Dan Titchmarsh explains his Scorpion Cargobike

K4RGO - Titchmarsh, with rosette

Freshly baked for Bespoked Bristol, the Scorpion has a number of innovative features. Dan, the frame builder who designed and built the bike, explains:

The design had been evolving for 12 years in the back of my mind after I rode as a courier and realized front loaders could be better. I started to design it properly in January 2013 as I thought it would be an interesting project to do for the show. It was evolved using solid modelling which took about 400 hours and then 12 solid weeks of making…
The weight of the flat bed version is 22kgs, and I could probably shave a couple of pounds off this as well. I can also alter the load bed size to client requirements and load capacity which can mean lighter tubes, possibly butted in places. I am also planning a super light version with butted tubes and a thin wall beam, a 24” front wheel (rather than a 20”), shortened load bed and wheel base with thin road tyres for road touring…
The frame is fillet brazed steel. I used straight gauge 17 S.W.G. CDS2 for the main beam and oval tubes on the swinging arm and the rest is straight gauge 18 S.W.G. Reynolds 531 barring the stays which are straight 20 gauge 531. It’s all brazed with Sif Bronze 101 wire and a gas fluxer, all but a little bit of TIG here and there.
In order to reduce rolling resistance I gave it active front suspension (swinging arm on rubbers) and passive rear suspension (S-bend stays and long seat post which flex slightly to absorb shock loads).The front suspension uses a polyurethane bump stop from a Mini. This gives a progressive rate and inherent damping. So no matter whether it’s unloaded or you have two rugby mates on the front there is still suspension. The travel is about 40mm. The pivots are rubber bushes in torsion which are pre-twisted to force the swinging up against the bump stop eliminating the need for an end stop rod. This also adds rebound damping!…
K4RGO - Titchmarsh, front hub
To save weight and increase frame stiffness and longevity, I went with hub centre steering. There is a substantial weight saving with not having to take a structural member up to a head tube. On a cargo bike when you could need to brake 250kgs + of all up weight going down a hill at 20mph this head tube needs to be held rigidly as the front forks act like big levers trying to bend it back (this can also cause brake juddering). As the main beam runs low under the load bed on my bike it is logical just to project the swinging arm forward to the hub saving a few pounds of weight in the process. This comes at the cost of a complex hub steering arrangement and increased unsprung weight. Unsprung weight is not really an issue on a road going pedal bike as the bumps are not coming big and fast enough and I enjoy an engineering challenge. Also there is virtually no flex under heavy braking as the load path is straight back to the beam along the length of the swinging arm. This means it can handle higher braking forces than a fork arrangement for a given weight of frame. I chose two arms rather than single sided after doing some stress analysis and mass calculations in CAD and it turns out there is quite a big stiffness to weight advantage with two arms. I did want single sided as it would make punctures easier to fix, but I rationalized that a puncture can be repaired without taking the wheel out as long as you don’t want to swap the tubes. It’s only five minutes to drop the wheel out anyway. The disc on the hub is a Hope 203mm saw tooth rotor with a 4 pot calliper. I sacked off the spider from the inside and used where the rivets were as my new PCD (pitch circle diameter)…
The steering is actuated by means of a push / pull cable. This has a weight benefit and has a more positive feel than a link rod (which would need to have a big kink in it which makes it vague on feedback)…
The over-centre gas ram prop stand is highly integrated into the design as it pivots on Nylotron bushes in a cross tube which is welded through the beam. This cross tube is joined to the down tube in a ‘T’ shape. This means that the down tube does not push on the top of the beam but instead transfers the load to the relatively unstressed side walls of the beam thus preventing the nucleation of a buckle and allowing a much thinner beam wall thickness to be used. The intersection between the triangulated rear end and the start of the cantilevered beam would otherwise be the Achilles heel of the frame…

Read the whole article here

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