Selecting the right frame size – Part 1: Points in space

I received a question about sizing on my new Mach 429sl. I did begin to respond to the comment but it got lengthy so I decided to write an entire post two posts on the topic. While the question came up in response to the Pivot Mach 429sl, the content applies equally to other frames.

Part 1 – This first post focuses on what we’re doing with and to the frame when we think of “fit”. This is sort of the baseline that will help some folks wrap their head around what’s going on with selecting frame size, what we can and cannot control, and how they impact the ride.

Part 2 – With this understanding, the second post will focus on questions you need to ask yourself that will help you zero in on the frame size that will work for you and how you want the bike to handle. So let’s begin.

Disclaimer: I am NOT a professional bike fitter. These are simply my thoughts on the topic. Caveat lector.

Decisions, decisions…

I just purchased a Pivot Mach 429sl in a size medium. I believe the frame is the perfect size for me even though I have not finalized the fit. The question of “What size frame should I purchase?” comes up all the time in different forums. If you haven’t ridden a wide variety of frames, and especially if this is your first relatively expensive frame, you could agonize over the decision for days or weeks. Plenty of folks have made the wrong decisions and then face the unfortunate prospect of having to decide to live with something that doesn’t fit right or sell it and lose a ton of hard earned cash. Fortunately, there are some pretty universal things to consider that should help make the decision a more informed one (if not easier).

 

The final frontier

The final frontier

Points in space.

The three points at which you contact the bike are of critical importance: the pedals, the saddle, and the grip on the handlebars. Imagine a rider profile on a bike. Assume the bike “fits” perfectly. Now take away the bike frame completely and focus on those three points. In a fully bespoke frame everything is built around those three points so the finished bike fits exactly as you want it to. If you’re getting a custom frame congrats! For an off the shelf production frame those three points in space do not change, but there are set frame sizes. What we’re trying to do is find a frame size that fits best within that space, and one that allows for the ride characteristics we’re looking for.

 

In a production frame there may be more than one size that will “fit” in that negative space. You don’t ride a bare frame though. The frame is merely the foundation and we add cranks, seatpost, and stem/bars of different sizes to allow us to ride the thing. We cannot change the placement of the bottom bracket (and by extension pedals) but we can change the position of the saddle and grips to complete the bike. This impacts your position on the bike (more or less upright). Frame size also impacts wheelbase, which can have some impact on ride characteristics.

Moving on to those three points. The order below is important. I’ll explain why at the end.

Foot position

Foot position is determined by the location of the bottom bracket and your crank arm length. Crank size should be determined by your leg length and be universal across most of your bikes. For my leg length 170 cranks work best if I’m actually going to pedal the bike. That’s a key phrase. On a downhill bike where I’m not actually pedaling much (compared to an endurance race bike) I could easily get away with shorter (say 165) crank arms simply to get more clearance. In general though, once you know your size the crank arms don’t change.

Saddle position

You have a frame. Feet are on the pedals, the position of the saddle is the second point to be filled in and fit to your body. What you’re trying to do with saddle position is gain the correct angles between your hips, knees, and feet when pedaling. Obviously the length of the seatpost is important – make it long enough for to get the correct angles.

The fore aft position of the saddle is important too, and this is where the specific frame geometry comes into play – especially the seat tube angle (STa). On my CX bike, which is bespoke, my saddle is dead center on the rails. On nearly every other bike I’ve had I need a straight post, and the saddle is slammed forward. This is entirely dictated by the position of my hips relative to the position of the pedals.

 

Hand position: Cockpit

Pedals are in place your hips are in the correct position on the saddle, now you can fit the cockpit which is the stem and handlebars. The stem and handlebar are two separate components but they impact your hand position as well as handling.

