6 speed groupset question

[brit_in_oz]

On my old repco frame i have Shimano RX100 components with a 26 capacity rear mech. I found a new cassette hard to find so i bought a HG50 12-28, but the top two spockets rub badly and cannot be used really. The rear mech cannot accomadate it, so i was wondering would a 6 speed from a MTB be ok to use, such as a Shimano Tourney RD-TX35 Rear Derailleur 6 speed?

[KGB]

Probably. I'd expect it to work.

The other option is to rebuild your cassette and mix mostly new cogs with the top couple from the old one. Thinking about it, this would be difficult/impossible on a screw on cluster.

Have you tried adjusting the b-screw on the derailleur?
A cheap shimano mtb cluster should cost little enough that its worth a try I guess.

[foo on patrol]

So you bought a six speed cluster but you are having trouble with the top two cogs, is that right? I would think that you have an adjustment problem or there is a spacer missing from those two. More inclined to think adjustment as I don't remember there being any spacer washer on them.

I just had a look at my old Regina and Shimano 600 clusters and the spacing is even all the way through! Are you cross chaining to far?

Foo

[mitzikatzi]

On my old repco frame i have Shimano RX100 components with a 26 capacity rear mech. I found a new cassette hard to find so i bought a HG50 12-28, but the top two spockets rub badly and cannot be used really. The rear mech cannot accomadate it, so i was wondering would a 6 speed from a MTB be ok to use, such as a Shimano Tourney RD-TX35 Rear Derailleur 6 speed?

My understanding is this will work. It is the shifter that makes it a six speed. Same pull just different amount of positions. In fact any 9 speed Shimano derailleur should also work.

[LG]

I have used RX100 rear derailleurs on 28 cog cassettes with no problem at all (52-39 chainrings). There should be enough length in the derailleur for the jockey wheel to fit under the cog. I'd assume the second largest cog on that cassette would be a 24, which is well within the manufacturing specs, so if you're having trouble with it there's likely to be an adjustment problem. If you've swapped out from a smaller cassette, it may be a case of the chain being slightly too short.

[KGB]

I was assuming the problem was that the 28t cog was too big for a short cage derailleur - causing the derailleur to basically run against the cog. The B screw is the one that adjusts the angle of the derailleur body against the hanger on the frame. Sometimes cranking this in will give enough clearance to run the bigger cog.

[brit_in_oz]

Thanks for the advice. i should have said the capacity is only 26 which i only found out later. The problem is 90% on the 28 but the 24 isnt perfect. i have been told perhaps its due to the spring tension dropping, but i have adjusted the B tension to the maximum and wont go any further. I have also put a new chain on at the same time so its not that.

[ldrcycles]

Weird one, I definitely would have put my money on a too short chain or b screw all the way out. If you can't get the RX100 to play nice then mitzikatzi is right, pretty much any derailleur will work just fine, I just built a bike the other day using a 7 speed shifter with a 9spd Deore, no worries.

[KGB]

True, generally speaking the derailleur just does what its told to do by the shifter (unless you go switching brands like from shimano road to sram mtb).