Brake Kit Upgrades for Racing Applications

Really great brakes are like an upgrade to your confidence on the track – more deceleration on tap means lower lap times thanks to being able to go deeper into corners, and less fade gives you the reassurance to do it corner after corner and lap after lap. But it’s not as simple as just slapping on a brake kit with the biggest rotors and largest calipers that will clear your wheels, and it’s even possible to have “too much” brake, making the car a chore to drive and slowing you down even when you’re not using them.

Today, we’re going to look at some useful information when it comes to putting together a brake kit for competition, and how to get the most out of the hardware you’ve selected. We’ll concentrate on circuit racing, including autocross, time attack, and open track days; while brakes for drag racing are also a complicated subject, they’re a different kind of animal thanks to the special demands of straight-line competition, and are beyond the scope of what we’re talking about here.

When it comes to how effective brakes will be, it all comes down to the interface between the rotor and the pad – everything else simply serves to support this relationship. At a microscopic level, a properly-bedded-in cast-iron rotor will actually have some pad material embedded in its surface, and pad compounds and rotor metallurgy both take this into account. Before using any new brake kit in practice or competition, you must make sure that the pads and rotors have gotten to know each other properly via the manufacturer’s recommended break-in process. This goes for spare pads as well; if you are competing in a form of racing where you’ll go through more than a single set of pads in a weekend (or a single race, for those running endurance events), you’ll need to make sure your spares are already broken in and ready for use.

…it’s even possible to have “too much” brake, making the car a chore to drive…

Speaking of pads, this is an area where a racer has a lot of opportunity to “tune” brake performance to their liking. One popular manufacturer of racing brake pads offers no less than  nine different compounds just for motorsports applications (and a similar number for high performance street use), with a range of different characteristics and performance trade-offs.

The first aspect of a pad compound to consider is torque – how much grip the pad can apply to the rotor, using the available pressure from the caliper. While this might sound like the start and end of the story, consider the fact that a broomstick jammed into the spokes of your bicycle will deliver more brake torque than you can effectively use.

Peak torque is an important factor for cars with a lot of grip from big, sticky race compound tires, a lot of downforce, or both. But in order to be useful, that torque has to be available in a controlled way, and pad compounds can be formulated to adjust how it is delivered. Cars with limited traction under braking can benefit from compounds designed with a linear torque delivery, to allow the driver to modulate braking short of lock-up (or ABS activation, in situations where that’s a factor.)

While this might sound like the start and end of the story, consider the fact that a broomstick jammed into the spokes of your bicycle will deliver more brake torque than you can effectively use.

You’ll also hear the term “bite” to describe initial braking force – cars with a lot of downforce and/or tire grip can take advantage of pads with a lot of it because the first moments of braking deliver the most deceleration, but quite often, drivers will describe brakes with high initial bite as “grabby” and there can be a steep learning curve before you become comfortable with the non-linear torque characteristics. “Release” is the flip side of bite, describing the brake feel as pressure comes off the pedal, and pads with good release characteristics are easier to modulate at the very edge of tire traction.

Heat tolerance and rotor wear are the two other major dimensions of compound selection. For situations like autocross or (to some extent) time attack where your brakes are going to start off cold and then be subjected to a large heat load, you’ll want a pad composition that is engineered to deliver consistent torque across a wide heat range, possibly at the expense of developing some fade if the heat input exceeds the ability of the system to shed it over a longer time period. For multi-lap track day use or wheel to wheel competition where it will be possible to get the pads and rotors up to a ‘working temperature’ and keep them there, a composition optimized to deal with heat that trades off poor cold torque is going to make more sense.

For high-end racing applications, carbon brake rotors and matching pads offer the widest range of heat tolerance without fading. In the past, these were competition-only parts due to their poor performance when cold, their lack of durability, and their sensitivity to pad/rotor contamination, but recent years have seen them make their way into many OEM applications in sports cars as well. These brake setups are far more reliable and capable than the track-only carbon brakes of the past, but they carry very steep initial costs and replacement of the consumable components is expensive as well. It’s not unheard of for some non-professional racers to ‘downgrade’ to metallic brake rotors and conventional pads to keep costs for a season of racing in check.

…a vented rotor will also be more rigid, ounce for ounce, than a solid one.

Since we mentioned consumable components, it’s definitely worth talking about rotors at this point. Once you begin to put laps on your car, you will quickly discover that it’s not just the friction material in the pads that wears out – brake rotors have a finite lifespan as well, and should be considered a component that requires scheduled replacement. There are numerous styles of rotors available, but the vast majority you’ll see that are suitable for competition are “vented” designs. These rotors have two friction faces separated by cast-in vanes that allow air to circulate from the hub center to the outside of the rotor, helped by centrifugal force.

