Frame Size and Geometry

Frame geometry

Typical frame geometry diagram from a bicycle manufacturer’s catalog

Typical frame geometry diagram from a bicycle manufacturer’s catalog

Frame geometry is a term used to describe measurements and angles of various bicycle frame members, and the way in which they are put together. Frame geometries differ from one bike category to another, and between different bike manufacturers. For the purposes of this guide, suffice it to say that if you have a hard time getting a good fit on a particular type of bike, you may consider another brand, or a bike in a different category, and fine-tune the fit by adjusting the relationship between the bike’s Points of Contact (seat, hands and feet) discussed here.

Frame Size

The length of tube #1 in the above diagram is also the size of the bike frame.

The length of tube #1 in the above diagram is also the size of the bike frame.

When we assess you for a bike fit at Cosmic Bikes, we typically start by getting you on a frame suitable for your height. But there’s a little more art than science even in this simple initial step.

The frame size is simply the length of the bike’s seat tube, measured from the center of the cranks to either the top of the tube (referred to as “center to top”), or to the point where the seat tube intersects with the top tube (referred to as “center to center”). Note that this measurement (a) doesn’t tell you anything even as basic as the inseam length required to clear the top tube of a bike, and (b) can vary quite a bit based on which of the above listed standards the manufacturer uses. It is simply the conventional —and somewhat arbitrary— way to refer to the size of the bike.

Standover height
This measurement will tell you how much clearance there is between your groin and the top tube of the bike, as you straddle the frame with your feet flat on the ground. Ideally, there should be 1 to 2 inches of clearance. With some road bike frames, you may be able to get away with a tighter clearance, but —generally— you should not buy a bike that presses into your body as you stand over it. This allows you to safely dismount if you come to a sudden stop.

However…

Extension or reach

Measurements #2 and #3 have to do with your extension and reach on the bike. #5 is the standover height.

Measurements #2 and #3 have to do with your extension and reach on the bike. #5 is the standover height.

Even more important than standover height is your extension or reach. This is because, while actually riding, you spend about 95% of the time extending and reaching, and only a very small amount straddling the frame. The extension refers to the forward reach from the saddle to the handlebars, and affects how upright you will be while riding. It can also be the hardest part of fitting a bike.

Individual torso and leg proportions are seldom equal. Long-legged riders often find that when the standover and seat height are correct, the extension from seat to handlebars is too long. Long-torsoed riders feel that a frame that gives them enough standover clearance doesn't provide enough extension, and makes them feel “crowded” on the bike.

Fortunately, most of the time these problems can be solved through minor alterations to the cockpit, saddle position and pedals, or Points of Contact.

Points of Contact

Although most people focus on saddle comfort, and sometimes on handlebar height, the bicycle actually offers three points of contact with your body: handlebar (your hands), saddle (your rear end), and cranks/pedals (your feet). The placement of these points in relation to one another is what needs to be adjusted to strike the right balance between your comfort and desired performance. Please note that only two of these points of contact —seat and handlebars— are stationary. Your feet are not. We’ll discuss below why this is an often-overlooked aspect of bike fitting, particularly on casual and less expensive bikes.

Links to other parts of this guide: