Opposed-Piston vs. Single-Sided Brake Calipers

Most passenger cars are fitted with single-sided calipers of one concept or another, but read any motoring magazine and you'll see that the more expensive, higher-performance vehicles are fitted with sporty “opposed-piston” calipers.

So, if opposed-piston calipers are better, why aren’t they fitted to every car?

The use of opposed-piston calipers on production vehicles seemed to be reducing when I started my career in braking. Single-sided calipers were the latest thing for all sorts of good reasons. Vehicles had evolved to have reduced aerodynamic drag, a lower hood line, smaller wheel arches and an increased windshield rake. This all resulted in smaller wheels and lower-profile tires. At the same time, there was a desire to move from a positive to negative steering offset to increase stability in the event of a sudden tire deflation.

Not having to package a caliper with pistons outboard of the disc helped greatly with packaging within these requirements.

Opposed-piston calipers continued to be used in motorsport. Their popularity started to increase again as they became more visible through more open wheel designs.

The question then is, which is the best concept?

The answer is dependent upon the application and whether the benefits of one concept over another will be significant.

Installation

• Negative-offset suspension is easier to achieve with a single-sided caliper.
• You do not need a brake hose from upright to caliper with an opposed-piston caliper, reducing hose length and improving system stiffness.
• Opposed-piston calipers take up less space inboard of the disc. This can help increase available steering lock (London taxis maintain a tight turning circle because of this).

Function

• Opposed-piston calipers directly load both pads.
• Opposed-piston calipers have positive piston retraction on both sides of the disc, reducing drag, and pads can be evenly loaded radially and axially.
• The housing design on opposed-piston calipers generally leads to a stiffer caliper with lower deflection and fluid displacement.

Fade resistance

• Single-sided calipers should be more resistant to fluid vaporization as the fluid behind the piston is further away from the disc and (more importantly) does not need to be transferred across the disc.
• Pad fade can be more of an issue on single-sided calipers due to the increased deflection seen at higher pressures as the friction coefficient reduces.

Environment

• Corrosion may cause the sliding features of a single-sided caliper to seize, leading to drag and fade.

Parking brake function

• Single-sided calipers allow for mechanical or electronic parking brake functionality without overcomplicated concepts.

Cost

• Opposed-piston calipers are generally more expensive than single-sided ones. This is more likely to do with the volumes that they are manufactured in.

Aesthetics

• Opposed-piston calipers are often painted and feature logos that look good in open wheels and contribute to a vehicle’s “performance” styling.

During the brake system calculations and concept phase of a project, the brake engineer will be able to recommend which concept of brake is best for the vehicle. This has to be done on an application-by-application basis to ensure that the most suitable concept is chosen.

Technical article provided by Alcon Components Ltd. Published by AlconKits with permission.

Related articles

• Improving Brake Feel
• Main Causes of Brake Fade
• Identifying Your Alcon Caliper