The writer is a long-time industry veteran based in Japan and he promises to continue to share his insights with Forkliftaction.com.
Weight distribution to the front and rear axles varies quite dramatically on counter-balanced forklift trucks when loaded or unloaded. An ordinary IC 2.5-tonne (5,000lbs.) loaded with 2.5 tonnes will have close to 6 tonnes of front axle weight distribution, while its rear axle weight distribution drops down to far less than 1 tonne - from almost 3 tonnes when unloaded.
This is precisely why this type of forklift is named and categorised as "counter-balanced". The lifting load is held by the service weight of the forklift, the front wheels as a fulcrum and the rear part of the forklift as the counterweight.
No practical suspension can absorb such a weight differential. Most popular rear axles comprise a rigid single piece, fixed to the chassis by centre pin supports, allowing the rear axle to swing so that all four wheels have firm contact with the ground.
The two front wheels' ground contact points and the centre pin define a forklift stability triangle as illustrated in Figure 1.
Should this combined center of gravity (C.G). stay within the profile of the triangle; the forklift is considered stable both longitudinally and laterally. The longitudinal stability "α" and the lateral stability "β" are illustrated in Figure 2.
Lateral and longitudinal stabilities are inversely related. The combined C.G. can be moved backwards most commonly by adding an extra weight on top of the counterweight. The longitudinal stability increases as the angle "α" increases and the forklift can lift more weight, as illustrated in Figure 2.
But, we must not forget if the combined C.G. moves rearward, the angle "β" decreases at the same time. In other words, increasing longitudinal stability also decreases lateral stability. When lifting height is high, say over 4 meters, the lateral stability dictates the lifting capacity. Remember, additional weight may appear to increase lift capacity, but it compromises lateral stability.
The use of double front tyres or expanded front axle tread are effective in increasing lateral stability without sacrificing the longitudinal stability as the base of the stability triangle is prolonged. If any attachment moves the load laterally, like a side shifter, wide tread or double front tyres can minimise the residual capacity reduction.
If the rear axle mounting position is raised, lateral stability is also increased since the top of the stability triangle is lifted (Figure. 3). This design now appears to be gaining popularity.
Lifting capacity of the truck is not only determined by these two stabilities. Structural strength of the forklift and lifting force are two other major factors which determine lifting capacity. However, adding weight is not a solution as it can overload the forklift, reduce lateral stability, void the manufacturer's warranty and accelerate steering linkage wear, including tyres. It is also not worth risking safety and liability.
I hope that this has shed some light on the issue of stability in counter-balanced forklifts, and believe the same logic applies to articulated forklifts. However, I am still puzzled about how this all impacts at maximum articulated position. Perhaps someone else can explain this for us all.