a trailer is part of a rental fleet and is underutilised, it&#;s not uncommon for the brake disc to become rusty. This is because a trailer can be stored for periods waiting to be leased, therefore, out-of-use maintenance is required to ensure the integrity of the disc and pads are not compromised, whereas for drums this is not an issue. Also, when leasing trailers, the make and model of the towing vehicle will vary, and discs are more sensitive than drums when it comes to the issue of trailer and tractor combinations. 

Most modern tractor units have Coupling Force Control (CFC) integrated into the EBS system. This is designed to give the optimum braking performance and compatibility between the tractor and trailer. From our experience and research, the CFC tends to be more sensitive, and in some cases, the trailer was found to take on more of the braking load than the tractor unit. This can result in the brakes running hotter and the discs and pads wearing out more quickly. The drum brakes&#; operating temperatures are also considerably lower, and this results in less thermal loading upon adjacent components such as tyres, camshafts, hubs, and hub bearings. 

MAINTENANCE COMPARISON: Brake shoes are sometimes sent for relining at the end of their lining life, which can bring down the cost of replacing the friction material in brakes. A disadvantage of sending the shoes away is that you may not receive the same shoes back. It is advisable to mark your own shoes so you know the ones you send are the ones returned, not someone else&#;s heavily aged shoe with no prior knowledge of its historical workload. Increasingly, operators fit new shoes with long-term operational reliability and economy for continued safety and longevity.

It is very simple to replace the pads on disc brakes and the back plate can be disposed of accordingly. The disc brake is much easier to configure during initial set-up and also after maintenance periods and pad renewals. There are more step and component checks on the drum brake when refitting new shoes compared to the disc.

SUMMARY: It has been highlighted that the disc brake is the more expensive option, both in terms of the initial purchase and the cost of replacement parts, yet it requires less maintenance if the brakes are used regularly. However, the drum brake has proven to be more resilient as it is better protected from the outside elements, and the life of the shoes and drum seems to extend beyond that of the disc and pads. While drum brake components cost less, remember there are more components to replace. 

Ultimately, it depends on the conditions the brakes will be subjected to and the workload encountered. There is no definitive answer when asking which brake setup to use, and opinions will always be divided.

**END**

16th January

For more information, please visit BPW Brake Pads manufacturer.

For further press information, contact Rebecca Wesley or Emma Makings-Hone at BPW Limited:

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BPW issues brake diagnostics advice

Roger Thorpe, engineering manager at axle and suspension manufacturer BPW, discusses some common trailer braking problems, and how to diagnose and rectify them

BPW&#;s technical services team is frequently called upon to help diagnose problems with trailer braking systems. Nowadays these are nearly all controlled by EBS (electronic braking system) and this can aid diagnosis of system faults.

Problem causes can be largely split into three areas: the foundation brake (disc or drum), the trailer brake control system (ABS or EBS) and the towing vehicle.

Very often the clues to the cause of the problem will lie in the condition of the friction material (brake linings or pads) or on the friction surfaces of the drums or discs. Sometimes the symptoms for both drum and disc brakes can be similar, but it must be borne in mind that as disc brakes run hotter than drum brakes, these symptoms will vary across the two systems.

Case 1: A trailer is presented for the annual MOT test and fails on brake performance; very often this is its first test. The driver has not complained about poor brakes, so the first thing that we would do is look at the condition of the friction material and friction surfaces. In most cases it will be seen that the friction surfaces are shiny, and that the friction material exhibits very little wear for the mileage that the vehicle has covered and also has a sheen on the surface.

This is a classic case of brake under-utilisation, which is sometimes referred to as glazing. With endurance braking (retarders and engine braking) included as standard on most modern towing vehicles, this is now a very common problem. Sometimes there are a couple of tweaks that can be done on the trailer EBS system, but very often the remedy lies in liaison with the towing vehicle manufacturer in order to get them to make an adjustment to the tractor EBS system.

People ask why this occurs at MOT time when the driver has previously not complained about poor braking. The answer to this question lies in the understanding of the braking pressures involved.

During normal driving the brake pressures delivered to the brake chambers are below 2 bar for 95 per cent of the brake applications. Even during an emergency stop the pressures are no more than 4 bar. During the brake roller test, the pressures are taken right up to 6.5 bar. Air pressure is directly proportional to friction surface pressure, and it is at these higher pressures that the under-utilisation effect takes place.

