Most people compare stack and reach measurements because that’s how bike geometry has always been done. But on a modern track bike, reach can lie to you.
From over 10 years of designing track bikes and navigating multiple generations of bikefitting trends and UCI rule updates, this blog shares some of the insights into what matters most when developing the geometry of a track frameset, the components that integrate with it,and how it responds to the bikefit.
What are we looking at
Lets first begin by outlining the some definitions of the various features of the track bike in context to this discussion.
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Bottom Bracket: The interface of the frame where your cranks rotate around. This is considered the origin for bike fitting and bicycle geometry
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Stem: The component that connects your handlebars to the frame. The stem is a translator to achieve a desired handlebar position
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Stack: The vertical dimension from the centre of the Bottom Bracket to the centre of the stems bearing surface at the top of the head tube.
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Reach: The horizontal dimension from the centre of the Bottom Bracket to the centre of the stems bearing surface at the top of the head tube.
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Front Centre: The horizontal distance between the centre of the Bottom Bracket and the centre of the front wheel axle.
The athlete comes first - the equipment is selected to achieve the athletes fit requirements
The frame is the core to achieve this. This is where all the components are bolted to. And if the frame isn’t the correct size, no matter what size stem you fit onto the bike - you may not be able to get the handlebars into the correct position for the athlete - or compromise handling, control and system aero.
The UCI also reference the BB as the origin for many component position limitations - such as saddle setback, and indirectly - handlebar positioning.
Why Front Centre Matters
Article 1.3.022 (figure 34 below) of the UCI ruling states that “In no case shall the front of the handlebars exceed the vertical plane passing at horizontal distance of 100 mm from the the axis of the front wheel spindle”
And in an earlier section; Article 1.3.016 (figure 8 below), it stipulates that the front-centre (measurement taken from centre of BB to centre of front axle) can have a maximum measurement of 650mm.
Therefore, the maximum UCI position a handlebar can be from the bottom bracket on any UCI legal bike (at maximum front centre) is 750mm (650mm Front centre + 100mm in front of the axle)
The Reach Paradox
Reach is the traditional indication of how long a bike is - and is functional on road bikes or older track bikes before the trend of increased length. However, when taking into account the modern track cycling trend of maximising handlebar reach out to the maximum 100mm (regardless of frame front centre length), the handlebar position no longer relative to the reach value, but to the front centre measurement. The distance between the handlebar clamp, and the top of the head tube is now a derived value. I.e this is the length of stem you need to achieve a desired fit.
There is a reach paradox that can be misleading whereby a larger size frame can have a shorter reach. This does not occur very often - but is most likely to occur on track bikes.
Hear me out. If you increase stack (taller front end) but keep the front centre and the steering geometry consistent, the top of the head tube moves up and also back along the steering axis. The result is the reach gets shorter even if the bike can place the bars in the same spot i.e 750mm from the Bottom Bracket.
As an example, we sometimes get asked why the XL sized Altias Fusion frame has a shorter reach than the L. Both frames are maximised out to the 650mm front centre measurement, the XL has a taller stack than the L. For taller athletes there are aero benefits from lifting up the head tube (and top tube) rather than having a positively angled stem. However, a longer stem is required to achieve the full 100mm in front of the axle handlebar reach. The XL Altias Fusion has a 10mm shorter reach than the L, but is a bigger bike - and more suited for taller athletes. Taller frames mean longer stems for the same handlebar reach.
Reach tells you where the head tube is. Front centre dictates where your handlebars are. The difference is the size of stem you need.
Setting up the Cockpit
When setting up a bike fit - or transferring a fit from one frame to another, it's critical to know the handlebar position relative to the origin (Bottom Bracket), rather than the top of the head tube. This keeps all the measurements consistent and relative - no matter what frameset you’re setting a position up on
Once you know your handlebars position, and identify the frames stack and reach numbers from the geo table, you have all the information needed to know what size stem is required to achieve your position.
The process involves a bit of algebra - but no stress, because we have created a calculator to manage that all for you. For those who are interested, the formula is below:
Stem X coordinate = (a - b) - c
Stem Y coordinate = (x + y) - z
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a = Total handlebar reach from the bb to the leading edge
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b = Handlebar reach from the stem clamp to the leading edge
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c = The reach of the frame
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x = Total handlebar stack from the bb to the centre of the grip
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y = Handlebar stack from the stem clamp to the centre of the grip
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z = The stack of the frame
The result is an Horizontal (X) and Vertical (Y) coordinate that a stem needs to span between the top of the head tube and the centre of the handlebar clamp. A secondary calculation behind the scenes uses some Trigonometry - introducing the head tube angle to translate those Horizontal (X) and Vertical (Y) coordinates into the most suitable stem and spacer stack required to achieve the desired handlebar position.
The full calculator is available up on the Performance Tools Dashboard
Conclusion
If you take one thing from this: reach is not a fit target on a modern track bike, it’s primary function for bike fitting is to distinguish how long of a stem you need to position your handlebars in the correct position. Once front centre is pushed toward the UCI limit, and the bars are pushed toward the 100mm limit, the ‘real’ fit target becomes handlebar X and Y relative to the bottom bracket. The stem is simply the translator between the frame and that target.
So when you’re comparing frames, start with stack and front centre. Stack tells you whether the front end is in the right height range. Front centre tells you the limitations of your handlebar reach relative to the bottom bracket.
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