Car Trailer / Top Box

This post is about a trailer design that I worked out years ago when I had a young family. We regularly filled the car when we went abroad and one of my ideas was to make a lightweight, small trailer that could be towed as needed and then converted to a roof box for short ferry trips, storage etc.


The trailer as originally designed was approximately one Metre long by one and a half Metres wide by half a Metre high, but could be made to virtually any size, the principle remains the same.

The trailer was to be very lightweight, to keep under the car roof rack carrying capacity, with removeable sides and ends, retractable tow bar and small diameter lightweight removeable wheels. The suspension units were originally Indespension ones, these were designed to be removeable, complete with the wheels. The hitch was to be a lightweight pressed steel one. The lights were a small removable trailer board.


The frame could be aluminium angle 50mm x 50mm x 3mm thick or galvanised 3mm steel, with a “top hat” section running centrally front to back, to provide support for the floor and also provide a square tube for the hitch tube to run in. The hitch is mounted on an aluminium 50mm x 50mm tube, this is located in the “top hat” section and could be either fully extended for road travel or fully retracted for storage or car rooftop use. Cross retaining pins and hairpin locking pins hold the two together. Two pins were to be used for safety, although one would probably be sufficient.

A wide piece of flat steel or aluminium runs across the trailer 150mm nearer the front than the centre line to hold the suspension units and locate the wheels on the trailer centre line.

The sides and ends are made from the same aluminium or steel section as the base and have small extensions that slot into retainers on the outsides of the floor frame in a similar manner to that used on lorries. At the tops of the frames are “antiluce” fasteners or similar, as used on trailers with a drop down tailboard, to hold the sides and ends securely together.

The floor and sides would be 4 – 6mm exterior plywood, well varnished and secured to the frames with aluminium rivets or nuts and bolts.

Either a fitted plastic flexible cover or a hinged solid top could be used as a top cover if required.

The suspension units are a square box section Indespension unit with either the same type of locking pin arrangement as the hitch unit, or nuts and bolts. These units also have integral plastic mudguards.

The trailer board would be mounted on the rear end with quickly detachable pins and clips, the wire would run round one side of the trailer to the hitch area and was also held in quickly detachable pins and clips to make removal and replacement easier.

Underneath the trailer are four “U” section retainers the same as those on rooftop boxes. These would be secured to a standard roof rack or rails when the trailer is lifted onto the car roof.  If required the roof rack could be mounted on the top of the trailer, used on the road and then easily removed for car top use.

hooks would be used to secure a canvas type top, or a locking plywood top with hinges and lock could be added.


When in Road mode the hitch unit was fully extended, the trailer board was in place and the suspension units were in the normalposition.

When rooftop mode was needed the trailer would be unloaded, the lights, ends and sides removed and the hitch unit fully retracted.

The suspension units, complete with wheels and mudguards, would  then be removed if required to reduce the weight or give more clearance for the car roof.

The trailer base would then be lifted onto the car roof rack and secured, the sides and top could then be replaced. The suspension units could be replace  depending on the clearance available. The light board could be replaced or stored in the trailer.

Note: drawings to follow as I have no photos in existence, I am also learning Sketchup, so the drawings may well use a combination of Sketchup and Inkscape drawings.

Author: trebor

I am a retired engineer who likes to keep busy with various projects. I am married with two grown up children and live in Yorkshire the north of England and have lived in my present house since 1978.

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