What do you think about my frame geometry?

A few reasons for the long head tube, none of them super good, so I may change it now someone's noticed. Need to give it some thought:

1. Ceeway sell headtubes in 160mm lengths so it would save me the hassle of having to cut it down exactly straight and face it. And yes I know you should face it anyway ;)
   
2. A longer headtube means I don't need to use stem spacers (got the stem the wrong way around on the drawing as well, would be vertical for normal use and facing up as shown for longer more relaxed rides) but still not have an enormous seat-bar drop.
   
3. Having a short headtube is all very well and good but puts more strain on the joints (or so I've heard but when you think about it, it's just reducing the size of the lever, so maybe it reduces the stress ... who knows.) and means that only someone with a correspondingly short headtube will be able to use your fork in future.
   
4. I'm going to experiment with thinner bamboo tubes than is normal; usually 40-45mm tubes are used for the main triangle; I'm going to try more like 35. I'm 80% sure this will be stiff enough if the joints are good (my slightly noodly thick-framed bamboo bike has taught me that the joints are the big deal). But no-one (that I can find) has done that before; I guess it doesn't have the margin of error for commercial builders and there aren't that many hobby builders. If it turns out to be awful a longer headtube would let me graft in a second TT like on some of the bigger Rivendell frames. It would still end up weighing a respectable amount, though more than I plan at the moment.
   
   

How do you mean, almost a polo bike? Is it the lack of trail and the fat tyres? I think fat tyres are great; wish that I could get some of the Rivendell or Grand Bois gucci ones, but I'm happy with the Specialized Infinity ones I've got at the moment, apart from the weight. On thinner tyres I'm just rattling around the whole time and the placebo effect of feeling sportier is not enough to convince me that I'm going faster ;) as for the lack of trail I don't know whether I should play it safer. I've read some very convincing arguments in favour of low trail bikes, though many of them have been from the point of view of tourers using handlebar bags and 650 wheels. High speed stability is not an enormous issue, but the question is whether it would be too twitchy at normal speeds. I could lengthen the chainstays, but the rear triangle is quite difficult when you're building with bamboo...

Thanks for your replies.