A NEW WAY OF LOOKING AT THINGS
Frames have been traditionally sized using either the seat tube length or top tube length. These conventions were carried over from road bikes.
Seat tube length would be used to give an indication of the stand-over height of the bicycle i.e. whether your crotch would clear the [typically horizontal] top tube and the top tube length would give an indication of how stretched out or bunched up you would be with your bum on the saddle and hands on the bars.
As road bike geometry is typically steep and fork length consistent, this was/is an adequate gauge for sizing and can still work well in most situations. Furthermore, the position of the rider is static on a road bike to optimise power and aerodynamics.
Modern mountain bikes generally have sloping top tubes and more aggressive seat and head angles so the previously used conventions for sizing are not entirely appropriate unless you still know how to do trigonometry.
Mountain biking is dynamic: the rider is constantly shifting their position on the bike for optimal climbing, turning and descent so there’s a lot going on at any moment.
REACH & STACK IN DYNAMIC RIDING
It’s not uncommon to see these measurements included in geometry charts and for a mountain bike, these are the dimensions you need to determine fit.
Both of these dimensions indicate your position in relation to the BB (where your feet interact with the bike) and the bars (where you hold on). These are important because your ‘dynamic’ positions (climbing, turning and descending) are of far more importance than your seated (cruising) positions.
Back-in-the-day with shorter forks, longer stems and higher bars, getting your backside over the back of the saddle was how you’d tackle the technical stuff. This was because the position of your hands in relation to the front axle was usually over it or just behind it, much like a road bike. Steeper head tube angles contributed to this further and all these variables came together to give you that wonderful OTB* free-flying experience.
A modern bike with a longer front-centre measurement (BB to front axle), longer fork, slacker head angle and shorter stem should position the hands significantly behind the front axle (as much even as 10-12″/254-304.8mm) allowing for more stability in fast swooping turns or tricky technical situations. If the reach is correct for the rider, it’ll position their weight correctly over the BB and they’ll ideally be hovering over the saddle with hands correctly positioned to weight the fork for traction and control, getting the best use from the suspension too.
Don’t get too hung up about stem length as most modern bikes are optimised for stems in the region of 50-80mm. This 30mm difference can certainly affect the bikes handling and steering but shorter stems are typically ‘balanced’ by the use of wider bars.
SIZE MATTERS FOR A.L.I.C.E.
A.L.I.C.E. does not follow traditional road bike sizing conventions so be sure to review the geometry charts and consider the kind of bars you plan to use.
Typical road-style bars have a reach of between 80-110mm (centre of bar clamp to the centre of the drops) with typically +30mm when the brake hoods are included and this doesn’t include the length of the stem.
COMPARISONS FOR ILLUSTRATION
Reach (Xa) on a medium sized Sirius 4G is 432mm.
With an 80mm stem (long for today but not entirely irrelevant) and a typical MTB bar, the effective reach (Xe) is 512mm (not accounting for stem angle, bar rise or back-sweep). Stack (Ya) is 626mm with a seat tube length (C-T) of 432mm.
Reach (Xa) on a small A.L.I.C.E. is 352mm.
With a 80mm stem and drop bar with 80mm reach, the effective reach (Xe) is also 512mm (not accounting for stem angle and hoods). Stack (Ya) is 576mm with a seat tube length (C-T) of 470mm.
Comparing a road bike to a MTB may be a little odd but the calculations above illustrate one way to size significantly different bikes and still get a good fit.
*over the bars.