coldass

I figure 3T are measuring reach not extension. In many ways this would be better as you know at a given angle your bars are in the same place. I found this table....

stemfit.jpg
https://www.habcycles.com/stemfit.jpg

waterrockets 

But every stem would need two reach measurements then. You can't assume how a reversible stem will be installed.

umd

Right, it's almost like they sized it at 100m reach when installed as +17, which is pretty silly since they market it as a -17 only stem (and is marked as -17, not +/- 17), whereas they market the 84D stem as +/- 6. The more I think about it, the more that is the only thing that makes the numbers work out correctly, other than just putting the wrong measurement on it. I should probably just write to them and ask, but it felt fine climbing with it today and descending was great, so I may just decide it doesn't matter in the end.

DaveSSS 

Every stem has several length measurements; the actual length parallel to the extension, which is the advertised length and two horizontal lengths, one for each of the two angles. That's nothing new, but apparently a lot of people don't realize that a 110mm by 84 degree stem has a horizontal length of 108mm and when it's flipped to 96 degrees it's 101mm.

coldass

Either way a stem is flipped reach will be the same.
And anyway you think of it, reach is way more important and useful a measure than a measure from center of steer clamp to center of bar clamp given stems have a variety of angles.
Stem extension at a given angle and reach is only important to gauge height.

So your right two measures are needed. I'd say three measures are needed to correctly asses a stem although only two need to be provided at a time:

Extension (c-c)
Reach
Angle

But again reach and rise or drop are what we want to know. I guess the reason extension length and angle mean are provided and we have to calculate this is because head angles vary. Still if a stem maker provides angle and reach this makes selection easier - given you are considering the same bike.

umd

I guess the confusion comes in when you don't know which length the manufacturer is using. In my case I think either they are measuring the horizontal length when it is at 117 degrees, or they are measuring the extension and it is just marked wrong.

How do you figure? That would only make sense if the steerer tube were 90 degrees. However because the steerer is angled back, the negative rise (relative to the steerer) puts the stem out further than the positive rise.

coldass

Yeah I didn't think it through considering head angle.

Can you measure your stem while on the bike and see what the reach is.

3T's website just says length.

umd

I had measured the extension along the top to be 110mm, along the centerline to be about 104-105mm, and along the bottom to be about 100mm. Since the stem is parallel to the ground (as my size has a 73 degree HT angle), the reach should be exactly equal to the extension.

DaveSSS 

You are confused. The head tube angle is not really relevant, since a degree one way or the other from 73 make too little difference to be of concern.

What you're missing is that reach is normally a horizontal meassurement. An 84 (-6) degree stem is measured along a line thorough the center of the bar clamp and PARALLEL to the extension, not horizontal. With that length definition, the horizontal length of a 110mm stem would be 110 times the cosine of 11 degrees, or 108mm. If the stem is flipped to 96 degrees, the horizontal length is shortened to 101mm.

On another forum, someone posted that fact that the stem length does not change when flipped, if you measure from the center of the bars, horizontally, to the center of the steering axis (at a taller height). What is overlooked in that statement is that the taller height also brings the stem closer to the rider.

Another fact is that it does not matter whether spacers are used under a stem or the stem angle is changed. For a given height increase, the bars will move closer to the rider by the same amount. In the case of spacers, it's the cosine of the HTA times the amount of spacer, or approx. 3mm for each 10mm of spacer.

umd

So bottom line Dave, is it looking like I got a 110 by mistake?

classic1

That will teach you for not buying American

stedalus

Resell it as a rare collectible like the 1955 double die cent.

DaveSSS 

3T might not adhere to the current "standard". Measure from the center of the steering tube clamp to the back edge of the bar, parallel to the extension and along it's centerline. A 110mm would normally measure 94mm. If it's less than 94 and more than 84mm, it's some where between a 100mm and a 110mm.

nogearCX

you could use the longer stem :-P

K&K_Dad

Why not measure the 100 at a point in which you know it's 100mm.. Use those same reference points on the larger. That way you're measuring them the same, and see what you get. But there's no way that the larger is 100mm...

Cleave

Hi umd,

What I didn't see anyone mention is that the current production processes for stems can have a lot of variability. I don't know why, but I have measured brand new Ritchey stems at my LBS and stems identified as the same length and rise were up to 5 mm different in length. I have two Easton EA90 stems, both marked 120 mm with 10 º rise/drop and one comes out at 120 mm while the other is 125 mm.

You may be seeing some of that manufacturing variability on your 3T stem.

wrobertdavis

I sure hope that's an indication of design version control and not actual manufacturing dimension variability. They would have to be manufacturing these things with stone axes by hand to be that sloppy. A stem and most parts on a bicycle are high precision parts.

Bob