Skewing chainline

Posted by: heathen

Anyone running non boost rings on a boost bike having problems with their chain guide? My chain guide would need to be filed to fit. As it is currently set up perfectly and has no shims.

I had to do a tiny bit of filing on the plastic of my guide to stop some rubbing. Then I just rode the bike. After a few rides, the rubbing was non existent.  Start with shims and spacers, then finish with a bit of filing.

Posted by: Taz123

Great comments everyone - I ended up ordering a NS Billet boost ring, but will change my BB spacing more inward. Should arrive today, so I hope to get some riding in in the coming days to see how it all feels.

I chose to go with a non-boost ring because you can space out with the BB.  If you start with a boost ring, you may not be able to get the ring far enough inboard to get an ideal chainline for the big cogs on the cassette.  From experience I have found you have more spacing flexibility adjusting the chainline with the BB than anywhere else in the drivetrain.

I hope you have good luck getting things aligned to where you need them.

Mounting a non boost chainring on a boost frame when riding a lot in the lower gears makes perfect sense and is possible. Better chainline, less wear on the chainring, on tension jockey wheel and the chain that is less crossed, drivetrain noise and drag are also reduced. This is my experience with the latest gen santa cruz bikes (nomad, megatower) running on 12spd in combination with sram cranks and a chainring not larger than 30t, a 32t could potentially fit but with minimal clearance. This set up combines well with a oneup bashguard without modification and without rubbing. Boost chainrings or cranks on boost frames are just a guarantee for frame manufacturer that large chainrings can be fitted without grinding a hole through the chainstay, with a small chainrings (30 and below) it's a non issue using non boost. As another user wrote i also used a 28t woltooth chainring on a 104bcd triple crankset a while ago to get a low enough ratio when cassettes were still small (40~42)and it had the added benefit of providing an excellent chainline for the climbing gears without creating any issue in the higher gears.

I had an email exchange with Blackspire about this when I was looking for a 52mm chainline boost ring and they told me that using a smaller than 55mm chainline on a Shimano crank was impossible. They said "The 55mm chainline with those cranks is designed to work with 148mm boost rear axles. We have this printed on a sheet directly from shimano. If you are considering that you are needing a different chainline you may want to consult with Shimano directly or a local bike shop with someone who can see your bike hands on and see what they say." 

I'm pretty sure that if the frame clearance is sufficient, it's entirely possible to use a 52mm chainline ring on a Shimano boost crankset. Maybe they won't advise it for liability issues if it doesn't work, but like other have mentioned, it's possible to make it work. I did not have issues with the 55mm chainline, but seeing the chain angle, it must wear out the bigger part of the cassette and chain much faster (which is the part I'm using the most). My One Up guide is installed pretty far outboard too and I don't see having the ring 3mm inboard causing any issues.

you guys are making 2x and 3x sound better and better ;-)

Posted by: taprider

you guys are making 2x and 3x sound better and better ;-)

Nah, then we would be debating the finer points of chainring ramp, how to fit multiple rings in a boost frame, and how to fit a front shifter and dropper remote on a single matchmaker to a brake lever!

Posted by: Poz

Posted by: taprider

you guys are making 2x and 3x sound better and better ;-)

Nah, then we would be debating the finer points of chainring ramp, how to fit multiple rings in a boost frame, and how to fit a front shifter and dropper remote on a single matchmaker to a brake lever!

Manual shift with dual N/W rings. Only need the Grannie for the brutal climbs and taking a few seconds at the top to breathe/shift is no big deal.

Bonus points for getting back to running a super short cage and lower unsprung weight.