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Chainline with a 148mm axle and a bash ring.

LugosLugos Posts: 3
edited September 2019 in MTB workshop & tech
Hi, I've looked at a bunch of chain line posts but not seen this exact thing, and I really want to make sure I'm ready to build my new frame when it turns up but I can't currently experiment. So I'd would love answers from anyone who has experience getting this to work, or a definitive no so I can go to plan B. Sorry this goes towards buying advice, but since I'm trying to avoid buying stuff and it is quite techie, I thought here was better.

I have RaceFace Respond cranks that can take a triple & I was running them with 1x12 Eagle GX on a 142mm hub with an AbsoluteBlack oval 36T 104BCD ring on the middle and a 40T sized round bash ring on the outside. I would like to reuse these cranks on a new boost frame. I'd like to keep the bash ring for a few reasons.

I've used a WolfTooth Boostinator to convert my rear wheel to fit, and this adds the 6mm to one side and is re-dished 3mm back so the cassette should be in the correct boost space. I have a Hope 73mm threaded bottom bracket conventionally fitted with one spacer on the drive side.

Boost offset chain rings don't exist for this, but since I just need an extra 3mm (Chainline from 49mm to 52mm from centre) could I not just add a 2nd 2.5mm spacer to the BB and be almost right? If it's that easy then why do lots of tables online say 104BCD to boost isn't possible? On a 68mm BB you would have 5mm of spacers on the drive side so there must be enough thread to be safe. Am I being an idiot or missing something? Is it the centre tube being too short the problem although the O-rings seem far enough in? Is there a better way to make this work I've not thought of?

I do have a plan B with my Eagle GX cranks, but I'll have to strip my currently only working bike, buy a boost offset SRAM chainring and lose my bash ring. Not ideal. Thanks for the help.

Posts

  • Welcome to the Forum Lugos.

    Sorry, but I can't help you in advance. You know more about your kit and what you've done to it than I do. All I can suggest is that you obtain a log and thin straight edge (like a one metre ruler for example). Then use it to press against the front ring and see where the same face comes when above the cassette. Use the BB spacers to move the crank accordingly, use extras if you have to do so. You can also move the ring in towards the frame by inserting washers under the ring. You may need longer bolts.
  • The RookieThe Rookie Posts: 27,748
    68 and 73mm bottom brackets are the same, so the thread is 'OK' for 73mm and 2.5mm longer than it needs to be for 68mm. Adding the spacer would take you into dodgy ground.

    The easiest way to run boost with a 104 ring is to move it to the outboard location on a triple (about 5mm) or the outer of a double. Both give similar chainline to a Boost specific setup.
  • Hi Steve & Rookie, thanks for your welcome & replies.

    Steve, that was going to be my plan if I had to wait until building before sorting the bits out. The frame requires a 2.5mm spacer on the drive side to get the correct line with a boost single ring according to manufacturer's spec so theoretically one more spacer should be 0.5mm closer than ideal but within tolerance. My concern is an extra spacer will move the BB cups a further 2.5mm apart and the tube in the middle would not be long enough, although it looks like the o rings could cope. Have you or anyone else reading this tried 2 spacers on a Hope 73mm BB or 4 on a 68mm?

    Rookie, I like your suggestion of moving to the outer ring generally, but it doesn't help in this case as I lose some of the reasons I want the bash ring. Never tried a bash in the middle, but that wouldn't help in my case.

    I have an untested theory based on your idea that might be worth writing here in case others who don't care about a bash ring are looking to run 104BCD rings with boost find this. If moving from middle to outer is a 5mm move out, then running the single BB spacer on the brake side instead of drive would move you back in 2.5mm, getting the chainline much closer to ideal and keep the BB cups the correct distance apart. Could save adding washers and buying longer bolts to bring back in.

    Regarding distance apart, I'm a bit confused by the first part of your reply. Yes many 68mm and 73mm BBs are the same, but conventionally you'd run a 2.5mm spacer brake side and 2 drive side on a 68mm rather than just the 1 drive side spacer on a 73mm. This is just to convert a 68 to a 73 by adding 5mm. My proposition that the engagement of the thread would be okay is because the 14mm or so that is threaded on the drive side would have 9mm of it biting in a 68mm BB with 2x 2.5mm spacers on, and if that's fine, then having 9mm biting in a 73mm with a 5mm spacer is just as good. 9mm of engagement must be enough, it would just be more typical that a 73mm would have 11.5mm.

    When you say dodgy ground with an extra spacer, do you mean because the cups will be too widely spaced rather than the thread? I'm not trying to be pedantic, just checking there's not something I've missed about the threads.

    Thanks again both of you.
  • The RookieThe Rookie Posts: 27,748
    Sorry I thought you were spacing the drive side cup out frther on a 68mm shell.

    Yes replicating a 68mm shell install on a 73mm will only have the issue of the between bearing sleeve possibly not fitting.
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