As a brand, Mintex had traditionally enjoyed a close relationship with motorsport but in the early 1990s the focus of its research and development was on the evolution of friction materials for road use. While the majority of Mintex's resources were dedicated to this development programme other specialist motorsport brake companies created new friction materials. These had higher performance levels than had previously been seen and were soon adopted at the highest levels in international rallying, CART and NASCAR. Mintex manager Terry Armstead recalled:

'These new high friction materials gained popularity very quickly and became used globally. It was decided at this time to commit to a new programme of research to develop a new high performance range of Mintex materials.'

To meet and exceed the characteristics of rival products, the technical requirements set for these new compounds were stability and performance at temperatures in excess of 800° C. Levels that had never before been necessary for Mintex friction materials. Project leader Ian Mortimer observed:

'Our existing range of competition materials had been engineered to operate at a maximum of 600-650° C and to produce consistent pedal feel. The F-Series materials were designed to retain the stability and positive pedal modulation of those earlier compounds, but also to operate at a high co-efficient of friction and much higher temperatures.'

Like its competitors' products, the new Mintex material has a ferrous and carbon base. Steel fibre and iron powder form the ferrous content of the friction matrix which works on the principle that rubbing together like materials - in this case ferrous-based brake pads and ferrous brake discs - produces the highest levels of friction. Carbon and graphite powders are added for high temperature lubrication.

After more than two years of intensive development, Mintex launched it's F-Series, a range of four new semi-metallic carbon-based material specifications.

Design Goals

After achieving the targets of performance at high temperatures, stability, repeatability and low taper wear, another design goal for the new material was enhanced release characteristics which translate into pedal feel. Essentially this is the ability of the material to react to a reduction in pedal pressure input and translate it into a reduction of torque on a proportional basis. This is a difficult characteristic to achieve in a racing brake system operating at high temperatures, but it is one that Mintex claims for its new materials.

Mortimer:

'What we have is a range of materials that offer high torque, where that is required, with a level of control not found in any other competitors pads. The drivers that have used our material have commented that it's substantially smoother than anything they have driven and offers superior deceleration rates.'

'We also benefit from a by-product of the superior release characteristics in those applications that are bias sensitive to the rear, such as Winston Cup, Trans-Am or any vehicle that has a live axle rear end or a rear application for front wheel drive. Such cars fitted with our material are able to utilise substantially more rear brake, if the car is set up for it...'

'We have also found that F1R and F2R are much kinder to the disc. If you can keep the disc in good condition, that is with a smooth polished surface, then the emmissivity (rate of emission of heat) of the disc is greatly improved and you can actually get temperature out of the disc faster. You can generate the same torque output but with a lower overall temperature. That's something we discovered in back to back testing of our material against those of our competitors. We were generating higher torque but at 100-200° F lower disc temperatures.'

Manufacture

A competition friction material is targeted at a theoretical efficiency of one, (where the amount of energy extracted is equal to the energy input) and performance is the highest priority. An OE pad for road use also has to cater for the additional considerations of pedal feel, noise and durability and is therefore typically made from a larger number of ingredients.

There are also great differences in the manufacturing processes. OE manufacture, which can involve huge production runs of 50,000 or more of the same item is highly automated with a high degree of quality control. Manufacture of competition pads is typically a more selective process yet quite conventional although the temperatures and pressures used in the process are greater than those for OE pad production and there are even finer tolerances.

F-Series competition pads are made from a blend of a number of individual raw materials, the names and proportions of which Mintex is understandably reluctant to divulge. This mixture is poured into a die and moulded under pressure that varies according to pad thickness. After a heat process to cure the resin contained in the mixture is completed and the pads are cool, they are machined with the required slots and chamfers to accommodate taper wear, before they are painted and sent for final face grinding of the working surface. The most important aspect of the manufacturing process is to ensure even pressure distribution across the pad when in use. There can be obvious variations in the aspect ratio (length relative to height) according to the intended application for a pad. On shorter pads, even pressure distribution is relatively easy to achieve. On longer pads it is even more important as a slight variation in material or pressure distribution can drastically affect performance. To minimise this, longer pads, such as those used in six-piston calipers, receive an initial face grind, before the pad is turned over and the backing plate is machined to ensure that it is parallel to the working surface of the pad. After this the pad is turned again and the final face grind is carried out.

Testing

Development testing of the F-Series materials took a little over two years and involved dynamometer testing of over 200 different compound variations.

This programme presented its own set of challenges. Mortimer:

'The facilities we had to test materials at our R&D facility in Cleckheaton, UK were limited. We didn't have a high-speed dynamometer testing capability and that was the biggest pitfall in early development. We couldn't test materials at the same rates and under the same regimes in which they would operate on a vehicle. In the back to back tests we did in the UK with materials from our competitors, the results were virtually identical. It was only when we got to test our materials on a high-speed dyno in America, where we were able to add higher inertia levels and higher temperatures, that flaws were revealed. We quickly discovered that unless you test under the conditions in which the material will be used, you don't get the full story.'

Dynamometer testing is based on the so-called 'matrix' test. In this, pads are bedded to a new rotor and the first several stops are used to put temperature into the system. Mortimer:

'Once the system is heated to the required level we do a matrix test over a range of speeds, temperatures and pedal pressures - high speed, high pressure; high speed, low pressure; low speed, high pressure; low speed, low pressure. The goal is consistent repeatable performance from the material no matter what speed, pedal pressure or temperature.'

