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Crowdsourcing – Roy’s S&S Coupler Titanium XACD Tandem: the design stage

Roy got in touch with me recently; he’s already kicked off his XACD experience with a custom made 29’er (more about that later), but has something quite different in mind for his latest project! I’ll let him explain…

Roy's latest CAD design for his S&S connected tandem - what do you think? (Click the image for a larger view).

Roy’s latest CAD design for his S&S connected tandem – what do you think? (Click the image for a larger view).

The tandem is a ‘big’ project in every sense of the word! It’s going to be an MTB/road bikepack machine. I currently use a Ventana full sus 26” and tow a BOB trailer where carrying capacity is needed. But, having discovered the wonders of 29” and no longer bombing down hills or winding through tight singletrack, I though it was time for something a bit different.

The idea is to lose the weight of the Ventana and create something that can carry most of the touring gear inboard in a more relaxed way (though a trailer might still be on the cards for ‘long’ tours).

The S&S couplers are so I can get it onto UK & Euro trains without too much hassle. For ti S&S are apparently available in 50.8mm, 44.5mm and 34.9mm diameter, so that’s what I’m working with until told otherwise. I’ll use them between the stoker’s and captain’s seat tubes, our US cousins unfortunately have to make them small enough to fly with which means breaking the frame into three parts and doubles the number of couplers!

Cunning S&S couplers use wedge shaped teeth to provide a joint that's stronger than the tubing it replaces!

Cunning S&S couplers use wedge shaped teeth to provide a joint that’s stronger than the tubing it replaces!

It’s just a work in progress currently as Roy says:

The rear brake mount position is very ‘provisional’ – I’ve not had a chance to do any measurements – but there are a few around like it and putting it there helps keep the Rolhoff box in the clear.

This one includes alternative seatstays that should allow for a 29 x 3” tyre. It seemed to be a good opportunity to keep up with latest developments in the 29’er camp, although I’m still in two minds about that as a 3″ tyre could be overkill for a ‘touring’ bike.

I’m looking into another version to include Surly Ogre style dropouts – moveable disk mounts, Rohloff dropouts, derailleur hanger, BOB trailer mounts, rack eyelets and (probably) a bottle opener and kitchen sink too!

Skyde currently offer an XACD made tandem which looks very much along the same lines as Roy’s design:

Skyde's tandem, also made by XACD looks very similar to Roy's design which is a good sign!

Skyde’s tandem, also made by XACD looks very similar to Roy’s design which is a good sign!

As Roy says:

It’s interesting to see how thick the transverse tube is on the Skyde. It looks like that may be to compensate for a relatively thin boom tube – the one on my Ventana is massive 65mm whereas the transverse is only 38mm.

That could be a good solution to having to restrict the boom on the ‘new’ tandem to 50.8mm for the coupling. I’ve also seen one frame that uses a variable width boom tube, but so far I’ve not been able to identify the couplers used, and not really sure if having a fat tube with two thin ends will really make much difference either.

I certainly think it’s looking pretty smart so far! I’d say Roy was along the right lines with the size of the tubing and general design. At least with an existing bike like Roy’s Ventana you’ve got something to base the main dimensions and angles on, with a little tweak here and there for personal preference.

The use of a ‘Santos’ style c-plate for the chainstay bridge is a smart move too, otherwise the chainstays would need quite a bit of manipulation to clear the tyre.

Equally I’m a fan of the inboard disk brake mount – but could Roy live with those uneven seatstays? And was he going to go for it with a truss fork?

Odd stays are not an issue – function first on this one!

Fork is still a BIG question – an of the shelf cromo do it all from Salsa is the current choice. But interestingly, looking at the latest from the US handbuilt show almost every tandem fatbike seemed to be running a truss fork. So, either they’re fine for tandems or (more likely I suspect) they’re fork-de-jour for custom builders to show off fancy welding!

Headset will depend on the above but I’m going to stick with 1 1/8th straight just to keep it simple.

At least with a fork you can try a few to see which you get on with best, but I would consider going with one of the new, larger, format headtubes. With a bigger tube you can always run a smaller fork if needed and the larger tube offers plenty of surface area to weld those large main tubes to.

What do you think of Roy’s design so far? Has he missed something obvious or does it look about perfect already? Drop me a line via the contacts page and I’ll pass your thoughts on!