notes : GENERAL
Measure your existing bike
Get a tape measure, or a yardstick. Metric is recommended, and will be used here almost exclusively.
Get a paper and pencil also.
Lean the bike or mount it level. Measure level as carefully as possible; most floors are not really perfectly flat OR level, so don’t assume. Know that five millimeters out of level will affect further measurements by up to five millimeters, so really just level the bike as carefully as you want to record your fit. The level I use is about four feet long and is calibrated to .5mm per meter, and it will easily measure a wheelbase. Whether you measure the level of the floor under the tires, or an imaginary line through the hub skewers, is up to you; shim up the lowest tire until the bike is perfectly level, and propped upright. You may leave the shim stack in place on the floor for future use.
Ascertain frame size. Frame size may be designated as Small, Medium, Large, etc. or by numbers (examples: 56cm, 23”) Different manufacturers use different measuring systems and styles, though normally the frame size refers to the length of the seat tube. If frame size is known, enter it on your piece of paper:
Frame Size: 61cm
If you don’t know for sure what the frame size is, measure it. Know that two styles of measurement are common:
1. Seat Tube Length c-t: 61cm
2. Seat Tube c-c: 58.5cm
Some companies have other ways of sizing bicycles, often involving imaginary points on the seat post or subjective terms like “extra large.” If this is confusing, join the club. I think the main reason manufacturers do this is so it’s easier to put more people on any one size-a “complete” size run thus consists of four, or maybe five, sizes, rather than the more traditional seven or more. One manufacturer which I will not name supplies their top-of-the-line road frames in only four sizes, effectively allowing maybe half of all customers to be well-fitted. The other half of us, extra-tall or extra-short, or long-legged, have to settle for second-best. Too bad on a $3500 bike. But I digress...
Your Fit Case
Now that we have the bike level and sized, proceed to recording your Fit Case. This is the set of dimensions of adjustments and components which determine how your body is situated on the bicycle.
Next, measure the length of your saddle front-to-back (usually about 270mm) and put a small mark in the center to serve as an index point. If you don’t want to mark your saddle, put a piece of tape onto it and mark that.
Saddle height is measured from the BB upward to that mark on your saddle. Crouch down and sight accurately at the level of the saddle. You can determine and set saddle height in a number of ways, which is beyond the scope of the present discussion.
Saddle Height: 81.0cm
Another saddle dimension is Setback. This measurement reflects the fore/aft position of the saddle as clamped on its rails by the seatpost.
You will need to make a plumb bob for this measurement; cut a length of string about three feet long, make a loop in one end and tie the other end to a small weight (a nut or washer works just fine). Slip the loop over your ruler, place the ruler end on the saddle center mark, and slide the loop to a point on the ruler where the string intersects the BB spindle. You may need a shorter or longer line depending on frame size and your style of sighting along the line. I like to hold the tape and string a few centimeters right of the bicycle’s center line (still touching and anchored on the saddle), and observe when the string lines up with the center of the crankarm fixing bolt. Or you can use a shorter string, put the ruler just a little to the right of the saddle mark (so the string clears the top tube), and sight down the string to the center of the BB shell. Either method is a little tricky; the string wants to swing back and forth, and sometimes bottle cages and the front derailleur interfere, and you have to hold the ruler steady, and not have had too much cappuccino, etc.
Saddle Setback: 23cm
Note that both Saddle Height and Setback are easily adjustable, and that ADJUSTING ONE WILL CHANGE THE OTHER. Again, there are numerous ways to determine either, which are beyond the scope of the present discussion. For the time being, we are simply collecting a series of numbers to establish a baseline, so that future adjustments can be made in a more deliberate manner.
Cockpit is next. Measure first the distance from your saddle center to the center of the handlebar where it is clamped in the stem.
Cockpit Length: 74cm
Cockpit Length can be changed by moving the saddle, but in that case remember that you are actually changing three dimensions (Cockpit Length AND Saddle Setback AND Saddle Height). To change Cockpit Length without affecting saddle position requires a different stem.
Now measure Handlebar Drop. You will need a long level again to make this measurement accurate. Place one end on the saddle, hold the instrument in a level position, and then measure the distance from the bottom of the level to the top of the handlebars. This is another difficult maneuver, one which is most easily accomplished with a helper: one person holds the level in position while another takes the measurement (note that many saddles have a pronounced dip in the center these days, making Saddle Top and Saddle Center as much as a centimeter different; properly, level should be measured from the actual line, but it might not matter that much just as long as you do it the same way every time).
Handlebar Drop: 6cm
Handlebar Width is next, and it’s fairly easy. Measure from one end of the bar to the opposite, center-to-center. I find it a little easier to measure right edge-to-right edge, because edges give a firmer index point than trying to get the exact center.
Handlebar Width: 44cm
Many other dimensions may be measured
and recorded for comparison, some more important than others:
Crankarm Length: 175mm is the length of the crankarm, BB spindle to Pedal spindle. Usually printed or engraved into the crankarms.
Q-Factor: 252mm is the distance between pedals centers, an imaginary line perpendicular to the frame. With the right side pedal at the 11:00 position, measure from its center to the seat tube centerline. Repeat for the left pedal, and add the numbers together.
Handlebar Depth: 16cm is the distance, center-to-center, from the top of the handlebars to the bottom of the handlebar drops.
Using these measurements, you can duplicate your fit case on two or more bikes. They will help study your style and technique on a bicycle, help you select frames and components as they become necessary, and provide a baseline to which further changes may be compared.