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Monday, February 27, 2023

The Tale of a Tailbox

 For more about 9 years I have been riding recumbent bikes and trikes which have an aluminium main beam as a frame, and a clamp-on unit combining the bike seat (otherwise known as a lounge chair) with luggage storage and improved aerodynamics. 


Since 2014 I have been riding this style of bike and trike with a more upright seat but aero tailboxes. Photo Melvyn Yap

Maria Parker leaned back for aero on a Cruzbike (Photo courtesy Cruzbike)


Some recumbent bikes like the Cruzbike Vendetta have the rider leaning far back to reduce frontal area, and so become aerodynamic. Instead of this, my aerodynamic method is to have a full aero tailbox behind my back, so that my body is not bluff and there is less friction-causing turbulence behind (this reduces the drag coefficient Cd). As well, I try to make my HPV’s “clean” with as few messy protrusions as possible.

Previous "lightweight" tailbox: Standard plywood tailbox with coreflute side-panels and lid.


The tailbox also carries stuff!  Although I have a car, I don’t use it for regular shopping or commuting, and my recumbents are load carrying workhorses.  Up till recently I had one basic plywood tailbox design for everything. Most of it was developed during study at Monash university. The box has swap out side panels and a lid which can be changed from plywood to corflute to make it lighter.

Like lots of changes, it takes an unexpected event to bring things on, and force changes which were ready and waiting come to the fore.  Winston Churchill summed it up by saying “never waste a good crisis”. I am thinking of covid 19, and the way it accelerated the ability to work from home as an example. But in the context of my tailbox, it was Jamie Friday stacking into my bike and damaging my tailbox which started things moving!

This happened in Ballarat in 2019, when we were on our way to the OzHPV challenge in Geelong.  I had organised 2 days of touring as a nice way to extend the OzHPV challenge holiday, and we took the train to Ballarat, rode on to Aireys Inlet and then to Geelong. Soon after starting out from Ballarat Station, I braked suddenly, and Jamie following close behind stacked into me, damaging the plywood ‘box. We were able to patch it up, both temporarily for the rest of the day, and (a bit better) for the next few days and the Challenge.  When I got home, I was left with a damaged tailbox which was a bit heavier due to the repairs. I thought that a bit later down the track, I could strip away all the damaged rear part of the box and make it lighter and more aero by replacing plywood with corflute and rounding off some corners.

Eventually I did this and results were good, with a 2kg reduction in weight compared to my standard tailbox. This wasn’t really enough for me though, I’m an engineer and if I make something, I want to be able to make the same thing at least as good again without too much effort, so that means doing drawings. The original boxes are based on 4mm CNC routed plywood and I got the same mob who cut the last lot of ply, Sean and Horn to make 2 new boxes. 

The wood for the job needs to end up waterproof, and it’s tempting to ask for marine ply. But 4mm marine ply can mean just about anything from low quality timber to high quality hoop pine 5 ply. Birch ply is reliable and I’ve found quality to be consistent. I bend the ply for the tailboxes, and its nice to have this happen reliably. Dropping the timber off for the routing was done by car, but when the routing was ready I was able to pick it up by bike, its a flat 5k ride each way, mostly on bike tracks.

Now a few months down the track, I have made both tailbox kits up into tailboxes. These days I am quite comfortable with 3d cad and home 3d printing, so I have put a few 3d printed parts into the boxes. These help make accurate curves on the corflute, make drilling into the tailbox stronger and removes some manual processes.

The boxes with extra corflute still carry luggage. Even though they hold less than the full plywood boxes, they still carry vastly more luggage than most standard bicycles and the luggage in the box to doesn’t add to wind resistance.

The great revelation about these new boxes is not that they are lighter and probably more aero than my all-plywood boxes. It is that I can relax and not stress about making the all-plywood boxes lighter or more aero, and just focus on making them better for load carrying. Similarly I don’t need to think about load carrying on the new lightweight boxes.

Trike in load-carrying mode with ......

raisable lid for carrying bulky items

Leaning trike with milk crate bolted to tailbox lid.

So I started improving the heavier box by using a plywood lid instead of corflute. The heavier lid doesn’t blow around in the wind much, so I can skip having to close it with a catch. With the plywood lid its simple to mount a milk crate on top, as it only takes one bolt. And lastly I have leather straps holding the front of the tailbox lid. The length of these straps can be adjusted, so if I want to carry bike wheels or other bulky items, I can raise the lid by up to 100mm without using tools.

I like the idea of the milk crate on top of the box. It adds wind resistance but this just makes it as aerodynamically bad as a standard upright bike! As well, I can pick up a milk crate wherever they are, so I don’t necessarily have to bring my own crate to carry more luggage.

There is a way to go with improving the lightweight tailbox. For now it doesn’t provide a fairing for my head, but I think this can be put in without changing the plywood frame or the lower part of the coreflute. Pictures of fairings at the recent world champs have been an inspiration.

Trike with trailer


The tailboxes seem to fit in with the modular trike and bike designs I am slowly finalising (see  ). The bike I have made is lighter, more aero but slightly more complex, while the trike is simpler, heavier and less aerodynamic. So the new tailbox matches the new bike, and they are parts for an aero speedmachine with luggage space, while the trike and timber tailbox go together for an urban load carrier which is still quite fast. I’ve mounted a trailer on the trike too, and the hitching arrangement ended up quite simple.

After my most recent ride in Cobram with the aero bike.

Tailbox interior with 3d printed shapers in white and water bottle holder.

My leaning trike designs are already on the internet as free plans in both aluminium and timber frame versions. Recently recumbent bike plans were added, and the new tailbox design has been put up in the last few days. Bike and new tailbox are variations on the aluminium trike design.

Regards Steve Nurse, 27/2/2023


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