Pusher axle Vs Tag axle – Bus

Scania Metrolink HD Chassis - Tag axle position

Pusher axle Vs Tag axle:

There is always a debate between pusher axle and tag axle, to know which better in terms of reliability and which has more advantages. This article will explain the basics of pusher and tag axles to tell which is better under what circumstances.

Before that lets understand the position of axles in detail

Pusher axle:

A dead, single tyre axle placed ahead of live axle is called as pusher axle. This type of axles are primarily provided to support the main axles and to increase the GVW of the truck or bus, when there is no need of twin tyre axles.

In buses pusher axles were first seen in Volvo 9400XL, a 13.7m coach and was India’s first multi axle coach. This was launched sometime in the year 2008, and it was an immediate hit primarily because of better comfort it provided to customers and an exceptionally better operating economics to fleet owner.

  • Advantage of pusher axles is they are simpler in engineering and are cost effective. Almost the same front axle can be used.
  • The biggest demerit of pusher axle is it can’t be steered making it only self centred. This means it needs to turn in accordance with the bus and it can’t be guided or controlled using steering. This makes it impossible to use it in much lengthier coach’s say at around 15m, which will definitely require a steerable rear axle if not the TCD of the coach will be very high and will have huge limitation to use in the regular roads.
  • Other demerit of pusher axles is its tyre wear. When other axles, say front and rear axle were giving tyre life of over 1.5 lks kms, pusher axle tyres were hardly touching a tyre life of around 1lk kms. This has something to do with wheel base and some physics. Since the pusher axle is a dead axle it need to accelerate and decelerate in tandem with the live axle which is just behind it, making higher than usual traction and eventually leading to tyre wear.
  • Angle and length of Propeller shaft: Due to shorter distance between engine and gear box, the angle of the prop shaft will be relatively larger and length of the prop shaft will be lesser. In case of tag axles, the longer propeller shaft, lesser joint angle results in smoother running conditions.

Tag axle:

A dead, single tyre axle placed behind a live axle is called as tag axle. This can also be air suspended with bellows. Mercedes Benz’s Multi axle coach O500 RSD 2436 launched, in the year 2010 is India’s first multi axle coach with tag axle. Post launch of this coach, started the first debate on which is better. Till such time, Volvo was selling comfortably until Mercedes triggered the market with demerits of Volvo’s way of axle configuration – pusher axle.

Tag axle

Let’s see them one by one:

  • Tag axle’s biggest advantage is it can be steerable. This primarily reduces the TCD and effectively makes the coach turn in tighter spaces. This even made multi axle coaches to go places where they weren’t before say Wayanad, Munnar
  • Less tyre wear as the dummy axle carries the weight of the engine only unlike pusher axles.
  • More complex and axle beam needs to accommodate the prop shaft and effectively leads to a costlier system.

Considering various parameters and in a view of standardisation Scania launched both their 13.7m and 15m, Metrolink coaches in 2014 with steerable tag axles only. Also Volvo when launched their 15m coach, Volvo 9400 PX in 2011 offered only steerable tag axles only. Though Volvo offers a similar configuration in 13.7m coaches globally for various reasons, it’s still retaining its older pusher axle in its 9400 XL 13.7m coaches in India. So going forward with the advantages of tag axles, pusher axles may be seen as a feature in past in case of premium rear engine coaches.

 

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