Bike Fit software mainstreams

Slowtwitchers, as a group, are almost as bike fit savvy as bike shops, as a group. Bike shop knowledge of fit ranges from a reliance on standover height (2 inches? It fits.) to a mastery of complex machinery and software costing tens of thousands of dollars. Slowtwitchers' knowledge of bike fit ranges from knowing absolutely nothing at all about bike fit to at least an understanding of the concepts behind the machinery and software. Because Slowtwitchers are on the savvier side of this issue than most, I asked Guru (division of Cannondale) whether I could allow Slowtwitchers to download, as a trial, a new software package it's launcing today (Interbike week). The idea is to let you kick the tires of a powerful program sold only to sophisticated bike fitters. Download details for you are further below and if you do download this package and have comments, criticisms, suggestions, accolades, let us chat it up over on our reader forum.

Very briefly, most of the software available today – and there is considerable, which I will mention below – focuses on the two main elements comprising a "modern" fit process: 1) the establishment of a proper position; 2) the matching of a complete bike solution to that position.

Prior to the mid 90s we (the bike industry) mostly relied on a position suggested by our morphology. If you measured your inseam, femur, torso, arm length, there were programs to help you get from these values to a bike geometry. One of the very best of these is still out there and still of some value, and it's called BG101, built by Martin Manning. This is a robust, full-featured program built in MS Excel that is designed to be used by custom frame makers, but if you nose around you'll find utility for this program beyond frame making. It's freeware.

Note the difference between this approach and the systems we often see today where your position is established while riding a fit bike. These fit bikes have been around for decades and they really were not made, in the beginning, as a tool aboard which you pedaled, under load, while your position was established. Rather, you could take something like BG101, or the Fit Kit system more than 30 years in use. You could, if you wanted, establish a position aboard a fit bike, maybe like the Waterford Fitmaster, and you'd have underneath you a "bike" that you could set up via quick releases that allow you to "build" a top tube of 56cm, down tube of 57cm, various seat and head angles and so forth. The bike, thusly built, is then measured and you match this against the bikes you have in stock (if you're a bike shop) or you give that data to a custom bike maker. This was all very analog. This stands in contrast to what we have today and it might help to differentiate between the traditional use of fit systems and tools and a more recent way of approaching bike fit.

There are 3 ways to parse between these older systems versus with what we've now got today (Retul, Guru, F.I.S.T., etc.).

1. When you take limb and torso measures and generate an "output" that's either an exact model (Cervelo R3, 57cm, 110mm stem –6° pitch), or a general parameter (any bike with a 58cm top tube and 57cm seat tube), you're relying on a STATIC fit process. When you actually pedal aboard a fit bike and the process of riding is incorporated into the selection of fit coordinates (seat height, cockpit distance, handlebar elevation) that's a DYNAMIC fit process. When we talk about software for bike fitting, it's important for you to determine whether the process the software relies upon is static or dynamic. If you don't have a fit bike and you don't intend on getting or using one, a piece of software that optimizes a dynamic process is probably a waste of money. Maybe the software only refers to taking fit coordinates and giving you back complete bike solutions. Software like this mates with any kind of process – static or dynamic – that outputs fit coordinates. (The software you have access to via this article is this kind.)

2. The software sold by Fit Kit will tell you the seat and top tube lengths of the bike you're looking for. It's designed to be used by bike shops that just can't or won't or choose not to move to an X and Y system of bike and fit metrics. The Waterford Fitmaster bike "looks" like a bike, with its seat and head tube members declined back from vertical, just like a bike you ride. This differs from fit bikes that adjust straight up and down and back and forth. Measurements like seat tube, top tube, head tube, head angle, seat angle all are based on what we'll call an ANGULAR approach to bike metrics. In order to know where the head tube top sits in relation to the bottom bracket, you must know the bottom bracket drop, seat angle, seat tube length, and top tube length. The other way to identify the head tube top relative to the bottom bracket is via stack and reach, and these are the X and Y (horizontal and vertical) distances between those two spots. Fit Kit, Waterford and others are angular in their approaches, and that's really the bike business up ‘til 10 or 15 years ago. A more recent approach is to look at dimensions like BB to head tube (stack and reach) or BB to handlebar clamp, or BB to armrests (for tri bike fitting) and this "X/Y" approach is the alternative to the angular approach.

