I got a NOS Shimano RD-6400 rear derailler and it came with cable housing that I usually see on brake cable housing with that thick wire coil inside. I always thought those aren't suitable for shifting because they are designed to compress under load. What's the deal here?

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  • At first, I wondered if you had a positron mech, which has a solid push/pull cable. But velobase.com/… says no, its normal 7 speed.
    – Criggie
    Oct 26 '20 at 0:52
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    What do you mean "came with"? Did you obtain a bike with this derailer and cable, or did you somehow purchase a "new" derailer and it came with the cable? Oct 26 '20 at 1:12
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    (And I'll note that the derailer is from roughly 1987, which may be before the "fancy" derailer cables became popular.) Oct 26 '20 at 1:13
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    Is it a very long piece of “brake housing” or a short one? Shimano sometimes specs a short section of special spiral wound housing for the final 180 degree loop between the chainstay and derailleur.
    – MaplePanda
    Oct 26 '20 at 3:22
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    @MaplePanda Added a photo. It's that rear loop that goes from the chainstay into the dreailleur. It sais SIS on the housing, so it's definitely meant for indexed shifting.
    – user430
    Oct 26 '20 at 10:51

It's a nice piece of history you have there. You will find that some of the newer generation Dura Ace final gear outer (the section closes to the derailleur) is sometimes supplied in this type, as is stuff right at the bottom of Shimano's range, IE Tourney.

The main advantage is that it is very flexible which is needed for that final curve. The reason it works with 7sp and the newer 11sp systems is that the cable pull between gear shifts is large enough for the "compression error" to be negligible (11sp uses a different pull ratio). The 9 and 10 speed systems benefited noticeably from the compression free housing we are more familiar with for gearing, as the cable pull between gears is very small.

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    This seems mostly correct however I'd note that the sequence was: d-a 7400 'Sis' outer , deore xt m730 'sis sp' d-a 7700 'sis-sp40'. It seems to me that the sis sp housing was better, in that the documentation for st-7400 which came out after sl-7400 and deore xt m730, specified sis sp or sis. In the catalogs it was sis for road and sis sp for MTB, both initially 6 speeds, but when d-a 7700 came out it used sis-sp40 which seems like it would be closer to the sis sp housing than the sis. So this 'sis' housing seems correct but perhaps less than optimal
    – thelawnet
    Oct 26 '20 at 13:03

I assume that you have a friction shifter? A friction shifter is one that does not click into an indexed position. You just adjust it until the derailleur is lined up where you want it.

Assuming this is the case, the brake cable housing is what you should be using because it was the stuff that was around when those derailleurs were designed. The compressionless housing, which is what the indexed shifters of today need, came about specifically for those shifters.

If my assumption is wrong, and your shifting is indexed, someone put the wrong stuff on there. If it is working for you I don't think you need to rush out and change it, but as soon as the shifting gives you trouble, you know why, and it is time to change it.

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    It sais SIS on the housing, so I'm sure it's meant for indexed shifting. I added some photos above
    – user430
    Oct 26 '20 at 10:52
  • 7400/6208 was the first generation of SIS, so this 6400 was definitely specifically designed for indexed shifting.
    – thelawnet
    Oct 26 '20 at 11:02

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