Stem

  • A longer stem stretches you out more. Duh.
  • Stem length impacts steering as well. The same degree of turn in the front wheel requires the handlebars to travel farther if the stem is longer. If however the stem is longer you’re likely to have more weight over the front wheel which should make climbing a bit easier. (If you ride a trail/all-mtn bike and have had the front end feel like it’s hard to keep on the ground when you’re on a steep climb you’ll understand this one.)

Handlebars

  • There are generalities and misconceptions. You really can size MTB handlebars to a person just as you can road bars; however, there is a range that will correctly fit every person. You’ve heard the person who says 780 bars (or 800 or whatever) are the best for everyone. What if I’m 5’4” and have narrow shoulders and short arms? Gene Hamilton of Better Ride give the best explanation of how to correctly fit bars here. Just know that we all have a range to work within. There is obviously too wide and obviously too narrow, but within those you have a choice in going slightly wider or narrower and it will impact the handling of your bike..

When and why the order is important

The order above is important when fitting someone to an existing frame. The reason is that you have the most flexibility in the cockpit setup. For any given frame, foot position will not change. Assuming this is a bike that will see a lot of seated pedaling, the location of your hips (saddle position) relative to the foot position should not change either, or if it does only by a couple of millimeters. (The obvious exception is growing children.)

I’ll repeat, saddle position is THE most important part of fit for any bike where you will do a lot of seated pedaling. The vast majority of injuries related to improper fit (think knee pain) are from an improper saddle position. Too far forward or aft, too high or too low can all place undue strain on the knee and lead to all sorts of injuries. This ONLY applies to bikes where you will spend a lot of time seated pedaling. Think of doing a 50 mile ride and assume it will take you 6 hours. If you spin the pedals at a moderate 80 RPM on average, that’s 4800 revolutions an hour or 28,800 knee bends over the ride! Have a slightly wrong fit for an hour and it’s not a huge deal, have the wrong fit for hours on end, day after day and you WILL have an injury. If you do this type of riding I’d highly recommend a professional fit such as Retul. (Yes, there are other elements that will come out in a detail fit such as cleat placement, varus/vargus, stem angle, etc. This is just the basics mkay?)

This is ONLY critical for long hours and many miles of seated riding. Close enough has been just fine for my trail bike. I rarely spend hours on end doing seated pedaling on that bike. Rides are usually only 1-3 hours and all day rides tend to focus on the downhill segments where you’re standing on the pedals and not in the saddle.

A quick note on wheelbase

The wheelbase changes based on the size of the frame. The larger the frame the longer the wheelbase. On some bikes this impacts the length of the chainstay as well. I’m going to make some HUGE generalizations here and say that in general a longer wheelbase is more stable at speed, but is less nimble in tight corners. That’s all for now as this has less to do with the fit of the bike than handling, and stability and steering are impacted by other frame geometry elements.

Ok that’s nice but how does that help me select a frame size?

These are the basics of fitting a bike. They’re important to understand when selecting a frame that “fits” right. They are NOT the only factors to consider when selecting a frame size. You also need to ask yourself a few questions to determine how you want that frame to handle (see part II). We have some wiggle room to get the right fit remember? I could theoretically get the proper fit (those three points in space in the correct spot) with three different frame sizes.

Assume a medium is perfect for me, I might still “fit” on a small, but I would need a much longer stem, and seatpost. On a large I would probably need a shorter seatpost, my saddle even more forward, and a much shorter stem. It is theoretically possible to get the proper “fit” in the sense that my hands, hips, and feet are in the right spots, but the way the bike handles will be wildly different on each frame size. If the medium handles the way I want it to, we would say that the large and small frame sizes do not “fit” me even though the three points in space are the same.

Knowing how a bike is properly fit to the rider is step 1. Now you need to determine how you want the bike to handle. Then you will be able to make a more informed decision about the correct frame size for your body size and riding. On to part II.

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One response to “Selecting the right frame size – Part 1: Points in space

  1. Pingback: Selecting the right frame size – Part 2: Know thyself | The Endurance Experience·

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