In addition to swapping big disc brakes for small disc brakes, another common brake system upgrade involves converting a vehicle originally equipped with drum brakes to disc brakes. Up to this point in the book, drum brakes have been ignored, as their poor thermal characteristics and non-linear torque output make them undesirable in high-performance applications. However, for this very reason, older vehicles originally equipped with drum brakes make ideal candidates for disc brake conversions.
Chapter 13 exposes you to some of the unique steps involved in the process of converting a vehicle with rear drum brakes to rear disc brakes. Because many of the brake system design factors are similar to those already covered in Chapters 11 and 12, they are not recounted here. Yet gain, balance, and thermal capacity still need to be primary considerations when upgrading your vehicle, regardless of its age or intended use.
Compared to the race-prepared Porsche 911 examined in Chapter 12, a relatively stock 1972 Chevrolet Nova may seem a bit understated. Although both vehicles were built in the same year, the Nova was much more likely to see time at the drag strip than at the racetrack. Its front disc and rear drum brakes were common for vehicles of the era, and were relatively well-suited for boulevard cruising and sprinting from stop sign to stop sign. However, improved braking performance shouldn’t be limited to vehicles with road course aspirations, and the owner of this particular vehicle had three distinct objectives in mind.
When this Chevy Nova was built in 1972, drum brakes had been replaced on most vehicles’ front axles by disc brakes, but drums were still commonly found out back. Primarily because of their low cost, low weight, and superior parking brake performance, drums can still be found in modern rear brake applications where thermal requirements are low. (Baer)
The Nova’s original rear drum assembly measured a diminutive 9.5 inches in diameter and used a 7/8 -inch wheel cylinder, common hardware to be found hanging off the ends of a GM 10-bolt rear end in 1972. While properly sized from a gain and balance perspective, the non-linear pedal feel of the drum brake design made modulation difficult at best when braking near the vehicle’s limit of adhesion. Thermally, though, the drum brake proved to be woefully inadequate, as no more than a couple stops from moderate speeds were enough to produce rear brake pad fade and a dramatic drop-off in brake system performance.
Therefore, in order to improve pedal feel and enhance the resistance to fade, the objective was to replace the old-school drum brakes with a pair of modern disc brake assemblies. Of course, the improved visual appearance of the disc brakes would score big points at the Saturday night cruise-in as well.
Solid axles, leaf springs, and drum brakes—all standard fare for the times. Like many cars of the era, this Nova’s drums were cast with fins on the outer diameter for enhanced convective cooling, but were still thermally inefficient even by contemporary standards. (Baer)
Picking The Right Parts
Picking The Right Parts
Since the owner desired to maintain the original 15-inch steel wheels, 12.0-inch diameter rear rotors were the largest that could be installed. Measuring 0.8 inches in thickness, the vented friction discs provided by Baer had substantially more thermal mass than the stock drum assemblies.
For primarily aesthetic reasons, the rotors were both slotted and cross-drilled. Although out of place on a competition vehicle, this rotor treatment provided a unique look that was desired in this particular application. It also helped that they matched the front rotor upgrade, which had been performed earlier.
Although two-piece rotors were initially considered for their lower weight and enhanced visual appeal, in the end, it was decided to use one-piece rotors to keep the budget reasonable. Curved vanes were within financial reach though, and were expected to further aid convective cooling, even during around-town use.
In order to examine the inner workings of a drum brake, it’s necessary to remove the drum as shown above. The design of the drum also dictates that cooling air is not able to reach the working elements of the drum brake during operation, resulting in thermal performance that is always inferior to conventional disc brakes. (Baer)
Drum Brakes 101
Although drum brakes and disc brakes may appear quite different at first glance, they both use similar principles to convert hydraulic fluid pressure into torque, while at the same time converting kinetic energy into heat. In both systems, pressure is converted into force, force is converted into torque, and energy is converted at a friction interface.
Drum brake wheel cylinders perform the same function as disc brake calipers. Located inboard of a rotating drum, two opposed pistons in each wheel cylinder are subjected to fluid pressure from the hydraulic circuit. Based on the piston diameters, this pressure is converted into a pair of linear forces acting against brake shoes, which function in the same capacity as the brake pads in a disc brake system.
A variety of designs exist to transmit and amplify the wheel cylinder linear forces, but all serve to expand the brake shoes out against the rotating drum. Based on the drum geometry, the sum of these forces is then converted into torque. While there are many ways to arrange the internal components to enhance gain, the end result is a pressure-to-torque relationship that is not typically as linear as that provided by a disc brake assembly.
Drum brake wheel cylinders convert hydraulic fluid pressure into linear force (yellow arrows) as a function of their piston diameters. Through various design techniques, this force is amplified (red arrows) as it forces the brake shoes out against the rotating drum. It’s important to note that the force distribution along the length of the pad (as well as between the leading and trailing pads) is never uniform, resulting in uneven friction material wear. (Delphi Corporation)
Acting much like a disc brake rotor, the drum’s temperature rises as kinetic energy is converted into heat at the friction interface. Unfortunately, the rubbing surface of the drum is located inside of the brake assembly, resulting in inefficient convective cooling. For this reason, drums typically contain fins on their outer diameter to enhance what little convective cooling is available externally.
There are many, many more design differences that differentiate drum brakes from disc brakes, but in summary they are both engineered to accomplish the same task. For the enthusiast, though, the poor thermal performance of drum brakes is reason enough to make them an inappropriate choice for any high-performance application.
Replacing the drum brake wheel cylinders were a pair of floating calipers with single 1.6-inch diameter pistons, also provided by Baer. While not as visually exciting as fixed multi-piston calipers, the tight packaging constraints of the steel wheels dictated the more compact floating design.
In contrast to the cast iron attachment brackets, the caliper bodies were fabricated from lightweight aluminum. A pair of slider pins served to locate the two relative to one another while simultaneously providing the required axial float.
Bolting Them On
The very first step in any drum brake service procedure is to remove the cast iron drums from the vehicle. Unfortunately, this can prove to be quite a challenge, since drums tend to firmly corrode themselves in place over time. However, this is not altogether different from servicing a vehicle with disc brakes, as you can usually break the drums free with some brute force and a mallet.
In this particular application, the corrosion was not significant, but because the brake shoes had machined their way into the drum diameter, there was a ridge on the inside of the drum that prevented its removal. For this reason, it was necessary to not only get out the mallet, but also to externally adjust the drum internal components to completely retract the brake shoes.