Everything You Need to Know About Converting Drum Brakes to Disc
It’s easy to take for granted all the advancements in automotive technology that are wrapped up in our modern daily drivers, but if you wind back the clock all these safety, convenience, and comfort updates were at one time an expensive option, or possibly not even available in the time period of your project car. The humble disc brake system is just such a feature we are referring to here. Developed around the turn of the 20th Century, disc brakes would not become a popular option until the early 1960s and were only seen mainly on sports cars of the time, both domestic and foreign. Even well into the early 21st Century you could still find vehicles with rear drum brakes, however it is the rare base model budget vehicle that isn’t 4-wheel disc brake equipped these days.
Manufacturers started offering disc brakes for their one two punch of better braking performance coupled with fewer moving parts for assembly line simplicity. Drum brakes may sometimes be lighter overall but are far more complicated to install (both from an assembly line standpoint and from a service standpoint), and are more susceptible to brake fade, contamination, and thermal breakdown of the shoes, springs, and other parts. Whereas disc brakes have fewer moving parts, run cooler, and are less susceptible to brake fade. For this very reason manufacturers slowly adopted disc brakes as the standard front brake system on vehicles moving into the 1970s and beyond. Today you can update just about anything on four wheels to run front disc brakes (at a minimum) or even 4-wheel disc brakes thanks to companies like Speedway Motors, where hundreds of disc brake upgrade kits are available for everything from pre-war hot rods to muscle cars, and classic trucks.
While we briefly touched on some of the benefits of disc brakes (and issues with drum brakes) in our opening copy, we wanted to take a moment to really explain in detail how each braking system works. Once you understand how drum and disc brakes differ in their function and ability to stop your car, you’ll fully understand our buyer’s guide’s intro when it says, “Why didn’t I do this sooner!”
We’ll start with drum brakes, as that is what many of you are considering replacing with modern disc brakes via a conversion kit of some sort. The drum brake system consists of a backing plate, which mounts the primary and secondary brake shoe linings, a hydraulic wheel cylinder, and mounting hardware/return springs that secure the brake shoes and wheel cylinder in place. The assembly is covered by a brake drum which is keyed to the wheels via the axles or hubs and wheel studs. When hydraulic pressure is applied via the brake pedal and master cylinder, the wheel cylinder’s internal pistons push outward on the brake shoes, which move outward and contacts the drum, applying friction force to slow the drum, which in turn slows the connected wheels.
Drum brakes certainly got the job done back in the day, but are well known for their issues, including retaining heat that can introduce brake fade, more complex assembly with more moving parts, brake “grabbing” when wet, and the need for regular manual cleaning of the drum brake assembly. Now consider today’s highway speeds, the increased performance we add to our classic muscle cars, hot rods, and pickup trucks, and it is easy to see why drum brakes simply are unsafe by today’s standards. See for yourself in this NHTSA report on the benefits of dual bowl master cylinders and upgrading drum brakes to disc!
Disc brakes, on the other hand, are a lot less complex in their function and are easier to service. They are less prone to wet weather “grabbing” and are self-cleaning as well. To top it off, they are lighter, have better stopping power, and can dissipate heat more easily due to their “open” design. All check marks in the right column and just goes to prove why most everyone wants to update to disc brakes on their classic drum brake ride. Thankfully, disc brake conversion kits/upgrades are pretty much a bolt on and go affair these days taking nothing more than basic hand tools and an afternoon of garage time!
While the drum brake applies outward force on the brake shoes to slow the brake drum, a disc brake system works more efficiently in a “C-clamp” manner by utilizing a caliper assembly with one or more pistons that is placed over the disc brake rotor (which is keyed to the wheels via the axle or hub, just like a drum brake). This clamping force is greater than that of what a drum brake can apply, and this is where the self-cleaning function comes into play, as the disc brake pads themselves scrape away old lining material, dirt, and debris to keep the brake rotor clean. The “open” nature of the disc brake system also means greater heat dissipation compared to the enclosed brake drum covering the drum brake components.
