Author : Lee Prescott
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 bike.
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.
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.
So 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 measure 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?
I think 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 designing a frame too. It is the perfect starting point though without any work the first contact point is already defined as 0,0.
Then by simply defining the relationship between the other two contact points by means of a vertical and horizontal dimension you have a rider’s position affectively captured. The specific points we use are the centre of the stem clamp and the top of the saddle, at the 70mm width point, as we have found this is a pretty good 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.
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 fine tuning.
By using this method one of the key benefits is that not only is your position always the same but also 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 keep in your centre of gravity and it’s 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 in a frame that is obviously too long or too short for them then the weight distribution is going to be incorrect.
The simple dimensions diagram that we supply our clients, following a fit, denotes the 4 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 etc. 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 following a fit 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. You can create your own X and Y axis by simply placing the back wheel of your bike (or even a Watt Bike etc.) 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.
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!
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