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I just received a new non-brand carbon frame and immediately noticed that the bottom bracket looks to stick out significantly more to one side than the other, and the rear bottom forks also seem to be have slightly different dimensions - both somewhat asymmetric along the center line of the frame.

Is there any reason for a frame to be designed like this? Does it have something to do with it being a disc brake compatible frame? Or is this more likely a manufacturing / design problem?

Here are some images:
enter image description here enter image description here

Thanks! I really appreciate any help + advice.

  • It's the chainstays that are asymmetrical, not the frame itself. If you want to be 100% sure whether the frame is symmetrical, run a thin rope from the rear dropouts (both left and right) to the head tube and measure the distance from the rope to the down tube. The distance on both sides should be identical (within the measurement error). – Mike Nov 9 '18 at 21:59
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    The magnitude of the asymmetry around the bottom bracket makes it very clear that this asymmetry is by design. You wouldn't produce such an asymmetry by accident. What you are seeing is a form that follows function, making the chain-stay go out of the way for the chain-rings. – cmaster Nov 9 '18 at 22:12
  • Mainly, with the molded frame there's no compelling reason to try to keep things symmetrical. With welded metal frames the manufacturing stresses are balanced better if the frame is fairly symmetrical, but the stresses are much reduced with a molded frame. – Daniel R Hicks Jan 4 at 3:44
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The bottom bracket shell does not really stick out more on the drive side, it's symmetrical around the center line of the frame. The non drive side bottom bracket shell and chainstay are just more built up.

On reason is providing clearance for the chainrings on the drive side.

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    Put in a rear wheel and you'll notice that the tyre is exactly behind the seat tube. – Carel Nov 9 '18 at 16:40
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Some bikes have a drive-side chainstay with a downward kink to minimize chain slap. Many bikes need to get creative with the path that the drive-side chainstay travels along the Y axis, in order to "thread the needle" between the rear tire and the chainrings, which can be a very tight fit. It's fairly common for the drive-side chainstay to be stouter, because that's what is most directly loaded when you're pedaling. Cannondale even uses asymmetric bottom brackets.

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    On a road bike this would be to reduce chain suck, rather than chain slap. Giant does this on their road frames rather than a metal plate attached to the frame to deflect the chain . – Rider_X Nov 9 '18 at 19:48
  • Cool, I did not know that. – Adam Rice Nov 10 '18 at 0:29
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Is there any reason for the rear frame and bottom bracket to be asymmetric?

The crankset, chain, and gearing are all on one side, making just about every standard bicycle asymmetric.

Given that there are different asymmetric forces on the parts of the frame because of that, there's plenty of reason for a frame to be asymmetric.

  • Thank you for the response. I understand that all frames will have some differences between the two sides to compensate for the spacing of the crankset, chain, etc., but in this instance it looks to be much more pronounced than usual. Would you mind commenting on the images I included? – aakzeman Nov 9 '18 at 16:30
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Or is this more likely a manufacturing / design problem?

Absolutely not. Carbon frames are made using moulds and jigs so every frame that a factory produces should be exactly the same shape, except for tiny variations caused by hand-finishing. This recent GCN video shows the processes involved in making carbon frames, though be aware that it promotes Look bicycles quite heavily: How are Carbon Fibre Bikes Made? | LOOK Cycle Factory Tour.

Asking if it's a design problem presupposes that it's a problem at all, and the other answers explain why it's not. And bear in mind that a manufacturer who released a product with something so obviously "wrong" about it would have had to make an enormous series of huge mistakes.

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A good question for the manufacturer! My guess is that it's to reduce flexion losses in the frame due to the force you exert on the pedals and chain. Putting them more in line and giving your forces less of a lever arm would reduce those losses, making the bike more responsive and faster with the same effort. There's a lot written about frame design, reducing flexion, etc.

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