Serpentine Belt versus Timing Belt: Understanding the Difference (2024)

Repair Topics > > Serpentine Belt versus Timing Belt: Understanding the Difference

Learn the difference between a Serpentine belt versus timing belt

They’re often confused, but there’s a huge difference between a serpentine belt versus a timing belt. While both are essential components, they serve different purposes and require distinct maintenance approaches. This article will explore the functions, differences, and maintenance needs of serpentine belts verus timing belts.

What a serpentine belt does

A serpentine belt drives accessory components like the alternator, water pump, power steering pump, smog pump, and AC compressor. It is driven by the harmonic balancer located on the front of the engine and tensioned by a manual or automatic tensioner. It’s referred to as a serpentine belt because it snakes around the driven components, driving some with multiple “V” ribs and others with the smooth backside of the belt.

This image shows a serpentine belt mounted on the front of an engine. Notice how the belt snakes around the components, driving some with the ribbed side of the belt and others with the smooth backside of the belt

Modern serpentine belts are made of durable rubber compounds reinforced with fiber cords. They feature a flat side that contacts the pulleys and a grooved side that provides grip and helps dissipate heat.

You can usually spot the serpentine belt easily when you open your car’s hood. It’s visible on the front of the engine, winding around several pulleys.

What a timing belt does

The timing belt, also called a cambelt, synchronizes the rotation of the crankshaft and camshaft(s), ensuring that the engine’s valves open and close at the proper times during each cylinder’s intake and exhaust strokes.

A sprocket or clogged wheel drives a timing belt from the front end of the crankshaft. You can’t see a timing belt under the hood because it is always covered with a timing belt cover. You’d have to remove the timing belt cover to examine it. Timing belts typically run without lubrication, except in special applications where the belt is made of a different material that can withstand submersion in engine oil.

The image below shows a timing belt on a Subaru boxer engine. The belt is driven by a cogged crankshaft pulley and tensioned by a hydraulic belt tensioner. It then wraps around the camshafts using the toothed portion of the belt and drives the water pump off the smooth backside of the belt.

Subaru timing belt routing

This image shows a typical timing belt with lateral cogs on one side of the belt

Serpentine belt versus a timing belt — Appearance

A serpentine belt is composed of multiple rows of small “V’s” that mesh with corresponding “V’s” in the pulleys.

A timing belt is composed of lateral cogs instead of “V”s”.

Serpentine belt construction

A serpentine belt is made from Ethylene Polypropylene Diene (EPDM) and other high-tech materials. EPDM is heat and crack-resistant and provides higher resistance to vehicle fluids and oxidation than older neoprene elastomeric materials.

The belt is constructed with stretch-resistant embedded tensile cords, usually made from aramid. The multi-ribbed side of the belt incorporates multiple “V” profiles that mate with corresponding channels in the component pulleys. The “V” profiles provide the “traction” to rotate the driven components.

The flat backside of the belt has an embedded fabric material to drive flat pulleys and resist wear

Serpentine Belt versus Timing Belt: Understanding the Difference (4)

Notice the linear rows of ribs that correspond to the tensile cord embedded in the rubber

Timing Belt Construction

Timing belts are precision components made through a multi-step process. The base material is typically a high-strength rubber compound, often HNBR (Hydrogenated Nitrile Butadiene

Serpentine Belt versus Timing Belt: Understanding the Difference (5)

This is an example of a double-sided timing belt

Rubber) or EPDM (Ethylene Propylene Diene Monomer). The reinforcing fibers are made from fiberglass or Kevlar and embedded into the rubber, typically running the length of the belt for strength.

The belt is placed into a mold that forms the teeth on one side. Heat and pressure are applied to cure the rubber and set the tooth shape. Tooth profiles are precisely engineered to match specific engine designs.

In some applications, cogs are molded into both sides of the belt.

Serpentine belt versus polygroove belt— there’s a difference

To qualify as a serpentine belt, the belt must drive engine components from both the ribbed

and backside of the belt. All serpentine belts are tensioned with an automatic belt tensioner that cannot be adjusted.

A polygroove belt is often mistaken for a serpentine belt because it looks like one. However, a polygroove belt is designed to drive components only from the ribbed side of the belt, not the backside.

©, 2022 Rick Muscoplat

Posted on February 9, 2022 by Rick Muscoplat

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Serpentine Belt versus Timing Belt: Understanding the Difference (2024)
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