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In trucking you need to know how tandem axles work for safe and efficient driving. Tandem axles distribute weight across a trailer so loads are hauled safely and legally. Knowing how to adjust them is a basic skill for any truck driver. Here’s a smart and simple guide to what tandem axles are, how they work and how to adjust them.
What are Tandem Axles?
Tandem axles are two axles close together on a semi-trailer. These axles support the trailer and stability while in motion. The positioning of these axles allows for better weight distribution across the trailer which is key to being balanced especially when hauling heavy loads.
Most modern semi-trailers have sliding tandem axles. These axles are mounted on rails under the trailer and can move forward or backward. Adjusting the tandem axles is necessary to get the weight distribution between the tractor and trailer axles so you don’t overload any one axle which can lead to unsafe driving or legal violations.
How Tandem Axles Work
The purpose of tandem axles is to distribute the weight of the cargo more evenly across the truck. By moving the axles forward or backward truck drivers can shift weight between the tractor and trailer axles. This is important because it affects how the truck handles and is weight compliant.
When you move the tandems forward more weight is put on the trailer axles which can take some of the load off the tractor’s drive axles. When you slide the tandems backward more weight is put on the drive axles which can be good in certain driving conditions or when crossing state lines with different weight requirements.
When the tandems are moved forward, more weight is placed on the trailer axles, which can relieve some of the load on the tractor's drive axles. Conversely, sliding the tandems backward shifts more weight onto the drive axles, which can be beneficial in certain driving conditions or when crossing state lines with varying legal requirements.
How to Adjust Tandem Axles: A Simple Guide
Adjusting tandem axles is a precise and careful process. Here’s a simple step by step guide to sliding tandems on a semi-trailer:
- Prepare Your Truck: Make sure the tractor and trailer are lined up straight. Clear the area around the truck. Do this on a solid level surface to avoid complications.
- Engage the Parking Brakes: Set both the tractor and trailer parking brakes.
- Air Pressure: Make sure the air pressure is max to slide.
- Lock the Power Divider: This gives you traction to move the trailer.
- Engage the Trailer Hold Switch: This switch is also known as the Trailer Spring Brake Override Switch. If you don’t have this switch use the trailer brake valve.
- Release the Slider Lock: Pull the slider handle out (or up depending on the trailer) and lock it in the retainer. If this is hard try using a tool or ask for help.
- Mark Your Spot: Mark the hole where the front locking pin will land after the tandems are slid.
- Reverse: If you have a manual transmission make sure the range selector is in low.
- Release Tractor Brake: Push the yellow button to release the tractor parking brake while the trailer brakes are still engaged.
- Slide the Tandems: Back up the trailer slowly. The tandems will not move while the trailer moves and the axles will slide into position.
- Re-apply Tractor Brake: Once the tandems are in position apply the tractor parking brake and get out of the truck.
- Check the Locking Pin: Make sure the front locking pin is in the marked hole.
- Release Slider Handle: Pull the slider handle out of the retainer and let it lock in place.
- Rock the Trailer: Put the truck in drive or reverse and rock the trailer to make sure the locking pins are fully engaged.
- Check the Pins: Make sure the locking pins are all the way through the holes and the handle is all the way in.
- Unlock the Power Divider: Disengage the Trailer Hold Switch and unlock the power divider to complete.
Why Adjusting Tandems Matters
Adjusting tandem axles is important for:
- Safety: Proper weight distribution makes the vehicle more stable and less likely to tip over due to overload or imbalance.
- Legal: Different states have different axle weight and spacing regulations. Adjusting tandems correctly helps drivers comply with these laws and avoid fines and legal issues.
- Economy: Adjusted tandems will improve fuel efficiency and reduce wear and tear on the truck and save you money.
Weight and Tandem Adjustment
When adjusting tandems remember that sliding them forward adds weight to the trailer axles and sliding them back transfers weight to the tractor axles. Each hole in the slider mechanism is approximately 250 pounds.
- To correct overweight drive axles slide the tandems forward. This adds weight to the trailer axles and reduces the load on the drive axles.
- To correct overweight trailer axles slide the tandems back and transfer some of the load to the drive axles.
Avoiding Mistakes
When adjusting tandems don’t slide them back too far or you’ll be violating the Bridge Formula which is the distance between the kingpin and the tandem axle group. Always check the most stringent state regulations along your route to be in compliance.
Understanding and adjusting tandem axles is a crucial skill for any truck driver. By following these steps you’ll make sure your loads are safe and legal and have a smoother and more efficient trip. Adjusting tandems properly is not only safer but also helps you comply with state and federal regulations and that’s the key to any trucking operation’s success.
