A transaxle is the combination of a differential and gearbox in one unit. What this does is to reduce the packaging of the components into one unit rather than two. So in effect it is a transmission axle or “transaxle”.
In a traditional rear drive car, the engine is at the front with the gearbox connected at the rear of the block converting the power delivery through a prop (drive) shaft to a differential at the rear that then splits the power to each rear wheel.
As with pretty much most of a car, the transaxle was first used in the very early days of motoring – in fact before 1900, De Dion Bouton in France was using one as part of its drivetrain. The transaxle can be used with many configurations – mostly where the power creation and delivery are at the same ends of the vehicle:
Rear or Mid Engine and Rear Drive
Early examples of this configuration appear to come mostly from Germany and certainly from Europe where rear engined cars were very common with Volkwagen, Renault and FIAT all producing this configuration. The transaxle was mounted in front of the engine, so the main weight was behind the rear wheels. This allowed for a compact grouping of components and this made the designs quite small, although the weight was at the rear, which can make the car swing like a pendulum – I know, I stuffed a Simca 850 into a wall after losing the rear on a greasy road when I was in my teens!
In later years when mid mounted engines were becoming more common in high end sports cars, they too used the transaxle for similar reasons: space was at a premium and with the advent of the transverse engine, it meant that packaging could be even smaller. When single seater racing switched to mid mounted engines, it was the pioneering work of companies like Hewland who made it successful. Probably every F1 car with a Cosworth engine would have had a Hewland transaxle hanging off the back of it!
Front Engine and Front Drive
This configuration was popularised by Sir Alec Issigonis and his team with the Mini, taking the transverse engine and adding in the transaxle into the sump, thus shrinking the size considerably, which was then copied by the designers of the mid mounted transverse engined cars.
Pretty much every modern front engine and front drive car will have a transaxle to deliver the power. This configuration puts most of the weight at the front and rather than act as a pendulum, the car will understeer when at its limits – remember, the front wheels are trying to do too many things at this point. Firstly they are trying to grip the road whilst delivering or scrubbing power and also they are trying to take the car in the direction that is being asked by the driver.
Front Engine and Rear Drive
This configuration has the engine at the front and a transaxle at the rear thus sharing the weight evenly across the vehicle. Many cars with this configuration have a near 50:50 weight distribution that should help the balance of the vehicle – although the usual caveat applies: it depends on the driver inputs.
Most very high end cars have this configuration: Ferrari, Maserati, Aston Martin and some AMG are good examples. The main image is that of a De Tomaso Pantera that used a ZF sourced transaxle mounted behind a Ford engine.
Front or Rear Engine and All Wheel Drive
With the packaging of the engine and transaxle getting more compact, the idea of all wheel drive could be considered as a practical drivetrain for passenger cars. Audi are famous for bringing the Quattro to the rally world although not the first to sell a production passenger car with all wheel drive: Other than true SUVs, that record should go to the Jensen FF with the Ferguson Formula system, which didn’t use a transaxle, instead it used three differentials! The Audi system uses a transaxle and a centre differential to help spread the power to where it is needed.
Another advantage of the transaxle, certainly in cars where the engine and drive are at the same end of the vehicle is that you no longer need a drive shaft. This reduces the weight and also helps to deliver more torque to the wheels. Whenever you add a component onto an engine, you lose a small amount of available power/torque as some is needed to turn that device. With compactness comes greater packaging options that mean that more technology can be added to a vehicle. Audi’s system is a good example, through its evolution it has got more complex and powerful.