And now for something completely different. Just in case you were worried frame design was looking a little conservative at the moment, here comes Adrian to save the day:
One good thing about XACD is that they will stretch as far as your imagination can go. Adrian had an idea for a radical design and by far the most cost effective way of seeing it brought to life was via XACD:
My reasons for the frame – the main one was to get the shortest chainstay I could, I wanted a short wheelbase bike to get a snappy 29er and to have clearance for 2.5 tyres without chainsuck problems etc. Elevated stays seemed like a good solution. Other positives would include it being belt drive compatible without having to split the seat or chainstays.
There were two main reasons for the long dropouts, to get the chainstays more horizontal for looks and to keep them short and stiff. With the long dropouts I could get the brakes mounted on them and away from loading just the chainstay or seatstay, so they both share the braking loads, also they would be changable to any present or future axle system.
I also think that most sliding dropouts only mount on 2 bolts which means it has a natural point through the bolt centers which it can flex with three it should be stiffer.
I’m certainly with Adrian on that last point, even though many sliding dropouts have a load bearing surface, the idea that the back wheel is held on with four slim bolts isn’t really that appealing.
Along with the practical aspects of the design Adrian also had an aesthetic ideal that gave his frame graceful curves that flow from front to back. He was aware though that the elevated stay design would lead to a compromise of stiffness around the bottom bracket area, but at the end of the day the only way of finding out for sure was to build one and test it…
I asked him how he felt it had turned out now he’s taken delivery of Mark 1:
It’s only been around the block as I need to get a bigger seatpost for it, but the quick spin proved it was very agile. The bottom bracket probably isn’t stiff enough to run a belt drive, it’s not as stiff as my Ragley TD-1, but it’s certainly ridable.
The dropouts are well over engineered hence it’s a little wide and heavier than I’d like, but the idea was to get a ridable first bike which it is.
In the 5 months it took to get built XACD were moving factory and I think the design stretched them somewhat. I have already had some ideas for V2 and can see after getting V1 there are ways of shedding some weight and increasing stiffness.
The quality is faultless.
Bike forum comments have shown it’s a real ‘Marmite’ frame; provoking extreme reactions of one sort or another which can only be a good thing.
I like the way Adrian has approached the build from an aesthetic point of view, he’s got a firm idea of how it should look even though it could have been done cheaper with straight tubes. With an elevated chainstay bike it was always going to be tricky to make the bottom bracket area stiff enough, extra bracing or having asymmetric chainstays would help, but may spoil the lines of the design.
One of the comments left on twentynineinches.com suggested that if we ignore the need for a front mech then the chainstay junction could be brought down lower to boost stiffness. The downtube could be made a larger diameter, and perhaps some subtle bracing could be added to the bottom bracket area.
An alternative bottom bracket shell might also be worth considering; a BB30 or BB86 shell would mean a larger tube which would be stiffer in itself and offer more metal to attach main tubes, or additional struts, to.
I like how the dropouts have turned out too, and it sounds like Adrian has already worked out how to thin these down to save weight for mark 2. It’s going to be a very interesting journey and I’ll keep you posted as and when we get more updates!
Have you had a frame made by XACD? If so I’d love to hear about it and maybe feature it in the Gallery so drop me a line via the contacts page.