Fall 2002 Forward to a Friend

A PIONEER in the development of front-wheel-drive technologies in Japan 40 years ago, Subaru has evolved and extended those technologies to its present symmetrical all-wheel-drive systems. The axle shaft is an example of a system component that is critical to a vehicle’s operation, yet given little thought by most drivers.

The front axle shafts transmit torque from the transmission and differential gears to the wheels and tires. Since steering and driving are among the functions of the front tires on FWD and AWD vehicles, increased performance and durability are expected of the axle shafts. So their development and engineering can be difficult.

“The front wheel axle shaft system is no longer unique to Subaru,” explains Atsuyoshi Nishizawa, the engineer in charge of axle shaft technology in Subaru Engineering Division. Of course, their components are not all the same, and Subaru axle shafts are packed with plenty of unique technologies.

The axle shafts – often referred to as “halfshafts” – are located on both sides of the differential, transmitting torque to the front wheels. Each halfshaft consists of a metal shaft with two joints – one on the differential end and another on the tire end.

With the symmetrical layout of the Subaru horizontally opposed engine, the lengths of the axle shafts are the same on both sides. This provides a great advantage, according to Atsuyoshi Nishizawa: “Many vehicles with inline engines placed transversely do not have the differential in the middle of the car, so the axle shafts are not of equal length. With our horizontally opposed engine, the differential comes naturally to the center of the car body. By extending equally on both the right and left, first of all, the basic balance is good. There are many advantages to this design, such as the elimination of the problem of the steering wheel tracking to one side (torque steer) during acceleration and the ability to build the system with fewer parts.”

Most axle shaft joints have six ball bearings. Subaru vehicles have eight.

Legacy and Impreza front axle shafts have a double offset joint (also known as a shudderless free-ring tripod joint) on the differential end and a constant velocity joint (also known as a high-efficiency ball-fixed joint) on the tire end. The basic job of both joints is to revolve while accommodating any differential gear vibration, tire steering or vertical movement and to transfer torque properly. In addition, the double offset joint prevents the axle shaft from transferring engine and transmission vibrations to the car body.

In cases when the axle shaft is pulled or pushed due to tire steering or vertical movement, the joints on the differential end slide horizontally while they smoothly convey the torque. Although other manufacturers’ cars also have a slide function, the Subaru movement structure incorporates three free-ring parts that allow the joints to roll as they slide. This design is called the free-ring tripod. It reduces friction and supports high-quality driving performance.

Most joints on the tire end of the axle shaft have six ball bearings, but the one utilized by Subaru has eight. Using eight ball bearings improves reliability and durability, and it helps to make the mechanism more compact and to reduce weight. The quality and durability of such important parts demonstrate the value found in every Subaru.

Each joint is liberally coated with grease. The joints are covered by boots to keep the grease from flying out as the shafts rotate. Previously, the boots were made from rubber, but rubber tends to crack when overexposed to heat and ozone. Now Subaru uses plastic boots developed to demanding specifications to cover the joints on the tire end, which are exposed to severe conditions.

Carrying on with detailed research and development is the only way to achieve higher levels of performance and to improve reliability and durability. But most importantly, says Atsuyoshi Nishizawa, “The joint design is made strong to prevent it from breaking.”

So, what ultimately moves Subaru drivers are not power-train components like the axle shafts, but the company’s human-oriented technological philosophy.

* Source: Subaru News, January 2003 SNS0302.