Even Pearly Whites Can Break

At first glance, it may seem that the lifespan of gearbox gears can be easily calculated. You just need to know the maximum forces, material properties, and number of load cycles – and that’s it, right? In reality, it’s far more complex. In practice, each gear tooth is subjected to a very intricate set of stresses: it deforms, “works” under pressure, and every aggressive racing run acts as a small but repeated endurance test.

Let’s take an example of the 3rd driven gear of a Mitsubishi Lancer gearbox. Under maximum load, the force acting on a single tooth corresponds to roughly two tons! The stress remains well below the material’s yield strength (the stress level a material can withstand without permanent deformation), so the tooth appears to be safe. The issue, however, is not just the stress itself – the tooth still deforms slightly, by mere hundredths of a millimetre. This deformation means that the teeth don’t engage exactly as mathematical theory predicts, which can lead to small collisions with newly meshing teeth. Externally, this manifests as a characteristic “whining” sound from the gearbox under load; the more serious consequence is the so-called cyclic excitation of the teeth, which dramatically reduces the fatigue strength of the gears. During these collisions, local stress peaks are formed, gradually weakening the material and potentially leading to cracks.

One of the solutions is the so-called profile modification of the tooth involute* – small adjustments to the tooth’s contact surface* geometry that shift engagement and reduce edge collisions. This technology, applied by grinding or wire cutting, significantly improves gearbox longevity. Still, it is impossible to precisely predict when a tooth will fail. Simulations show that a tooth with a microcrack up to 5 mm can still safely transmit load, but once the crack reaches 6 mm, breakage becomes likely.

In racing conditions, the theory of material fatigue plays a crucial role. Each tooth is exposed to hundreds of thousands of load cycles. For example, at an engine speed of 5500 rpm, the driven gear of 3rd gear rotates about 3740 times per minute. Each tooth engages once per revolution, which means that in one hour, a tooth experiences more than 200,000 cycles. After several hours of hard racing, the number of cycles approaches one million – and it is around this threshold that fatigue effects begin to appear.

Fatigue damage often manifests as the formation of microcracks in critical areas of the tooth, such as the root, where the profile curvature is smallest. These cracks gradually grow until they reach a critical size – at which point the tooth fails suddenly and breaks off. The fracture surface then shows a combination of smooth and rough areas: smooth sections result from gradual crack propagation, while rough ones are formed by instantaneous breakage.

In racing practice, it is therefore accepted that even if a tooth is theoretically overdesigned, one cannot rely on calculations alone. It is recommended to:

• Inspect the gears after every two to three competitive events.
• Periodically replace the most exposed gears, even if they appear undamaged.
• Monitor deformations and microcracks, as they indicate fatigue damage in progress.

Despite advanced simulations and calculations, the lifespan of gear teeth is always, to some extent, a matter of probability – even perfectly designed components sometimes fail. On the other hand, through regular maintenance, inspection, and replacement of the most stressed gears, the risk of failure can be dramatically reduced, ensuring that every race is finished without issues.

In conclusion: design and calculations are essential, but racing reality is unpredictable. The exact lifespan of a gear cannot be determined – we can only minimize the risk and maximize the chance that the gearbox will last all the way to the finish line.