Lister 5/1 Diesel

Drawbar Trailer Design & Build

Designing & Building Our 6-Wheel Drawbar Trailer Page 12


08/06/11

The brakes on a trailer are usually of the overrun type, that is, the weight of the trailer pressing against the towing vehicle applies the brakes when the towing vehicle is slowing down itself. 99% of braked trailers have this system in the lower weight ranges. Over 3500kg and they have to have power brakes by law. There are other practical limitations as far as the towing vehicle weight limits.

In our case we have a Land Rover Discovery 2, with a maximum towing weight of 3500kg for the trailer and its load. Our new trailer was designed to operate at or below that weight. The basics are three axles, some linkages and a coupling rated for the trailer gross weight. Only it isn't that simple!

On our trailer, the two rear axle brakes are controlled by a single pull-rod that is activated by the trailer coupling. The front axle is similarly controlled, but it needs less pull to operate than the two rear axles combined. That is where the balance-beam comes into play.

Brake balance beam for the brakes.

The braking effort is apportioned on a 2:1 basis by the distances from the pull-rod going to the coupling, going off to the right, to the rear brake cable at the farthest end, and the front axle cables at the bearest end. The front axle cables have a compensator in their link to the bar, so that inequalities in the cable lengths etc are automatically catered for. Thus a 120lb pull on the coupling rod would be apportioned 80lb to the rear brakes (4 drums, 20lbs each and 40lb to the front brakes (2 brake drums 20 lbs each)

That's the easy bit!

The front brake cables are easily hooked up, flexible cables from the plate by the balance beam go to the individual drum backplates. As the drawbar moves up and down while towing, the cables much have sufficient slack to prevent the drawbar movement tightening the cables and applying the brakes. In our case the pivot point of the drawbar is close the the anchor point of the cables.

Brake cables, front, rear front, left to right. Lighting & power cables coiled round pivot.

The rear cable is what caused the problem. It has to go under or over the front axle, which itself rotates about the steering pivot, so some interesting situations arise if you put the cable in the wrong place. We had three goes at it before we were satisfied that it was safe to use and couldn't cause an accident, something borne out in the trips we have done with it. The brake cable passes immediately underneath the centre of the front axle and back to the first cross-member on the chassis behind the frame step. It is a long run and the cable is heavy, so a swinging support is provided to keep the cable straight.

Cable swinging support. Can move side to side but not back to front.

Rear cable anchor point, stainless steel M10 threaded rod joins here with adaptor.

View under the chassis to the rear brake compensator on the front axle beam. Equalises 1 into 4.

Once we had got the geometry correct and the brake linkage cable sorted out (thanks to Hindle Controls) we got everything cleaned up, secured what we could of the kitchen framework, put the repaired seat cushions back in, put curtains up at the windows, and loaded everything ready for the following morning. I had a brief run round the block, but we were due out the following morning at 5am to catch the 9am ferry from Harwich, and we needed to know it would be OK for the trip.

Interior after we had fitted the cushions and curtains.

There was some trepidation about whether the thing would work, as the steering pivot and brake linkages all looked a bit foreign to most people, but we expected it to be OK, and weren't going to be going that fast. We did try and get full European breakdown cover, but found that few would cover a trailer of that size and weight. On the Thursday evening, we loaded the engine and all our kit and left it parked outside the factory for the early morning start.

Trailer outside the factory 1.

09/06/11

Trailer outside the factory 2.

Trailer outside the factory 3.

Trailer outside the factory 4.

We had an uneventful trip to Harwich, meeting our son in his Land Rover who had gone the previous evening in his Land Rover Series III. Loading on the ferry was fine, and we had an excellent trip. The trailer worked well, nothing needed repair or adjustment and the steering assembly was fine.

Trailer at the show site in Nuenen. The Eastern Bloc truck behind is fitted out with camping gear. This was the morning we left for home.

When we got back home, we had to start on getting the bathroom and kitchen sorted out!


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