I will start by saying this. The below thoughts are certainly my opinion, formed from fitting bicycle frames to great people like you. The way I describe fitting a rider to a specific geometry below works for me, and I welcome your feedback if you think I am way off base here or if I am missing something. Thanks to Kyle Milino - for inspiring me to write this post.
First off. What exactly is "reach"? It is present on every geometry chart, and published for every bicycle. However, how is it measured?
Reach is defined as: "the horizontal distance from the centre of the bottom bracket to the top middle of the head tube". As below, the relationship between Reach and Stack can be seen.
Reach changes; as the wheelbase changes, the BB location changes in relation to the head tube, as the front center grows larger, and to a lesser degree as the stack increases or decreases.
A few words on stack.
Stack is defined as: "the vertical distance between the bottom bracket and the top center of the head tube".
For mountain bikes specced with a certain size of suspension fork, stack becomes a bit of a moot point. This is because whether you are riding a size small, or a size XXL - and the fork length stays the same (as say - 160mm travel) - your stack doesn't change very much. Stack becomes dictated by the length of the headtube (which is why road bikes made for taller riders look so ridiculous - but I digress), and you will see headtube length grow as the size of the frame increases.
The geometry chart below belongs to a popular 2021 suspension bike, and I've used it to make a point. That if the fork length stays the same - stack and head tube length are coupled (red). Furthermore, that reach and wheelbase are linked, if other geometry factors remain the same (blue).
Now, because I build steel mountain bike frames, I want to keep my frames as light as possible (while, obviously, keeping them robust). The head tube on a steel bicycle is a major source of the frame strength, and as such it is one of the heavier components. Therefore I want to keep the head tube relatively short, and because of that I prefer my customers adjust their "actual stack" (where their hands contact the bicycle) primarily by using more headset spacers, a different stem, or more rise on their handlebars. So for what I am doing, the stack is pretty much a constant (most of my MTBs are specced with a 160mm fork).
We can park stack for now, assuming it is a function of the length of fork chosen by the customer, and move on. It is a constant. If I build a run of frames in 3 sizes, the stack will only vary by ~10mm per frame between small/medium/large.
Now, back to reach. I am going to use a case study from the design process, involving a recent customer, to further the discussion.
Jenna and her Quest
Jenna M reached out to me late in 2021 and a conversation started. Jenna has been riding her entire life and had some pretty specific asks. She is 5ft 11" tall, and like myself - was drawn to custom bike frames more from need then want. Us taller folks struggle to find bikes that feel right, as we exist on the outliers of what the mainstream industry offers.
Her input was abbreviated and listed below:
Customer Jenna M. Edmontonian. MTB enthusiast.
Use: primarily single track in Edmonton area - generally smooth and flowing; moderate climbs; moderate drops; some stunts; pump track
Style of riding: moderately aggressive style with a focus on agility and climbing - I want a bike that I can easily throw around on stunts and twisty trails but that’s also nice to climb with.
Reason I want a custom frame: I’ve never found a bike that fits my whole body well. Either the reach is too much or there’s not enough room for my legs (CK Note: She is 5' 11, as mentioned above).
I realized that Jenna would likely be happy with a slightly "toned down" version of my Gnartail geometry. She isn't really dealing with extended technical steeps, and she wants to be comfortable climbing and just comfortable in general. However, she still wants an aggressive and modern mountain bike (drops/stunts/pump track). Prior to this blog post we had settled on some basic geometry parameters:
66 degree head tube angle (Gnartails are 63 degrees).
75.5 seat tube angle (Gnartails are 77 degrees).
75mm BB drop.
29 inch wheels.
I built a few models for her - but it wasn't really working like it should have. I struggled a bit, with fitting Jenna to a bike size. I had no frame of reference, and the only custom womens frames I had built previously were for shorter individuals. Although BikeCAD is a powerful tool it wasn't delivering what I wanted. I needed help.
I needed a mentor to get Jenna her dream bike. I needed a Guru.
A bike fitting Guru.
Enter Flexion Bike Fit - and Owner Cody Osborne
Fortunately I knew such a Guru. In Jenna's home city of Edmonton too.
Cody Osborne is Flexion Bike Fit (www.flexionbikefit.com). This guy knows geometry, having both the education (a lot of related education!) and experience (over 250 fittings and counting) to back up his offering. I reached out to Cody for some help with finding a perfect fit for Jenna, and he agreed to give me assistance in the matter.
