This is a really interesting topic as to why two bike fits can be different. Some of it comes down to our understanding of what these systems, such as Retul, actually do and how the fitter might make use of them. It also comes down to the modern world’s belief in computerised systems as being the best and correct.
I use the STT 3DMA system. I have customers ring me and ask if I use a computer in the fitting as often they think it’s better than not using one. Sometimes they chuckle when I tell them that I write their reports up on a computer, does that count? I then ask them why they asked that unless they already hung up.
Basically, these systems are measurement devices. Some of them are more accurate than others for example, due to the sample rate. On top of measuring, some of them compare a taken measurement against a database of accepted measurements and angles, such as knee max. This will help an inexperienced or rushed fitter develop a fit for your average person. Not average as in performance ability but rather, average biomechanically.
You can read several books listing knee angle ranges and also see them in these software systems settings. Knee max angles can range from 138 to 144 in the literature I’ve read. Now that’s quite a range isn’t it and obviously could lead to different saddle positions. Then take into account that our joints aren’t exactly consistent in where they pivot and that other points of articulation, such as ankles, help absorb differences in knee max. Now, if that ankle has a prescribed range of angles in the system too, you have a compounding difference in the range of positioning. Oh and then the marker sticker falls off and goes back on the knee in a slightly different position!
So if a fitter just chooses to utilise the system with its own defined range of angles, they might stop as soon as they hit the green range. What happens if they start with a high saddle and come into the range from above vs starting low and coming in from below? What does the system know about the cyclist’s flexibility and injury history (and is that important)? @mcneese.chad has already commented on the ability of fitters and how we all develop with experience.
A fitting protocol should be about so much more than the angle has been achieved, tick. It’s about working with the cyclist and helping them gain the ability to sense the difference in their body and self-select the best position given what their individual requirements are. Even when the cyclist has self-selected their position, there is a window within which, their position can be altered without them being able to tell. The range of this window differs between individuals. How many of you can tighten a bolt to 5Nm exactly every time? You can’t do that consistently, so how consistently can you find the same bike fit position?
Also bear in mind that riding a bike is a compromise from one day to the next in terms of position for loads of reasons such as:
- Slower endurance rides = less power = shorter, higher reach. All-out race effort = higher power = lower longer reach.
- Changes in muscle length due to more exercise, ageing, nutrition etc
- Our mental state
- Different/worn shoes and cleat position
- Changing fitness level
- Temperature
I hope the above covers off why there could be a difference in position between fits.
So why did I invest in the STT system given the limitations of a computerised system?
- It enables my customers to see what I can see when they’re pedalling on their bike and visualise the situation. It aids discussion.
- It gives confidence to the cyclist in the outcome of the fitting.
- There are some people that aren’t able to listen to their body within an acceptable length of time for a bike fit session and determine their own best fit. Fitting them within a range of known good avearge angles is a good start for them.
- The STT system output looks bloody cool. It is the full body in 3d at 100Hz.
- It’s a marketing tool as some customers buy because of it.
There have been other interesting threads on the TR forum about using apps and online services to determine fit. Given the above, I guess you have to ask yourself are you average enough for them to get you in a good place and then when you start deliberating the position, was it worth it?