Trailers - Design & Build

B. FENDERS

Posted on by Guilherme Campos
Figure 4.7. Fender styles and shapes commonly available.
08
Apr

Although their primary purpose is to deflect debris and water thrown off by the tires, fenders greatly improve the looks of any trailer. They also often serve as a handy seat or platform. Fenders are not generally regulated by law but some states do require them under certain circumstances. In California, a gross weight (trailer plus cargo) of 1500-lbs or more requires a trailer to be equipped with fenders. Many trailers below this weight are seen without fenders … as could be expected. Fenders are made in a variety of shapes and sizes but can be categorized as either trapezoidal, rolled, bent, half rolled or special formed. Figure 4.7 illustrates some of the more common types available. 

Finding a fender to fit over the tire may sound easy, but like everything else, a few traps can turn simplicity into frustration. The biggest challenge in selecting fenders is finding the style, shape and size to fit properly over the tire. The answer is not always as easy as it looks. The diameter of the tire must be known along with the height of the fender mounts. Most moderate size 14-in and 15-in wheels and tires will fit a fender 33-in across the bottom. Single axle fenders of this size are commonly roll-formed from a piece of metal which stretches out to a length of 44-in to 48-in long, and are rolled to finish widths of 6-in, 7-1/2-in or 9- in. A fender to fit a 14-in wheel may then be designated as a 7-1/2-in x 44- in x 33-in (width x stretch-out x length across the opening). 

Figure 4.7. Fender styles and shapes commonly available. 
Figure 4.8. Fender mountings that have worked well typically support the base at either end of the fender with some sort of brace. Without this support, bending moment applied at the top center can easily twist the fender outward and down. Welding of the fender to its brace can be tricky because of the material’s extreme disparities. A little practice may be necessary to avoid a series of holes. Fender supports are generally of much heavier material than the 16 or 18 fenders.
Figure 4.9. Fender mountings that have NOT worked well typically support the top or bottom from the side only. Anything that supports the top and lets the bottom flop, lets it do just that. Although the fender weight is light, bending loads applied at the top easily twist the methods shown.

The final fender height will be determined by the position at which your fender braces are mounted. Some thought should be given to this while you’re still drawing and certainly before you weld them in place. Figure 4.8 shows a few examples of fender mountings I have found to work well for me, while 4.9 shows a few that haven’t worked well. 

Fender clearance problems are often just as difficult to sort out:

  • A. Unloaded trailer-note straight axle. Sometimes axles are precambered with a center bend to give the wheels a positive camber.;
  • B. Loaded trailer-note negative camber of wheels.
Figure 4.10. Axles are typically “positive cambered” in the center to accommodate the addition of basic weight which generally brings the wheels and tires close to vertical. Additional loads can then bend the axle as shown in B. This puts the top of the wheel/tire closer to the trailer frame and the upper fender down closer to the tire. These changes must be accounted for when fenders are installed. Clearance problem areas to check:
  1. Edge of fender/tire tread;
  2. Top of tire/underside of fender;
  3. Body side/inside sidewall of tire;
  4. Tire tread/fender brace.

Sometimes more so since the tire moves in unexpected directions not just up and down. The actual movement of the tire in relation to the fender depends mostly on the suspension style and how it is mounted. A standard style spring with shackles in the back will move the tire slightly back as it also moves up; with shackles in the front, it will move forward and up. A torsion suspension will also move both directions following the circumference of its circle (usually back and up). This slight movement. must be accounted for when defining the fender clearance required. The rear (or front) fender brace may need to be moved rearward (or forward) to accommodate the slight horizontal movement. 

Figure 4.10 illustrates other common problems. First of all, the clearance from the top of the tire to the underside of the fender (point #2) must be sufficient to allow a fender to clear the tire when a bump is encountered. A commonly used dimension for this is 3 to 4 inches or about a fist height. Fender braces must be set to provide this clearance. Secondly, the inside top of the tire moves inward toward the frame as the load increases. This is called a negative camber on the wheels. When a bump is hit, this movement is even greater. At least 1/2 to 3/4-inch should be provided for this clearance (point #3 & #4). 

Finally, fender width must be enough to cover the tread of the tire (point #1) and not cut its edge down through the tire tread or sidewall. In severe cases of inside tire interference, wheel spacers, Figure 4.12, can be used to increase the offset dimension by about 5/16-in. In some cases to ensure full thread engagement of the lug nut, the use of longer studs may be required. A choice of different wheels with an offset further to the outside are also a possibility. Remember though that too much offset can create interference problems with the outside fender flange-and impose excessive wear on the bearings. 

Figure 4.11. Fender with potential clear-ance problems. Unless this trailer has an unsprung axle, a small bump will easily put the fender down on top of the tire tread.

The tire and fender shown in Figure 4.11 is an example of a clearance far too close for my comfort. If this is not an unsprung trailer, this tire is in for some rough treatment. Even if it is unsprung, at 60 mph the tread on a tire can grow up to an inch in diameter causing interference with the underside of the fender. With the tire’s existing negative camber and the body roll from bumps, the outer fender edge is poised to cut the sidewall. 

Figure 4.12. To gain additional clearance from the inner wall, wheel spacers can be used for spacing a wheel out from the trailer body. Be careful here, too much spacing can weaken the attachment.
Guilherme Campos

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