Cody Osborne. Bike fitting Guru. Look this guy up for pain-free riding! Photo credit - Jay Garland
Cody had Jenna visit his very well appointed studio, and they worked together to find a comfortable and anatomically beneficial setup that Jenna could live with. I wrote to Cody recently and stated that he provides "proprioceptive and musculoskeletal fit, tailored for maximum performance". He didn't correct me so I must be close.
I am decent at bike fitting, but Cody is a real master. He has all the tools, the know-how and also carries saddles, bars, pedals - etc (to really dial it in).
Without his work on this, Jenna and I would likely still be struggling.
Flexion Bit Fit World Headquarters. Jenna M on the rivet.
After the session concluded, Cody sent me the following (invaluable) output:
The output of a professional bike fit. Truly invaluable stuff.
So, I now know that the geometry stated above works for Jenna. She is comfortable within the parameters listed and if I can retain the key relationships above I can deliver her a bike that feels as good as it can under her. One key take away above - "Saddle tip 2 bar - 560mm". More on that below.
KRUCHification
Cody's product is great for a bunch of reasons, but for this exercise it is great because the product he offers is agnostic. It applies to mountain bikes, road bikes, cruisers - you name it. The task now was to take his excellent work, and apply it to the deliverable for Jenna. To do that, I had to bring his data into BikeCAD and start modelling the eventual outcome. I had to "KRUCHify" the data, taking a universal set of parameters and modifying them piecewise into a mountain bike geometry that I know works for my customers. While doing so, the critical pieces that are "Jenna-specific" needed to stay constant.
My first step was to import the data into BikeCAD and see where we landed (the head tube angle was set and the BB drop was set as per our previous discussions). I also knew her preferred seat height. As below:
Step 1: Import the bike fit data into BikeCAD
Note that the big 29er wheel interferes with the seat tube, and that won't stand. For fun - I bent the seat tube in the modelling program to accommodate the big wheel (like early 29er bikes all did) and left everything else alone:
Step 2: Experiment - leave all parameters alone but bend seat tube to accommodate tire
Sure, this would work for Jenna to a degree - but the goal isn't to build her a 29er with 2010 geometry! The goal is to build her a modern, progressive trail bike that will stay relevant for at least a decade.
To do that, I rotated the rider forward using the bottom bracket as a pivot, while maintaining the relationship between the BB and the handlebars.
Step 3: Rotate rider forward, preserve BB to bar relationship and seat height
You can see what the result is - the front center grows from 500mm to a whopping 561mm, and the seat post is no longer bent while the rear wheel has been tucked in. The wheelbase has grown as well.
The reach has expanded, from 445mm to 540mm - but the relationship between the rider and the bars hasn't much changed. To finalize the design - I grow the head tube back to a reasonable length and verify the "Saddle tip 2 bar - 560mm" measurement as mentioned above and shown in yellow below:
Final step: Grow head tube back to a reasonable length and verify saddle tip to bar measurement.
Above is the final design. This geometry offers some performance gains over traditional configurations, as listed below:
Longer wheelbase = more stability at speed and greater bump compliance.
Upright seating position = climbing comfort and increased pedalling power.
Upright seating position = easier to hammer out of the saddle, move on/off seat.
Rider forward (increased front center) = more weight on front tire.
Rider forward = decreased need to move body rearward while descending.
Rider forward = more natural descending position.
Higher seat to lower bar ratio = better descending position
Retention of tucked rear wheel = a more playful, spirited ride.
*Note that if all you were going off was reach - and you knew that a reach of ~445 was your sweet spot number...you would assume that this bike (reach of 506mm) was way too big for you. You would be wrong.
And that is my point.
One Caveat
There are advanced mountain bike riders who buy frames based on reach alone. This is because of the relationship reach has between the pedals and the handlebars.
If you think about it, when the saddle is dropped - your seat tube angle becomes irrelevant and your bike is being manipulated by only four touch points.
My opinion? That my "KRUCHified" geometry actually makes descending a more natural experience as the feet have been moved further backwards and under the body (instead of out front), which allows for more advanced riders to use a higher degree of "body english" while descending - and places more weight on the front tire (better braking control on steeps).
Thanks for reading my blog! I welcome your comments or input. Please DM me or better yet - write a response below.
KRUCH