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Troubleshooting Article Archive:  March 2005          
Damaged Carrier Bed Locks:  Fix the Cause, Not the Symptom
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.