Slipping chain on 1x retro build

[FiveAlpha]

I have built-up an old, 90s steel MTB frame with original and spare parts. For simplicity (and because the original Alivio one-piece chainset was shot), I have added a single ring up-front. The problem I have is the chain slipping and also coming off the chainring.

I have tried 2 different 7-speed cassettes, the chain is new (and correct length), and the rear mech has no play in it. Does anyone have any ideas what the issue could be?

Thanks

 

[MT62]

Is the chain slipping in the front or rear? If in the rear and your cassette/chain is OK, I have no idea what can cause that.
As for the chain coming off, do you have a NW chainring? From my experience, the 1x7 build can work even without NW, but you need to have correct BB length (as for chainline), a small difference there can result in chain falling off at lowest/highest gears. Also, adding a FD without cable and setting it just to guide the chain can help this.

 

[FiveAlpha]

Thanks - the chain is slipping at the rear. Didn't think I'd need a NW as it is only 7 gears and it worked across all 7 when on the middle ring, originally.

 

[ishaw]

Re:

Have you mounted the chain ring in the outer or kindle position. Sounds like chain line could be the cause.

 

[FiveAlpha]

It's a different bottom bracket - designed for single ring (though for a 73mm BB on a city bike).

 

[Markybeau]

Re:

Are the cassettes and the chainring also new?

 

[FiveAlpha]

No, not new, but not worn. I thought the slipping could be the cassette, so changed it, and it still slips.

 

[MT62]

Ye, NW shouldn't be necessary (I used to ride 1x8 without NW, chain fell like twice in 1/2 a year of daily city riding), it just needs more exact BB length then. Isn't it possible that the BB for citybike has shorter chainline? Not sure, but sometimes it happens, so if it was less than 47,5mm, I can imagine the chain falling off when you are on the smallest cog in the rear. I would just check how the chain crosses on both outer positions, and if it is straight when miggle cog is shifted.
Also, does it slip on all cogs? If so, I don't know why, if only on some, it could be slightly worn cassette.

 

[FiveAlpha]

I will have another look at the BB length - thanks. The slipping is annoying - it's a hack, so I don't really want to spend €10 on a new one, but might just do that (and a new BB).

Cheers

 

[shedobits]

Re:

All good advice, I would just add to spend 5 seconds checking that the rear mech springs back with some force, if the spring is weak or just binding the chain can unship from the front. Hope this helps.