Between which points are you supposed to measure components such as seatposts and stems?

I'm plannning to upgrade both in the near future and although I could go for a long seatpost and cut it down, I'd rather know how long its going to be in the first place. Also, I can hardly cut down a stem!

Is the stem measured from centre of steerer to centre of bar (seems sensible)? Or is it length of the 'straight bit' ?!

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.....but it still has to work!

Centre to centre:

Thanks.

Regarding the seatpost, is it end of post to rail clamp?

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.....but it still has to work!

Yes, most posts are measured from rail center to post end.

p.s. Impresive photo admin, hold the tool with both hands AND take the photo...

p.p.s. I'm going to have to get one of those stem measurers - another cool tool to have in the tool box, why didn't the rep tell me about that?

To be honest these aren't my hands.
I guess they belong to Jo Klieber (chief of Syntace).

That's nice picture from Syntace, but the this stem has nearly or 0 degree angle. What about, say 30 degree or more? The difference between lenght measured parpendicularly to the steerer and lenght along the stem can be quite huge even if both are taken center to center.

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Best regards,
Rado bladteth Rzeznicki

"That's nice picture from Syntace, but the this stem has nearly or 0 degree angle. What about, say 30 degree or more? The difference between lenght measured parpendicularly to the steerer and lenght along the stem can be quite huge even if both are taken center to center."
I think you got your wording mixed up in your message. The length along the stem is equal to the length measured perpendicularly to the steerer tube I think what you going at is to get the horizontal displacement of the stem with reference to a right-angled line drawn between the ground and the pivitol axis of the stem-steerer clamp. To do this, you can drop a length of string with a mass attached to the end off of the center of the stem-handlebar clamp. Then take a ruler and measure the distance to the pivitol axis of the stem-steerer clamp. If you like the mathematicl practice, take a protractor and measue the angle the stem makes with the horizontal and take length of the stem as you would normally do. Then use some trigonometry (in this case, use cosine to obtain the horizontal displacement) to get all the measurements you want.
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All right, let's make it clear once and for all. Please, take a look at the picture below and tell me, which distance is stem length. Blue line is head tube axis, green one (perpendicular to steerer axis) is 75 mm long, red one a little less than 90 mm. Angle is 30 degrees. Thanks.

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Best regards,

Rado bladteth Rzeznicki