Custom means you really can change anything – but how do you know what size and thickness of tubing to use? Titanium bikes started off with tubing sizes similar to steel frames which made them very light but also gave titanium a bit of a flexible reputation! XACD is pretty old school if you leave them to it which is why I decided to run with some larger diameter tubing for ‘Love You Long Time’.
Starting at the backend we have 19mm diameter seatstays and 22.2mm diameter chainstays. From research that seems to be pretty common, although some people use 19mm for the chainstays too (Ragley by the looks) and others beef ’em up to 25.4mm (Van Nicholas)!
I liked the way the stays on ‘Ti-Bride’ looked ‘right’ so I left things as they were. In both cases the tubing is 0.9mm wall thickness which appears to be the standard gauge for bike tubing.
Seat tube wise I wanted to run with a 27.2mm seatpost so I stuck with XACD’s standard 31.8mm diameter tubing, again with 0.9mm wall thickness. This tube is then shimmed down with a 100mm long sleeve to 27.2mm. Some manufacturers spec a 34.9mm tube here for extra beef, if you’re a big lad then you might want to do the same. Although the larger seatpost you’ll need might give a slightly harsher ride.
Now for the two biggies; downtube wise it looks like most contemporary designs work with a 44.5mm diameter tube (Ragley and Van Nicholas for example) which is slightly ovalised at each end so it meets the head tube and bottom bracket correctly. I was even able to specify the distance over which the tubing is tapered (200mm rather than the initially suggested 100mm) for a more subtle change in size. It’s interesting to note than even though I chose a design which would need quite a bit of extra tube manipulation and mitring I wasn’t charged any more – so don’t be shy in coming forward with your own designs, it might not be as expensive as you think!
For the top tube I went slightly larger than XACD’s standard with a 34.9mm diameter, again ovalised so it butts against the seat tube correctly.
For both the top tube and down tube we’re using 0.9mm wall thickness tubing. It is possible to ‘butt’ titanium tubes, that is to vary the thickness along the length of the tube so you save weight in the middle of the tube while still keeping the ends thicker for strength. But due to the problems of working titanium tube this ends up being really expensive to do, and often only saves a tiny amount of weight. Given the extra cost and the fact that no-one’s quite sure of the technique XACD use to butt tubing I thought I’d play it safe and see what happens – it’s also cheaper that way!
And that’s about it, I’ve no idea about the bottom bracket and head tube so they’ve been left well alone! XACD run with a normal 68mm bottom bracket shell and a standard 1 1/8″ headset headtube.
Et Viola! We’ve just put together a recipe for a contemporary trail ready titanium hardtail which should have decent stiffness while still retaining some titanium comfort wihout any of the flex problems of designs that go too light.