Towards the Conversion Kit

From Corker to Cruzbike, or Easybiking to Cruzbiking: Part II - The Conversion Kit
Edited by Dennis Perry

In the second part of the Cruzbike story, we look at why John started off in this direction. In his words …

“There plenty of reasons documented for why an easybike (recumbent) is better. The two primary reasons are less drag and better ergonomics. These factors are so important that recumbents hold all speed records, by healthy margins. (In fact, the margins are so healthy that they have been banned from most bicycle races.)

But why front wheel drive? All easybikes involve a long, long chain, long cables and often complex handlebar systems. The feet are at the front, but the power has to be transferred to the back wheel. The hands are often a good distance from the front wheel. So driving the front wheel simplifies things and this delivers a weight saving.

The Torp is around 17-18 kg, the Corker is around 13-14 kg. One common criticism of recumbents, right or wrong, is that they are heavier. The Corker is not heavier. Although there are three additional structural members (four if you include the steering tube extension) compared to a regular safety bike, the main frame is smaller for a given person size than a comparable safety bike frame and weighs no more.

Stiffness of frame is important. On a rear wheel drive, the chain constantly works to flex the frame as you peddle, eating up your power. On a front drive, there should be less of that flex as the whole drive unit is very compact. On the Corker, it takes the form of a tetrahedron, as does the rear section of a regular safety bike.

Also, because the feet turn with the front wheel, they do not interfere with each other, and this removes a significant design constraint. You can use nice big wheels.”

Together, these comments show the emerging Cruzbike philosophy that is evident in the early days of the Conversion Kit. Specifically, the simplicity of the front-wheel drive, and the stiffness of the tetrahedral shape of front forks and chain stays connecting to the head stem / front fork head.

At this point we are probably not seeing much more than where Tom Traylor left things, but appearances can be deceptive.

In the Information and Communications Technology field, which is my day job, there was a similar event in cryptography, specifically, the development of public-key encryption. The first published paper was written by Whitfield Diffie and Martin Hellman, and gave rise to the principle of the Diffie-Hellman key exchange. However, classified papers written by James H. Ellis and colleagues at Government Communications Headquarters (GCHQ), a British Intelligence agency, showed the concepts surrounding public-key encryption had been worked out several years earlier, but were classified as secret.

When Diffie finally met Ellis, many years later, the exchange is reported as follows …

"Tell me how you invented public-key cryptography". After a long pause, Ellis replied "Well, I don't know how much I should say. Let me just say that you people made much more of it than we did."

And so it is with the front wheel drive recumbent concept we know as the Cruzbike. We can now move on to the inventive part of the process. The next stage in this development process is documented in material on FADER.

We left Part I of the story musing about creating a comfortable riding experience on a Corker. John said he had an "ah-ha" moment when he saw his first dual suspension mountain bike.

On 4 May 2004, there is a working document that describes the development of a Front Articulated Drive Ergonomic Recumbent (FADER) - the adaptation for a Y-frame dual suspension mountain bike. John Tolhurst lays out the following “Design Values”, that make so much more of Traylor’s idea. It recognises the economics of the situation, that plague recumbent bike designers, and it recognises the value of a reversable conversion.

“The purpose of this design is to provide a conversion kit that can be applied to a Y-frame dual suspension mountain bike frame to change it to a recumbent style, without any permanent modification to the mountain bike so that at any time, the conversion can be reversed. This strategy would deliver a low cost way of providing a recumbent bicycle. By respecting the integrity of the donor bike it is then possible to upgrade the donor bike at any time by trading in the widely established market for new and second hand mountain bikes. None of the parts of the original bike are altered, although the seat and possible handle bar are put aside. It is possible to have any quality of fittings and components, any selection of gears, any colour and so on and to upgrade these components at any time.

The second design value is the quality of the conversion parts, expressed as durability and strength well beyond the anticipated loads and usage. Weight of components is not an important consideration, as the weight of the recumbent bicycle will largely be determined by the quality of bike that is converted. Durability, especially through the development process, is the driving value so that all converted bikes are safe to ride.”

