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I bought a 1990's steel mountain bike a few weeks ago and I have some concerns about the design of the rear dropout, pictured here:

Close-up image of bicycle rear dropout

It seems to me that an ideal dropout design would redirect upward force from the wheel axle toward the ends of the seat and chain stays. The axle mount should be positioned closer toward the center of the bike, and below the chain stays so as to distribute upward force between them. Here's a diagram showing how I expect force to be transferred to the frame by the wheel axle when the bike lands from a jump:

Force diagram

For maximum load-bearing capacity, I would expect the dropout to redirect the force of impact perpendicularly to the tip of the seat stay. However, here the angle is about half that, so half the force is applied to the seat stay as torsion force instead of compression force. This isn't ideal because the soldered joint isn't as strong as regular steel, so it becomes a stress point when torque is applied. While that's still the case when compression force is applied, the joint is less vulnerable to compression force due to technical reasons I can't explain.

Almost every other dropout design I've came across at least tries to redirect the force so that it meets the seat stay directly. Here is an image with some examples of other rear dropouts:

compare and contrast

In these examples, upward force is always directed toward the base of the seat stay, whereas the chain stay of my bike extends in a way that pushes the axle mount further to the rear, off center from the truss that connects the chain stay, seat stay, and axle. I think this means that most of the force is transferred perpendicularly to the upper truss member, which connects to the seat stay at an angle and exerts perpendicular force that's not balanced out by opposing force from the lower truss member.

In the examples, while some axle mounts are positioned further away from the stays than others, the dropouts in those cases are long and flexible enough to provide structural support that can absorb some force of impact. In my bike, the dropout is both off center and not long enough to provide structural support by itself, so I think most of the force is transferred directly to the seat stay at an angle.

I don't think the chain stay handles much load either because it sits slightly below the axle mount point and so doesn't receive much compression force.

Finally, the dropout functions to securely join the seat stay with the chain stay, but the lower truss member doesn't form a very smooth curve between the stays, creating a stress point at the chain stay.

Is my analysis correct? Does any of this have a significant impact on the stability and strength of the bicycle?

  • I do want answers, but I also think your answers are inaccurate. For example, it's commonly accepted that traditional bike frames tend to fail at the joints, except in the case of butted tubing. Also not all bikes have thickly welded joints, including this one. –  Feb 27 '21 at 06:37
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    Bridges are built with diagonally slanted beams, because that's how you achieve stiffness. Mechanical Engineering 101 (literally). – Hot Licks Feb 27 '21 at 12:52
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    This is my last comment. Thanks for all the input, but I've now made up my mind on this matter. –  Feb 27 '21 at 08:32
  • It's like arguing that bridges built using diagonally slanted beams are as viable as those built using straight vertical beams, because the loss in structural strength constitutes a compromise rather than a design shortcoming. –  Feb 27 '21 at 08:29
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    Answers go in answers please, comments are for clarifications and improvements. This question is about the design of this dropout. If you're going off onto tangents about frame designs, welding vs brazing, etc then thats a separate question. Also disagreeing with an answer should be done with a downvote or a comment to improve that answer. – Criggie Feb 27 '21 at 08:27
  • I just found it strange that almost no bike I could find with similar profiles opted for this kind of compromise. –  Feb 27 '21 at 08:21
  • I searched the entire internet for an explanation of how this all comes together to make the ideal bicycle, the closest I could come to was this article, specifically the second section –  Feb 27 '21 at 07:58
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    You don't need recent metallurgical magic for the welds to be stronger, they are often stronger because there is simply more material after welding. Look, do you actually want answers or do you just want to argue? – whatsisname Feb 27 '21 at 06:36
  • I meant welded. To the best of my knowledge I don't think TIG welding from 1993 actually strengthens the material. Otherwise there would be no selling point to Reynold's "air-hardened" tubing. As for load-bearing, my complaint was that an objectively better design could've easily have been used, but wasn't. It's a nitpick, but I think it's valid. –  Feb 27 '21 at 06:24
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    "For maximum load-bearing capacity" - that's not an important design criteria, it only has to be appropriate for the loads the rest of the frame can handle. – whatsisname Feb 27 '21 at 06:22
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    "the soldered joint isn't as strong as regular steel" - it's not soldered, it's welded. You are right it's not as strong as, it's stronger than the tubing. – whatsisname Feb 27 '21 at 06:16
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    With regard to chainstay forces, the BB is lower than the axle, so the force applied by the axle pulls on the chainstay (ie tension) rather than pushes (compression). Remember that the weight on a bike is between the axles, not directly above either, so it wants to sag in the middle. That means compression of top tube and seat stays and tension on downtube and chainstays when at rest. Try drawing a free-body diagram of the system. . . –  Feb 25 '21 at 14:38
  • @Morgan087: Just from my own garage: an 80s era Puegot https://i.imgur.com/eEnasmk.jpg and a 2007? Surly Cross-check https://i.imgur.com/dvE49h7.jpg While both have horizontal dropouts instead of vertical, their interfaces to the stays are the same. I have another bike with virtually identical dropouts as well but was too lazy to take a picture. Look how rusty that cross-check dropout it, yet it's been like that for thousands of miles. That's a good dropout. – whatsisname Feb 25 '21 at 02:27
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    What is the yellow line supposed to represent? –  Feb 28 '21 at 13:20
  • @ojs I'm fine with threaded stem, it was a limitation of its day. But in my view there's no practical reason not to position the axle closer inward, or extend the seat stay more so that it sit above the axle. By compression I meant compression force, and chainstay refers to the horizontal tube at the bottom. I wasn't expecting an answer as much as input about what I personally considered to be suboptimal alignment. –  Feb 25 '21 at 01:20
  • @whatsisname If you can find a photo of another steel bike with the same dropout design as this one, please share it –  Feb 25 '21 at 01:01
  • @cmaster I think that's what I meant to say. The hypothetical concern is that if the bike lands from a high enough drop the force of impact might be enough to dislocate the dropout from the seat stay, or at least deform the frame. I have no practical experience though to back this up, so I'll take your word for it. –  Feb 25 '21 at 00:38
  • Have you ever looked at the girders of a steel bridge? – Hot Licks Feb 25 '21 at 00:38
  • @Andrew I was referring to the force of gravity in both cases. Compression force against the chain stay can occur when the axle mount is positioned below the chain stay, so that the wheel axle pushes up against the chain stay to an extent. –  Feb 25 '21 at 00:18
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    "because they seemingly suggest a lack of rudimentary understanding of physics by the designers. Unless I'm mistaken" - When something is utilized in a gazillion bikes over many decades, the likelihood of it being the latter is about 100%. Bikes using drop outs just like these are still produced by the tens of thousands. – whatsisname Feb 25 '21 at 00:10
  • While you are right that there are torsional forces on the ends of the seat- and chain-stays, the bike is obviously a steel frame with steel dropouts. Steel is very robust, and the dropout has quite a significant thickness. As such, those torsional forces are simply irrelevant, the material is strong enough to absorb them. –  Feb 25 '21 at 00:09
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    I'm totally confused by your positioning descriptions. Could you please post some pictures with arrows indicating where forces are acting and what you mean by e.g. "contacts the seat stay from off-center"? –  Feb 25 '21 at 00:09
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    You also need to specify which forces you are concerned with. You seem to be talking about the downward force from gravity, but then you mention compression of the chainstay which only occurs due to pedaling force. Weight on the bike results in tension on chainstays –  Feb 24 '21 at 23:28
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    Bike design is super complex and the definition of "ideal" is really subjective. Design trends are not necessarily driven by what is the "strongest". for a manufacturer "ideal" is cost effective while for the consumer aesthetics can be a strong driver along with practicality (mounting eyelets etc). This blog post has some relevant points https://www.google.com/amp/s/www.bikehugger.com/posts/dropouts-good-aesthetics-and-bad-design/amp/ –  Feb 24 '21 at 22:07
  • @ojs It's a mid 90's bike so the stem is threaded. I don't think rack clearance was the reason, just sloppiness. –  Feb 24 '21 at 21:57
  • @Daniel I'm not saying that the overall design pattern is wrong, or that there's some catastrophic deficiency, but that the proportions aren't ideal compared to what's seen on almost every other bike with the same drop out design. Case in point, latter models by the same manufacturer would adopt a number of refinements. The chain stay would be shortened and the axle mount would be moved further in to line up better with the seat stay. –  Feb 24 '21 at 21:42
  • This was a bit difficult to understand, but to me it looks like the dropout is designed to give good clearance for rack support that's screwed to the eyelet on top. There are a lot of sub-optimal designs in bicycles, I'd guess this one has threaded stem too. –  Feb 24 '21 at 21:37
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    It's a bog-standard design that's been used on good-quality bikes for at least 50 years, maybe 100. Speaking as an engineer I see nothing wrong with it. – Hot Licks Feb 24 '21 at 21:29
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    @Morgan087 Unlike this dropout design, threaded stems have actual practical problems. They were still standard for about 100 years. It sounds like you're using the words "compression", "chainstay", etc in non-standard meanings. An understandable description of the problem might increase the chances of getting an answer, –  Feb 25 '21 at 01:12

4 Answers4

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An ideal dropout design would......

OK, already there you've gone into the weeds. There is no ideal dropout design. Engineering real world things is a balance of tradeoffs. You have to balance the mechanical aspects of the dropout like you mentioned, but you also have to factor in the price of the drop out itself, the material properties of the dropout itself, as well as the cost of manufacturing utilizing said dropout, plus some other things*. The importance of each of those aspects is going to vary depending on what kind of bike and what kind of price point you are aiming for.

And in practice, the mechanical aspects are pretty much the least concern. The forces at play trying to rip rear axle area are so small relative to the strength of pretty much any dropout is insignificant. Breakage is pretty much always due to corrosion, or a defect in manufacturing rather than by being under-spec'd.

Now to be fair, the style of dropout is cheap, both easy to make the dropout, and ease in frame fabrication. Those dropouts can be stamped out of sheet metal, or a bit better forged in a single step. Their slotted ends can be fit into slots in the tubing in a jig, then welded into place. The slotting interface also buys you a little wiggle room in tolerances of the cut tubing. On a more expensive bike, those curves and angles would likely be smoothed and filleted. But not to avoid stress concentration, but for aesthetics.

Sure, you could have a minimalist dropout that puts the tubing interface as close as possible to the dropout, such as these. However, that requires more manufacturing sophistication. When using dropouts like those, your tubing lengths need to be absolutely dead on correct. Tighter tolerances means more expense. Bicycle fabrication may not be as glamorous as F1 racing or space travel, but it is surprisingly sensitive to small angles, and with thin walled and lightweight tubing, getting those tight tolerances is remarkably demanding.

Look at sites such as framebuildersupply, paragon machine works, or many other suppliers for other examples of dropouts, and you'll find a huge variety, including drop outs just like the ones in your picture.

* For "some other things" - look at these: https://salsacycles.com/culture/reasons_to_love_the_alternator_dropout think of how much torsion is going to be on the dropout-stay welds. Think about how much more complicated making that dropout is. Yet, those are on the Salsa Fargo's because of the versatility they provide, which is prioritized due to that model bikes intended usage.

whatsisname
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  • "easy to make the dropout, and ease in frame fabrication" - the latter in particular means easy to turn out cheaply and reliably – Chris H Feb 25 '21 at 09:57
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In Edit #2, OP argues that the dropout does not redirect the force applied by the axle upon vertical impact in line with the seat stay. This statement neglects the role of the chainstay in resisting the torsional force.

Let's start with the OP's diagram of the force applied by the axle to the dropout: original

This force vector can be deconvoluted into two vectors (drawn in white) in line with the seatstay and chainstay respectively, such that their vector sum (within the accuracy of my drawing skills) is equal to the original red force vector. enter image description here

What we see is that the seatstay and chainstay can completely offset the red force arrow by applying forces directly along their respective lengths with no contribution from a torsional force. The seatstay is only under torsional force if the axle is not aligned with the length of the seatstay or if the chainstay is able to elongate significantly. Since tubes of metal are very resistant to compression or elongation, the axle does not move significantly from its static position when under load. Thus, the optimal design of a dropout for the limited purpose of resisting gravity (which is only one aspect of the role of a dropout) is any design that properly positions the axle at or very near the intersection of the seatstay and chainstay if they were extended. Furthermore, the dropout must be able to resist compression along the line of the seatstay and tension along the line of the chainstay. The nearly triangular design of the dropout truss system ensures that it resists these forces in a very similar way that the large triangle of the frame does. These are great examples of why triangles are the strongest polygons with respect to load bearing, which is why the two-triangle bicycle frame design is so elegant.

Obviously some dropouts, such as the cantilevered one shown in another answer, are not optimized for weight bearing, as they are intended to allow movement of the axle under load to provide some shock absorbance.

  • First, I didn't really understand what torsion force meant and you're right about chain stay offsetting the torsion force. Second, I understand the principle behind triangular constructions and how they're rigid. Third, I was probably referring to impact force resistance instead of load-bearing capacity (forgive my poor command of terminologies), such as when landing from a jump. In this case, the brazed joints seem less able to resist bending force from sideways than parallel compression force. I base this assumption on my experience building a bridge from tooth picks and Elmer glue. –  Feb 28 '21 at 05:52
  • @Morgan087 - impact from a vertical drop is just a rapid increase in the magnitude of the load. It doesn't change the directions of the forces. –  Feb 28 '21 at 11:50
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    It's interesting how it looks like axle is not at the intersection of seat and chain stays until you draw the lines. The angle between the stays is unusually small, and this leads to the unusual dropout design. –  Feb 28 '21 at 13:16
  • The low angle seems like the main cause of this issue, in that this particular style of dropout isn't commonly used for frames with flattened arc between the stays. Later models changed the dropout design a bit and appeared to user a seat stay that didn't narrow toward the tip, which I think are noticeable improvements. Though admittedly the difference isn't as dramatic as I originally thought, since the overall dropout shape and the frame geometry didn't change that much. –  Feb 28 '21 at 18:39
  • So, is your problem the hole between the axle and seat stay? Again, using standard English would make your message more comprehensible. –  Feb 28 '21 at 18:54
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The dropout is a single stamped piece of steel, and that shape has been used on millions of bike for decades. No the dropout design is fine.

Normally the top line of the derailleur would be in-line with the seat stay, visually looking like a continuation. This would leave the axle hole more sloped rather than the up/down look it has now.

I do think the frame looks odd, but as you point out its a 90s bike so has survived 20-30 years in this configuration. Assuming there are no cracks, rust, or dents then it should continue to work fine into the future.

If you look at
enter image description here
From Mountain bike ID help the "cantilevered" style of rear dropout was a thing in the 90s.

Cannondale embraced it in both road and MTB for years, and my early 90s road bike works fine with such cantilevered rear dropouts.

If you don't like the look, sell the bike and get one you do like. Or just ride more and don't look backward :)

Criggie
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  • That's an entirely different drop out style though. I'm comparing the drop out to others of the same/similar style, and it's evidently less robust. –  Feb 27 '21 at 04:49
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After much pondering, I've finally arrived at the conclusion that the dropout design on this bike is in indeed subpar for reasons that have been discussed at length elsewhere, but mostly from an aesthetic standpoint in practice, and not to the degree that I initially surmised. Still, it does reflect poorly on the manufacturer's sense of craftsmanship.

(Thanks to everyone for all their input, though I don't think there's a single best answer... just differences in perspectives ;) And I wouldn't want to be accused of favoring the answer that most closely aligns with my own biased opinion. I don't trust my ability to objectively evaluate other posters' answers, which all raised a good point or two, so for fear of engaging in favoritism under the naked guise of rational objectivity, I've decided to just announce my subjective verdict.)

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    You're not obligated to accept an answer. You're also not obligated to vote on the answers that are given to your own question, if you feel incapable of evaluating them. It's fine to wait for a more expert community to help identify the most useful ones. It's fine to accept an answer later on when you have more information, or to change the accepted answer. But this meta-conversation is not an answer. – Air Mar 03 '21 at 18:33
  • Okay thanks for the info. I've unchecked my own answer and I'll keep this in mind for the future. –  Mar 03 '21 at 19:13