Calculation Method: (Well, I ‘think’ I done did’er right but buyer beware……..)

from http://www.giant-bicycles.com/en-US/...in/2314/32252/ (med size frame)
1) get angle C ,standard head tube angle 71 deg.
2) get length of line b. wheel base length, 42.5 in – 1079.5mm

from http://www.foxracingshox.com/fox_tech_center/index.htm (keep digging to get to the drawing)
3) get length of line a, axle to crown for Fox 32mm forks, 100mm fork-470mm and 140mm Talas-510.9mm

from http://www.analyzemath.com/Geometry_...alculator.html (Also handy for checking/finding all your bike angles, all you need is a tape measure)

use SAS calculator to find angle B and length c -- So both of these ‘calculated’ variables are ‘fixed’ by the frame itself and can not change. They are then used to calculate the different HTA’s for each axle to crown (AtC) fork length.

(Note: My 'angle A' and 'line a' convention may not match up with the online calculator's, but just cut & paste as needed if you want to do your oun calcs)

Ta da--Using fixed angle B and length c and the SAS calculator find the Head Tube Angle for:

71 deg -- Stock 100mm fork, 26inch wheel (470mm AtC) Sharp XC bike
70.38 deg -- Stock 100mm fork, 650b wheel (482.5mm AtC) Std XC bike
70.33 deg – Talas set at 100mm , 650b wheel (483.4mm AtC) Std XC bike
69.34 deg – Talas set at 120mm , 650b wheel (503.4mm AtC) Std Trail bike
68.36 deg – Talas set at 140mm , 650b wheel (523.4mm AtC) Relaxed Trail bike

(I added 12.5mm to each AtC for 650b front wheel vs std 26 wheel)

So looks like an Anthem X frame with an adjustable Fox Talas 140mm fork and 650b front wheel would give you ‘three’ bikes in one!
1) 4X4 standard handling XC race bike (70.33 deg HTA)
2) 5X4 Trail/XC race bike (69.34 deg HTA)
3) 5.5X4 relaxed Trail bike for that rocky downhill stuff or your trip to Moab (68.36 deg HTA)

And all with a 27in front wheel to ‘roll’ with.

You could buy a frame & build it up, or buy any model, X0, X1 etc and sell the std 100mm fork on ebay


Comments??????????

Note:
BB height for a std Anthem is about (I am guessing) 12.3 to 12.7, if this is so then the increase in BB height shouldn’t be an issue unless you try to do a steep climb in the 140 mm range.