Occasionally we still come across cases where the vent plug bungs have not been removed when the brake chambers have been fitted to the foundation brake. This is especially troublesome with disc braked axles but also causes problems with drum braked axles.

Depending on which plugs have been left in place there can be either a pressure retained in the low pressure housing, causing a binding brake, or there can be a residual vacuum in the housing which has to be overcome by air pressure before the brake can be applied. It is this latter case which will manifest itself as under-utilisation.

Note that sometimes the bungs may be left in on only one or two wheel stations, causing the problem on those corresponding brakes.

Case 2: The customer complains of braking issues, normally poor brake performance. The trailer is two or three years old, and in these cases the problem has usually been reported by the driver.

The clue here is that the brakes have performed well until now, and the owner is asked what has changed. Is there a new driver or a new towing vehicle or is the trailer being used on different types of work? The normal response is that nothing has changed, but that the brakes have been re-lined a little while ago.

Further investigation discovers that different or non-original linings have been fitted and this has upset the brake balance. During design, the coefficient of friction of the friction materials is balanced to the metallurgy of the friction surfaces, and any deviation in these two components can cause problems.

Don&#;t forget that, during the manufacture of the trailer, a set of brake calculations is performed using the known parameters of the original equipment specifications of the friction materials. If this is changed then the brake performance will also change.

Case 3: A customer may have suffered with under-utilisation and has overcome the effect of this by fitting a different type of brake lining which has a higher coefficient of friction. All may have been well for the first few weeks or months, but then problems arise.

The heat generated by the higher coefficient of friction may cause the surface temperatures to rise on the friction material and friction surfaces, giving rise to surface cracking. Some surface cracking is normal and it is removed continually by the natural action of wear in the friction components; indeed this is evidence of a healthy braking system where the friction surfaces are a dull pewter colour and the friction material is a matt grey colour.

If cracks are generated too rapidly they become larger and join up, eventually forming long cracks which propagate through the brake disc or drum. In these cases the friction material will start to disintegrate because the resin that bonds the material together has been drawn out and burned off.

The remedy here is to replace the friction material and the drums or discs if they are too badly damaged, and then to investigate why the under-utilisation occurred in the first instance.

Case 4: The customer will call to say that they are unhappy with the life they are getting from the friction material or, worse still, that the brake linings or pads are burning out with excessive heat.

Normally these cases demand a visit from one of our engineers, ideally to see the vehicle before material has been replaced. The engineer will check the condition of friction surfaces and material on all wheel stations. If the issue occurs only down one side of the trailer, it may suggest a problem with one of the modulator valves in the braking system.

If the issue is only on one wheel station, then this may point to a problem with the brake chamber (remember the bungs) or the foundation brake itself. If the problem is uniform, then the engineer will look at the data in the EBS system.

Brake Calculation: The brake pressure control settings in the ECU will be checked, and if BPW has carried out the brake calculations, the parameters will be checked against those on the calculation sheet. The onboard data will be downloaded and checked for evidence of overloading, speeding, ABS cycling, roll stability activations and prime mover ESC (electronic stability control).

ABS cycling, where the ABS system continually sends test pulses down to the brakes because of high cornering speeds, can be a problem. This can be resolved either by driver training or liaison with the manufacturer of the system to eliminate any programming issues.

In cases where none of the above checks reveal a reason for the high component wear then you may have to resort to more high-tech testing to determine tractor/trailer braking balance.

Testing Developments: We used to perform these tests statically by installing pressure gauges and regulator valves onto the tractor/trailer combination, and monitoring pressures in the complete combination. With the widespread use of EBS, the braking pressures balance out dynamically when the vehicle is in motion, and we have developed our own testing equipment to measure this. ECE regulations require that the EBS prime movers must allow static threshold testing, but BPW have found this unreliable.

The towing vehicle and trailer are installed with pressure gauges, temperature gauges and accelerometers, and data is fed back to a laptop computer in real-time as the vehicle is driven on a typical route. Later, the data is analysed and imbalances in braking identified. When the cause of the problem is established, we can then work with the tractor manufacturer and resolve the issue.

Further information on the wear and damage characteristics of both disc and drum brakes can be found in the &#;Technical&#; section of the BPW website.

www.bpw.co.uk

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