Originally Posted by
B1KE
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.
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)