We’ve explained the differences between drum and disc brakes and all the great reasons why you’d want to run disc on at least the front of your hot rod, muscle car, or classic truck, but the real question for most is how the conversion is accomplished. While we do not have the room within our buyer’s guide to go into detail about every make and model conversion offered (that’s what the product instructions are for!) we can provide an overview of the conversion process to give you a rough idea what you’re in for. But don’t worry, all the kits we sell provide everything you need to get the job done with full instructions, so think of this as just a primer.
So many early muscle cars and certainly pre-war hot rods were built with drum brakes as the primary front braking system it is a solid bet if you pick up a project vehicle it’s still going to be rolling on drums (and they’ll quite possibly be seized up from sitting too!). Obviously, the first step will be to remove the old drum brake components. For the front brakes we are focusing on that means removing the drum with bearing hub, and then the backing plate with brake shoes, wheel cylinder, and hardware can usually be removed as a complete assembly. Disc brake kits require their own flexible brake hose that differs from a drum brake hose, so remove the old hose at the frame rail connection and at this point you should have a bare drum brake spindle to clean up and prepare.
The typical drum to disc brake conversion kit provides an adapter and specific sized wheel bearings that allow a popular disc brake rotor to install on the original drum spindle. Once the rotor is fitted the disc brake caliper is mounted via a custom caliper adapter bracket. So, for many a drum to disc conversion kit if the drum brake spindle is in serviceable condition, you’re good to go. For many hot rod applications, we do offer replacement drum spindles that are a modern forged spindle. The forging is stronger than the original cast spindle, plus any clearance or machining normally required for the disc brake kit to allow it to be installed is already built into this new spindle, making it an easy swap. These spindles are the perfect answer if you’re building from scratch or if your original spindles are damaged from a bad wheel bearing, bent from an accident, or other such issue.
Moving into the muscle car and classic truck drum to disc brake conversion it can quite often require replacing the drum brake spindle for a disc brake spindle. If this is the case our disc brake conversion kit will either include the required spindles or will provide details on where to source disc brake spindles for your conversion. However, we do offer several drum to disc brake conversion kits for muscle cars that retain the drum brake spindle as well, making the swap an easy one providing your spindles are good to go. Many of our replacement spindles are offered in a drop option, providing a 2- or 3-inch drop, which produces the lowered stance many want without affecting suspension ride quality and handling.
Rear disc brake conversions install typically like the popular front disc brake conversion kits, though many will argue their effectiveness on lighter cars and pickup trucks. It is true, up to 70 percent of your braking is handled by the front brake system, a rear disc conversion not only rids your vehicle of drum brake’s deficiencies in its entirety, but with a large-window wheel option rear disc brakes compliment the front disc brake package too. For the minimal additional cost (generally a slip-on rotor with brake caliper and adapter bracket), we highly recommend the addition of a rear disc brake conversion to your build. This way you’ll never have to mess with the intricacies of a drum brake system and their inherent problems ever again.
You have decided that your hot rod, muscle car, or classic truck deserves a disc brake conversion and you’re scrolling through the offerings from Speedway Motors and see many caliper options, including single piston, and multi-piston offerings. A single piston caliper is the most used caliper in both production and retrofit disc brake packages. The single piston caliper uses a floating design, where the caliper is installed on a pair of guide pins and the caliper moves on said pins when brake fluid pressure is applied to the single internal piston. This movement is how the two disc brake pads clamp down on the disc brake rotor.
It is a simple, yet effective, design. However, the floating caliper mounting system is not without its problems. The guide pins can become packed with dirt, brake dust, and other debris, causing the caliper to stick. The pins can become so bad that they will rust and corrode to the caliper, seizing it in place and preventing full brake application. Furthermore, the floating design has a lot of inherent flex in it, reducing clamping force on the brake rotor. For a cruiser-style build or a build on a budget a single piston floating caliper disc brake conversion will do, but if you’re looking for the best braking and highest performing brake conversion, then it must be a non-floating caliper setup.
