How to Replace Damaged Tandem Fenders: Exact Fit, Tools, and Installation Steps

Tandem-axle trailer fenders endure more stress than any other fender type due to long-span length, heavy loads, and vibration from two wheels. Over time, they may crack, bend, or fail at weld points—especially on equipment and utility trailers.

This guide provides a complete, step-by-step method for replacing tandem trailer fenders, ensuring proper fitment, strength, and long-term durability.

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1. Signs You Need to Replace Your Tandem Fenders

You should replace your fenders if you notice:

• Cracks near welds or mounting points

• Tire rubbing marks

• Visible warping or bending

• Rust deterioration (steel fenders)

• Holes or corrosion (salt exposure)

• Fenders shaking or vibrating while towing

Any structural failure can lead to fender separation, which is a major road hazard.

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2. Tools and Materials Needed

Tools

• Angle grinder or cutoff wheel

• Drill and bits (if bolt-on)

• C-clamps or welding magnets

• Tape measure

• Socket set

• Level

Materials

• Replacement fender (usually 72×10×17 or 72×9×17)

• Backing plates (recommended for welding)

• Welding machine (MIG or stick)

• Zinc primer or rust-preventive coating

• Hardware (if bolt-on): stainless or zinc-coated bolts

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3. Determine the Correct Replacement Fender

Most tandem trailers use standardized sizes. Identify:

Correct Length

✔ 72-inch fenders fit the vast majority of tandem utility, equipment, and car hauler trailers.

Correct Width

• 9-inch width → for 15–16” tires

• 10-inch width → for 16–17.5” tires (industry standard)

Correct Height

✔ 17-inch height aligns properly with most tire diameters.

Correct Material

• 14-gauge steel → best for heavy-duty use

• Diamond plate steel → added rigidity

• Aluminum → only for light-duty or boat trailers

If in doubt, measure your existing fenders’ length, width, and mounting points.

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4. Remove the Old Fenders

If Welded On

1. Use an angle grinder to remove weld beads.

2. Pry or tap the fender away from the frame.

3. Grind down remaining weld material.

4. Clean the trailer frame surface until smooth.

If Bolt-On

1. Remove the bolts with a socket set.

2. Detach the fender from brackets.

3. Inspect brackets for wear or cracks.

If brackets are bent or rusted, replace or reinforce them before installation.

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5. Prepare the Trailer Frame

Before installing the new fender:

• Clean metal surfaces

• Remove rust

• Ensure mounting points are square and level

• Verify axle spacing (33–36 inches standard)

A clean mounting surface ensures strong welds and proper alignment.

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6. Test-Fit the New Fender

Before welding or bolting, test-fit the replacement:

Ensure:

• The center teardrop aligns between the tires

• Vertical clearance is 2–4 inches

• Side clearance is 1 inch on each side

• The fender sits level (use a level tool)

• Ends of the fender are centered over each tire

Make adjustments before committing to welding or drilling holes.

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7. Weld-On Installation (Recommended for Steel Fenders)

Step-by-Step

1. Clamp the fender securely in place.

2. Tack weld at 4–6 anchor points.

3. Re-check alignment after tacks.

4. Complete welds along the mounting surface.

5. Install backing plates if not already attached.

6. Grind welds smooth if needed.

7. Paint with rust-preventive coating.

Why Weld-On Is Ideal

• Eliminates vibration

• Strongest possible attachment

• Prevents shifting under load

• Ensures even stress distribution

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8. Bolt-On Installation (Common for Aluminum or DIY)

Step-by-Step

1. Mark and drill mounting holes in the fender.

2. Align with bracket holes.

3. Use stainless or zinc-coated hardware.

4. Add lock washers or nylon lock nuts.

5. Tighten evenly across all mounting points.

Bolt-On Tips

• Do not overtighten aluminum—may crack.

• Use anti-seize to prevent galling.

• Check bolts periodically for loosening.

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9. Final Safety Checks After Installation

Verify the following before towing:

Clearances

• 2–4 inches above tire

• 1 inch side-to-side

• 3–4 inches between tandem tires

Structural Integrity

• Welds fully cooled and inspected

• Hardware securely tightened

• No interference with suspension

• Fenders do not vibrate when shaken

Wheel Movement

Turn tires lock-to-lock (if steerable), compress suspension slightly, and ensure no rubbing occurs.

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10. Optional Upgrades During Replacement

Consider upgrading to:

• Diamond plate fenders for added rigidity

• Thicker gauge steel (13G for extreme-duty)

• Backing plates to reduce flex

• Rust-resistant coatings

• Improved brackets (bolt-on systems only)

Replacing fenders provides an opportunity to increase overall trailer durability.

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Conclusion

Replacing tandem trailer fenders requires determining the correct size, removing damaged components, preparing the mounting surface, and installing new fenders with proper alignment and clearance. Weld-on steel fenders remain the strongest and most reliable option for heavy-duty applications, while bolt-on aluminum versions suit lighter-duty or boat trailers.

Correct installation ensures safe towing, prevents tire interference, and extends the life of the trailer.