Overview

Use cases for this Bike Geometry Calculator web app:

The app is made for recording and comparing bike fits and frame geometries, it's not designed to make bike fits based on body measurements (which is a bad idea anyway, as good bike fits are made using dynamic evaluations).

How to use

Function reference

Measurements buttons

The measurement buttons are toggle buttons, red = off, green = on. Each button enables a set of measurements, and several can be combined although the chart can become a bit crowded.

Geometry control buttons

Function buttons

Demarcations

Measuring a bike

To measure an existing bike, take it inside and put the rear wheel to a wall and hold it exactly upright with a bike stand, trainer or just some furniture. The floor and rear wall will serve as perpendicular reference planes when measuring.

For measuring tools get a measuring stick or a stiff folding rule, and a bubble level (see the measuring technique section for additional tips).

Enable only the Measure button to show the XY measurements with the wall and floor, and start measuring. It will be easier if you do things in a certain order:

  1. Inflate the tires and make sure that the vertical height of the front and rear hub is exactly the same. Add spacing under the tire of the lower hub if necessary. Enter the resulting hub Y.
  2. Measure back hub X and offset X (that is how much the tire is off the wall, if any).
    • This will indirectly set the wheel radius, which is defined as the outer radius of the wheel including the inflated tire, without any compression from the weight of the bike.
    • As the tire is compressed a little from the bike weight, hub Y is typically a few mm less than back hub X if the tire touches the floor and wall.
  3. Measure/estimate offset Y, which represents the ground level.
    • Offset Y is only used to calculate how high the pedal axle and bottom bracket is off the ground when riding. If the bike is mounted on a trainer the tires float above the ground and needs compensation. Hub Y minus offset Y should be the hub height when the bike is ridden. If you want conservative numbers for that, estimate how much extra the tires are compressed when riding and add the corresponding to the offset.
    • As the wheel is drawn as a perfect circle in the chart it's normal that the tire can drop a few mm below offset Y (or the floor line).
  4. Measure saddle length, saddle front to center, and measure/estimate saddle angle.
  5. Measure/estimate stem angle (often printed on the stem).
  6. Measure handlebar diameter (31.8mm on most bikes).
  7. Measure crank length (often printed on the cranks).
  8. Measure bottom bracket X/Y.
  9. Measure seat tube top center X/Y.
  10. Measure head tube top center X/Y.
  11. The head tube and fork is tricky. Here's why: while you can measure head tube bottom X/Y, the distance is too short to safely get an accurate head tube angle. Wheel base is easy to measure accurately though, but we need fork rake, which is hard to measure. Here's a couple of methods to go forward:
    • Method A: if you can get the fork rake in advance:
      1. Measure head tube length.
      2. Measure headset bottom stack, headset is often invisible on modern bikes, but there's usually a 1mm gap or so.
      3. Enter fork rake.
      4. Measure front hub X.
    • Method B: measure head tube bottom center but use it only as a guide and adapt to other known data.
      1. Measure head tube bottom center X/Y. Sanity check that head tube length matches.
      2. Measure headset bottom stack.
      3. Adapt fork rake with trial and error until front hub X matches the measured value. Sanity check the resulting fork rake, for most road bikes it should be in the range 40 to 50mm.
  12. Measure handlebar back X and handlebar top Y.
  13. Now with handlebar position established, we want to check that stem length, stem angle, stem stack, headset spacers stack and headset top stack all match up.
    • The stacks can easily be measured. If you don't have the stem angle, you can adjust it until all matches up.
    • In the chart the stem will is anchored in the middle of the stem stack. Some real stems are not, so you may need to use a fake stem stack to get the stem aligned properly.
    • Note that when you change the stack and stem angle values the handlebar position will move, so when finished check that it is where it should be.
  14. Measure hoods rest top X/Y.
  15. Measure saddle back X and saddle top Y.
  16. Measure saddle stack and measure/estimate seatpost setback.
  17. Adjust top tube position to match the real frame with seat tube top to top tube center and head tube top to top tube center measurements.

The bike is now fully measured, and you can enjoy all automatically derived measurements. It's a good idea to go over some derived measurements, like top tube length, and see that they match to double-check the accuarcy of the measurements.

The measured chart usually does not exactly match a geometry chart from the manufacturer. While this can be due to measurement errors, it's also likely that the manufacturer has rounded some values (like tube angles) or measured with slightly different anchor points. A different headset or fork can also affect frame rotation and thus yield different angles.

Measuring technique

Below follows a few tips on how to make accurate measurements.

Use a bubble level

If the ruler cannot be held directly against the point measured, use a bubble level to make sure you get the measurement exactly at the accurate height. In the image the measurement cannot be seen until the bubble level is removed, so we place and hold a fingernail exactly where the bubble level and ruler meet. Of course, we also take care to avoid bending the ruler and make sure to hold it perfectly vertical (using a bubble level for that is generally overkill though, just use gravity).

The three images above show what can happen if a bubble level is not used, here when measuring saddle top Y. The ruler is placed the same way in all three images, but in the first two we look by eye from the side, and due to the ruler is some distance away from the saddle (as there's no room to put it directly against while still having it vertical) and the saddle top is rounded, the result depends on from which height we look at the ruler. The first looks like 998mm, the second 996. In the third image we use a bubble level and put a fingernail where it meets the ruler, and there we get an accurate measurement (994mm).

Derive measurements

Sometimes deriving a measurement is more accurate than trying to measure directly. The example above shows when there is a hollow crank axle so there is no clear center point of the bottom bracket. We then measure to the edge of the hole, which the ruler can be put directly against, to get an accurate measurement. Then we measure the diameter of the hole using a vernier caliper or a ruler, and add the radius to get the bottom bracket center measurement.

Tube centers

It can be helpful to put some masking tape on the tube ends and make a pen mark at the center to have something to aim at when measuring. Looking from above the head tube / seat tube centers can be seen by eye, a vernier caliper can be used to get additional visual guidance where to put the pen mark.

Hoods reach

If you measure a complete bike you normally measure hoods reach just by measuring the hoods XY coordinate. However if you want to measure it separately, here's how: the image shows a technique with two rulers, one is put over the hoods where the purlicue rests, and the other is used for measuring the distance to the front of the handlebar. A fingernail is placed and held exactly where they meet so we can remove the ruler and see the result. Then radius of the handlebar is added (usually 16mm) to get the final measurement.

Copying a geometry chart

Manufacturers' geometry charts often lack crucial measurements, and many times the charts are poorly or incorrectly drawn so it's not clear between which points the measurements are made. Sometimes you need additional information to be able to make a complete chart, or you have to live with that some measurements are not known.

Due to the large variability in chart quality and content a step-by-step guide how to copy a chart will not work for all. However if the chart indeed is detailed, here's a suggested order to copy the measurements:

  1. Seat tube angle
  2. Head tube angle
  3. Bottom bracket drop
  4. Chainstay length
  5. Fork rake
  6. Stack
  7. Reach
  8. Head tube length

As the bike needs to stay connected when changing a measurement some related measurements will automatically change, but if you use the order above all those measurements will stay at the value entered. If you need to use a different order or other measurements you may need to revisit old measurements to see that they are still the same.

When copying or editing geometry charts it's often useful to change the geometry control "Head Tube: Fixed Top" to "Head Tube: Fixed Bottom".

Here follows some general tips how to deal with charts that are less detailed:


© Copyright 2018 — Anders Torger.