Hey everyone,

Sorry if it's a noob question but I was wondering if anyone has a moment and can help me out.

Can anyone explain what I believe to be called shims (circular parts with numbers +4, +2, 0 that go on the inside part) of the stem do. Right now the stem I have has some inserted.



1. What do they do?
2. How do they effect the position?
3. What does removing them do?

They affect the angle or "rise" of the stem. They raise or lower the bars with a smaller movement to the front or back by about 1/3 the amount of the change in height. Removing the shins sets the stem angle to it's base value.

Unless I'm mistaken (quite possible) shims are simply adaptors that allow you to fit a stem, seat post, etc that wouldnt otherwise fit. They work in some applications but are not ideal - the right fit without the shim would be ideal of course.

I don't think that is the case here. See my post.

Actually they are not stem size adaptors, they are used to "fine tune" the angle of the stem as described in post#2 above.

Thanks for your help. The stem I have now has a 12 degree rise(not sure what that means) but I want to get it as horizontal as possible to extend my reach a bit when I take off the spacers I have the "shim" on the inside, I have the shim inserted and one side of the circular shim says +4 and on the other side of the same shim it says -4. How do I insert this and whch way to achieve the most horizontal angle?

If I take out the shim completely the stem does not fit on the steerer tube.

Thanks for your help, how do I go about fine tuning the fit because the shim says -4 on one side and +4 on the other side. It seems like the only way to insert it is from the top or the bottom.

You know you can turn the stem over and make it -12. The shim can be used to either raise or lower the stem further. +4 raises the bars, -4 lowers it. Keep in mind the -17 is horizontal when your head tube angle is 73 deg.

There should be a way to use the stem without the shim. It is supposed to be a 4 position system.

Thanks for the reply, I'm learning a lot.

The head tube angle on my bike is 72 currently. Does the +4 or -4 come into effect depending on whether I flip the stem up or down? Because I only have one shim labelled +4, -4, where as in the specialized picture it comes with 3 different ones.

Four or six combinations. + or - 12 and + or - 4 or no shim so + or - 8, 12, 16

This stem is sort of unusual, most stems don't have these adjustable shims. Basically, the stem is a little oversized, and the hole through the shim that fits the steerer is angled from top to bottom, either 4, 2, or zero degrees. (The top of the shim is thicker on one side, and thicker on the opposite bottom side.) A pretty slick design.


You need to have one of the shims installed. The photo shows +4,-4 and +2,-2 and +0,-0.
If you are missing the other two (is it a used bike?), you should be able to contact Specialized for replacements.

EDIT -- do you have that 17 degree stem in the photo? or one that's labeled 12 degrees? The discussion below is similar, just start with -12 instead of -17.

With the correct spacer, and flipped down, the stem can be set to any of these angles:
-17 -4 = -21 degrees
-17+4 = -13

-17 -2 = -19
-17 +2 = -15

-17 +0 = -17


So:
72 degree steerer on your bike. The "-17" degree angle is measured downward from 90 degrees straight out of the stem, which is +18 degrees from horizontal, since the steerer is tilted back. (90-72=18)

Then, starting from the bike's +18 degrees, a -17 degree stem is a 1 degree rise from horizontal, essentially flat.

See this Stem Comparison calculator, it's a lot clearer to see the stem angles. (I couldn't explain it until I looked at the calculator!)

You can plug in the 72 degree steerer, the stem angle, the number of spacers, and the stem length to compare two different settings. It will tell you how much the bar position will change by changing angles or spacers.

Flipping the stem
If you flipped the stem with the -4 spacer, it would be +18 +17 -4 = +31 degrees upward. Way up!

Most "flipped" stems are closer to 6 degrees, which would make them +18 -6 = +12 degrees, or +18 +6 = +24 degrees. Both of these could be usable for different riders.

Look at most modern bikes, you'll see that the stem angles up a little or a lot. It's rare now to see one horizontal, those are more often on racer's bikes.


Example:
Here's the stem, I guessed it was 110 mm (a typical length)
The red stem is -17
The blue stem is -21 (with your -4 spacer)

Me too, lol!

All of that is explained in the stem's users manual btw.