3. Finally – and this is important when you choose software – there are 2 parts to the fit process. Part 1 is the establishment of fit coordinates (seat height, handlebar elevation, etc.). Part 2 is where you match those fit coordinates to complete bike solutions. Images here are of the Guru fit report that show, among other things, your complete bike solutions.

Many people have gone to a fitter and gotten a fit report that doesn't look anything like what's pictured here. That report might just list fit coordinates. And that's fine. That's the most important part of a fit report. A report from Retul might contain 2 sets of metrics specific to your body: first, a bunch of angles. Your knee angle was 140°. Your closed hip angle was 50°. And so forth. All that is fine, but is of no practical use to you. It's of use only to the fitter, who was using body angles to help drive the fit process. Contrast this to Fit Kit, which uses limb and torso lengths as drivers. You would only use body angles as drivers during a dynamic fit process, as these are angles your body achieves during the process of pedaling.

The second part of a Retul report shows you your fit coordinates. These are really important. Saddle height, saddle nose or clamp setback (or set-forward), cockpit distance, handlebar elevation, aerobar pad placement versus the saddle or the bottom bracket, these are all fit coordinates. These tell you how to set your bike up, and they'll also help you determine what new bike to get.

The third part of a Retul report – using its Frame Finder software – is the matching of these fit coordinates to complete bike solutions. Very granular solutions: frame model, size, and front end set-up. Only X/Y-based systems can give you this.

This third part of the report, this is now in vogue with fit systems companies and they've proliferated. They use stack and reach and some trigonometry and, presto, complete bike solutions. This software delivers solutions to you based on X and Y to the handlebar clamp. Purely Custom has developed this and it's marketed both by Purely Custom and Trek (its Precision Fit system). Retul has this, as noted. Guru, the division of Cannondale Sports Unlimited, has it as well. The image below is an example of this kind of software output. It's very compelling.

Quick thinkers might realize that you could pry this part of a fit report out and offer it stand-alone. Let's say that you generated a set of fit coordinates, either through a static or dynamic process, or because you already know your fit coordinates and you like your position just fine. Let's say you're a mechanic of a pro cycling team and you know every rider's position, you just changed sponsors, and you need to know what bikes to order from your new sponsor. You could use this very powerful software.

Guru has just split this part of its software off, it's a new package called Guru Bike Discovery, and I have prevailed upon them to let our readers – who, after all, are not that much behind the dealers they go to in fit sophistication – take it out for a test drive. Here's the download link to Guru Discovery and, alas, as of now it's Windows only (it's an exe file).

Once you download this, use the following:

User Name: admin
Password: admin321

Go to to see the web support on this, with videos and whatnot.

This will give you a week or so of use before your access obliterates.

This software is brand spankin' new, I think it's only getting released today, so, Slowtwitchers and bike shops will each see it for the first time.

Bear in mind this is not functionality unavailable elsewhere. This is no different in theme than what you'd get from Purely Custom and Retul, just, realize what's happening here. This software was built as an adjunct to dynamic fit processes. The hard-charging companies selling expensive fitting tools were those who had their feet hard on the accelerators and produced powerful software like this. But this software does not require the expensive tools. The fit tools were for Part-1 of the process: generating a good rider position using a dynamic process. Part-2 is determining what bikes match that set of fit coordinates generated during Part-1 of the process. But if you don't need Part-1 – if you already have a position generated – this software is of great utility.

You can also find out most of what you need to do by just going to, the stack and reach tables, and using our handlebar-to-head-tube-top calculator, stem calculator, etc., but these software packages like Guru Discovery integrate all of this into a single click.