How Suspension Bolts Work

"The Wages of Fear"
Based on the novel by Georges Arnaud, "Wages of Fear" will make you think twice about how cushioned your ride is.
Image courtesy

In the classic 1953 French thriller "The Wages of Fear," a group of desperate men t­ackles one of the more dangerous jobs imaginable: transporting highly unstable nitroglycerine across brutal jungle roads to extinguish a burning oil well. Watching the film, you can't help but join the characters in feeling every bump in the road. After all, one bad pothole isn't going to spill their coffee; it's going to blast their truck halfway to Peru.

Chances are your towing jobs aren't quite this life-and-death. But whether you're hauling livestock or linen, you're going to want a reasonably smooth ride. Proper tires and tire care both play a big role in this, but most of the duty falls to the trailer and vehicle suspension systems. Using an array of springs and shock absorbers, these systems keep every bump in the road from transferring directly to your seat. To learn more, read How Car Suspensions Work.



This is where suspension bolts enter the picture. These thick metal rods have spiral grooves on one or both ends, much like a screw. This feature allows them to slide through metal plates or other vehicle parts and screw firmly into nuts on the other side. In this way, they connect the suspension to the rest of the vehicle or wheel.

In this article, we'll look at the different kinds of suspension bolts and how to use them. Who knows? If you're ever stranded in a dreary South American town, this article could save your life.



Types of Suspension Bolts

shackle bolt
Shackle bolts lock shackle links into place between the suspension system and the vehicle or trailer.
Image courtesy

Attaching wheels to a car body is easy when you're playing with those building blocks for kids. Connecting a real suspension system to a vehicle or trailer, however, isn't as simple. Different connections involve the use of different bolts. Below, we'll examine the main types of suspension bolts you'll find in a suspension system.

Shackle bolts: Shackle links are one way automakers connect the vehicle body to the wheels. As you can see in the adjacent image, one side of each shackle link hinges to the vehicle and the other to the suspension. Nuts screw down over the ends to prevent the bolts from sliding off, and grease keeps these parts moving freely.


U-bolts: These horseshoe-shaped bolts loop over or under a vehicle's axle, while the ends screw firmly into nuts on the other side of a metal plate. Carmakers use this arrangement to fix bow-shaped leaf springs snugly against the axle. To learn more, read How Leaf Springs Work.

Spring eyebolts: While U-bolts attach the center of a leaf spring to the axle, eyebolts attach the ends of the spring to the vehicle or trailer body. These simple bolts slip through round eyes at either end of the bow-shaped leaf spring, as well as through round hanger holes on the frame.

Spline bolts: When you want a bolt to fasten something without rotating, then this is the hardware for you. Spline teeth extend from the sides of the bolt, securing it. Frequently, they're used to lock shock absorbers in place.

Equalizer bolts: Equalizer hitches apply leverage across the trailer tongue and towing vehicle, distributing heavier loads more evenly. Equalizer bolts simply secure these hitches. Some models feature a cotter pin, which slides through the end of the bolt and the castle screw to keep the nut from sliding off the end.

On the next page, we'll explore how to use suspension bolts.


Using Suspension Bolts

These simple U-bolts play an important role in holding vehicles and trailers together.
Image courtesy

If you're installing suspension bolts to hold your suspension system together and connect it to the vehicle or trailer, then congratulations. You are using suspension bolts for their intended purpose. Just remember that certain bolts are designed for very specific parts of the suspension. If you need to replace a bolt, you'll want to make sure you have the right size and variety of bolt required.

If you decide to replace suspension bolts on your own instead of taking your vehicle or trailer to a professional garage, make sure to take precautions. Before you disassemble even part of your suspension system, be certain to support the vehicle or trailer body off the ground in the same way you would if you were changing a flat tire. Never place jacks or supports on the suspension system itself. You want to elevate the heaviest part of the trailer or vehicle (the body), so you can work on the much lighter suspension.


Bolts designed to rotate, such as shackle bolts, require greasing so they can move. As with any lubricated mechanical part, occasionally you may need to reapply the grease. On many modern vehicles, these parts are sealed effectively for life, but older vehicles and trailers sometimes require greasing through vehicle or grease zerks (also known as grease nipples or grease fittings). These tiny holes can accommodate the tip of a grease gun. If your vehicle or trailer features these fittings, you'll want to apply more of the manufacturer-recommended lubricant regularly.

Explore the links on the next page to learn even more about towing.



Lots More Information

Related HowStuffWorks Articles

More Great Links

  • "4Crawler Offroad Custom Spring Shackles." Oct. 13, 2008. (Oct. 16, 2008)
  • Fuller, John. "How Leaf Springs Work." Sept. 25, 2008. (Oct. 16, 2008)
  • "The Suspension Bible." Car Bibles. (Oct. 16, 2008)
  • "Trailer Suspension Maintenance." Cerka Industries. 2008. (Oct. 16, 2008)