7

I bought this PRO (Shimano) LT handlebar stem. However, I'm having difficulty decoding the instructions in which to tighten the handlebar clamps.

The stem itself has this piece of cardboard in between the top handlebar clamping bolts enter image description here Which I assume is telling me to tighten the 2 top bolts first, so that there is no gap at the top of the clamp. Then proceed to tighten the bottom 2 bolts on the bottom to torque spec.

However, when I look at the manual, it says

  • In case of 4 bolts: tighten always crosswise and evenly.
  • In case of additional instruction is added between stem body and stem cap, follow this instructions carefully.

The visual instructions show no mention of tightening the top bolts first. enter image description here

I'm familiar with the method above, but not so much the instructions on the yellow cardboard. How do i satisfy both conditions, in which I tighten things crosswise and evenly, while following the instructions on the clamp to tighten the top two bolts first so that there is no gap?

I've always been taught that gap differential in handlebar bolts is a bad thing. Is this just an exception to that rule?

Here is the actual stem. There are no markings on the bottom bolts. enter image description here

Anyone know what I might be misinterpreting here? I'll be sending an email to the manufacturer as well, but just wondering if anybody had any insight.

Here is the relevant visual instructions in the manual enter image description here

The full text instructions: enter image description here

EDIT:

So I've found some information on it. These two videos mention the existence of "zero-gap" stems.

A procedure with a zero-gap stems:

An article on stem variations:

https://www.mbr.co.uk/buyers_guide/best-mountain-bike-stems-356559

The reason they seem to exist, reduces the risk of uneven tensioning, as there are only two bolts to worry about.

The procedure seems to be to tighten the designated side fully to spec (or just tight), then tighten the opposite side to torque specifications.

Improve this question
4
  • 6
    "In case of additional instruction is added between stem body and stem cap, follow this instructions carefully." says pretty explicitly that the extra instructions on the card should be followed. I wonder why they didn't just update the main instruction sheet.
    – ojs
    Jan 8, 2021 at 17:58
  • yeah, i figured. The issue i suppose is that the extra instructions are fairly bare bones, which I feel like gives enough leeway/ambiguity for some sort of error for those unitiated (such as myself). Just wanted to make sure and inform myself, as I don't want to take any risks when it comes to the headset. Also the phrase to "Always" tighten 4 bolts evenly threw me off a bit. Thanks for the response!
    – andoofthewoods
    Jan 8, 2021 at 18:17
  • @ojs That main instruction sheet is likely supplied along with every stem and handlebar model Shimano makes, not just this one. See how it is titled generically "stem/handlebar". Saves them money and logistical complexities.
    – MaplePanda
    Feb 9, 2021 at 0:31
  • 1
    @MaplePanda I get it. As an engineer I'm just wondering how it can be cheaper to include both generic and model-specific instructions instead of just the model-specific one. Especially when the model-specific one is die-cut piece of cardboard when a paper printout would work.
    – ojs
    Feb 9, 2021 at 7:26

5 Answers 5

Reset to default
14

Here are a few diagrams I drew in Autodesk Fusion 360 to explain how these zero-gap stems work.

According to the instructions provided with the few zero-gap stems I've encountered, you should tighten the "no gap" bolts to full torque first, then alternate between the lower bolts to fasten the handlebar. That makes sense according to the diagrams because that ensures the upper tabs are solidly clamped together to form the C-shaped clamp. If you're confused by my weird terminology, take a look at the diagrams (especially #3 and #4). You'll see what I'm talking about.

This is a normal stem. The left piece (the stem's faceplate) evenly tightens in a straight line towards the right piece. Assuming it gets properly torqued, the faceplate will travel in a straight line to the right as the bolts are tightened.

enter image description here

Next, this is what would happen if you tried to use a normal stem in a no-gap fashion. As you can see, the two pieces are no longer parallel, and the bolt holes are no longer aligned properly. That's not good. If you were to tighten the bolts while the stem was arranged like this, you run the risk of damaging your stem and handlebars because the faceplate will be forcibly bent into an oval shape so that the tabs (and thus bolts) are parallel once again. Obviously, you cannot properly clamp a round bar with an oval-shaped clamp.

enter image description here

The next picture represents a zero-gap stem installed correctly. Note how the upper tabs are flush with each other and are lined up parallel; you're effectively turning the two separate pieces into one solid C-shaped clamp. When you tighten the lower bolt, the top joint will stay still. Instead of the faceplate moving in a straight line towards the right like how it would with a normal stem, it is now deflecting in a circular manner.

enter image description here

And finally, this is what it would look like if you tried to use a no-gap stem like a normal stem. You can see how the tabs are completely out of alignment. Like before, the faceplate might get deformed into an oval shape if you try to tighten it down.

enter image description here

My guess is that with a zero-gap stem, you're basically trying to replicate a classic quill stem's one-piece clamp without the hassle of also having to remove the brakes and shifters when removing the bars. The entire clamp should function as a rigid unit after you tighten the "no gap" bolts.