There’s some debate about just how effective this circulation actually is – the amount of air being moved is small, and it’s likely that the biggest advantages of a vented rotor design over a solid disk are increased mass to act as a heat sink, and improved dimensional stability. In the same way that a box girder resists being deformed better than a flat plate made from the same amount of material, a vented rotor will also be more rigid, ounce for ounce, than a solid one.

You’ll also see a lot of debate about drilled and slotted rotors. In theory, both allow trapped gasses and debris to escape from the interface between the pad and the rotor, but in recent years drilled designs have fallen out of favor to some extent as many manufacturers and users believe that holes provide starting points for cracks in the rotor surface without offering a significant difference in performance over slots that don’t completely pierce the rotor faces. This impression wasn’t helped by the flood of cheap “drilled” rotors that came on the market that were simply some small ‘manufacturer’ taking an OEM part and throwing it on a mill to put some holes in it without any consideration for the overall strength of the rotor. You will also see debate about the advantages of a two-piece rotor assembly over one that is cast as a single component. Primarily, these claimed benefits fall into two categories; one, a rotor assembly with a separate disc is less expensive to maintain because the friction surface can be replaced independent of the mounting “hat,” and two, if the rotor is designed to “float” on the hat (which is quite typical of motorcycle applications, but less so in four-wheel vehicle designs) distortion of the rotor from expansion and contraction is reduced.

Finally, there is the question of caliper selection. Once again, there is a huge range of possibilities in terms of piston count and other features, but for most sportsman-level racers in economical classes, the biggest bang for the buck will come from an upgrade from factory “floating” calipers that have a single piston and rely on the entire housing shifting on its mounts to apply even pressure to the rotor to a design with opposed pistons. Most cars with sporting aspirations will come from the factory with opposed-piston calipers, which can benefit from an upgrade to designs that offer staggered piston size for more consistent pad wear, additional pistons to spread clamping force over a larger swept area, or calipers and matching mounting brackets that allow an upgrade in rotor size to take full advantage of the room available within larger-diameter wheels.

We’ve obviously just scratched the surface of this topic, but hopefully we’ve provided a jumping-off point for further research into competition brake upgrades. Remember – a car that won’t start is just an inconvenience, but a car that won’t stop will ruin your whole day.

On a Dime: Brake Tech – Brake Rotors

Brake Rotors

“Come on,” you’re probably thinking, “those break rotors are just big slugs of metal cut to fit my car.” Nope, break rotors are another very complicated part of your brake system. However, there is a huge misconception on how they should be designed. Those cross-drilled rotors you have are pretty much junk.

How Are Break Rotors Made?

Rotors are typically made of cast iron known as grey iron—a type of cast iron with graphite in the mixture and sometimes other compounds such as copper, silicon, or other materials that bond with iron. Early front disc brakes and many rear brakes today are a solid disc. However, these discs can have trouble with dissipating heat fast enough. This is where the invention of the vented disc brake came in to fix that issue.

Both types of discs are molded, but vented discs are done in a procedure known as sand casting. The veins of the vented rotor are made of a separate sand core. It’s placed between the cope (top portion of a mold) and drag (bottom portion of the mold) and the metal flows into the mold.

Those cross-drilled rotors you have are pretty much junk.

Once the metal cools, the core is removed by hammering it out, using air, or various other methods of removal depending on how the sand cast was made and bound. After that, the rotor is then machined for vehicle fitment before final surface finishing and coating—if a coating is being applied, that is. Drums are usually made in a very similar way with molds.

Rotor Faces

Rotor faces come in four distinct types: solid, slotted, cross-drilled, or slotted and drilled. How does each of those work and what are the advantages of each? We answer that in this rotor article.

Solid Face Rotors

A solid face rotor will be the most rigid and can dissipate heat very well. It can take a little more abuse and can also be resurfaced easily from “warping”. It’s the simplest design that all OEs take advantage of because it doesn’t require extra machining or complex work to build or mold it. While it’s simple, it’s still very effective in most high-performance brake systems where pad gassing and debris clearing isn’t an issue.

Slotted Face Rotors

A slotted faced rotor is designed to keep some of the rigidity and heat dissipation of the solid rotor but create a space for gasses and incandescent materials to be wiped away from the friction lining. Gasses come from the natural breakdown of the adhesive that holds the brake friction to the brake pad as it heats up from use. This gassing creates a bearing surface, like how an air gap works, and creates a form of brake fade because the gasses can’t be compressed. The slots transfer those gasses away from the friction and rotor surface along with the incandescent materials to improve braking performance in high-performance applications. A street car normally won’t see this, but if you track yours then you will and is why a slotted rotor is an excellent choice.

Cross-Drilled Rotors

A cross drilled rotor has holes drilled straight across each rotor face that also feature chamfered edges to reduce hot spots at those drill points. This design is for maximum degassing as the venting of the rotor helps pull those gasses away from the rotor surface. The problem you start to encounter with a cross drilled rotor is the reduction of surface area for cooling. This can cause heat stress cracks at the drill points and a loss of rigidity overall for the rotor.

