5 minute read
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.
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I’m sure even the most inexperienced fitters amongst us will have come across a situation where a client has come in to your fit studio complaining that they just can’t seem to get their position right, even though they’ve transferred their dimensions from their old bike, which was comfortable, onto their new one.
When you then delve into this, it becomes apparent that they’ve made the normal mistake of measuring up the seat tube to get saddle height and then adjusted the bars to some relationship with the saddle. This method of measuring the bike is effectively chain dimensioning (see box).
My background is in industrial design and I spent my formative years working at a parallelogram drawing board (yes, I know, showing my age). I was always taught to create engineering drawings to the old BS308 standard. This standard took a very dim view on chain dimensioning due to its lack of accuracy and the fact that discrepancies in your measurements accumulate as you work through the chain.
Many years ago, when we started to formalise our fitting protocol at the studio, it didn’t even occur to me to measure a rider’s position in any way other than XY coordinates. I simply measured the riders position in the same way that I would annotate it on a drawing. It was only several years later that I discovered that tools such as the Purely Custom XY device existed. But isn’t this just over complicating the process of measuring a bike you may ask?
FUNDAMENTAL PRINCIPLE
The fundamental principle that underpins the process of getting one bike to fit like another, is that it is the rider’s position that needs to be measured and not the bike specifically. Really, all we trying to do is locate two points in space from a datum.
The obvious datum point is the centre of the bottom bracket, although I would say this as this is usually the datum you use when “By using this method, one of the key benefits is that designing a frame, too. It not only is your position always the same, but is the perfect starting your weight distribution point, because without any across the bike will work the first contact point tend to be pretty similar.” is already defined as having the coordinates 0,0.
Then by simply defining the relationship between the other two contact points by means of a The simple dimensions diagram (image below) vertical and horizontal dimension, you have a that we supply our clients following a fit denotes rider’s position effectively captured. The specific the four XY co-ordinates along with bar width, points we use are the centre of the stem clamp crank length and saddle width. Yes, we add some and the top of the saddle, at the 70mm width other detail into their report such as bar shape point, as we have found this is a pretty good and drop, saddle type and so on, but the basics approximation of where the middle of the Pubic Rami will sit, i.e. we are locating where the pelvis is, not where the saddle is.
MIRRORING OUR FIT APPROACH
What I love about this method is that it mirrors the way I generally approach a fit to begin with. I normally concentrate on getting a rider’s power centre set up by adjusting the relationship of the saddle to the bottom bracket, and then adjust the upper body to suit the intended goal of the fit by placing the bar in the right place. This tends to get the athlete into a pretty decent position before finetuning.
By using this method, one of the key benefits is that not only is your position always the same, but your weight distribution across the bike will tend to be pretty similar, due to the fact that your weight distribution in relation to the bottom bracket will always be identical, keeping your centre of gravity in its ideal location.
Obviously, the caveat here is that the bike frame needs to be the right size. If a rider is trying to get comfortable on a frame that is obviously too long or too short for them, then the weight distribution is going to be incorrect.
SIMPLE DIMENSIONS
The simple dimensions diagram (image below) that we supply our clients following a fit denotes the four XY co-ordinates along with bar width, crank length and saddle width. Yes, we add some other detail into their report such as bar shape and drop, saddle type and so on, but the basics are all there on one simple diagram. We do the same for TT bikes, just using two points in space for the bars (back of arm pad and tip of extension), which are defined in exactly the same way.
One of the questions we get asked a lot about the dimensions we supply to our clients is ‘how do we replicate this on to a different bike at home without any of your fancy tools?’. In fact, we’ve been asked it so often we created a YouTube video to explain the process of transferring these dimensions. The simple answer is all you need is an upright wall and a tape measure (above). You can create your own X and Y axis by simply placing the back wheel of your bike (or even a Watt Bike) up against the wall. And then by taking reference measurements from the wall and the floor to the bottom bracket, you can infer the other two contact points.
FUTURE STANDARDS
I’d like to think that some of the bigger manufacturers would start to think about this method a little more when they are describing their bikes. Canyon seem to be leading the way on this with their Stack+ and Reach+ system for bar position. It is also a pretty simple task to define the saddle position range in identical terms. I actually did this for the last production frames I designed for our own bicycle range. Just imagine looking at geometry charts that instead of being filled with largely irrelevant dimensions such as head tube length, you only have to check two position ranges, one for the bars and one for the saddle.
Lee Prescott
IBFI Vice President and owner of Velo Atelier, Warwickshire, UK lee@ibfi-certification.com
