All About Brakes

[skylight]

Here are some views from the other side:

Those Poor Rotors!
by James Walker, Jr. of scR motorsports

Let's look at some common rotor `modification' and `performance' upgrades that you may have been exposed to and try to separate the marketing from the engineering...

Super Sizing

Bigger rotors will make your friends think you are cool, bigger rotors look sexy, but bigger rotors do not stop the car. What a bigger rotor will do is lower the overall operating temperature of the brakes - which is a GREAT idea IF your temperatures are causing problems with other parts of the braking system. Take for example a F500 racecar - a small 800 pound single seat formula car. While the brakes are certainly much smaller than those found on a 3,000 pound GT1 Camaro, that does not necessarily mean that they need to be made larger. In fact, swapping on a GT1 brake package would probably do more harm than good - that's a lot of steel hanging on the wheel that needs to accelerate each time the `go' pedal is pushed. So, the motto of this story is bigger is better until your temperatures are under control. After that point, you are doing more harm than good...unless you really like the look (and hey - some of us do!).

Crossdrilling

Crossdrilling your rotors might look neat, but what is it really doing for you? Well, unless your car is using brake pads from the 40's and 50's, not a whole lot. Rotors were first `drilled' because early brake pad materials gave off gasses when heated to racing temperatures - a process known as `gassing out'. These gasses then formed a thin layer between the brake pad face and the rotor, acting as a lubricant and effectively lowering the coefficient of friction. The holes were implemented to give the gasses `somewhere to go'. It was an effective solution, but today's friction materials do not exhibit the same gassing out phenomenon as the early pads.

For this reason, the holes have carried over more as a design feature than a performance feature. Contrary to popular belief they don't lower temperatures (in fact, by removing weight from the rotor, the temperatures can actually increase a little), they create stress risers allowing the rotor to crack sooner, and make a mess of brake pads - sort of like a cheese grater rubbing against them at every stop. (Want more evidence? Look at NASCAR or F1. You would think that if drilling holes in the rotor was the hot ticket, these teams would be doing it.)

The one glaring exception here is in the rare situation where the rotors are so oversized (look at any performance motorcycle or lighter formula car) that the rotors are drilled like Swiss cheese. While the issues of stress risers and brake pad wear are still present, drilling is used to reduce the mass of the parts in spite of these concerns. Remember - nothing comes for free. If these teams switched to non-drilled rotors, they would see lower operating temperatures and longer brake pad life - at the expense of higher weight. It's all about trade-offs.

Slotting

Slotting rotors, on the other hand, might be a consideration if your sanctioning body allows for it. Cutting thin slots across the face of the rotor can actually help to clean the face of the brake pads over time, helping to reduce the `glazing' often found during high-speed use which can lower the coefficient of friction. While there may still be a small concern over creating stress risers in the face of the rotor, if the slots are shallow and cut properly, the trade-off appears to be worth the risk. (Have you looked at a NASCAR rotor lately?)

Too cool!

Last year we bought 4 rotors. Two were bone stock, and two were subjected to a process know as Cryogenically Treating - one of the high-tech buzzwords floating around the paddock. The rotors were run back-to-back on the same track on the same car on the same day with temperatures taken to make sure that they saw the same level of heat. Following the track session, the parts were removed and we had them literally dissected by a materials lab.

The testing conducted included surface hardness, grain structure analysis, density, and surface scanning with an electron microscope. Guess what - after seeing the heat of use, the rotors looked identical in every regard. This is not to say that there is not a benefit from treating other parts which see lower temperatures and/or have different material properties, but treating our rotors on our car showed no tangible benefits (note that it didn't seem to hurt anything either). Come to your own conclusions, but in our case, we'll pass.

Summary

So, what's the secret recipe? Again, there is no absolute right or wrong answer, but like most modifications, there are those which appear to be well-founded and those that `look cool.' If ultimate thermal performance is your goal, look to what the top teams are running (relatively large, slotted rotors). However, if `image' is your thing, break out the drillpress - and be prepared to replace your brake pads on a regular basis.

[skylight]

Does a disc need to be cross drilled and slotted?
Not necessarily. But in order to create a street performance disc suitable for the widest range of applications, we've decided to incorporate both the features into every DBA Longlife Gold manufactured rotor.

Let's talk about cross drilling first.
When the friction surfaces of a rotor are smooth and flat, there is no means of escape for the gases and dust which build up between pad and rotor. This is not a huge problem in normal motoring, but is an important consideration in street performance applications. These contaminants tend to "wedge up" and reduce braking performance. The bigger the pads and the higher the braking temperatures, the more likely the problem.

