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The distance between the plane of the largest sprocket and the plane of the smallest sprocket in a Sora cassette is about 34 mm.

When the rear derailleur cable is untethered, the spring in the rear derailleur brings it outside of the plane of the smallest sprocket. This must be somewhat the case, since even when the chain is on the smallest sprocket, the cable must be a little bit taut.

Yet as you see in the image below, the cable-untethered position is several millimeters outside the smallest sprocket. This means that the cable must first have quite a bit of tension to bring the pulleys under the smallest sprocket, and the tension at the largest sprocket must increase by that initial tension.

rear derailleur at rest outside plane of smallest sprocket

Can a rear derailleur reach the largest sprocket if the cable is already quite taut on the smallest sprocket?

In other words, does the rear derailleur have a range of motion that exceeds that needed to move between the planes of the smallest and largest sprockets?

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    Something is wrong with the wheel. In that picture, the nut between the cassette and frame protrudes a lot more than would be normal. With knowing the history of the wheel, hard to know why - what is the non-drive side like, is the wheel centered in the frame? Is it original wheel for bike - Road 9 speed (130mm dropout spacing) in MTB frame (135mm dropout spacing)...
    – mattnz
    Sep 11, 2022 at 5:45
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    Did somebody replace the thin lock nut of the cup&cone bearing with a full size nut? This big, ~1cm thick nut between sprockets and frame looks wrong.
    – Michael
    Sep 11, 2022 at 7:19
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    @Michael good spotting - yeah that's messed up. Someone's used a regular hex nut where there should be much less space between 11T cog and frame. Its 9 speed so probably a 135mm OLD. I suspect frame is damaged and extra wide, or the wheel is not running very centered, or the hub came from a 142mm (?) OLD bike, or as Mattnz says 130 in a 135 frame is more likely. High limit screw needs to be set before that chain drops off the outside.
    – Criggie
    Sep 11, 2022 at 10:08

3 Answers 3

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This question is asked in terms of the range of motion relative to the distance between the planes of the small and large sprockets. The answer to that is basically yes, if it has a low limit screw, it will have at least enough range of motion to exceed that distance, otherwise there would be no need for the low limit screw. (There are some bad, old derailleurs that don't have a low limit screw, based on the principle that it's not needed if the total cable pull and the total distance between those planes are known numbers).

However, phrasing the question this way may not be the most useful for what you're really trying to understand. One can also view any given rear derailleur as having a minimum outer position and maximum inner position relative to the mounting surface of the derailleur (the derailleur hanger on most modern bikes, or the face of the dropout for a derailleur with a claw type hanger).

The picture appears to show a 9-speed cassette mounted on a hub whose drive side locknut protrudes much further from the lockring than any such hub would come stock. This creates a situation where the center plane of the largest cog is much farther away from the mounting surface of the derailleur hanger than usual. It would not be surprising if a given derailleur couldn't reach in far enough to get to it, even if the limit screw were completely removed.

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  • Your answer suggests a simple test. Say the planes of the big/small sprockets are separated by 34 mm, and say the total range of motion of the rear derailleur is 40mm, then for ideal placement, we should have 3 mm on either side of free play. It would appear that that free play on either side is also an indication whether we got the wheel set up correctly for that derailleur.
    – Sam7919
    Sep 11, 2022 at 12:51
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    @Sam dropouts vary in thickness, particularly between steel and everything else. That's the biggest need for a range of motion wider than the cog to cog spacing. Many steel frames (particularly ones with stamped dropouts) locate the small cog near the very extreme end of the range. There's nothing wrong with that. Sep 11, 2022 at 22:46
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Yes, typically. There is a little variation in the spacing of wheels and where the cassette is positioned.

That's why there are the high and low limit screws, to dial in that range of motion to match your cassette.

Additionally, you set that before tensioning the derailleur cable.

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As per comment stream - that nut is throwing everything off.

It is possible the derailleur can't reach the lower gears (bigger tooth count) at all based on the loss of range.

If its your bike, I'd back out the Low limit and see if you an get all the way over there. If not, replace that fat nut with a much thinner one, or see if it should be on the other side of the wheel.


Ideally the chain would rest hard on the high limit stop precisely over the smallest cog, and the gear wire should have low tension but also not be loose. If its loose then ferrules can back out or barrel adjusters move by themselves/vibration/etc so you always want a preload on them.

At the other end the gear wire will be under normal tension, and the low limit stop will NOT hold anything because the derailleur wants to pull outward away from the low stop.

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  • "the low limit stop will NOT hold anything": Of the millions of views that the front/rear derailleur videos from Park Tool have received, I alone am responsible for at least five views each. I cannot memorize every single step along the way, but I keep wondering if I can decipher the logic to be able to deduce the steps as I go along. It's still work in progress. But you've just pointed to one element: Whether we're adjusting front or rear derailleur, we have to start with the limit screw on the smaller/smallest chainring/sprocket, where the derailleur cable has least tension.
    – Sam7919
    Sep 11, 2022 at 12:46

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