Almost all road cars have a synchromesh gearbox but many race race cars have a dog ring gearbox. Why, and what is the difference?
Changing gear works by sliding different gears up and down shafts. This engages different gears to allow different combinations and produce different overall gear ratios.
In a dog ring gearbox there is no synchromesh to even out the difference in speed between one gear and another. Synchromesh gears have a collar or cone that works like a clutch and as the two gears move towards each other the friction between the cones causes the gears to rotate at the same speed. The small dog teeth then engage to lock the two gears together.
In a dog ring gearbox the gears do not have this cone which causes the synchronisation of gear speeds. Instead the gears have fewer, much larger dog teeth which mean that gears are either fully engaged or fully disengaged. To change gear the driver has to match the speed of the gears manually by double de-clutching or by changing gear at the correct speed. To minimise wear, gear changes should be completed as quickly as possible to prevent the dog teeth clashing. The image below shows a synchromesh gear on the left and a dog ring gear on the right.
A common misconception is that dog ring gear boxes have straight cut (spur) gears. In fact both synchromesh gearboxes and dog ring gearboxes can have straight cut gears or helical gears. Straight cut gears transmit more power so they are always used for racing, but straight cut gears are very noisy which is why road cars tend to have helical gears. In the image below the top pair of gears are helical and the bottom set are straight cut or spur gears.
The advantages of having a dog ring gearbox are that gear changes can be completed very quickly by briefly blipping the throttle with minimal use of the clutch. This has obvious benefits on track because the engine will be powering the wheels for a larger proportion of time. However, dog ring gearboxes are difficult to use on the road in traffic or around town and without quick gear changes at the correct engine revs the dog teeth with wear out quickly. Synchromesh gearboxes are much easier to use, allow slower shifting and tend to last longer.
The only road legal production car with a (optional) dog ring gearbox is the Abarth 695 Biposto. The 695 Biposto is the most extreme version of the Abarth (Fiat) 500 with the 1.4 litre turbo engine tuned up to 190bhp and 250Nm of torque. The car weighs just under a tonne at 997kg and accelerates from 0-62 in 5.9 seconds. It has a carbon fibre front splitter and rear diffuser, a stripped out interior and Akrapovič exhaust. The car is rather expensive for what is essentially a tarted up Fiat 500, costing upwards of £30k. Oh, and the dog ring gearbox is an £8500 option! But if you want a stock production car with a race car gearbox, this is your only option. Check out this one for sale for £46k!
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