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Sunday, August 10, 2014

Small at the back!

Evelyn Hamilton on an early recumbent.  The brown rear box has storage.  The image is from a 1939 set of cigarette cards from Player's.  More details about Ms. Hamilton is here.
A modern small wheel recumbent moulton conversion from about 1996.  Don Thomas organised and competed in the Canberra HPV races, and this image is from an exhaustive catalogue of bikes sold when the Canberra Bike Museum closed in 2008.

One of Ian Humphries' bikes from about 1997.  Ian went on to found Flying Furniture cycles. Photo fom Huff magazine available here

Myself and Lloyd Charter, also from 1997

Flying Furniture Tour, a modern commercial recumbent with a small wheel at the back.  The corflute tailbox works as storage and an aerodynamic aid. Link

My 22" wheel model recumbent  circa 1999. (Canberra Ozhpv challenge)
Newly minted / unpainted Zeica on a Melbourne bike path
Zeica underseat steering / small at the back bike at Mordialloc Beach Road Cafe.  The fairing is a bit of a hotch potch and made from recycled corflute.
A 2014 wooden bike with Lloyd and Lauren at Corryong Airport

Wheel come full Circle: Michel Kater's converted Moulton.  Where's the tailbox Michel?

Seat is very appropriately made from an old bike sign.


This post is about how part of my bike design came along.   My bikes are simple and I like to carry things on them in an efficient way.  Included in my designs is an aerodynamic box above a small wheel on the back of the bike.  This idea is absolutely not mine!

The top picture and link shows that the principal of the thing was well understood in 1939.  Unfortunately I wasn't aware of 1939 recumbents between 1987 and 1997 when I first started tinkering with recumbents,  so I had to work it out for myself.

My first recumbents were not fast, long distance machines and it took from 1986 (when I made my first unrideable machine) till 1997 for me to have bikes good enough to compete with or ride 200k's on.  I was privileged enough to visit Canberra OzHpv races in the  late 1990's and saw a wide range of human powered vehicles there. Amongst them were ones by Ian Humphries, Don Thomas and Wayne Kotzur which had small wheels.  Ian Humphries was beginning to put tailboxes on the back of his small wheel machines and within a few years, he was making and selling bikes like this commercially, amongst them the "Tour" and "PBP Special".

My own bikes of the time still had big back wheels but were slowly evolving.  I attempted to build fully enclosed tailboxes for bikes with the  large rear wheels and failed miserably.  My building skills were not great and from memory I was using bike spokes and recycled black corflute material to make the 'boxes.  I hated making the cutout which is necessary to clear the back wheel.  The cutout was hard to build, and it intruded and ruined the beer carrying capacity of a perfectly good storage area. By about 1999 I had a bike with smallish (22") wheels and a "complete" tailbox, perfect for the OzHpv Challenge which included a shopping race.  I came 12th overall in a field of 40 or so, quite encouraging.

So what are tailboxes all about? Here is a little bit of recumbent bike theory, and I'll start with a bit about recumbent bike drag.

Recumbent bikes have wind drag which is a function of CdA, where "A" is the area of the bike as seen from the front, and Cd is the drag coefficient.  Lower either of them and you have a bike which is automatically faster on the flat and downhill.  Some tactics for lowering CdA are:

  1.  Keep the bike low.  If bikes are very low, the wheels merge in with the body when seen from the front and area is reduced.
  2.  Minimise messy exposed cabling: the cabling dramatically increases the drag coefficient, Cd.
  3.  Lean the seat back.  A bluff torso measures about 30cm x 66cm, but by leaning back the area facing forward can easily be reduced by a factor of 2/3 to 30cm x 44cm, and leaning even further back is possible.
  4.  If you don't have a front fairing, arrange the steering so the arms are tucked in to your body and not seen from the front or thrust straight forward so only the shoulder areas add to the front profile.
  5.  Add aerodynamic front and or rear fairings to minimise the drag coefficient.
  6.  Cover the front and rear wheels or minimise the spoke count and profile to minimise wheel drag.
  7.  Etc. etc. etc. the list goes on.
While all of these things improve drag, there are some drawbacks to implementing them, for example very low bikes are hard to get in and out of and can be hard to see in traffic, leaning way back can be uncomfortable, fairings add weight, and front wheel discs can adversely affect steering.

We're actually only concerned with #'s 3 and 5 here. The aerodynamics of an upright or moderately leaned back back bike can be dramatically improved by a tapering tailbox.  The tailbox changes the seat and rider from a high drag coefficient bluff body shape to a low drag coefficient "ice cream cone" shape.  The tailbox can double as a storage area and help the bike to be seen.

Have a look at this link, where Pete Heal's round Australia, tailbox equipped rocket machine is shown in wonderful detail: it shows that having a small wheel at the back is by no means a prerequisite for having a good tailbox on a recumbent bike. But for a home bike builder like me it makes building a good one with lots of load space much easier!


Steve Nurse