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BikeTwins
How it works

Four readings, in three passes

Twenty minutes with a tape, a plumb line and a spirit level, and I stop guessing about your bike. Every reading has its box on the bike form, so you can type it as you read it rather than remembering four numbers on the way back inside.

Drawn
Giant TCR L · a worked example, not yours
One reading
good to about 5 mm with a plumb line; two averaged, ±3 mm
Bar reach
from the bar's spec sheet if it says; otherwise I assume 75 mm and say so
Where they go
the tape tab of the bike form, as you read them · Enter the readings
floor 2 · 913 mm4 · 1003 mm1 · 500 mm3 · 220 mm middle of the barmiddle of the railsbottom bracket floor 2 · 913 mm4 · 1003 mm1 · 500 mm3 · 220 mm barrailsBB

The two points, and where the four readings go. The orange crosses are the two points everything else is solved to: the middle of the bar where the stem clamps it, and the middle of the straight part of the saddle rails. The dots on the floor line are the tape marks under them.

Drawn from a Giant TCR L, so the numbers on it are a worked example, not yours. One square is 100 mm.

You need

A tape measure in millimetres, a spirit level or a phone level app, a plumb line (string and a nut) or a laser line, masking tape and a pen. A flat, level floor and a wall. Tyres at your normal pressure. The bike as you ride it: the stem and spacers you use, the saddle where you have it.

Level the saddle before you start. I model the saddle top as a fixed height above the rail centre, and a nose-down saddle breaks that quietly rather than loudly.

  1. Pass one, on the floor

    readings 1 and 3

    Stand the bike level and upright, ideally in a turbo trainer with both wheels held, or against a wall with a level on the seat tube. Drop a plumb line from three places: the bottom bracket, the middle of the round part of the bar where the stem clamps it, and the middle of the straight part of the saddle rails. Mark each one on a strip of masking tape on the floor. Stand on the non-drive side so the line hangs clear of the tyre and the bottle cage.

    1 · Bar mark, forward of the BB mark
    500 mm on the drawing
    3 · Saddle mark, behind the BB mark
    220 mm on the drawing

  2. Pass two, up from the floor

    readings 2 and 4

    Now the two heights, floor to the same two points. Take each one twice and use the average. One plumb-bob reading to a floor mark is good to about 5 mm in careful hands, and mine are no better than yours; averaging two is what gets it down to the ±3 mm I quote on the answer. Two readings more than 3 mm apart mean a third.

    2 · Floor to the middle of the bar
    913 mm on the drawing
    4 · Floor to the middle of the rails
    1003 mm on the drawing

  3. Pass three, the two that are easy to get wrong

    The rails first: on many saddles they are not straight, the midpoint hides under the shell and you cannot hang a line from it. Put tape on the rail at the front and the back of its straight section, halve the distance between the two, and plumb from the tape rather than from the saddle. What's this?

    Then the bar: if your tops are not round, measure to the centre of the clamp bolt circle instead, and say so in the notes on the form so I know which it was.

    Saddle height, BB to the saddle top
    along the seat tube, for the seatpost check
    Your bar's reach, if the spec sheet says
    so hoods line up across bars

What comes out

the two points, from the bottom bracket
500 / 630bar clamp, from the bottom bracket
-220 / 720saddle rail centre

The tape tab asks for the bottom bracket's height off the floor too, and takes it off your two readings. On this frame it should come out near 283 mm: a 350 mm wheel less this frame's 67 mm of bottom bracket drop. If your tape says something very different, the bike is not upright or the tyre is soft.

On the tape tab of the bike form you type the floor readings as they are; I subtract the BB height and the BB mark and keep the two points. Both bands are then the tape's own error, ±3 mm. If you also know the bar's reach from its spec sheet, hoods line up across bars; without it I assume 75 mm and say so.

Nothing about YOUR bike is assumed afterwards. The bikes you compare against still carry whatever their makers do not publish, mostly the headset cover height and the stem clamp centre, and every answer says which.

The band is a straight sum of everything I had to assume rather than a clever average, so it errs wide on purpose. What's this?

Worth writing down too

The stem's length and angle, the spacers under it, the headset cover height at the steerer axis, the seatpost's offset, and the bar's make and model. They are not needed for the two points, but they let me check the forward model against your measurement and tighten it for everyone.

What is a steerer? What is a headset cover? What is bar reach?

Enter the readingsWhat do these words mean?