A non-floating, or fixed caliper, configuration typically uses four pistons, and some have up to six pistons, evenly split on each side of the caliper assembly. A four-piston caliper will have two pistons on the inboard side and two pistons on the outboard side and utilize either an internal or external fluid crossover for the brake fluid to apply pressure to all four caliper pistons at the same time. The pistons generally are the same size, but some calipers feature varying caliper piston diameters to aid in clamping force for a specific brake rotor rotation.
The fixed caliper is preferred in a performance braking package due to the thicker mounting bracket and more robust caliper assembly. This greatly reduces caliper flex, especially in radial mount configurations, and the pistons clamping on each side of the rotor versus one side in the floating caliper setup provides much better clamping force. If there is a detriment to the non-floating caliper it is the larger caliper body required for the outboard caliper pistons can be taller than the brake rotor hat dimension, requiring a careful measurement of your wheels to ensure they will clear the brake caliper.
Many original drum brake master cylinders are of the “jelly jar” design with a single reservoir that feeds all four drum brake assemblies. If there is a failure of the hydraulic fluid circuit, like say a brake line or fitting fails, it affects all four brakes and once the reservoir is empty you will have zero pedal and no way to stop your car. This is an extremely unsafe condition and even if you are considering sticking with your four-wheel drum brakes, we urge you to at least upgrade to a split circuit front/rear master cylinder on your vehicle.
Even if your drum brake setup is using a split reservoir, it will not be compatible with a disc brake conversion. For starters, the two brake systems work under different fluid pressures utilizing different master cylinder bore sizes (learn about master cylinder bore size and its effect on braking in this Toolbox guide. Drum brakes use approximately 400 psi to apply, whereas disc brakes use 900 psi and higher to apply. The drum/drum split master cylinder will not have the right bore size, nor enough capacity for the disc brake’s fluid volume. Furthermore, the drum/drum master cylinder is designed to provide the same brake fluid pressure to all four wheels, so when upgrading to disc brakes in the front you will not have enough pressure, requiring multiple pumps of the brake pedal to get any sort of brake application from your discs. Therefore, it is critical to use the correct disc/drum master cylinder with any swap, or a specific disc/disc master cylinder if going 4-wheel disc brakes on your ride.
Finally, most drum brake master cylinders include a built-in residual pressure valve in the outlet fitting(s) of the master cylinder, which is a 10 psi valve in the case of drum brake use. Disc brakes generally use a 2 psi valve, usually only if the master cylinder is below the level of the calipers (such as a hot rod under floor frame mount setup like our illustration examples below). An attempt to use such a master cylinder means your disc brakes will have 10 psi of line pressure applied even without the brake pedal depressed, which will cause brake drag, overheating brakes, and eventually brake lockup due to such heat. Learn more about residual pressure valves, proportioning valves, combination valves in our Toolbox guide.
We all know that going fast takes money. Well, so does stopping fast. It can be said that you can pay for convenience, or you can pay for time. Meaning, you have the option of taking the time and finding all the individual bits to piece together your disc brake conversion kit, possibly saving a few bucks, or you can pay for the convenience of an all-inclusive disc brake conversion kit that has every piece you need, right down to the brake hoses and fasteners that will get your conversion done quickly and without hassle. In all honesty, our kits rarely cost more than piecing your brake package together, and for the popular applications our buying power and in-house manufacturing mean quite often our disc brake conversion kits will save you money (and time) over searching all over for individual pieces. That said, pricing will vary depending upon kit contents. We have basic front disc brake kits for Mustang II spindles for under $200, while our GM muscle car front disc brake conversion kits with spindles are in the $400-$500 range. Higher end front brake kits, such as Wilwood brakes with aluminum non-floating four piston calipers, drilled rotors, and more, are usually just north of $1,000. Of course, these prices can change but are listed from the time we published this buyer’s guide.
When it comes to your disc brake conversion kit choices, you won’t find more options anywhere else, from street to race, with plenty of great features and all inclusive. Plus, we offer pedal kits, brake master cylinder and boosters, and more to convert to power disc brakes as well. Make your hot rod, muscle car, or classic truck safer and enjoy the drive!