Building A Simple Race Car Trailer, Part I

Sometimes we bucks-down racers spend all our money on the race car and then have to borrow a trailer to get it to the track. This is an all-too-familiar scenario, so I sat down to devise a simple open trailer that will safely transport my beloved car at an economical cost. In this first article, we’ll cover the basic construction of the frame and running gear.

Getting Started

The needed parameters called for a 16-foot long bed with a dovetail rear to lessen the need for long or steep ramps. The objective was an open bed with runners on each side because often I don’t unload the car to change the oil and check for loose bolts. With this setup, you can also lie under the trailer and scan the bottom side of the car for damage.

It is my belief that a trailer needs springs, two axles and brakes, although many are built without them. Keeping cost in mind, mobile-home axles work very well. In many parts of the country they are abundant, but you should check with local agencies to make sure they are legal to use in other applications. Several states have laws to the effect that a mobile-home axle can be used only for its intended purpose on a mobile home. Most often, mobile-home axles are left intact by the installers.

The axles on this trailer were obtained by crawling under a friend’s mobile home and cutting them out ... with his permission, of course. All the mobile-home axles I’ve encountered have a capacity of 6,000 pounds or greater and have matching springs. That’s 12,000 pounds of capacity for two axles, more than enough for a 3,000-pound car on an 1,100-pound trailer. In choosing mobile home axles, you will find some with brakes. Get at least one of these if possible. You’ll be glad you did if you find yourself in a tight spot.

Next, find five 14-½-inch mobile home tires and wheels (one for a spare). The tires are bias-ply nylon tires made for trailers. Their load capacity is greater than 15-inch steel-belted radials, and they tow better, according to those who know. If your budget allows and you feel you must have 15-inch wheels, tandem-axle assemblies with springs and brakes are available from several sources. There should be at least one trailer-axle dealer in your area. Be prepared to spend $350-plus for a pair of 2,500-pound axles, (more for heavier ones) and then put on some good used tires.

In many cases, the mobile-home trailer tongues are also underneath. If you don’t mind using a 2-5/16-inch ball, one of these can be made to work. That will save a few more bucks.

I like high, forward-mounted brake and taillights. The lights on this trailer are up high on the tire rack. They are very visible from any angle and don’t get knocked off, a constant problem when mounted low and at the rear. It’s a good idea to use reflective tape on the rear of the trailer. In Texas, where this buildup was done, there has never been a problem with this arrangement. If the gendarmes in your area are more critical, add some lights at the rear, protecting them as best you can.

Some tools and equipment will be needed for construction. A cutting torch, a 14-inch cutoff saw and a welder are the basic items. A hand-held angle grinder will let you clean up a lot of rough edges too. You will need excellent welding skills. If you doubt yours, hire the welding out.

A bill of materials appears below for the items needed to construct the trailer. There may also be a few pieces of scrap in your shop you can use. Now let’s build it.

The Axles

For our purposes, the outside width of the trailer is 98 inches. This allows a bed width of 78 inches with a tire clearance of at least an inch. Mobile-home axles are too long in the beginning, and notice the crown in the center of the axle. This flattens out with the weight of the mobile home. Cut out this section to achieve the proper overall width. With this accomplished and the ends re-welded, the axle should be straight. Weld it back together after grinding a bevel on both ends. This will aid in weld penetration. Before you weld the axle back together, don’t forget to align the spring pads with each other. I used 0.030 wire in the Millermatic (see sidebar) for this entire project, making two weld passes around the axle to fill the bevel and leaving a built-up bead.

Using the piece removed from the center of an axle, make two eight-inch, half-round pieces. One of these was welded to the bottom of each axle joint for additional strength.

Remove the springs and turn them over so the spring will be below the axle. This will lower the trailer about 3-½ inches. Hopefully you found axles with the shackles and pivots to make it easier. The axles are finished for now.

Fabricating the frame The perimeter frame is made of 4x4x¼-inch steel angle on the sides and front. The pieces are cut to make outside measurements of 16 feet by 78 inches. Cut the corners at a 45-degree angle so they fit together smoothly. A piece of four-inch channel forms the rear-most crossmember. It is installed flanges out.