enter image description here

Improve this answer
7
  • Nice drawings MP! You suggest fully torquing the top no-gap bolts before installing the bottom gap bolts. Would it make any practical difference to start threading the bottom gap bolts before the top bolts are fully torqued, as long as the top bolts have already seated and closed the upper gap?
    – Armand
    Jan 9, 2021 at 6:35
  • I wonder about the purpose of these no-gap clamps. Is it so that the clamp area inner diameter can be made a bit smaller to eliminate the possibility of a loose mounting due to undersized bar diameter? Or is it that people have difficulty mounting all-gap clamps with equal gaps and this is a way to prevent them from even having to try?
    – Armand
    Jan 9, 2021 at 6:40
  • @Armand No, it shouldn’t matter if you screw the bottom gap bolts in in a few turns as long as they aren’t interfering with the top bolts at all when you tighten those. As for the purpose, I think they clamp more evenly. With a normal clamp, you have pressure hotspots at the two gaps because you’re basically trying to deform your bars into an oval shape. With these, it’s a more even, circular clamping action. However, this benefit can be negated by people who inadvertently install them incorrectly.
    – MaplePanda
    Jan 9, 2021 at 18:48
  • 1
    thanks for the diagram and clarification on torquing the top bolts. It's very informative. Greatly appreciated!
    – andoofthewoods
    Jan 11, 2021 at 6:01
  • 1
    @MaplePanda Re: "I originally thought you meant just screwing the bolts in superficially without them contacting the faceplate at all in order to not lose the bolts or something" Yes, that is what I meant -- we're all good here I think.
    – Armand
    Feb 9, 2021 at 5:05
6

The card is telling you to tighten the top bolts until the gap is closed, then tighten the bottom ones. It does not mean to torque the top bolts to final tightness before tightening the bottom ones.

Improve this answer
1
  • thanks for the clarification!
    – andoofthewoods
    Jan 8, 2021 at 18:32
5

You should follow the instructions on the yellow cardboard. The manual contains the normal default procedure of tightening crosswise and evenly, but the second bullet point, "In case of additional instruction is added between stem body and stem cap, follow this instructions carefully." is telling you to override this normal procedure if other instructions are given.

The yellow cardboard is giving you these override instructions, which also match the markings on the stem itself for additional certainty ("close gap").

Improve this answer
1
  • yeah, i figured this was the case, I just wanted to confirm my assumptions about the procedure in yellow cardboard, as I was totally unfamiliar with it before this!
    – andoofthewoods
    Jan 8, 2021 at 18:33
3

It is an 'eternal' and more of a philosophic question. Both variants are out and both seem to be true and not so. It is the same on the motorcycle front. The picture of the actual stem says clearly that you should close the gap at the top. I would follow that procedure. Screw the bolt in just until the plate touches the opposite side, hand tight, not with the extra quarter of turn. Then tighten the bottom plate to the recommended torque. That way the top bolts, the plate and the part of the stem behind the bar will act as a single element and the plate plus the bolts act as elastic elements.

If you tighten them cross-wise, the bar is compressed into an oval shape and has to take up more of the clamping forces.

(This is merely an opinion, but I think it is mathematically impossible to evenly tighten 4 bolts crosswise with the same torque. Five bolts, like on the wheel of a car, yes, but not 4! Bolt 1, torqued, bolt 3, torqued, but then bolt 2 or 4 are breaking the crosswise rule! Whereas with 5 bolts it's 1-3-5-2-4.)

Improve this answer
5
  • yeah, the reasoning behind the design makes sense. I'm just completely unfamiliar with it this design and the exact procedure, as it's only a basic diagram. Will follow the procedure you outlined!
    – andoofthewoods
    Jan 8, 2021 at 18:21
  • would you know how this zero-gap design would affect the torque I should set on the bottom bolts? Just curious, as I'd usually tighten it to 4-5 Nm for normal 4 bolt designs with even spacing with each bolt. The max torque on this stem seems to be higher at 7 N m
    – andoofthewoods
    Jan 8, 2021 at 21:15
  • 1
    @andoofthewoods It probably takes more clamping force to bend the clamp enough to keep the bars in place in comparison to a regular design. Not a problem.
    – MaplePanda
    Jan 9, 2021 at 0:35
  • Two other no-gap stem instructions say to tighten the bolts on the no-gap side to the specified torque, then tighten the bolts on the other side to the same torque. FSA and Easton
    – Gene Pavlovsky
    Mar 5, 2021 at 14:31
  • @andoofthewoods late reply about torque, but i got same exact stem. maybe it's the bits i have but tightening to 7nm starts to round out the 4mm alu bolts it shipped with. 5nm seems to hold it in place.
    – Sander
    Jul 27 at 20:56
2

Two other no-gap stem instructions say to tighten the bolts on the no-gap side to the specified torque, then tighten the bolts on the other side to the same torque. FSA and Easton

Improve this answer

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.