Dynamometer testing is also used to conduct high temperature wear tests which start with a minimum brake temperature of 600° C. Such tests are useful in simulating the most rigorous conditions likely to be encountered such as in the NASCAR Winston Cup series races on the 0.5-mile Martinsville oval. The cars brake every 11 seconds and typically maintain brake disc temperatures consistently above 700°.C. for three hours or more on cars that weigh 1.5 tons. Mortimer:

'That's probably the most extreme use that brakes go through in racing.'

Friction material development eventually comes down to feel, a nuance that is only discernible in track testing. Mortimer:

'The dyno will get you 80% of the way to the final material; at that point you have to get on-car testing going. You need feedback from the team and driver. That fine tuning for consistency is one of the biggest challenges.'

The Mintex F-Series Product Range

F1R

An ultra-high friction material suited to the highest forms of racing where high mechanical grip is combined with high levels of aerodynamic downforce. Used in CART, Formula 3000 and similar series. F1R is claimed to offer high friction output at all temperatures coupled with a level of feel and control never before offered in a racing brake pad.

F2R

A medium torque material designed for high speed, high horsepower applications with little overall aerodynamic downforce. Currently being raced in NASCAR Winston Cup and Craftsman Truck in the USA, TVR Tuscans and (as of 2001) ASCAR in the UK.

F4R

A user-friendly material that has been engineered and developed for high-end rallying and tarmac club racing. Many of the cars competing in the UK National Saloon Car series use F4R. This being a series where vehicle weights are relatively high but brake discs and calipers small.

Using the F-Series Material

Mintex F-Series friction materials are capable of running at high temperatures but it should be born in mind that the use of such material could put extra strain on the other parts of the braking system. It doesn't matter how good a friction material is if the fluid in the system is boiling and the driver is getting a spongy pedal. It is critical to ensure that proper cooling is utilised. In applications that are exposed to high temperature it's imperative that the brake fluid - dedicated high temperature racing fluid as recommended by the caliper manufacturer - is replaced regularly. Weather, also has an influence. In wet or high humidity conditions, fluid should be replaced every day during the race weekend. The hygroscopic nature of brake fluid means that it will absorb atmospheric moisture which significantly reduces the boiling point of the fluid. In the CART series the teams change the brake fluid for every practice and qualifying session and for the race. Competitors in other series may find that they are able to change fluid less frequently.

For Group N or production based classes using production based calipers, it is strongly urged that mild-steel brake pistons are removed and replaced with stainless steel or titanium alternatives. Cutting slots in the face of the pistons so that air is able to vent to atmosphere is also recommended. A production piston is typically hollow and if it is not ventilated for use with the F-Series material the air inside the piston becomes superheated from the backplate of the brake pad and this heat transfers to the fluid.

Follow the supplied Bedding Guidelines and not bedding procedures based on old technology. Failure to do so could result in brake component overheating and possible failure. Bedding Mintex F-Series material requires 12-24 short hard stops while gradually building temperature. To do this without overtaxing the system, the stop should be done in a low gear to ensure that the energy input is limited. Accelerate to high rpm and apply the brakes in a short hard snub. Once a predetermined lower rpm level is reached the driver should accelerate back to high rpm immediately and repeat the procedure a further 8-10 times. After this a lap can be driven to test the brakes. If necessary the entire process should be repeated. On returning to the pits the brakes should be allowed to cool and the system bled before further use. It is stressed that when carrying out the bedding process, the brakes are not dragged or the car taken from very high speed to very low speed or a complete stop. To do so may result in damage to or failure of the brake system.

Mintex and the Motorsport market

In 1925 Scandinavia Belting Ltd changed its name and the Mintex brand name was born. Under its original name the company had been a supplier to the automotive industry from its earliest days and its biggest customer was the Ford Motor Company which specified Scandinavia transmission linings for its epochal Model T.

Following the patriotic name change, a competition was held to come up with a new brand name for BBAs brake linings. Fortunately the winner was Mintex (standing for MIN-eral TEX-tile) and not Brakefast or Stopit (other genuine entries). Competition success soon followed, Bentley's victory at Le Mans in 1929 with Mintex brake linings, was the first of 11 wins in the classic 24 hour race.

In Formula 1 Mike Hawthorn and Jack Brabham between them won three World Drivers Championships on Mintex brake linings. While ferrous brake discs were still in use, the company remained a supplier to Formula 1 teams, but eventually carbon-carbon brake discs and pads took over the F1 market in the 1980s. In rallying Mintex established a strong reputation and went on to sponsor a round of the British series and eventually its own championship.

'Then motorsport began to take to the world stage and the component business began to expand. In 1989 a new friction material compound, based on a carbon composite material, was introduced to the motorsport market. It had much higher performance characteristics than anything else available with its stability at high temperatures and almost twice the co-efficient of friction of conventional competition compounds. This new material, manufactured in the USA, created a new standard at the higher levels of international motorsport.'

The emergence of this new genre of material propelled Mintex and other manufacturers into a fundamental change in research and production practices. We see the results today.