The drill holes (which are sometimes called "gas relief openings) provide an exit route for the dust and gas. The holes are also commonly labelled "cooling holes" because of the improvements they make in this area. Better cooling means less fade during repeated heavy brake application. Obviously, the holes reduce mass. They also help dissipate water when driving in poor weather.

What does slotting do?
Slotting increases the bite of the pads. This helps pull the car up quicker. The process doesn't involve removing as much metal as cross-drilling, so it doesn't result in as great a weight saving. However, slotting is even more effective than cross-drilling in combating the problem known as "out-gassing". This is when, at very high braking temperatures, the bonding agents used in some brake pads produce a gas. Under extreme conditions, this gas can create a pneumatic cushion between pad and rotor, giving a driver a normal pedal feel but reducing the amount of friction being generated. The slots pump away gas and restore full contact.

The "micro-shaving" effect of the slots also serves to de-glaze the pads (this is why the edges of the slots on DBA Longlife Gold rotors are not chamfered or "radiused"). It also tends to even out the wear across the brake pad faces, increasing the effective contact area. This can extend rotor life.

Do the slots need to be left-handed and right-handed to work properly?
Conventional wisdom once said so - and also dictated that all slots to be swept back from the centre in the same direction. However, with computer modelling we've managed to get slots on DBA Longlife Gold rotors to work in combination with the cross-drilling. This enables us to use a minimum of four grooves arranged in a mirror-image pattern. Such an arrangement saves the buyers the additional expense and inconvenience of having non-interchangeable left-handed and right-handed rotors.

Is a cross drilled-and-slotted disc brake rotor weaker?
Removing any metal from a rotor can potentially weaken it; working in the DBA Longlife Gold rotor's favour is Disc Brakes Australia's policy of safe, conservative designs (our discs are even over-engineered, some suggest), and the basic strength of the cast-iron alloy we use.
Some potential buyers have expressed concern about the likelihood of cracking. This is reasonable, as even standard factory rotors can suffer cracking under extreme use or abuse. Proper bedding of both rotors and pads should minimise the risk of cracking. Logic suggests that a drilled surface is even more exposed to the problem.

The weakest or most crack-prone part of the rotor is the outer edge, which in the normal course of expanding and contracting with heat, endures more movement than the centre of the disc, which is obviously smaller and is reinforced by the central hub or "hat". This is why DBA Longlife Gold road discs do not follow the practice of some racing rotors which have holes right at the outer edge or run slots off the edge of the disc.

Reports from a few owners confirm the occasional appearance of very small cracks around the holes. These were expected; they are caused by localised stresses and in no way detract from the reliability, durability or "stop-ability" of the disc. Some rotors have been returned with more serious structural cracking; however the number equates to a ratio of just 18 rotors in every 10,000 shipped. And of these, most were used in motor sport applications, something they were not designed or warranted for. To be frank, most of these owners would have destroyed the standard rotors under the same conditions; they were simply asking too much of their standard braking system and should have upgraded to bigger calipers and purpose-built motor sport rotors.

How much metal do we take away with cross-drilling and slotting?
On a typical big Aussie six we take away 180 grams from each disc, or less than 2 per cent of its total weight. Multiply this by four and you have a total vehicle weight saving of 720 grams.

Although this is the sort of gain that a Formula 1 team would spend thousands of dollars attempting to achieve, it is not enormously relevant with a conventional car in normal road conditions. That's one reason we have not over-stressed the benefits of lower mass in our marketing. The other reason is a fear that people will mistakenly associate "lighter" with "less safe".

Can I expect dramatically improved braking?
DBA Longlife Gold rotors will deliver better braking performance than conventional discs, but in most cases the improvement should probably be described as "significant" or "worthwhile" rather than "dramatic".

There are several reasons why it is difficult to quantify exactly how much performance improvement DBA Longlife Gold rotors can bring in typical road applications. It varies with the model of car, its kerb weight, the type of pads used, the type of braking system into which the new rotors are being incorporated and the inherent effectiveness of the standard rotors being replaced. Testing every type of vehicle under all conditions is clearly not possible. Nevertheless, we have tried a wide sample in varied situations. We have also interviewed as many owners as possible and are yet to talk to one who hasn't been pleased with the improvement.

To get an additional expert view, Disc Brakes Australia commissioned respected advanced driving instructor (and three-time NSW Hillclimb Champion) Peter Finlay to conduct a series of independent tests. Finlay - the proprietor of Nationwide Defensive Driving School - is comparing the stopping distances and general brake performance achieved with DBA Longlife Gold and conventional "factory" rotors under identical conditions.

