Birth of a fat tandem

I’d listened to my friend and team rider Dan blathering on for some years about getting a fat tandem. He’d had various builders lined up to do it who all fell by the wayside for what were ostensibly a variety of reasons, but I suspect ultimately came down to the fact it’s a damn difficult thing to build. Even more so because he wanted to make it singlespeedable….

See, Dan and his wife and redoubtable stoker Jo are dyed in the wool singlespeeders and liked nothing more than powering past hapless fools labouring uphill, while on a tandem, with one gear, and a fat front wheel. See, for a few years they had been doing just that on a bodged and spliced together old Dawes tandem with a fat front fork. However this was always meant to be a stop-gap until they could finally find someone to build their dream machine.

It was at Battle on the Beach 2015 when Dan and I got chatting about the current status of their fat tandem project and the difficulties he was having in finding someone to bring it to reality. A few cogs started whirring in the back of my head as I recalled that an aluminium framebuilder in the Czech Republic I had worked with on a few projects had also built fat bikes, tandems and singlespeeds. Surely we can just bosh all of these elements together and sprinkle them with a bit of Singular design magic and we’ll have the bike of the Treby family’s dreams?

Well, initially it seemed as though it might be just that simple. I contacted the guys at Duratec to give them a broad outline of what we had planned and whether they might be willing to take such a mad project on. To say they jumped at the opportunity would be exaggerating more than a little…. But they are adventurous and resourceful folks and they seemed up for a challenge.

Now I have designed quite a lot of bikes in my time, but never a tandem. I have never even really ridden a tandem in anger. However being an enthusiastic student of bicycle frame design I had read various pieces over the years which referenced tandems and I had formed a few ideas about what might work well and what wouldn’t. How they rode, steered, and what kind of geometry was needed to do ‘good things’. Fundamentally, tandems are huge long things and giving them any capacity to go around corners is not easy… Particularly I was inspired by the French Herse racing tandems of the 1950’s which used very low trail numbers in order to compensate for the extra weight on the front wheel and enormously long wheelbase. Would these things apply to a fat tandem? Who knew?

No-one it seemed… Everything I could find about geometry for fat tandems (which was not an awful lot…) made them pretty slack. Fundamentally geometry on a par with a modern fat bike. Even most things I could find on modern mountain and even road going tandems seemed in most cases to replicate the same numbers you’d expect to find on their ‘single’ equivalents. A few companies made the case for low trail tandem geometry and from what I could find they were particularly well regarded for their handling. This supported my feelings that we needed to keep the trail as short as practical as well as trying to keep a lid on wheelbase.

With my thoughts and opinions somewhat reinforced it was time to hit the drawing board – or in my case CAD program. I do all of my initial designs on a rather wonderful piece of software called BikeCAD, designed by a Canadian bike nut and programming boffin by the name of Brent Curry. He provides a ‘light’ version of this java based software on his site for free, then there is a paid version which gets all the bells and whistles and ability to design tandems as well. It’s a remarkably intuitive and easy to use bit of software, but very powerful for designing bikes for everything but the minor details. I bought the ‘pro’ version of the software in 2006 when I started dabbling in designing bikes properly for the grand sum of $300, it might even have been canadian dollars… Since then there have been numerous improvements and updates, none of which I have had to pay any extra for. I guess Brent is not going to be the next Bill Gates….

Anyhow, tandem design…. Dan has ridden a large Swift for quite a few years and it fits him like a glove. Jo rides a small Hummingbird which also fits her quite well. So fortunately I had a pretty good baseline to work from in terms of fit. I also had some pretty solid ideas on the geometry I wanted to get the handling we were after so that was also sorted. But tubing for a tandem? Frame lay out? All these things were new to me and I’m glad to say the builders were a great help in that respect as they were able to draw on their experience of tandem fabrication.

My initial design used an ‘open’ double triangle design, with a top tube running straight from the head tube to the rear seat cluster, and a normal front triangle just joining the front and rear frames by means of the bottom ‘boom’ tube as they are called. Dan had asked for a nice plain design and my philosophy is always to keep it simple and use nice straight tubes. However the recommendation was to use an extra ‘downtube’ connecting the headtube to the stoker’s bottom bracket shell. This seemed to make sense to offer some more reinforcement and torsional rigidity for a bike which was going to have a lot of rubber on the ground and force going through it.


So I put together a drawing with this configuration, suggested tube diameters, and the geometry we needed to fit Dan & Jo – along with my thoughts as to geometry. A few revisions and back and forth with the builder and we had something we thought would be pretty workable. We wanted to use a suspension fork up front so went with a 44mm ID head tube. And to get as much tyre clearance as we could in the back we opted for a 190mm rear hub spacing with the plan to fit a 4.8” tyre on a 100mm rim.


