Tech Nugget: How a Dog Engagement Gearbox Works

If you follow any forms of professional motor racing you would have most likely heard the term ‘dog engagement gearbox’. Ever wonder what it means? Well we can assure you it’s got nothing to do with a four legged ball of fluff, but instead refers to how the gears inside the gearbox are selected by the driver. When we change gears, the relative speed of the shafts inside the gearbox must be matched to allow the selector to engage the next gear. In a road car gearbox this is done via a synchromesh system that relies on friction of a synchro ring to match the shaft speeds and allow the next gear to engage.  

The synchromesh gearbox is smooth, easy to drive and quiet which makes it ideal for road cars but in race use the shift speed is slow (forcing shifts quickly results in worn synchros), and synchromesh boxes don’t work well at very high engine speeds. The solution is dog engagement. In this sort of gearbox each gear has a set of ‘drive dogs’ which protrude from the side of the gear. The selector that slides between gears to select the gear that we want is called a dog ring and has matching dogs on both sides. When the driver wants to change gear, the dog ring moves across and the two sets of dogs engage. The drive dogs have a slight back cut or taper on them which means once they are engaged, the torque of the engine prevents them disengaging.

Dog engagement gearboxes offer several advantages for race use. Firstly the gear changes are significantly faster than a conventional gearbox and it’s possible to shift without using the clutch - All that is needed is a slight interruption to the engine torque to allow the dogs to disengage. This torque reduction can come from the driver lifting off the throttle slightly but many ECUs now provide gear change ignition cut to help speed the shift. This function cuts the ignition when the driver wants to change gear, allowing shifting at full throttle.  

The downside of a dog box is that they are noisy, with each shift accompanied by a solid ‘clunk’. They also must be shifted aggressively or excessive wear on the drive dogs will result. This makes them generally unsuitable for road car use. 

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