Bicycle frame design explained - article comments


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To prevent article pages from being miles long, but preserve all the useful questions and answers provided over time, I've decided to copy/paste the website comments to the forum - and "move" further discussions here.

These are the comments from the article:
Bicycle frame design explained
    22/08/2022 at 14:54
    If you were to draw a freebody diagram, I think you will find that the chain stay and the down tube are subjected to tension (stretching force). While I am not expert on bikes, I am reasonably certain that would be the case. In addition, the downtube will be subjected to torsion due to the offset of the pedal force from the bike mid plane and will be reacted by the forces on the handle bar (which is again offset from the centre plane). That I think explains why the down tube is usually the most heavy member (because it is subjected to a tension force and an additional torsion).
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      Relja Novović
      22/08/2022 at 16:38
      Good points. I agree. My reasoning:
      I think that chainstays see tension most of the time, except for the right chainstay when pedalling really hard.
      Seat tube is in compression when the rider is seated, and under tension when they are standing on the pedals.
      Tubes of a larger diameter see some torsion as well, which is one of the reasons for them having a larger diameter (yes, wall thickness does vary, but still, the overall stiffness and strenght do increase with greater diameter, with most tube designes). Especially the downtube – as you noted.
      I would argue that the highest loads are tension and torsion loads (and the tubes facing those are of a greater diameter). However, it’s fair to note that I have no way of accurately measuring that, especially since it only makes sense to measure while a rider is riding, jumping, sprinting etc, so take this with a grain (or a bucket 🙂 ) of salt.
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    Paul Lindsay
    13/10/2022 at 20:07
    What criteria determine the shape and location of rear derailleur dropouts? why are there so many designs of replaceable dropout ears?
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    21/10/2022 at 17:03
    too many newer bikes do have expendable parts,now i am not saying all new bikes are trash but too many bikes have parts that break or wear out fast which is why i ride old school retro style bikes,once you fix them up good you do not have to work on them for a very long time,those older bikes are ten times more reliable then some of these newer ones
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    23/10/2022 at 12:40
    why would they even make alloy rear derailer hangers when all previous bikes had them built into the rear steel dropouts,that was one step backwards for bikes,another step backwards for bikes was moving over to all sealed bearings,while it may be good for the manufactures bottom line its no use to bike riders,try even finding the right sealed bearing for your bike when they wear out,then try fitting one,i had a friend bring me a bike not even 10 years old and all sealed bearing were done including the headstem.
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      Relja Novović
      25/10/2022 at 06:28
      Hi Mike,
      Regarding RD hangers, steel can be bent back more times than aluminium. For aluminium (and carbon) frames, as Paul said, I think a separate hanger is a good idea in terms of durability.
      Also, the separate hangers are designed to be weaker, so they “give in” first in case of an accident – protecting the rest of the frame, and even the rear derailleur from the impact.
      My biggest gripe is that manufacturers didn’t bother to accept one (or three for that matter) hanger shape standard – so finding a spare, matching one is difficult. That’s the biggest nonsense.
      Regarding cartridge bearings – they have pros and cons. I very much prefer cup and cone. My video on cartridge bearings vs cup and cone.
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      13/12/2022 at 06:13
      i switched over to a sealed bottom bracket and found they do run alot smoother and alot less work,but when it comes to wheels and headsets i still use cup and cone system,easy to work on those and easy to find spare bearingswhen i was using cup and cone bottom bracket i found it too fiddly to adjust and no matter what type of grease i used there the grease would allways move away from where it was needed
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    Paul Lindsay
    24/10/2022 at 17:15
    Your dropout comment is valid perhaps for steel, aluminum and titanium frames, but for carbon fibre a separate solid dropout which can be threaded is mandatory.
  1. Nic
    27/04/2023 at 17:14
    This is an edit to the post I made a few minutes ago with a different logic. I think it would be best if you only “publish” this one: Are 1) seat stays assymmetric in shape and 2) do the drive side chainstay and the drive side seatstay sit more outboard (from the bike center / bb Center / wheel center) than their non-drive side counterparts? Assumption for my conclusion: the wheel sits in the center of the bottom braket and is perpendicular to the bottom bracket. My conclusion is that both (1) and (2) must bet true because 1) the center of the rear wheel is closer to the non drive side dropout than to the drive side dropout (see pictures above the headline “2 Visual Test” on and 2) the wheel is / should be centered between the seat stays near the seat tube (see first picture on the same parktool URL above below the headline “2 Visual Test”). Is there a flaw in my thinking?
    Context: I’m designing a custom bike for myself with the rear rack welded to the frame. Ideally I would want to have the same distance between the tire and the inner edges of the rack on the drive and non drive side….
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      27/04/2023 at 19:22
      Hi Nic,
      Most frames have symmetrical dropouts (equal distance from both the longitudinal and transverse frame axes). With such frames, the rear wheel is dished so that the rim’s centre is right between the hub’s “locknuts” (quotation marks are because many modern hubs don’t have classic locknuts, just the hub ends that face the inside of the frame’s dropouts). See picture 22 in my Bicycle wheel building basics article.
      There are however “offset frames,” where the drive side dropout is a bit further to the right (i.e. placed more outwards from the frame’s longitudinal axis compared to the non-drive side dropout). That is done to help make the rear wheel stronger, as it allows spokes coming from the drive-side flange to not have such a steep angle, thus making the drive and non-drive side spoke tension more similar (less drastic difference in tension), plus it provides a better bracing angle for the drive side spokes.
      For the bike to steer nicely, the rims (and the tyres) need to be in line with the bike’s longitudinal axis. Hence, if your goal is to have the same distance of the tyre from both dropouts, it would make sense to make the dropouts symmetrical (i.e. to not make an offset frame).
      As English is not my native, and this topic is a bit complex for me to explain with words, I asked Grammarly’s new AI tool to read what I wrote, and make it easier to understand. This is how Grammarly’s “robot” rewrote my reply – in case it’s easier to understand (I’m curious to hear if it really was helpful):
      Dear Nic,
      I hope this message finds you well. I wanted to bring to your attention an interesting fact about bike frames. Most bike frames have symmetrical dropouts, meaning that the longitudinal and transverse frame axes are equidistant from the rear wheel. In this case, the rear wheel is dished so that the rim’s center is right between the hub’s “locknuts.” You can find a visual representation of this in picture 22 of my article on Bicycle Wheel Building Basics.
      However, there are also “offset frames” where the drive side dropout is a bit further to the right, placed more outward from the frame’s longitudinal axis compared to the non-drive side dropout. This design is meant to make the rear wheel stronger. It allows spokes coming from the drive-side flange to not have such a steep angle, making the drive and non-drive side spoke tension more similar and providing a better bracing angle for the drive side spokes.
      I hope you find this information useful. Let me know if you have any questions or if there’s anything else I can assist you with.
      Best regards, [Your Name]
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    28/04/2023 at 07:40
    Hi Relja,
    Interesting AI tool. I can also say that, although English isn’t my first language, I understood the text you wrote perfectly well. And in my humble, non native speaker, opinion your English is very good (and very precise).
    On the topic: …I also had assumed that the wheel is in the center between the dropouts. But the picture from the parktool website made me think again. I downloaded the picture from the parktool site and measured the distance from the wheel centre to each of the dropouts. Here is the result:
    So either this is a wheel made for an offset frame or, I guess more likely, the camera of the parktool employee was not positioned over the wheel center. It could also be that, with my measurements, my location of the wheel center wasn’t precise. But the white dashed line is rather too far to the left anyway. If this was corrected it would only increase the difference between right and left side distance to the dropouts.
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