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Context

Even major bicycle corporations have the occasional product recall on handlebars.

It's easy to see why. The bike itself is a gentle, light, and nimble ~10 kg (for a road bike) object, whereas the weight of the brute riding it is anywhere from 50 kg to 100+ kg, and if the cyclist doesn't rise before hitting bumps, the frame is subjected to considerable stresses. The stresses are most worrisome on the handlebar (and the seatpost).

The handlebar can be roughly modeled as a beam with two 20 kg weights at each end, and a 40 kg supporting load at the center.

But that's without any impacts. Either way the stresses spike at the stem-handlebar interface, and their starting values are already nontrivial because the handlebar must be firmly clamped at the stem.

Carbon on bicycles is, as cyclists like to repeat often and in unison, not a spontaneously explosive material. Still, anyone who has followed a serious bicycle race will have witnessed live the spectacular ways that carbon frames can fail. You don't want that to happen to your handlebar.

This is one of the reasons why the bicycle industry has moved on mountain bikes from a 31.8 mm diameter for the stem-to-handlebar connection. On mountain bikes that diameter is now used only for lower-market bicycles. The trend is moving towards 35 mm diameter at the stem-handlebar interface.

Statement

The stems on the market are of two designs.

The dominant one is an open-front design.

open-front stem

The alternative is a closed-front design.

closed-front stem

Either way, the backside of the stem (the one on the side of the cyclist) is always open.

Especially for a carbon handlebar since the recommended torque of stem bolts is often much lower than that for an alloy handlebar, it would seem that the closed-front design is superior. It distributes the grasping stresses on a larger area. (A special gritty paste is usually required to make a firm connection possible using lower bolt torques.) Indeed, it would appear that a closed-from-both-sides design would be even better.

Why is the open-front bike stem design so widespread despite that it may be inferior to the closed-front design?

The only explanation I gather why the open-front design suffices is that nothing is rigid. Even steel ball bearings, which we imagine are a rigid object, do not generate stresses that shoot towards infinity. When loaded, they behave just like a rubber ball. They can be squished.

Here also, the two pairs of bolts will generate the dominant stresses on the two narrow vertical bands—regardless of whether the center section is present in the stem. If it's present, it will take relatively little load, and it can hence be discarded. The presence of the mid-section may simply be cosmetic.

Can you confirm/refute and/or elaborate?

Future Questions

  • Normally shims are only needed to use a larger-diameter stem. Might shims be used to better distribute the load and enable rougher riding with less concern of using dental insurance?
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    $\begingroup$ I don’t think it makes much of a difference. The biggest problem is the stress riser right at the edge of the clamp. $\endgroup$
    – Michael
    Oct 7, 2022 at 13:24
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    $\begingroup$ This question really seems like it's comprised of very leading or opinionated or speculative statements wrapped in a veneer of curiosity. Consider revising to: "Are their any distinct advantages between open or closed-faced stems in the context of mountain biking?" Otherwise, elements of your question (e.g., "but they are inherently inferior to stems with a closed-front design") come across as assertions. $\endgroup$
    – Paul H
    Oct 7, 2022 at 18:20
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    $\begingroup$ @PaulH Is the term "speculation" ambiguous? Would it be better if I had used "hypothesis" to remove any chance this is considered an assertion? $\endgroup$
    – Sam7919
    Oct 7, 2022 at 19:28
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    $\begingroup$ Tolerances are not as fine as some people think. Difference between open or closed, carbon or alloy are swamped by the burden the lawyers of handlebar manufacturers put on them to over-engineer for cack handed gorillas with spanners. This ain't aircraft engineering where to do up a bolt you need a 5-year training course and 25 sheets of paperwork. $\endgroup$
    – mattnz
    Oct 8, 2022 at 5:20
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    $\begingroup$ I am unsure about whether carbon and aluminum components need such distinct service procedures and/or characterization in general. My usual assertion that CF is not a self-igniting high explosive seems particularly relevant here. $\endgroup$
    – MaplePanda
    Oct 8, 2022 at 6:05

1 Answer 1

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For starters, I think you have to differentiate between "theoretically inferior" and "practically inferior". Judging by the hundreds of thousands of bikes with open-faced stems out there which are safely zipping along, I don't believe it's really an issue in practice. Stem/bar failures generally are caused by corrosion or grossly incorrect torquing procedure in my experience.

Regarding the actual stress distribution, I would imagine the removed material doesn't play a huge role in clamping the bars. As you noted, the stem side of the bar clamp is universally open. Hence, you're going to be limited by the maximum contact stress on the backside of the stem, regardless of whether the faceplate is open or closed. Stems made by Bontrager for example have two separate "face plate" bands, which is a design that seems to support this theory. Also, as Michael noted, the stress risers on the edges of the stem are of far greater concern.

I also don't think it's necessarily "safer" to reduce the clamping pressure on carbon components. So long as you're within the safe zone so to speak, it doesn't really make you extra safe to lower it even more. In fact, high preload force tends to make bolted joints more fatigue-resistant, so there may actually be some benefit in keeping clamping pressure at a moderate level.

As a point of interest, I have the same stem as the one in your first picture (albeit in black) on my bike right now. No issues.

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