What Is a Retarder and How Does It Work?

What is a retarder for?

A retarder provides frictionless or low-friction deceleration. It engages when the driver lifts off the throttle or uses the retarder lever, converting the vehicle’s kinetic energy into heat to reduce speed.

This means the service brakes (linings, drums) are used less; brake fade and wear are reduced on long descents. On heavily loaded trucks and buses, the retarder is critical for both safety and operating cost.

How does a retarder work?

An electromagnetic retarder (e.g. Telma) works through a magnetic field between a rotor and fixed coils. When current is applied, an opposing force (eddy current) forms in the rotor and the shaft slows; the energy dissipates as heat.

A hydraulic retarder (e.g. Voith) works through oil flow between a rotor and stator. The resistance from the oil slows the shaft; heat is removed via the cooling system. Neither system contains a wearing lining, so maintenance cost is low.

Retarder types: Telma and Voith

The two most common retarder brands in Türkiye are Telma (electromagnetic) and Voith (hydraulic). Installation, control unit and spare parts differ by brand and application.

KMQ Otomotiv repairs and fully overhauls Voith hydraulic retarders in its own workshop, and supplies original spare parts for Telma electromagnetic retarders. Knowing who does the work matters as much as correct parts and calibration.

Frequently asked questions

Does a retarder replace the brakes?

No. A retarder is an auxiliary slowing device; it reduces the load on the service brakes but does not replace them. Service brakes are required for emergency stops.

Does a retarder require maintenance?

A retarder has no wearing lining, so maintenance cost is low. Periodic inspection, oil checks (hydraulic type) and checking electronic connections are still recommended.