This document was revised three months later, still as a working draft, with an expanded section entitled “Utilise existing production volumes”. Interestingly, we see the first use of the expression “CRUZ bike design”. 

The document still has the running title of FADER, but it appears this FADED as a suitable name.

“Development of a recumbent bike that is technically as evolved and refined as the many regular bikes on the market and costs about the same is impossible - because there are about 5000 regular style bikes are sold for each one recumbent. The economies of scale are simply not there. A solution would be to adapt an existing bike frame to a recumbent!

It is difficult to convert a regular bicycle to a rear wheel drive recumbent because a full size turning front wheel severely limits the positioning of the pedals. And you have a great long chain going almost the entire length of the bike The front wheel drive configuration used on the CRUZ bike overcomes this design constraint because when steering the pedals and front wheel turn together. Tom Traylor first patented the front wheel drive bicycle in 1982.”

The CRUZ name came from a combination of pragmatism and whimsy. CRUZBIKE was an available Internet domain name, and John always referred to riding a motorcycle as “cruising”. Later, Kim Tolhurst settled on “cruzbiking”as his name for riding a Cruzbike. Again, this domain was available on the Internet, and deliberately registered in the AU domain.

“The CRUZ bike design is a conversion the Y-frame dual suspension mountain bike frame. No permanent modification to the mountain bike is required and so the conversion can be reversed at any time. And the mountain bike can traded in or upgraded at any time.

Design aesthetics is important for most people. The regular bicycle frame pre-dates living memory, so strange looking recumbents bicycles with little odd ball front wheels are seen as novelties, not serious bicycles. But the CRUZ bike is a proper looking bicycle, with two full sized wheels and a regular bike frame.”

The following sections from the August 2004 working draft shows the development of the Conversion kit as a series of design challenges to be overcome. The donor frame was an Optima Cyclone, a mountain bike that meets the requirements that John outlined.

The Optima donor frame

“Frame Selection

Front wheel drive relies on consistent contact of the front wheel with the pavement because a sliding front wheel is exceedingly difficult to control. Also, on any recumbent, suspension on the rear is desirable because it is not possible to stand on the pedals while going over bumps. Bikes with dual suspension can have high-pressure tyres fitted to significantly reduce rolling resistance. Fortunately, dual suspension mountain bikes with a good range of gears are available at reasonable prices. A further criterion is that the frame slopes down from the head stem so that a recumbent style seat can be fitted at a reasonably low centre of gravity. Fortunately suitable y-frame dual suspension mountain bikes are available purchased through internet outlets at very reasonable prices in large markets such as Europe (£110) or the US.(USD 200). “

Note: These are 2004 price estimates.

“Adapting a Y-frame dual suspension mountain bike to a front wheel drive recumbent

The key to adapting the bike is solving the problem of swapping the wheels, allowing for a drive assembly to be added to the forks while retaining the action of the suspension.

Accommodating the drive wheel at the front

Fitting a drive wheel to telescopic front forks is a challenge because the telescopic arms must stay parallel in order to operate correctly, but the distance between the dropouts is around 25 mm too narrow to accommodate the drive wheel. Secondly, the forks tubes clash with the gear cluster of the drive wheel. The solution is to turn the forks 180º. This places the fork dropouts around 9 cm further back and creates room for the drive wheel without compromising the original geometry and handling of the frame. A second important constraint is to keep the drive axle the correct distance from the brake pads, so that the original braking system can be used unaltered. This necessitates lifting the drive wheel around 25 mm. The wheel must be lifted in the direction of the steering axis in order to preserve the original trail of the frame geometry. Dropouts are designed to position the drive wheel correctly.”

The solution of flipping the forks around was a breakthrough moment that occurred when John was studying the suspension forks on a mountain bike, noting that they couldn’t be splayed, otherwise the suspension wouldn’t operate. He saw how a beam, and later, a bracket, could be added to the forks, once they were flipped.

