Lately a
series of flatbed-type tow trucks has come into my shop with the same
problem: damaged bed locks. The bed locks, sometimes called
deck hold-downs, are stationary anchors located right behind the truck
cab. They serve to hold the flatbed deck down when you've rolled the
deck all the way forward. Like other equipment disorders, bed lock
problems tend to recur when you only fix the visible symptom and ignore
the underlying cause. The underlying cause of most bed lock problems
is in a completely different area of the truck: the subframe tilt
function and its geometry.
First, some words and what they
mean. The truck chassis is your truck frame, which largely
stays stationary during equipment operation. The subframe is
the steel framework above the chassis which tilts when you tilt the deck
down to the ground, or to the level position. The subframe supports
the deck, but does not move forward and back with the deck. The deck
is the flatbed, which slides forward and back on the subframe and, when
all the way forward, slides under the bed locks, right behind the truck
cab.
When you tilt the deck to level
on a properly built new truck, the tilt cylinders draw the subframe
downward toward the truck chassis until they firmly hold the subframe down
on top of the truck chassis, parallel to it, with zero clearance (see top
photo). Over time, however, the triangular geometry of the tilt
cylinders, subframe and truck chassis becomes distorted just a little, and
when you tilt to the level position, the tilt cylinders end up fully
retracted and can go no further, but where there used to be zero
clearance, now there's a 1/2-inch or greater gap between the bottom of the
subframe and the top of the truck chassis (see bottom photo).
Because the subframe pivots at
the back of the truck chassis, that 1/2-inch gap near the tilt cylinders
turns into a 1-inch gap at the forward end of the deck, and instead of the
deck sliding smoothly under the bed locks, the deck comes in 1 inch too
high and hits the top of the bed locks, bending them forward or breaking
them off.
Correct the underlying cause,
instead of just the symptom, by restoring the original geometry of the
subframe tilt function so that it draws the subframe down firmly against
the truck chassis as before. Some older flatbeds come with tilt
cylinders that have threaded adjustable rod-end clevises that can be
turned to shorten the retracted length of the tilt cylinders. On
other models, the mounts where the tilt cylinders attach to the truck
chassis have to be tightened, modified, or moved forward a little.
Keep in mind that the angles of the parts involved form what your
high-school geometry teacher would call an "obtuse," or somewhat
flattened triangle, so all you're going to move anything is only a
fraction of an inch to get the desired result.
As a last resort, it may be
necessary to move the mounts where the tilt cylinders attach to the
subframe backward. If so, make sure to maintain vertical clearance
between the mounts and moving parts such as the winch fittings and deck
hardware.