The why and how of
XY Photo by Eduard Delputte on Unsplash
Measuring positions with XY co-ordinates is the gold-standard of measuring contact points in bike fit. UK fitter and IBFI Vice President Lee Prescott explains the concept.
I
’m sure even the most inexperienced fitters
drawing board (yes, I know, showing my age). I
amongst us will have come across a situation
was always taught to create engineering
where a client has come in to your fit studio
drawings to the old BS308 standard. This
complaining that they just can’t seem to get their
standard took a very dim view on chain
position right, even though they’ve transferred
dimensioning due to its lack of accuracy and the
their dimensions from their old bike, which was
fact that discrepancies in your measurements
comfortable, onto their new one.
accumulate as you work through the chain.
When you then delve into this, it becomes
Many years ago, when we started to formalise
apparent that they’ve made the normal mistake of
our fitting protocol at the studio, it didn’t even
measuring up the seat tube to get saddle height
occur to me to measure a rider’s position in any
and then adjusted the bars to some relationship
way other than XY coordinates. I simply
with the saddle. This method of measuring the
measured the riders position in the same way
bike is effectively chain dimensioning (see box).
that I would annotate it on a drawing. It was only
My background is in industrial design and I spent
several years later that I discovered that tools
my formative years working at a parallelogram
such as the Purely Custom XY device existed. But isn’t this just over complicating the process of
Chain Dimensioning is when dimensions are drawn from one feature to a second feature, then a third feature is located relative to the second feature.
measuring a bike you may ask?
FUNDAMENTAL PRINCIPLE The fundamental principle that underpins the process of getting one bike to fit like another, is
