How to Install Poly Fenders on a Semi Truck
A DIY-fender installation is doable. Poly Fenders are universal and will fit any semi truck model. You just need to measure your fender-to-tire clearance correctly and bolt the poly bracket mounts into frame.
Here are some tips.
First, deflate the air suspension so you can get an accurate measurement of the clearance.
Remember to maintain about 1.5" space between the tires and fenders, and 4" gap between the mounted fender and the top of the fifth wheel. Meaning: 5" of total clearance - top to bottom.
Minimizer Single Fender Installation
Tools You Should Have
- Socket and wrench set: Tightening mounting hardware.
- Torque Wrench: Final tightening
- Drill: Fender mounting holes.
- 3/16 inch drill bit: Pilot holes.
- 7/16 inch drill bit: Final bracket holes.
- Measuring tape: Clearance and centering.
- Marker or paint pen: Hole marking and fender labeling.
- Level or straight edge: Fender alignment.
- Wheel chocks: Truck safety.
- Safety glasses and gloves: Drilling and handling protection.
Hardware Checklist
- Poly fenders: Tire coverage
- Straight or offset mounting tubes: Frame-to-fender support
- Plastic mounting brackets: Connect tubes to fenders
- 5/8 inch frame bolts: Tube-to-frame mounting
- 3/8 inch carriage bolts: Bracket-to-fender mounting
- Flat washers: Load spreading
- Lock nuts: Vibration resistance
Safety Tips
- Park on a flat surface
- Set the parking brake
- Chock the wheels
- Keep hands clear while deflating or raising suspension
- Check behind fram rails before inserting bolts or drilling
- Avoid air lines, wiring, fuel lines, brake components, and suspension parts.
Check Fender Fitment & Clearance Before Installation
Confirm fender fitment: Match the fenders and mounting kit to the tire size, axle spacing, suspension type, and truck configuration.
Choose the mounting tube style: Use straight tubes when the frame mounting area is clear. Use offset tubes when air lines, brackets, suspension parts, or other components block the mounting path.
Inspect the frame mounting area: Locate usuable frame holes and check both sides for wiring, air lines, brake components, and other obstructions.
Measuring suspension clearance: Check tire movement with the suspension deflated and at normal ride height before setting the final fender position.
Check fifth wheel clearance: Make sure the fenders remain clear of the fifth wheel, slider assembly, mounting brackets, and nearby components.
Review kit specifications: Verify the required drill sizes, hardware dimensions, and final torque values for the for the specific fender and bracket kit.
Troubleshooting Fitment Issues
| Problem | Likely Cause | Recommended Fix |
|---|---|---|
| Fender sits too high | Brackets mounted too high on the tube | Lower the bracket position when clearance permits. |
| Fender rubs the tire | Insufficient suspension travel clearance | Reposition the fender or inspect the suspension stops. |
| Fender looks uneven | Tube angle or bracket placement mismatch | Loosen the hardware, realign the fender, and retighten. |
| Bracket does not sit flat | Incorrect tube position or fender corrugation mismatch | Reposition the bracket along the tube. |
| Fifth wheel clearance is too tight | Low fifth wheel height | Use taller risers or reassess the fender setup. |
Installation Steps for Mounting Polycarbonate Fenders
Step 1: Add Spacers on the Tires
With the suspension deflated, place a spacer on top of each pair of drive tires.
A 1-inch spacer is preferred.
This helps set the right tire-to-fender clearance.
Step 2: Set the Fenders in Place
Place each poly fender on top of the spacers.
Set one fender over each pair of tires.
Label each fender before removing or moving anything.
Use labels like:
- Left front
- Right front
- Left rear
- Right rear
This keeps each fender matched to its position.
Step 3: Center the Fenders
Adjust each fender so it sits centered over the tires.
Try to keep the fenders level and even.
Small differences are normal because poly fenders have some flex.
You will fine-tune the alignment later.
Step 4: Choose Your Mounting Tubes
Decide whether straight or offset mounting tubes work best for your truck.
Use the frame holes when possible.
Offset tubes work well when air lines, suspension parts, brackets, or other obstacles are in the way.
Attach the plastic mounting brackets to the tubes.
Leave the brackets loose for now.
Position the brackets so they line up with the fender corrugations.
Spread the brackets as far inside and outside as practical for solid support.
Step 5: Mount the Tubes to the Frame
Mount the tube to the truck frame.
If you are using offset tubes, insert a 5/8 inch bolt through the tube and frame hole.
Add a flat washer and lock nut.
If you are using straight tubes, insert the bolt from inside the frame into the threaded insert in the tube.
Keep the hardware loose enough for final adjustment.
Step 6: Mark the Fender Holes
Place the tubes and brackets over the fenders while the fenders are still sitting on the spacers.
Set the brackets as low as practical.
Make sure the fenders still look centered and level.
Mark the bracket holes on each fender.
Step 7: Drill the Fender Holes
Before drilling the fenders, confirm the mounting tubes are level, brackets sit squarely on the fender corrugations, and each fender is centered over the tire pair. Once drilled, the mounting holes will determine the final fender position.
- Remove the fenders from the tires
- Drill a 3/16 inch pilot hole at each marked location
- Then drill each hole out to 7/16 inch
- Keep the drill straight
- Don't rush
Step 8: Bolt the Brackets to the Fenders
Attach the brackets to the fenders with the supplied 3/8 inch carriage bolts.
Leave the hardware slightly loose at first.
This gives you room to adjust the fender position.
Step 9: Reinstall and Align the Fenders
Place the fenders back over the tires.
Adjust the tubes, brackets and fenders until each fender sits centered and level.
Check all clearances again.
Look for contact points around the tires, frame, fifth wheel, suspension, air lines, wiring and brake parts.
Step 10: Raise the Suspension and Tighten Hardware
Raise the suspension back to normal ride height.
Check the fender position one more time.
Make sure nothing rubs or binds.
Tighten all bolts once the fenders are centered, level and clear of moving parts.
Step 11: Final Inspection
Walk around the truck and inspect each fender.
Make sure:
- Each fender is centered over the tires
- The tire clearance is correct
- The fifth wheel clearance is correct
- The brackets are tight
- The frame mounts are secure
- No air lines, wires or suspension parts are touching the fenders
- All hardware is tightened evenly
After the first short drive, check the hardware again.
And that's it. You know how to install a polycarbonate fender on your semi truck.
Shop for Semi Truck Poly Fenders at Big Rig Chrome Shop
Upgrade your vocational truck for jobsite road spray protection or customizae the style of your highway-hauling big rig with durable poly semi-truck fenders from Big Rig Chrome Shop. Explore multiple color finishes, designs and styles including half-fenders, full fenders and quarter fenders made from lightweight poly material.
