Trying to work out the geometry and critical dimensions of your frame is the hardest part of getting your own build done.
The best starting point is measuring up a current favourite bike – and then maybe factoring in the latest trends from magazine reviews and manufacturers spec sheets. A big problem in trying to copy a design is that there are so many ways of measuring a frame; virtual or actual, with sag or without sag, and then you’ve got to factor in the effects of different fork lengths and tyre sizes too.
Also working against you is the way some magazines decide to measure frames, MBR think you can work out how well a frame fits without giving you any top tube dimensions – the cocks! Manufacturers are sometimes a bit cagey with their figures too, and some seem to be quite different compared with measurements from magazine tests (I’m thinking Cotic here in particular).
Broadly speaking I was after a general purpose ‘trail’ hardtail with excellent singletrack handling that would take a 120mm travel fork (length of the Fox F120 QR15 I’m using here is 490mm with an offset of 39mm). It should be fairly roomy front to back and be designed to work with a 70mm stem and bars of around 26″.
If you’d like to compare I’m a fairly average 5’10” tall with a 32″ inside leg (trouser size!), my XC background means I like to be nicely stretched out too!
Working from ‘Ti-Bride’ I realised that I’d not made the head angle slack enough, especially as I’d designed it to run with a short 70mm stem (which speeds up the steering). The steering was pretty quick and the front wheel had a tendency to ‘tuck under’ when cornering tightly on the limit.
Looking at tests of current best hardtails like the Wyte 19, Orange P7, Cotic Soda/Soul, Charge Duster, Ragley Ti etc. the frame angles were coming out at an average of around 67-68 degrees head angle and a 71-72 degree seat angle so I went with something similar.
Calculating the head angle on Ti-Bride was a bit tricky as I didn’t know the length of fork that was going on it. Working backwards from a 100mm travel fork I used Brant’s rule of thumb about how much slacker the head angle became when you stick a longer fork on (about a degree per inch of travel). Either way I thnk it ended up at 69.5 with the longer fork on – which was too steep!
Ti-Bride currently has something like a 73 degree seat angle and I’ve got the saddle fairly well back on the rails, having a slightly slacker seat angle should put the clamp back in the middle of the rails where it should be.
There are various schools of thought as people like Brant want a steep seat angle to put more weight forward for better balance when climbing. I think that the location of the saddle should only be govered by bio-mechanical factors – ie. getting your knee in the right position over the pedal axle. For me, anyway, it made more sense to go slightly slacker.
Chainstay length was fairly easy too, I went with 425mm (16.73″). Ti-Bride wants to wheelie fairly readily and I didn’t fancy making it any shorter which would have exageraged that!
Seat tube length (or frame ‘size’) – I went with 440mm (17.3″) measured from the centre of the bottom bracket to the top of the seat tube. Ti-Bride is 20mm larger and looks a bit of a gate! I knew I could go lower as I’ve got a 400mm post and my full sus frame measures up at around 17″. A smaller frame should be lighter and stiffer.
I think top tube length should be measured as a virtual horizontal (from the virtual start of the top tube, in the middle of the head tube, horizontally back to the centre line of the seat post). It’s just the best way of working out how ‘stretched’ you’re going to be on a bike. 595mm (23.4″) for me is spot on, could be considered long by most folks but remember we’re going to marry it up with a fairly short stem.
Bottom bracket height is another key factor (measured from the centre of bottom bracket to the floor) . I don’t like going too high as it starts to feel less stable at speed, I picked a drop (from the centre line of the wheels) of 15mm which should translate as a fairly typical 12.5″ bottom bracket height with 2.1″ size tyres.
And that’s about it, for head tube length I wanted to keep the front end fairly low so I spec’ed 110mm which is what Ti-Bride came with. This would result in some overlap between the top tube and the down tube but nothing extreme, I trusted the frame builder to plumb in the various pipes in the best order!
See, dead easy – you can check the finished result on the drawing I’ve attached on this post…