Technology: everything you need to know about connecting rods
A connecting rod is the connection between the crankshaft and the piston. The French name is also popularly used: 'de bielle'. It is subject to heavy, fluctuating loads and must therefore be of very high quality.
The connecting rod moves rapidly up and down in the cylinder to move the piston up and down. The best examples are made of forged steel. The structure of the material is then changed in such a way that the connecting rod becomes a lot stronger. Another production process is that the connecting rod is milled from a block of high-quality steel and then given the correct strength through a heat treatment.
Size matters
A long connecting rod relieves some of the pressure on the piston and cylinder. Compared to older four-strokes, modern engines have a very short stroke and a large bore. This allows higher speeds but places more stress on the piston wall, piston rings and piston pin. An advantage is that a short stroke provides more engine braking when closing the throttle.
Aftermarket crankshaft manufacturers move the so-called 'big end' from position between the crank webs in order to play with the stroke of the piston. Extending the stroke ensures better filling of the gas mixture. You then need an extra gasket under the cylinder and the cylinder head must be further milled out to create more space.
Driven mass
All the moving parts in an engine block create a kind of gyroscope effect when cornering. In other words, it takes a certain amount of force to overcome this and force your bike into the corner. The upper parts of an engine in particular, such as the piston and the connecting rod, provide this effect. The lighter these materials are, the less pronounced the effect. Connecting rods are therefore best as light as possible without losing their durability and strength. The less driven mass, the better.
An engine that rotates at approximately 12.000 revolutions per minute produces approximately 200 piston movements per second! Think about this for a moment. It places an enormous load on the connecting rod and its bearings. Two-stroke engines have needle bearings at the top of the connecting rod. Four-stroke engines have twice as many piston displacements, which causes more wear. Modern four-strokes now have bushings instead of needle bearings. These bushings are much more durable than needle roller bearings. Some buses have a coating, others are made of bronze.
Did you know that manufacturers nowadays no longer move the piston directly opposite the crankshaft? The manufacturers currently use an 'offset' of a few millimeters towards the exhaust. The constant stopping and starting from rest of the piston puts pressure on the crankshaft, connecting rod and the piston itself. By moving the piston a little from the centerline you will reduce friction. This friction mainly occurs during the power stroke because at that moment the piston is pushed against the cylinder wall with great force.
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