One hinge, a moving target
The dock floor is at a fixed height forever. Trailer beds are not: a loaded van squats, an empty one rides high, air suspension moves several inches under the same load, and a flatbed is somewhere else again. Everything a leveler does follows from that single mismatch, and the useful way to look at a dock is not one number but the range of numbers it has to cover.
At the defaults — a 48 in dock, beds from 42 to 52 in, a 72 in deck and a 16 in lip — the plate spans 88 in and the grade runs from 6.8 per cent downhill at the low end to 4.6 per cent uphill at the high end. That is 11.4 points of swing across one working week at one door, off one hinge, with nothing adjustable about it.
Grade along the plate, not along the floor
The plate is a rigid thing of fixed length hinged at one end, so its length is measured along the slope rather than along the ground. That makes the rise over run a little arithmetic rather than a division: the run is the square root of the plate length squared minus the differential, and the grade is the differential over that run.
At these angles the difference is small — a 4 in differential over 88 in of plate gives a run of 87.9 in rather than 88 — but the shape of the calculation matters more as the differential grows, and it is the difference between an angle and a percentage grade that most people are actually after. Four inches over 88 is 2.6 degrees and 4.6 per cent, and those two numbers are not interchangeable.
The gap eats the lip, the slope does not
This is the result that changes how the dock gets used. Swing the bed height across the whole 10 in range and the lip resting on the trailer floor moves by about a tenth of an inch, because at two or three degrees the lip is lying almost flat and gives up almost nothing horizontally. Now add an inch of gap — a deeper bumper, a trailer that has not closed up, a driver who parked at an angle — and the lip on the floor drops by a full inch.
At the defaults, 16 in of lip becomes 11.8 in on the trailer floor at the low end of the range: 0.2 in lost to the slope and 4 in lost to the gap. A dock where the lip is not reaching properly is nearly always a parking and bumper problem wearing a slope costume, and this is the arithmetic that separates the two.
There is an exact point where that stops being true, and it is worth knowing because it says how safe the shortcut is. The lip left on the bed is the run of the plate less the deck length less the gap, so an inch of gap always costs an inch, while an inch of bed height costs the tangent of the plate angle. Those two are equal at 45 degrees and nowhere else. On an 88 in plate that is a differential of 62 in, over five feet, so anywhere near a real dock the gap outweighs the slope by a factor of four or more, and at the default 4 in differential by a factor of twenty.
What the limit window means, and what it does not
Put your equipment supplier grade figure in and the page turns it round: at 15 per cent over an 88 in plate, the differential that reaches the limit is 13.05 in either way, so bed heights from 35 to 61 in stay inside it. That is a geometry conversion of your own number, not a recommendation. What a pallet jack, a tug or a rider truck can actually manage on a slope is the manufacturer figure for that machine, and it varies with load, with wheel type and with whether it is going up or down.
Deliberate omissions
There is no capacity here of any kind. Deck ratings, lip ratings, hinge loads and what a trailer floor will take are all published figures belonging to the manufacturer and the carrier, and none of them can be inferred from a slope. The break angle where the plate meets the dock floor and where the lip meets the trailer is not modelled either, and it is a real clearance problem for anything with a long wheelbase.
The largest thing left out is not geometric at all: a trailer that moves while somebody is driving on the leveler. Restraints, chocks, communication between the driver and the dock, and what happens when a trailer creeps forward are site procedure. This page names the hazard and stops there.
Questions people ask
How do you work out the grade on a dock leveler?
The differential between the trailer bed and the dock floor, over the run of the plate. Because the plate is rigid and hinged, its length is measured along the slope, so the run is the square root of the plate length squared minus the differential squared. At the defaults a 4 in differential over 88 in of plate gives 87.9 in of run, 4.6 per cent grade and 2.6 degrees.
What trailer bed height range does a dock have to cover?
Whatever turns up at that door, which is the point of measuring rather than looking it up. The defaults use 42 to 52 in against a 48 in dock, and that 10 in range alone swings the grade by 11.4 points — from 6.8 per cent downhill to 4.6 per cent uphill. Measuring a handful of trailers over a week gives a far more useful range than any published figure.
Why is the lip not reaching the trailer floor?
Almost always the gap rather than the slope. Every inch of bumper projection, or every inch a trailer fails to close up, takes an inch straight off the lip resting on the floor. Slope takes almost nothing: swinging the bed height over a 10 in range moves the lip engagement by about a tenth of an inch, because at two or three degrees the lip is lying nearly flat.
What grade can a pallet jack handle on a dock leveler?
That is the manufacturer figure for the specific machine and this page does not state one — it asks you for yours and converts it. Put in 15 per cent over an 88 in plate and it comes back with the differential that reaches it, 13.05 in either way, which on a 48 in dock is bed heights from 35 to 61 in. What that machine really manages depends on the load, the wheels and the direction of travel.
Does the leveler length change the grade much?
Directly and in proportion. The grade is the differential over the run, so a 6 ft deck and a 16 in lip give 88 in of plate, and going to an 8 ft deck gives 112 in and cuts the grade by about a fifth for the same differential. It also buys a little more lip reach at a given angle, though not much — at these angles the lip is nearly flat either way, and the gap remains what decides the reach.