Forklift stability principles

Forklift stability. Longitudinal stability

Fork lift trucks are designed to be safe when properly used. The manufacturers all test their machines to the standards laid down by one of three organisations and these standards are the same in each case.

forklift longitudinal stability seesawTo make a truck safe so that it doesn't overturn in a forwards direction the manufacturers use a principle very similar to that used in a child's see saw as anyone who has been on an approved training course will tell you.

Using the see saw analogy we can see that in the first picture above, the see saw is in balance. It will only be in balance if the people on it are the same weight as each other and also sit the same distance from the tipping point which is known as the fulcrum. If, (say), the person on the right is heavier than the person on the left then obviously the see saw will tip over. This could be corrected if the person on the left moved further away from the centre or fulcrum. It is exactly this principle that is used in the design of a fork lift.


forklift stability diagramAs can be seen from this picture the load is carried very close to the front drive axle whilst the distance to the rear counterweight of the truck is longer. This allows the counterweight at the rear to counterbalance the load at the front with just a little bit "left over" as a safety factor. It should be noted that the safety factor is very small and you are advised to read the section on stability testing to get a clearer picture. If you are a truck operator don't be fooled into thinking that you have a lot of safety factor - you don't!

Take another look at the picture on the right where it says "Load Centre". This is arguably one of the most important factors that an operator should be familiar with before he/she attempts to pick up a heavy load. The standard load centre nowadays is 500 mm (20 ins) as it is assumed that a standard load will be 1000 mm. It was not always like this and the original load centre in the UK was 24 inches (600 mm) assuming a 48 inch long load. I mention it because there are still a great many trucks around with the 'old' load centre. Some trucks even quote both load centres in an attempt to assist the operator.


forklift tipover forwardsIt follows from the above that every load should be hard up against the front face of the forks, (often erroneously called the heel of the forks), because any gap here will increase the dimension on one side of our truck see saw and it can tip over!

I'll finish this page by listing the things that can cause the truck to overturn in a forward direction. They are:

  • Overloading the truck - Check the capacity plate
  • Having a gap between the load and the forks as described above
  • Picking up a load that is longer than it should be
  • Tilting the mast forwards when laden and elevated at or near full height
  • Driving too fast and hitting an obstacle that stops the truck dead
  • Incorrect truck operation on slopes and gradients

Apart from forward or longitudinal stability, a truck can tip over sideways. This type of accident is even more common and details of lateral stability can be found by clicking here.