The frame will be built upside down so there won’t be any overhead welding. Set the framerails flange-down on cinder blocks. This gets them off the floor. Measure corner to corner in an X pattern. Tap the pieces around until this measurement is equal both ways. Now the frame is square. Put a good tack-weld in all four corners before welding it solid. Check the corner-to-corner measurement again. If it is off a little, rap it with a hammer.

Cut seven pieces of 1-½-inch square tubing to 7-7½ inches to fit inside the frame. The inside of the four-inch angle, as with all angle iron, has a radius. The tubing can be cut straight and then radiused on a grinder for a good fit. Being willing to take a short cut, I cut the tubing to 77.25 inches and did not grind a radius. The Millermatic sewed up the gap like Betsy Ross putting stars on a flag.

Notice the photo showing the locations of the square tube crossbars. The center crossbar is on the centerline between the two axles. This should be 18 inches behind the center of the trailer bed. The next two are away from center under the mounting points of the spring hangers. One more should be placed halfway between the front spring mount and the front of the frame. In the rear, 40 inches from the rear would be about right.

Now that the basic frame is together it must be notched for the dovetail. The dovetail is the drooped section at the rear of the trailer. This enables the use of shorter ramps. The dovetail line should be 48 inches from the rear end of the trailer. There needs to be a drop of five inches measured at the rear. Using the torch, cut the vertical flange of the four-inch angle 48 inches from the rear. Remember, the frame is still upside down. I placed two jackstands five inches higher than the cinder blocks under the rear channel. Standing on each side of the frame at the notch, my weight bent it down properly. From scrap, cut a diamond 4x7x¼-inch thick. Place it on the outside of the frame over the notch. Weld it inside and out.

The remaining piece of four-inch channel should be cut in half. The two resulting pieces will make the tongue. You should, at this time, have selected a hitch. Some are made at different angles. The tongue sides should be at the correct angle for your hitch. Consider it a minimum to have 36 inches from the front of the trailer to the center of the trailer hitch ball. More would be better. Some good welds here will give you peace of mind for a long time. With the basic frame done, set the axles on the frame and hook up the spring linkage. Place a two-inch block between the axle and the frame to set the bump-travel limits. Measuring carefully, align the axles parallel to each other and square to the trailer frame. Weld the spring hangers in place.

Next, the axles must be removed so the frame can be turned over more easily. Turning the frame over is an operation that requires great care. Use an engine hoist to raise one side of the frame to about a 70-degree angle. At this point attach safety chains to heavy objects on both sides of the frame. I used the rollcage of our race car for one attachment point. The other chain ran from the trailer frame to the trailer hitch on the tow vehicle. Step by step, I let one out and tightened the other. It gets a bit dicey when the frame goes over-center and the tension goes from one chain to the other. Keep yourself out from under the side coming down. Use the engine hoist for the final lowering just like the initial pick-up. If you don’t, the frame will skid sideways during the last few feet of lowering. This sort of surprise will make you wish you had taken those dancing lessons when you were young.

With the trailer right-side up, weld all those places that will be covered up by the purling now before you forget.

The Runners

Two eight-inch purlings need to be cut to fit on each side. This makes a 16-inch-wide runner. Measure inside the frame from front to rear and add a quarter-inch to the total. You will have to cut a V notch for the dovetail to take up the difference. This V notch should be five-sixteenths-of-an-inch wide at the flange side and near zero at the top face. When bent, it will fit right down in the frame. Weld the first purling in next to the outside of the angle flange. Weld it to the angle flange and also to the 1-½-inch square cross members. Then add the next one, welding it to the first and to the cross members. Depending on your purling supplier, it might stick up slightly over the angle. Mine did, so I got some three-quarter-inch angle to cap it off at the front.

The purling can be slippery at times. My cure for this is to rake it with the welder. Medium amperage and a medium wire feed works well. This leaves small weld beads similar to very coarse sanding paper stuck to the surface. Foot traction is much better than with diamond plate, and it’s a lot cheaper. At this point, your trailer should look like a trailer. In the second segment I will show you how to build a pair of $12 fenders, a slide-in Superwinch mount, where to put the M&R tie-downs along with a tire rack with a deck. And I won’t forget to show you the lightweight, swing-away ramps that stay attached to the trailer. Have fun building.

E-mail Sleepy at: sleepy.gomez@prodigy.net