With modern adhesives and pad construction, the requirement of a cross drilled rotor has been reduced to the point that they aren’t used that often. This includes professional motorsports. The exception is environments where having high rotor surface temperatures are needed for brake pad friction effectiveness or where the rotating material just needs to be removed. In other words, you don’t need a cross drilled rotor on your daily driver. The brake temperatures won’t be high enough for pad degassing and the pads you are using don’t need that much temperature to operate.

Slotted and Drilled Rotors

The combination of slotted and drilled seeks to gain the advantages of both: the maximum degassing of a cross drilled rotor and the wiping of the friction surface of the slotted rotor while also retaining some of the rigidity from the slotted rotor design. However, if you’re not experiencing any degassing issues with solid rotors, you’re not gaining much in terms of performance from switching to either version. You’ll also lose surface area that helps with cooling your brake rotors.

…if you’re thinking about getting those drilled or slotted rotors, you may want to reconsider.

Both a slotted and cross drilled rotor will be slightly lighter, but only by a few grams at best. Unless you’re in a Formula Car or maximized the reduction of the weight of your tires and wheels, losing weight at the rotor isn’t going to be of much use to you. It can be detrimental if you don’t buy a high-quality slotted or drilled rotor.

Losing Weight with a Two-Piece Rotor

However, if you want the maximum rigidity but want to reduce weight, you should consider a two-piece rotor with an aluminum hat, as you see here. The aluminum hat reduces the weight of the rotor significantly since that large mass of metal is of a lighter material. You also gain the ability to change rotor faces and material without changing the rotor hats and this type of hat can allow you to work with a custom design by just changing the hat instead of the whole rotor. This does come at a price increase over a solid hat and rotor but if you’re going for maximum lightness, the price usually isn’t a concern at that point.

How a Rotor Cools

Again, rotors come in solid disc or vented disc, with most front rotors being vented. The venting design is a centrifugal (radial) fan type, where—in the simplest terms—the blades create a low-pressure area on the outside of the rotor as it rotates. The high-pressure area between the blades flows in to fill in that low-pressure area, which then creates a low-pressure area behind that to pull in more air. Again, that’s oversimplifying it. Changing the angle of the blades can increase efficacy but will make the rotors directional. There are also multi-blade designs that direct airflow for better hot spot cooling.

So, if you’re thinking about getting those drilled or slotted rotors, you may want to reconsider. If you’re simply going for the looks, we can’t argue against it. If you’re going for performance, consider staying with a solid face rotor and finding other ways to either reduce rotational weight or brake cooling.

On a Dime: Brake Tech – Theory and Warping

Theory and Warping

Your brakes are possibly one of the most important parts of your car or truck. However, it’s probably one of the least well known after the shocks. Let’s talk about the basic theory of your brakes and discuss what “warping” really is.

You need to stop or slow down for that next corner but letting off the gas won’t slow you enough in many cases. In those cases, you need to get on the binders. When you hit your brake pedal, fluid is sent from the brake master cylinder to your calipers and/or drum wheel cylinder to move a set of pads or shoes against a rotating surface.

Those pads and shoes are fitted with a friction material that clamps down on that surface to take kinetic energy, in our case that is wheel rotation. That then turns that kinetic energy into thermal energy from the friction between the friction material and the rotor or drum surface. This friction causes the wheel to slow until it is stopped.

Well, they don’t warp like a wet piece of board does.

While your tire’s traction will determine how effective your braking is, the coefficient of friction of the brake liner will determine how much bite the pads or shoes will have on the rotors or drums. That thermal energy is then radiated away by airflow over the surface area of the rotor or drum.

Discs or rotors of the disc brake system do an equal amount of the hot work of the brake system, but they also do more than just transfer heat. Their face designs help the pads do their job, but what about the issue of rotors “warping?” Well, they don’t warp like a wet piece of board does. What’s happening is that the pads are leaving some of their friction material on the rotor surface under harsh braking.

Notice that “warping” is in quotation marks here. Your rotors do not warp in the sense that wood warps when it gets wet. Instead, what’s happening is that the brake friction material is transferring unequally to the rotor face. This can happen because of unequal temperatures on the surface of the rotor, a hotter spot on the rotor will transfer more friction material onto the rotor surface than the colder spot.

…what’s happening is that the brake friction material is transferring unequally to the rotor face.

This creates an uneven surface that transfers into the brake calipers and creates the judder and vibrations associated with “brake warping.” When a technician resurfaces the rotor, they are removing that access material along with the rotor surface to create an even face again.

That’s not to say a brake rotor can’t warp, but if it does there’s a whole host of other problems going on and usually, the rotor will crack and break before that warping happens.

Now that we’ve covered that, how about those rotors or brake pads?