Two-post vehicle lifts typically comprise two upright columns – a master or powered column plus an auxiliary or slave. Some columns are connected by a structural overhead beam, which helps keep the columns rigid under load (not to be confused with light-weight cable trays, which do not).
Various configurations are available:
- Originally, most lifts had the columns squarely facing each other with front and rear arms of equal length. Vans, trucks, 4x4s and rear-wheel drive vehicles typically have their centre of gravity near the centre of the vehicle, so the most even weight distribution is achieved when the mid-point of the vehicle is aligned with the columns. Unfortunately, this makes opening car doors difficult (trucks and vans have their doors further forward). This is often known as a symmetric lift but symmetry can also refer to: the relative positions of the columns; the lifting arms; and the positioning of the vehicle’s mid-point in relation to the columns.
- Since two-post lifts were introduced, front-wheel drive vehicles have come to dominate the market. However, their centre of gravity is not the mid-point of the vehicle but around the dashboard. To take account of this, lifts are now often manufactured with the front arms shorter than the rear arms (sometimes known as asymmetric arms or semi-asymmetric lift), so that the centre of gravity of the front-wheel drive vehicle can be placed approximately between the columns. The mid-point of the vehicle will then be behind the columns and provide better door access. A further development is to rotate the columns up to 30 degrees to make door opening even easier (sometimes referred to as asymmetric columns or as a truly asymmetric lift).
Each column is fitted with a pair of carrying arms that are pivoted at the column and of adjustable length (usually by telescopic means). This provides some flexibility when positioning the vehicle on the lift and may allow, for example, a van to be lifted on an asymmetric lift.
At the free end of each carrying arm there is a height-adjustable pick-up plate fitted with a rubber mounting pad. A two-post lift achieves ‘wheel-free lifting’ by aligning the pick-up plates to four jacking points on the underside of the vehicle body. This improves accessibility for certain tasks, for example, taking the gearbox and/or engine out of most front-wheel drive cars requires the removal of the front drive shafts, wheels etc to drop the unit out from below. This would be much more difficult to do on 4-post lift.
A typical two-post pick-up pad is between 100 and 125 mm diameter (4” to 5”), so adjustment and placement of the pick-up plates and mounting pads is critical to ensure the elevated vehicle is properly supported and stable. Accessories (eg low ground clearance skirts), or vehicle configuration (vans, 4x4s etc) may make it difficult to place the pads on the designated points. Adaptations need to be mechanically sound and securely fixed and manufacturers supply a variety of pad extensions for this purpose (see photo 1): blocks of wood are most unlikely to satisfy both criteria. Stability of the elevated vehicle can be affected if the rubber pad is worn, missing or contaminated by oil or grease. Adjustable screw threads are also subject to wear and damage. Where fitted, they should be self-braking and prevented from unscrewing (eg fitted with a circlip or similar).
Photograph 1 showing van supported on a symmetric two-post lift
Also see HERE for further related post on 2 post lifts…