There’s nothing that screams high-performance more than an oversized brake rotor sitting behind an open-spoke wheel wrapped in the widest rubber possible. No self-respecting automotive enthusiast would be satisfied with a 10-inch rotor tucked inside an 18-inch wheel. Bigger is always better, right?
Well, yes and no. There’s much more to selecting the proper brake rotor than finding one that fits. Certainly the big-brake touring-car look is desirable, but selecting the wrong rotor can actually compromise overall brake system performance. It’s time to find out what it takes to get the best of both worlds.
Although rotors are available in a variety of different shapes, sizes, and materials, they all share a common purpose—they must first absorb then dissipate a vehicle’s kinetic energy during braking. While this rotor may be horribly undersized for a road racing application, it may fit the bill perfectly for a boulevard cruiser. (Randall Shafer)
A Rotor Refresher
Although discussed separately to this point, the rotor actually performs two functions in the brake system. First, the rotor acts as the primary heat sink during the conversion of kinetic energy to thermal energy. This is where a majority of the vehicle’s kinetic energy ends up during a typical braking event, and back in Chapter 1 you learned to estimate the temperature rise of the rotors by using the following equation:
Rise in temperature (degrees F)
= kinetic energy (ft-lb) ÷
weight of the brakes (lb) ÷
77.8 (assuming cast iron)
The rotor’s second function was covered in Chapter 3—it is also responsible for converting the brake pad friction force into wheel torque. Because the brake pad friction force occurs at a fixed distance from the center of the spinning rotor, the resulting wheel torque was calculated as follows:
Wheel torque (ft-lb) =
brake pad friction force (lb) x[rotor effective radius (in)
÷ 12 (a conversion factor)]
Although these tasks are quite different in nature, heat absorption and torque generation occur simultaneously. In a competitive environment, the rotor is continuously compressed with thousands of pounds of caliper clamp force, generating thousands of foot-pounds of wheel torque, all while sustaining operating temperatures well over 1,000 degrees F, lap after lap after lap. It’s not easy being a rotor!
During track use, rotors are squeezed with thousands of pounds of clamp force, twisted by thousands of foot-pounds of torque, and heated to over 1,200 degrees F. Heavy cars with large engines such as these only make the demands that much more intense. (Wayne Flynn/pdxsports.com)
Like other parts of the brake system already discussed, a typical rotor can be broken down into several discrete components. Therefore, before going any further, it’s once again necessary define the terminology.
The friction disc is essentially the working component of the brake rotor. It’s responsible for providing a mating friction surface for the brake pads, as well as supplying the thermal mass necessary for thermal energy absorption. Consequently, the friction disc experiences the highest operating temperatures of any brake system component.
Friction discs are usually made from cast iron due to its inherent strength, energy absorption characteristics, and temperature robustness. Other materials can be used in select racing applications (more to come on this topic later in this chapter), but when comparing cost to performance, cast iron simply can’t be beat.
The friction disc is where all the action takes place. Because of its ideal mechanical properties and reasonable cost, gray cast iron is the predominant material of choice for nearly every friction disc today. The hazy film shown covering this friction disc is a coating to prevent corrosion before installation. (Randall Shafer/StopTech)
The rotor hat, also known as the mounting bell, serves to locate and attach the friction disc to the vehicle’s wheel hub or spindle. In doing so, the torque generated in the friction disc is transferred by the hat to the hub, through the wheel, and ultimately to the tire contact patch.
The rotor hat can either be integral to the friction disc, or it can be a separate component assembled to the friction disc. In either case, the hat also provides the primary mechanical heat transfer path from the friction disc to other vehicle components at the wheel end.
In the quest to further reduce rotating inertia, it’s possible to use rotor hats made from exotic alloys. The hat shown above (already attached to a friction disc) was machined from a magnesium alloy. While cost prohibitive, it provides the lightest rotor assembly possible. (Randall Shafer)
Integral rotor hats are made from the same material as the friction disc—cast iron in most cases. Discrete rotor hats can be made from a variety of materials, with aluminum alloys being the most common due to their low weight and relatively modest cost. In more exotic applications magnesium alloys can be employed, but these are beyond the budget of most automotive enthusiasts.
The rotor hat couples the friction disc to the wheel hub. In many production vehicle applications, it’s integral to the friction disc, but in high-performance applications, it may be a separate component. The hats shown here are machined from billet aluminum in order to reduce rotating inertia. (Randall Shafer/StopTech)
Two-piece rotors allow for relative movement between the friction disc and hat at temperature. Specialized fasteners are used to provide this freedom while simultaneously transmitting thousands of foot-pounds of brake torque. (Randall Shafer/StopTech)
Since most rotor hat mounting methods allow for some unrestricted axial motion, they may rattle around when cold. Although not present on the racing rotor shown, anti-rattle springs are typically employed on street applications. (Randall Shafer)
Most two-piece rotors are designed to allow the friction disc to thermally expand and contract radially without binding or distortion during use. For this reason, specialized mounting hardware must be used to constrain the rotor axially without preventing radial movement at high temperatures.
Unfortunately, most designs of this nature also permit some axial movement of the friction disc when cold. While not an issue for a dedicated racecar, this can lead to unwanted noises and vibrations on a daily-driven vehicle. Consequently, some rotor manufacturers take the extra step of installing an anti-rattle feature to alleviate this annoyance condition.
Some two-piece rotors may contain a thermal break to reduce the flow of heat from the friction disc to the mounting hat. This particular design uses a conical washer made from an insulating material to act both as an anti-rattle spring as well as a thermal break. (Randall Shafer/StopTech)
The effective radius is the lever arm that converts the brake pad friction force into torque. As shown above, it’s not the same as the rotor radius, but rather the distance from the center of the rotor to the center of the caliper piston (blue). (Randall Shafer/StopTech)
When using a two-piece rotor, it’s possible to install a thermal break, or insulator, between the friction disc and the rotor hat to reduce the flow of heat into vehicle components at the wheel end. Similar to the thermal barriers used in brake pad design, these devices are most typically used when there is a concern with wheel bearing longevity. However, the heat must go somewhere, and higher friction disc temperatures are normally the accepted tradeoff.
Rotor Mounting Tips
When installing brake rotors, it pays to take a few extra moments to properly prepare the mating surfaces and components. A little care upfront can prevent brake-induced headaches down the road.
1. Be sure that the hub mounting face is free of deposits, corrosion, or other junk that may have accumulated over the last 50,000 miles. A piece of Scotch-Brite and a bit of elbow grease should be all you need to ensure a clean mounting face. Be extra sure to inspect and clean the areas around the wheel studs.
2. If you are reinstalling used rotors, the same preparation comments go for the front and back surfaces of the rotor mounting hat. No crud allowed!
3. Inspect the mating wheel mounting face to ensure that it is flat and free from debris. A quick hit of Scotch-Brite should take care of any light corrosion you may need to address.
4. Finally, when mounting your wheels, tighten the lug nuts in the manufacturer’s recommended sequence (usually a star-shaped pattern) and tighten them progressively to their appropriate torque. Over-tightening is a guaranteed way to distort the rotor, which can ultimately result in unwanted brake vibration.