High performance cars can come fitted with brakes that feature four- or even six-piston brake calipers, but is that want you want for your classic car?
Brake calipers come in a lot of different sizes and styles, but they all do the same thing. They push friction material – that is the brake pads– into the spinning brake disc, slowing the rotational speed of both the disc and the wheel it is attached to. More torque means more stopping power. Also, by using the floating caliper design instead of an opposing piston design, engineers can avoid wheel clearance issues and other robust design features of the floating caliper.
Some background on the most common type of brake caliper uses just one piston in what is called a ‘floating’ setup. The brake pedal presses fluid down either rubber or braded brake lines and into a cylinder inside the caliper, where it pushes the brake piston out and onto the disc.
That is one side covered, but floating calipers can move inboard slightly when that first pad strikes the disc. The first pad’s contact effectively then pulls the other pad onto the opposite side of the disc, creating the necessary grip to haul your speed down as far as needed. Lift off the brake and everything returns to normal.
This kind of setup works fine for ‘normal’ cars and warmed-up hatchbacks, provided it is lubricated and maintained properly. For everyday driving there is no real reason to upgrade, other than for aesthetics. But better systems exist, for where there is a demand. Heavier and/or faster cars need brakes that can handle more heat, more often, and deliver even more force to a disc with even more control at the brake pedal. That is what multi-cylinder brake calipers are for.
With a single-piston caliper, the footprint of pad is limited in some ways to the diameter of the piston. The diameter can only grow so large in relation to the rotor and brake pads. If the pad is too long, it flexes. When the pad flexes, it creates an uneven friction coupling on the rotor’s face. The uneven friction can create unwanted noise. If the flexing is severe enough, the friction material can separate from the backing plate. An example of this is the early Chrysler Neon. Also, there is a direct relationship between the diameter of the rotor and the diameter of the single piston caliper.
With dual- or twin-piston brake pads, the pads can be larger with even pressure distribution. A larger brake pad has several benefits. First, a larger pad will absorb more initial heat (less thermal shock). Second, a larger brake pad will have better wear characteristics and a longer life due to even pressure distribution. Third, a larger pad can absorb more unwanted noise and vibration because it is able to cover more of the rotor.
Another advantage is in drag and fuel savings. With dual- or twin pistons, it is possible to have more aggressive piston seals that can pull back the pads more effectively after the driver removes his foot from the brake pedal.
As well as twin-piston floating calipers that work in the same way as above, you can get twin-piston fixed items that feature a piston on both sides of the disc, squeezing equally and – the theory goes – offering a greater feel of control at the bite point. On top of that there’s potential for more initial bite and more even wear because both pads should contact the disc at the same time. A twin-opposed-piston setup works well for racy but still lightweight cars.
With more speed or weight comes the need for even better brakes. Sports cars need a wholesale upgrade with larger brake discs as well as more brake caliper pistons. Reason one is for heat management. All that extra speed and weight creates much more heat at the pads, and excess heat is the brake system’s nemesis.
Larger calipers with more pistons have larger brake pads, which resist temperature build-up better than smaller ones made of the same material. Larger rotors also shed more heat from their greater surface areas.
Reason number two is that a four-piston caliper, with two sets of opposed pistons, can be set up to squeeze the front two pistons slightly before the rear pair, which gives more modulation and feel to a good driver; useful for wet roads. The more gradual force input at the bite point can also be less destabilizing if you must brake mid-corner, although much of that comes down to specific setup.
Reason number three why multi-piston brakes are better is the simplest. More pistons equal more force. You already know there is more friction material in a bigger pad, but more of it is being pressed against the disc at full strength. In short, multi-piston calipers can stop you faster on the road and let you brake later track – but only if your tires have enough grip.
Of course, larger pads are way more expensive to replace and the best brake upgrades can cost an arm and a leg to begin with, so the investment is one to consider carefully.
SERVICE CONSIDERATIONS
Dual-piston and multi-piston calipers can be serviced in the same manner as single-piston calipers with a few exceptions:
Look for uneven pad wear. If the pads have tapered wear, it could mean a problem with the caliper. Tapered wear could mean that the slides are binding or that one piston could be exhort more force than the other, or both conditions could be present.
The slides and bushings play a critical role on dual-piston calipers. If there is too much play, the caliper could rotate on the slides. This will cause uneven pad wear.
The pistons on dual-piston calipers may be difficult to retract. Both pistons must be pushed back at the same time and rate with special tools. If unequal force is used, it is possible to scrape the pistons against the bores. This can damage the finish and coatings on the pistons and bores. Also, it can damage the seals.
The question you need to ask yourself is the type of driving you do worth the added expense of the brake system.
