Everyone Falls "Between Sizes" (to some extent)
Sizing a rider to a bicycle must take into account rider proportions, and use them to find the best frame size and optimal placement of points of contact: saddle, handlebars and pedals. The trick is that what is "best" is highly subjective, and dependent not only or the riders physique, but his or her physical condition, age, flexibility, personal preference, and many other difficult to measure factors.
Overview of Bicycle Fit Considerations
However, finding the best fit on a bicycle is almost always bound to involve some compromises. Even if you order a premium custom bike made to your exact proportions and specifications, chances are you will have to make some choice between comfort and performance, speed and stability, aerodynamics and viewing the scenery.
With stock bikes typically available in bicycle store showrooms, this is even more true. Each bike manufacturer makes several frame sizes of each bike model (usually between 3 and 5 frame sizes). Those sizes are based on certain assumptions about the rider's size and proportions, and these assumptions tend to correspond to a male physique more than the female.
If you’re a woman, you may wonder if you need a “woman specific” bike, until you realize that assumptions made about the average female physique (whatever that is) don’t come any closer to meeting individual women’s fit considerations any more than assumptions about riders in general. Women are generally shorter than men, but not all women are short, and there certainly are some short men. It’s been said that men have longer torsos than women, but the length of torso may or may not have anything to do with the length of the arms, shoulder width, relationship between thigh and calf length, or a zillion other things that affect how you fit on a given bike. The truth is that most of us —male or female— in one aspect or another, fall outside of the "average" build criteria, and need to modify existing bikes to achieve better fit.
But first: WHY?
On top of all of all the factors already mentioned, each rider’s intended use for the bicycle may differ from what the manufacturer envisioned.
So, even though there are several aspects of bike fit, before we get to them, you should first ask yourself what your purpose is for getting the bike. This is not about getting in touch with your inner cyclist. Physical fit considerations will be different for a person intending to go on a long bicycle journey, where long days in the saddle make comfort and stability paramount, from someone who wants to compete, where weight and agility may outweigh comfort. Those who ride primarily for fitness will have different needs from those who ride for relaxation, and still different for urban commuters. The trickiest scenario of all may be having one bike that needs to be a jack of all trades: commuter by day, fitness machine on weekends, and a touring rig during vacations. It’s a good idea to carefully consider your reasons for biking, and make sure they get included along with other fit considerations.
For these types of bikes, look elsewhere
This guide is intended to help with fitting standard upright bikes for uses such as those mentioned above. This includes hybrids, city bikes, road and gravel bikes, touring bikes, some off-road bikes, and variations on the above. These guidelines may not apply to downhill mountain bikes, triathlon or time trial bikes, comfort cruisers, crank-forward bikes, and certainly not recumbent bikes. Although they also don’t specifically apply to folding bikes, some of the suggestions may be used to optimize the fit on an otherwise one-size-fits-all folder.
What this guide will cover
In the following sections we’ll shed some light on different aspects of bicycle fit. Please note that the links on this page will take you to chicagobikeblog.com, which is the official blog of Cosmic Bikes, where we've been compiling useful information for many seasons.