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Replacing stem - Stack height question

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evan the cdn

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Discussion starter · #1 ·
I want to replace the stem on my Specialized Sirrus Comp with a shorter one. I'm trying to figure out what stack height I need. The factory stem measures 40mm tall but it was installed with the shim that can adjust the rise up and down by 4 degrees. On the shortest end of the shim it measures about 1.5mm high which effectively makes the stack height 41.5mm (I think).

When installed, should the cap tighten down on the stem or the steering tube? I'm assuming it should be the stem to keep the spacers from rattling but I thought I would ask just in case. Should I be looking for a stem with a stack height of 42mm?

Here's the shim.

 
#2 ·
Your stem cap MUST tighten down on the stem.
What this does, is press the stem down, which presses down on your spacers, which then presses down on the top of the headset. This pushed the bearings into the races, and makes the headset "tight". If the stem cap butts into the steerer, your headset will likely be, or at least become loose very quickly.

MOST stems will probably work. If the stem "stack" you are looking at is smaller, you may need to add another spacer under your new stem. I would say 2-3mm should be the gap from the top of your steerer, to the top of the stem. This usually allows the stem cap to tighten down properly. If you have more than a 7 mm gap between the top of the steerer and the top of your stem, you definitely want a 5mm spacer installed. (Above or below the stem - just to give the cap room to snug up.)
 
#3 ·
Depends on how much shorter the steerer tube is compared to the top of the stem stack.

The worst case is if it's only 3mm shorter, meaning once you exchange the stem, the steerer will only be about 1mm shorter, meaning the stem cap will not work correctly.

If you don't want to trim the steerer tube to compensate, one way to address this is to exchange one of the spacers for one that has an additional 2mm of material. Or just buy another 2mm spacer.

40mm is the most popular stack height. I wouldn't compromise stem choice based on this.
 
#4 ·
I think the Specialized stem shim alters the angle of the stem (and the bar height), not the actual stem stack height.
You need to know what the base angle of the stem and then where you had the shim set to figure out what angle your stem was.
The stack height of the new stem doesn't really matter as you will fit it with shims, either under or over the stem. You need to decide on the stem angle you want to buy. And of course shortening the stem with the same angle will lower the bars.

Play with this to see what happens:
Stem Comparison Tool | yojimg.net
 
Discussion starter · #5 ·
Thanks for all the replies.

Slash5 said:
I think the Specialized stem shim alters the angle of the stem (and the bar height), not the actual stem stack height.
You need to know what the base angle of the stem and then where you had the shim set to figure out what angle your stem was.
The stack height of the new stem doesn't really matter as you will fit it with shims, either under or over the stem. You need to decide on the stem angle you want to buy. And of course shortening the stem with the same angle will lower the bars.

Play with this to see what happens:
Stem Comparison Tool | yojimg.net
Click to expand...
I was thinking about leaving the Specialized shim off because the steerer tube on it's own measures 1-1/8" which seems to be the most common size. If I were putting the Specialized shim back on it would mean I'd need to find a stem for a thicker steerer tube.
 
#6 ·
evan the cdn said:
Thanks for all the replies.

I was thinking about leaving the Specialized shim off because the steerer tube on it's own measures 1-1/8" which seems to be the most common size. If I were putting the Specialized shim back on it would mean I'd need to find a stem for a thicker steerer tube.
Click to expand...
You can only use that shim with a Spec stem that's designed for it. It will not work with any other stem. There's a good chance you will need a new, normal top cap also, as Spec uses a non-standard oval top cap to compensate for the shim.
 
Discussion starter · #7 ·
Joules said:
You can only use that shim with a Spec stem that's designed for it. It will not work with any other stem. There's a good chance you will need a new, normal top cap also, as Spec uses a non-standard oval top cap to compensate for the shim.
Click to expand...
Good point. The cap is not round. I guess something like this Headset Spacer Kit would get me the cap plus the spacer so that I can use a standard stem.
 
#8 ·
evan the cdn said:
I want to replace the stem on my Specialized Sirrus Comp with a shorter one. I'm trying to figure out what stack height I need. The factory stem measures 40mm tall but it was installed with the shim that can adjust the rise up and down by 4 degrees. On the shortest end of the shim it measures about 1.5mm high which effectively makes the stack height 41.5mm (I think).

When installed, should the cap tighten down on the stem or the steering tube? I'm assuming it should be the stem to keep the spacers from rattling but I thought I would ask just in case. Should I be looking for a stem with a stack height of 42mm?

Here's the shim.

View attachment 1063026

View attachment 1063027
Click to expand...
I have one of those Specialized stems here. What length stem are you looking for?
 
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