Tow hitch for The Fire Bike, part 1 – 21 Jun 2016.

As I’ve already mentioned, I recently bought a trailer to tow behind my bikes. My first idea had been to build myself a single-wheeled trailer but the chance to buy this one at a good price from a friend was too good to miss. It would have cost me more to put something together myself.

I want to tow it with The Fire Bike so I needed to come up with a design for a hitch. A number of people had suggested just welding a bracket to take a tow ball onto the pannier frame. Now this sounds fair enough and, although the rear loop looks to be at the right height, the frame is only designed to take the weight of the two small metal panniers.

The first thing I did was to offer up the trailer to the back of the bike and spend some time looking at what I’ve got.

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I needed to do some calculations but, the only guidance I could find regarding tow ball height relates to cars. The tow ball height (to the centre of the ball) on a car should be between 350 and 420mm with the vehicle laden. While trailer coupling height ( to the centre of where the ball would be) should be between 385 and 455mm with the trailer level and laden. From this it looks like I should aim for the trailer coupling to be around 35mm higher than the tow ball. However, these figures allow the extremes  of a trailer coupling 105mm above the tow ball or 35mm below it.

Bearing this in mind, I got my wife to come out and measure the height of the underside of the loop on the back of the pannier frame, while I balanced on the bike as best I could, to give a “laden” condition. The measurement was 315mm. The tow ball I intend to use will add 55mm giving a height to the centre of the tow ball of 370mm. With me off the bike this goes up by 40mm. Now, if I were to carry a pillion passenger the laden weight and compression of the suspension would increase. I thought about this and decided I am unlikely to carry a passenger and, if I did, I would increase the pre-load on the rear shocks to compensate.

I then got the trailer level and propped it in place. The height of the centre of the trailer’s coupling (where the tow ball would be) is about 420mm. I got this figure with the trailer unladen so it will reduce a little when the suspension is compressed.

All this means that, if I had a bracket welded to the back of the pannier frame to take the tow ball, the trailer coupling would be 50mm (or less when loaded) above the tow ball. This should be alright.

I removed the panniers from the frame.

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Then the frame itself.

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You can see that, if I just hang a bracket for the tow ball on the back of the pannier frame, there is no straight through pull to the rear footrest mounting. The panniers themselves, being made of steel, would stiffen the frame but I think modifications are needed. My idea was to add a piece between the inner and outer lower loops and perhaps a diagonal on each side to triangulate the frame like this.

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It was actually about two weeks ago that I took the pannier frame down to a local engineer. He agreed with my ideas and had previously seen the panniers fitted on the bike. I asked him to copy the frame so that I could store the original. He said, in that case, he would make the lower rails from solid bar rather than tubing. I’m not especially concerned about the increase in weight and I’d rather have it over-engineered than too flimsy. I’m not sure how long Eifion will take making the new frame/tow hitch. I’m waiting for his call.

Meanwhile, The Fire Bike doesn’t look too silly without the panniers.

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