I need a new front derailleur for my Shimano Acera T3000 gear shift. I have to choose between:

They differ only in the chain stay angle they are made for. The info I find on what the "chain stay angle" is seems partly contradictory or unclear. So I want to make sure I got this:

The chain stay angle is the angle between chain stay and seat tube - is that correct?

Or more precise the angle between AB and AC with:

  • A: bottom bracket axis
  • B: axis of rear wheel
  • C: "middle point" at the top of the seat tube

Here's my frame with measured angles:

enter image description here

of the angles in question only the one between seat tube and chain stay (68°) falls into the interval [63,69] ... so, it has to be that one, I suppose. From my mechanical, geometrical intuition I don't see why that angle is relevant for the FD. It should be ∠(KK',KC) in my opinion!?

  • Ahaaa ... "The derailleur must fit this angle or risk the cage hitting the chain stay." - that's interesting! So, it isn't about the chain but about distancing the FD from the CS. – Raffael Jun 1 at 10:02

Nominally the chain stay angle is the angle between the chain stay's centerline and the seatpost.

The Seatpost angle is between the seatpost and the horizontal.

enter image description here

However there is some slight discrepancy between the two

  • Chain stay angle (Actual) This uses the centerline of the physical chainstay

  • Chain stay angle (Rear axle to BB) This uses a straight line from the BB axle center to the rear wheel axle center, and depending how the chainstay is routed or how the rear derailleur hanger is laid out, this might be different, or it might be so close as to be identical.

On this example, the difference is small, and may not be enough to push your choice from 63-66 to 66-69 degrees.

enter image description here

Realistically, if your seat tube is quite vertical, you'd want the 66-69 degree derailleur. If your seat tube is more raked-back then the 63-66 would be better. If you're in the middle, either will work adequately.

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    The last picture was exactly the one confusing me. It's stupidly labled. But thanks for clarifying what it means. Now the question remains - which angle is Shimano referring to. As the second picture indicates - the difference can be anything from insignificant to large. – Raffael Jun 1 at 6:57
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    Mechanically the only angle relevant to the FD should be what you refer to as the seat tube angle because this is also the angle between the top chain line and the seat tube where the FD is mounted (assuming that the top chain line is also "horizontal" or parallel to an even floor where the bike is standing - I assume this is the case always for physical reasons). This is so unnecessarily confusing ... – Raffael Jun 1 at 7:42
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    @Raffael yes - its horribly overly complex. Honestly I suspect either FD will work fine, its more of aesthetic thing. – Criggie Jun 1 at 11:31
  • the link from the comment under my question suggests that it is about keeping the FD away from the CS. – Raffael Jun 1 at 11:49

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