Minimum Wheel Base Defined
Weight load testing for a four post lift is very similar to the weight load testing for a two post lift.
For a two post lift, the weight capacity rating is based upon the vehicle’s weight to be evenly distributed across the four arms. EG: A 10K lift will lift a vehicle weighing 10,000 lbs… BUT the weight distribution should be about 2500 lbs on each arm. (4 x 2,500 = 10,000.)
Weight capacity load testing for a four post lift is perhaps just a little more confusing.
A four post lift has two runways, not four arms. When the maximum rated capacity test is performed on a four post lift, it is necessary to have an equal amount of weight on each parallel runway. Most vehicles would have equal weight distribution from side to side.
EG: a Toyota Camry® would have shorter wheel base and weigh much less than Chevrolet Suburban.
Some manufacturers advertise different percentages of lift capacity ratings for their 4-post lifts based upon the MINIMUM wheel base length of the vehicle. Some 4-post lift manufacturers publish a “disclaimer” (see below). When the wheel base of the vehicle gets shorter, the maximum weight lifting capacity of their 4-post lift diminishes.
Minimum wheelbase @ rated capacity: 160”
Minimum wheelbase @ 75% rated capacity: 140”
Minimum wheelbase @ 50% rated capacity: 110”
Minimum wheelbase @ 25% rated capacity: 90"
It is important to understand how 4-post lifts are tested. One of the more difficult tests for the 4 post lifts to “pass” is the weight limit stress test on the inside jack rails.
Obviously, the strongest point of any four post lift is near the end of the runways, where the cross tubes are attached to the columns. The weakest point of the runway is in the middle of the runway. The middle of the runways is also the weakest point for the welded jack rails.
When weight testing a four post lift, the (manufacturer’s advertised) minimum wheelbase is the deciding factor as to where to put the maximum weight on the runway rail. The maximum amount of test weight will be placed at the minimum wheel base specification as measured from the front of the lift runway. An advertised SHORT minimum wheelbase requires that the maximum amount of test weight be placed nearer the center of the runway than an advertised LONGER minimum wheelbase.
In the real word, shorter cars and pick-up trucks weigh less than longer wheel base vehicles. However, forklifts, skid steers, and other types of construction equipment may have very short wheelbases and may actually have a total weight much more than that of longer wheel based pickup trucks and SUVs.
A forklift or skid steer also has a lot of weight (back counterweight) concentrated in a very small area so the “stress” on the runway is exaggerated where the weight is concentrated.
Some things to remember:
A longer advertised minimum wheel base makes it easier for a four post lift to pass the jack rail test.
However, if you are working strictly with cars and pickup trucks, the advertised minimum wheel base should not be a factor. You can put a short wheel base Camry® on your 12,000 lb four post lift and not worry about runway deflection. Manufacturers provide disclaimers (maximum weight capacity limits diminish when wheel bases get shorter) so that customers will understand that commercial vehicle four post lifts are not designed to raise and support short wheel based heavy material handing vehicles (forklifts… skid steers… etc)