Since many heavy truck and trailer platforms are built with optional suspensions to meet the specific needs of the vehicle, interchanging the OE part number is the most accurate way to find a replacement unit.

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Yes. Since most shock absorbers are stored on their side prior to installation, priming the unit before the installation is recommended. To prime the unit, you should hold it up-right and vertical, then cycle the unit by fully compressing and extending the unit 2 to 3 times. Once the unit is primed there should be no lag in control or dead spots while cycling the unit through its complete stroke. Now the unit is ready to install.

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Some drop axle shocks, like Monroe part number 66502, have a spring integrated onto the shock body. As stated in the instruction sheet included in the product's packaging, these shocks can still be primed in the normal manner however, instead of completely compressing the shock you will only compress it about 2 inches which will sufficiently allow the piston to cycle within the internal oil. This process should be repeated 3 to 4 times. The shock will extend slowly if properly primed. Also keep in mind that these shocks have a directional sticker on the body which signifies which side of the reserve tube should be mounted facing up.

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Yes. Since most air spring suspension rely on the shock to limit extension travel, the shock absorber should be designed with a hydraulic limiting lock-out feature. This prevents the shock from pulling apart due to repeated topping of the unit. Monroe Commercial Vehicle dampers utilizing hydraulic lock out can be indentified by a double cross symbol after the extended length in the mounting and length chart. Review the mounting and length chart here.

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Each axle should have matching shock absorbers installed on it however, if one shock fails prematurely and it is being replaced with an identical part number, both shocks do not need to be replaced.

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The majority of shock failures on commercial vehicles are due to one of three things. 1.Excessive side loading of the damper. 2. Excessive compression travel. 3. Excessive extension travel. To eliminate excessive side loading inspect the suspension for worn bushings or linkages and replace components as required. Compression travel is typically limited by an jounce bumper. Inspect the jounce bumper to make sure it is not smashed or missing, replace as required. (Note: Some jounce bumpers are contained within air springs.) After the jounce bumpers are replaced, the compressed link of the shock absorber should be checked to ensure the shock absorber is not going to limit travel on compression. On vehicles with leaf spring suspension, the suspension extension travel can be checked with the wheels hanging. The weight of the axle should be supported by the leaf springs and not limited by the shock absorbers. On suspension systems which utilize air springs, coil springs, or torsion bars, a special shock absorber should be used which is designed to limit extension travel.

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Yes. We offer a number of different shocks for drop axles. The most popular part number is 66502, however the best way to indentify the correct unit is to interchange the OE part number of the unit removed from the vehicle. To use the product interchange click here.

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Gas-charged shock absorbers solve many of today's commercial vehicle ride control problems by adding a low pressure charge of nitrogen gas to the damper. The gas serves several important functions; however, the primary function of gas charging is to minimize aeration of the hydraulic fluid. The pressure of the nitrogen gas compresses air bubbles in the hydraulic fluid which reduces the adverse effects of oil foaming. Foam affects performance because it can be compressed - fluid can not. With aeration reduced, the shock is able to react faster and more predictably, allowing for quicker response time and helping to keep the tires firmly in contact with the road surface.

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If the vehicle has been driven recently, initial inspection can start by checking the temperature of the shock absorbers. If one shock absorber is much cooler than the rest, it will require additional inspection. In order to thoroughly inspect a commercial vehicle shock absorber, Monroe recommends disconnecting one end of the damper, cycling it in and out 3 to 4 times by hand in the up right position. During the last cycle, if the shock absorber exhibits dead spots or if the unit is binding or feels gritty at any point during its travel, it should be replaced. Keep in mind that not all commercial vehicle shock absorbers are gas charged, so checking the extension rate of the rod is not an accurate test.

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If the vehicle is being operated off pavement, the damper should be inspected every 50,000 miles. For on pavement use, the inspection period should be no longer than 200,000 miles.

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When it comes to durability of air suspension components, one of the most critical, single items to inspect is the shock absorber. Shock absorbers designed for use on air suspensions, utilize a special "hydraulic lock-out" which limits the extension travel of the suspension. This feature prevents the air spring and other suspension components from being overly stressed by the weight of the axle, wheels, and tires. If you are seeing repeated failures of one or more air springs the shock absorbers may be internally worn, damaged, or the incorrect length for the application.

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