During his most recent tests, using Sydney's Oran Park Raceway as a closed and safe environment, a current series Holden Commodore fitted with the DBA Longlife Gold product averaged a theoretical efficiency of 97 per cent over ten stops from 100 km/h. It recorded a best stop of 38.6 metres. The average stopping distance from 100km/h was 40.4 metres. This compared with 42.6 metres (90.25 per cent theoretical efficiency) for the same car tested under similar conditions with standard rotors.
On both occasions, the car was fitted with similar brake pads, tyres and wheels. Finlay was the sole driver throughout and supervised the testing procedures to ensure a valid comparison.

The Finlay report noted that the DBA Longlife Gold rotors provided greater consistency from stop to stop and delivered good pedal feel. The ability to pull up the car 2.2 metres, or 5 per cent shorter is very encouraging. Such a distance could easily be the difference between an accident and a near miss. However, it can't be classified as a universal gain. Some cars may demonstrate less of an improvement with DBA Longlife Gold rotors, while other cars or perhaps other testing conditions might reveal a bigger performance gain.

We feel, for example, that the DBA Longlife Gold advantage would have been increased even further if the speeds were higher and the cool-down period between stops was reduced. On the other hand, the way the tests were conducted and the speed chosen - 100km/h - is entirely relevant to everyday road requirements, as was the choice of Australia's best selling car for the test.

Are there trade-offs?
When a car-manufacturer designs a disc rotor, their aim is to please the "average" owner, placing high priority on such things as quietness, durability and low production costs. When enthusiasts change their cars to improve performance they always have to accept some trade-offs. After all, there's no such thing as something for nothing.

In this case, the main trade-off for improved braking is a higher purchase price. The extra cost is due to there being much more production work in manufacturing cross drilled-and-slotted rotors. The specialist nature of the product also adds to the expense, reducing production economies of scale. However, we've done our best to keep the price premium as small as possible, using low-volume manufacturing techniques. Customers consider that the additional cost of the new DBA Longlife Gold rotors is justified by the performance increase, and are also won over by the appearance of the new rotors. In an emergency situation, every buyer will appreciate the extra stopping power even if they rarely drive hard enough to utilise it.

As any driver knows, the ability to pull up as car even one metre sooner can save a life.

Something to note about slotting: as the rotor wears, the slotting becomes shallower and therefore less effective. The DBA Longlife Gold slots are tailored to each rotor type, but typically they are about 1.5mm deep x 3mm wide. By the time the rotor has reached "minimum thickness" (we are careful to point out that the slots are not intended as a wear indicator!), there is not much slot left. To make them deeper would risk weakening the rotor.

[skylight]

In contrast, the holes remain effective in "out-gassing" for the whole life of the disc. As with slots, the pattern is unique for each rotor type; typically we have around 36 holes per disc face, each with a diameter of 6.5mm. To counter the tendency for cracks to form between holes, we normally put just one hole per vane on ventilated discs. This ensures the vane rib will act as a barrier between the holes. Each hole is chamfered around its opening to reduce pad abrasion and provide a smooth transition between hole and friction area (the latter to help combat surface cracking).

The feedback from our customers suggested that in certain circumstances a clicking noise occurs. We'd never noticed it; further research showed it is the sound of the pads running across the holes.

Are drilled-and slotted- rotors hard on brake pads?
A more powerful engine uses more fuel, and it stands to reason that if brakes do more work, or generate more energy, they must suffer more wear. In the original planning stage we predicated a 10 per cent increase in pad wear. After all, the holes and slots would be abrasive on the pads and, with less metal-to-pad friction area, we expected more as well. Yet the on-road results have surprised us. In normal usage, pad wear is generally no greater, and rotor wear may be extended.

Testing on high-mileage taxis over the past 12 months has shown a totally unexpected benefit. One leading taxi company using DBA Longlife Gold rotors on its fleet has reported that they can last up to 150,000 kilometres with no machining.

This is three times the company norm, and has been achieved without any additional pad wear (the pads continued to be changed at the normal 25,000km intervals). The reason for the improvement relates to the shaving effect of the slots, which ensure improved contact between pad and rotors, plus the lower running temperatures.

Should high-mileage DBA Longlife Gold rotors require machining, this must be performed on a modern, high-speed, low-feed brake lathe. The correct procedure is to machine from the hub to the outer edge, taking 0.25MM (.001") in each pass. A word of caution: Machine in the one direction only; when feeding back, withdraw the tools and recommence from the hub to avoid tip damage. Repeat until required finish is achieved.

Cross-drilling and slotting removes not only weight, but also friction surface (or "swept" area). The figures are relatively small and the enhanced performance shows that the reduction in swept area is more than made up for by the improved ventilation that cross-drilling and slotting achieves.

For the record, the four disc rotors on a typical big sedan have a total swept area of 351.5cm2. With the cross-drilling we remove 22.9cm2, while the slots take away another 5.4cms. That's a total of 28.3cm3, or 8.1 per cent of the surface area.