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I'm building a (non-standard) bike and that means mounting a front derailleur in relationship to the incoming chain line (rather than selecting a seat-post angle visavis chainstay angle, etc.) So I'm wondering about the typical angle between the chain (on of the crank) and the derailleur mounting post (usually a seat tube). I've measured several bikes here (also non-standard, some standard) with my digital angle finder, chain in the middle of a triple ringset, and I get values from 68 degrees to 75 degrees. All shift fine. I'm wondering if there's a best practice reference here. Most of the seat-post angle stuff I see is in relationship to the chainstays, not the chain.

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    If you are using a mountain type drive train you could consider a bottom bracket mounted derailleur such as Shimano Deore XT Front Derailleur FD-770-E. – mikes Apr 29 '19 at 0:00
  • That's a good solution because it's adjustable. I actually had machined some road-crank-length, BB-mounted brackets (emachineshop) in order to fit a prior non-standard bike that had nowhere to put a post. For the current build, a post will be simpler. – WPNoviceCoder Apr 29 '19 at 12:42
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There is not a fixed angle between the front derailleur and the chain, because the angle of the chain relative to the bike changes depending in what front and rear sprocket it's on.

Chain stay angle range is a parameter specified for front derailleurs. As an example, see this Shimano specifications page for road groupsets. Chain stay angle is the angle between a line drawn from the bottom bracket axle center and the rear wheel axle center, and the seat tube (or a line from the BB axle center to wherever the front derailleur is mounted if there is no traditional seat tube).

If you place your front derailleur on a line at that makes an angle to the chain stay line in the specified range, you know the chain will be at an acceptable angle to the cage in any gear combination.

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    Having had a non-standard bike that used a derailleur mast, I can confirm it was in the range Shimano prescribes. – Adam Rice Apr 28 '19 at 23:44
  • @AdamRice ????? – Argenti Apparatus Apr 28 '19 at 23:45
  • The point of the question is that it's not possible to measure a chainstay angle as the chainstays are nowhere near the front derailleur (It's a recumbent) thus it was necessary to try to get an angle from the next possible constant, the chain. Sure, the chain's not completely constant, but it's constant enough to be useful, as opposed to a non-existant chainstay. The actual answer is below. – WPNoviceCoder Jul 9 '19 at 21:00
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The answer is rather simpler than I thought. The angle between the mounting tube and the chain (not the chainstays, and not the chainstay angle, since chainstays aren't horizontal, and aren't relevant to the recumbent chainline) is about equal to the seat-post angle on a standard bike, since the top of the chainline on upright bikes is generally about = to the horizontal from which the seat post angle is measured. That means when using a road front derailleur on a bike with a significantly non-standard chainline (e.g. no chainstay angle to measure) then use an angle of about 71 degrees between the post and the chain line. If it's a mountain front derailleur, something slacker to match a mountain bike's seat-post angle.

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  • This worked. The bike in question had a chain line meeting the horizontal boom at a 27 degree angle. Mounting the derailleur post or "seat tube" at a 45 degree angle on this boom yielded an tube-mount-to-chain-line angle of 72 degrees -- what you'd get on a regular road bike measuring the level chain to the seat tube angle. With a road triple, the cage matched the crank rings well, and it could be adjusted close to the rings, as on a normal bike, and shifting was flawless within minutes of install. – WPNoviceCoder Jul 9 '19 at 21:05

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