No6: Stack and reach
Why traditional bike sizing doesn’t mean a thing
Why is it that you can be a size nine in one brand of shoe, but a size 10 in another? And why isn’t there a standardised sizing system to tell us whether or not a shoe will fit before we try it on?
It’s a similar quandary for bicycles. Not only do brands have different ways of sizing frames, but the most common method isn’t particularly effective at indicating whether a bike will fit or not.
Traditionally, frames are sized by the length of the seat tube, usually measured from the centre of the bottom bracket to the top of the tube itself. As with shoes, this figure is little more than an arbitrary number, so most bike manufacturers also provide a lot more detailed sizing information, packaged up in the geometry chart. Except then there’s a danger that data overload will confuse anyone without a proper understanding of bike fit. That’s where stack and reach comes in.
Getting down to it
Stack and reach values, first championed by Cervélo, simplify frame geometry down to just two measurements that focus on what matters most in bike fit – the position of the front end. Stack is the vertical height between the bottom bracket centre and the top of the centre of the head tube. Reach is the horizontal distance between the same points.
‘It’s not like these are new metrics – they’ve always existed,’ says Tom Sturdy, head of education at the Bicycle Academy. ‘If you’re designing a frame you can’t do it without those measurements. But the industry hasn’t done a great job of explaining them to the consumer, which is a shame because stack and reach are certainly a more accurate way of defining the size of a bike than traditional measurements.’
Their benefit is twofold: firstly, they allow a bike fitter or framebuilder to accurately match your measurements to an ideal bike set-up irrespective of any stated frame size. Secondly, they allow precise comparison between bikes.
The latter makes stack and reach a valuable tool if you’re in the market for a new bike and want to compare the fit of, say, a Trek Émonda with that of a Specialized Tarmac. For argument’s sake, let’s compare the size 56cm for these two very popular choices. The Tarmac has a given stack of 565mm and a reach of 395mm. The Émonda has 577mm stack and 387mm reach.
Thus the Trek’s front end position is 12mm taller vertically and 8mm shorter horizontally. That means (assuming equivalent bars and stems) the Tarmac has the more aggressive riding position, which may sway your choice of bike depending on your riding preferences.
Yet there is another issue. ‘The main failing of stack and reach is that they only refer to what is happening in front of the bottom bracket,’ Sturdy says. ‘If two frames have the same measured reach but one has a slacker seat tube angle [putting the seat further back], that creates additional reach, which isn’t taken into account. We’re seeing a trend towards slacker seat tube angles in search of more comfort and it throws up a flag that the front end looks short [in its reach value] but once built that won’t be the case. That’s the caveat.’
He adds that you need to consider stem length and handlebar dimensions with off-the-peg bikes too. ‘These can alter reach a lot. To my mind a slightly more accurate way to represent the fit would be to measure stack and reach from the centre of the bars, not the head tube. Or even more useful would be to reference the actual contact points, but maybe that will come in the future. For now though, stack and reach is still the most concise way to sum up whether a bike will be in the right ball park.’
‘The industry hasn’t done a great job of explaining them, which is a shame because stack and reach are a more accurate way of defining the size of a bike than traditional measurements’