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I have an older Batavus bike, with a Shimano Nexus 7 speed, which won't shift into the lower gears.

What I mean by that is; the dial will display the gears 1 - 3 but they all feel the same when pedaling. The higher gears are fine (4 - 7), there's a noticeable difference when switching between them. This is obviously a problem when going uphill, as the lowest gear it can go to feels like the 3rd or 4th (not sure).

Adjusting the red casing lines does not work either. They get misaligned as soon as I shift gears back and forth.

Looking at the cable, it seems that it is not the original shifter cable that comes with the bike.

Could a brake cable cause this kind of problem, when used in place of the shifting cable?

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    It's unlikely (yet not impossible) that a brake cable was installed. They are thicker, but most of all, the end bits have a different shape. It's possible to use a file to adapt them, hence not impossible. If replacing the cable is an option for you, i'd suggest doing that, since what you describe sounds like a cable not running freely to me.
    – Burki
    Commented Sep 19, 2023 at 15:39
  • Looking at pictures online, the end that attacches to the dial (near the handle) does not seem to be like mine. When adjusting the lines on the casing, the user is supposed to pull on it then twist, then push it back into place; whereas with mine I can just twist it right away, which means that there’s no locking mechanism in place, so every time I switch gears, even if I start from the perfect position, it’s always going to miss. Atleast that’s my guess so far. The rest of the bike is well maintained (no rust, tires are barely worn) so I doubt it’s an issue with the hub itself
    – Andrei
    Commented Sep 19, 2023 at 19:56
  • Have you tried detaching the cable to see if the inner cable can move freely in its housing?
    – Berend
    Commented Sep 20, 2023 at 15:57

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Installing a brake cable may cause shifting problems because brake cables (and their hosings), in addition to being thicker (hosing is usually 5mm vs 4mm on gear hosing, cable is usially 1.5--1.6mm vs 1.1--1.2mm), are slightly flexible---cables stretch a bit more, and especially hosings compress a bit more. It is because with braking, they need to bear more weight but precision is less important (you basically adjust braking power using the feel in your hand and by how the bike feels). A couple mm difference can occur at the length of some 1--1.5 m and you would feel it.

That being said, if all three lowest gears are not working, this is not likely to be a problem. It is because lowest gears on Shimano systems are the ones where the cable has the most slack, i.e., lowest tension. So assuming the cable moves freely in the hosing, the problem is not the cable.

To diagnose, take cable nut (what Shimano manuals e.g. https://si.shimano.com/en/pdfs/sm/IHG-INTER7/SM-IHG-INTER7-003-ENG.pdf call "inner cable mounting bolt") off from the cassette joint, and take the hose off as well. See if the length appears correct (you can measure it using a ruler - see dimensions in section "Installing to the cassette joint" of the manual I linked above), and see if the cable moves all the way back and forth as you switch between lowest and highest gear on the shifter handle.

If that looks okay, the problem is either with the cassette joint (e.g., it is broken or stuck) or in the gear hub itself. For starters, take the wheel off, take the cassette joint off, clean up everything that seems dirty, reassemble, try again.

If this does not help, and if the cassette joint (Shimano part CJ-NX10 or CJ-NX40) is clean and not broken, the problem is likely in the hub, for example broken or stuck spring inside the hub. Fixing that is big professional repair (although there are some manuals and videos around and some questions on this site that could help with that).

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