There was also an excursion into the work on rake and trail done by Tony Foale on his BMW motorcycle. This showed there was no optimum value for rake and trail, and his experiments included one setting with the front forks reversed. Armed with this knowledge, John could confidently proceed with the fine-tuning of the front-end, and move on to the rear wheel set up.


Images from Tony' Foales paper on rake and trail

“Accommodating the idle / front wheel at the rear

The rear dropouts of a mountain bike are around 125 mm wide while the idle wheel requires around 100 mm. Also, because the drive wheel is lifted in the front forks around 25 mm, the idle wheel must also be lifted to preserve the original head stem angle. Additional axle slots are provided to further lift the idle wheel in the rear dropouts and each slot effectively lowers the head stem angle by around 1º. Many front wheel drive designs utilise head stem angles down to 65º. As well as pure handling geometry, the head stem angle is one of the factors that determine the lever arm length of the pedals and hence the torque applied to the steering from pedalling. Dropouts are designed to position the idle wheel correctly, ensuring the axle stays the correct distance from the brake pads.

Design of the drive frame

The drive frame has two lower arms leading to the bottom bracket and a single tube rising to the head stem. The single upper tube has a telescopic adjustment to accommodate different leg lengths, is positioned to take the front derailleur and connects to a head stem extension tube. To allow the suspension to operate, the drive frame is mounted on bearings, where the frame attaches to the fork dropouts, where the upper tube attaches to bottom bracket and where the upper tube attaches to the stem extension tube.

Stem extension tube

The stem extension tube fixes with a wedge nut into the head set in the usual way, it provides a tube for the original stem to fix and allows it to be set at the required height. The stem extension also carries a mounting bracket for the drive frame to be attached.

Seat design

For comfort, seat webbing strung between two parallel guides is perhaps the best solution, especially in warm climates. From an aesthetic viewpoint, a sculptured hard seat is a better solution to a soft surface for a number of reasons. A hard seat can be designed to accentuate the traditional narrow form of the standard bicycle and easily conforms to the expectation that the components of a bicycle either follow or are positioned very closely to the centre line of the structure. A solid seat also has the stiffness characteristics that are expected in bicycle components. For these aesthetical and cultural reasons a solid seat design has been developed.

Many construction methods exist for forming a hard seat. This solution employs sheet aluminium where a 3 cm strip along each edge is bent to create flanges on each side. The shape of the flat sheet seat is specifically designed so that by bending the flanges, a three dimensional shape is created that has right cupping profile to be comfortable against the buttocks and back and is wide enough to provide adequate support.

The lower part of the seat is mounted directly on the frame and the upper part of the seat is supported by an attachment to the seat post. “

John credits his studies in architecture, training him to take a systems approach to problems. Thus it was always a design goal to produce a reversible conversion.


The Optima with reversed front forks in Martin Arnold’s workshop (Logo Trikes)

The previous photograph shows the beam construction that would later be replaced by a bracket, and the telescopic tube attached to what would become the new bottom bracket. The original crank is still visible in the original bottom bracket. The concept, like all good ideas evokes the incredulous “why didn’t I think of that” response! It just makes sense.

Description of the front wheel components

The beam in items 8-9-10 turned out not to be a reliable solution, and was replaced by a bracket in the final design. The Conversion Kit is the entry-level for the cruzbiking experience, and the kit components, as finally designed, are all set out there.

The Conversion Kit is the subject of US Patent application.

The essence of the Cruzbike is captured in this material, which shows how the initial Design Values gave rise to an ingenious Conversion Kit. It’s a small step to see how the Sofrider arose as a fully assembled “converted” Cruzbike, from an almost stock donor frame.It's interesting to muse on how the original mountain bike was derived from a modified heavy cruiser bicycle. Now we have the Cruzbike Conversion Kit being developed from a modified mountain bicycle. From Cruiser to Cruzbike via a dual-suspension Mountain bicycle. Interesting?

I had the pleasure of owning the first Sofrider in Victoria, Australia - bought from the website - without the benefit of a test ride. The story of how that led to the building of a Cruzbike Silvio is the subject of my own story.

Back to Part I.