Then we needed to think about the drivetrain. Traditionally tandems have a timing chain on the non-drive side attaching the front and rear cranks. This therefore requires a ‘drive side’ crank arm in both locations, but threaded for a left hand pedal. These are readily available for standard width bottom brackets, but not for the 100mm wide one of a fat bike. The options were either to get very creative with tandem chainsets and super wide square taper bottom brackets – or just run the timing chain on the right. A drive side timing chain is not an unprecedented thing, I recall first seeing one at the Singlespeed World Championships in Napa in 2007, and I’m sure they pre-date that. But a singlespeed tandem is a fairly rare beast. However with the advent of single ring drivetrains it’s starting to become a more common thing, at least on off-road tandems.

For some time I played around with the idea of trying to use a standard width captain’s bottom bracket and crank set which would run a drive side timing chain to a rear chainring in the inner position of a wide fat-bike chainset. Then run the drive chain off the outer ring of the rear cranks to the freehub. However we came to the conclusion that while this would be fine in the singlespeed setup, chainline would likely be excessively compromised in a multi-speed arrangement. So we opted for two standard fatbike chainsets, the front one with a single 32 tooth chainring in the outer position, the rear with two 32 tooth rings in both the outer and inner positions. The inner would then be used to drive either a single cog on the back for one-gear madness, or a wide range 10 or 11 speed cassette.

That meant we would need two extra wide 100mm eccentric bottom bracket shells. On paper and in theory not a problem at all. In practice somewhat more difficult. The builders had never made an eccentric for a fat bike, and as such didn’t have any suitable shells. Moreover the tubing they had for making standard eccentric shells was a little on the thin side and we weren’t 100% confident it would stand up to the rigours of tandem fatbike singlespeeding. So we needed to have some shells made up specifically – out of a solid lump of aluminium billet!

A whole lot of umming, ahhing, hemming, and hawing then went on over cable routing options and placements, rack mounts, bottle cage mounts etc etc. But in the end which stuck with a nice and simple arrangement of full length outers all the way and minimal other braze ons.

Drawings were signed off, money was paid and the waiting game began! And continued, and dragged on. This being an unprecedented build a few issues cropped up during manufacture. None of which were irresolvable, but all of which took some thinking about and therefore time. Meanwhile Dan was getting antsy – ‘will we have it for this event?’ I hope so, was the response but a couple of these timeframes came and went. He wasn’t being impatient, after all they had waited four years to get this project off the ground, but now that it seemed to be getting so close they understandably just wanted it to be here!

Finally the day came and we had some pictures of the as yet unpainted frame come through mocked up with some wheels and a fork. Jaws dropped, superlatives were used, in some cases expletives. It wasn’t even painted yet but it looked amazing! A shiny silver beast for two backs.


Wonderful, it looks as though everything fits and all the bits are in all the right places. Now just for a nice thick layer of Singular blue powdercoat and to find a big enough box to ship it over. Shipping details were sent and forwarded on to Dan. By this point he was climbing the walls, I think he must have been checking the tracking updates hourly. One morning I had an excited call from him ‘it’s here it’s here!’ – the UPS man was saying it was out for delivery that day. Sure enough an enormous box soon arrived. I was excited to see it too so I hurriedly unpacked it and gave it a once over. It looked to be beautifully made and up to the excellent high standards I’ve come to know I can trust from these guys. Everything was bang in alignment and round and straight as it should be. Dan turned up shortly thereafter having jumped in his car and blasted down the motorway from Stourbridge. He just about managed to squeeze it in the back of the Land Rover and shot off back home to get it built up.


Sure enough there were a couple of little things which meant the build took a bit longer than hoped – the right size chainrings were needed, the super long tandem derailleur cable and the frustrating small things that can often hamstring a hugely anticipated build. Nevertheless Dan’s mid-build shot precipitated huge enthusiasm – I won’t say it quite broke the internet but it quickly became my most ‘liked’ ever photo on Facebook.

Finally all of the pieces came together and about 9 months from the first drawing Dan and Jo were able to get out for a ride on what Dan dubbed in a flash of divine inspiration, the Toucan.

Overall this was a hugely satisfying project to work on. It was something completely new for me and made me learn a lot of things along the way. Collaborating with a good friend and a hugely skilled fabricator to make something unique is the ultimate reward for a bike designer.

Of course, the questions quickly cropped up ‘when are they going into production?’ How can I get one?’. Well they won’t, and for the moment you can’t. However once the Trebys do a bit more testing and I’m sure we’ve got the geometry and details dialled I hope that we will be able to offer these amazing beasts on a custom basis.


“This bike is absolutely awesome, rides way better than we could have ever asked for. Cheers Sam for helping our fat tandem dream come to life”


Sam Alison is the Founder and Designer at Singular Cycles

Author: Sam Alison

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1 Comment

  1. Are they available yet 1-11 configuration ?
    Thank you

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