Does the rear derailleur have a range of motion that exceeds that needed to move between the planes of the smallest and largest sprockets?

Question

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.

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.

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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.