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Today's question - a query regarding my newly installed (and still un-ridden) cassette/chainring chainline:
In the ideal world (presumably) the chainline would be straight from any given cog on the cassette to the chainring, but in practice because cassettes have width, the chain enters/leaves the chainring at a bit of an angle depending on the gear. For my set up the angle seems noticeably greater for the higher gears (bigger cassette cogs) than the lower gears. The below pictures illustrate this (albeit not perfectly).
When I installed my bottom bracket the instructions were very clear that there should be a single spacer on the drive side of the bike. I'm wondering if it would be sensible to remove this spacer (or put it on the other side of the BB) in order for the chainline to be more even across the cassette - i.e. it would move the chainring closer to the centre of the cassette.
Is this a terrible idea? Am I getting too worried about the chain angle and risk of slipping/rubbing on the chainguide? (I've not had a 12 speed before).
Opinions/expertise please 🙂
When you're on that big cassette cog, drop the front down to the inner ring...
He would if he could.....
Yes i would move the spacer to the non-drive side.
Is it a boost rear axle? Whats the measurement from centre line of seat tube to chainring? Thats the chainline measurement, should be about 52mm from memory for boost?
I did exactly what you are suggesting for a non-boost crankset (old Deore triple!) on a boost frame. When it was nicked it was running a road Ultegra BB in that mode (even more wrong) and I had yet to die in a ball of flames.
My current bike (boost frame and dedicated 1x boost cranks) has a 28T chainring, which seems mad, but means that most of my riding is toward the 11T end of the cassette, making the chainline more betterer more of the time.
The standard chainline for Shimano or Sram, boost or non-boost, indeed puts the front single ring a bit more outboard than the centreline of the cassette cogs.
A problem with wide chain angles on the smallest cog is the drive part of the chain fouling gear n-1, which is obviously not going to be a problem with the biggest cog. This may have something to do with the design decision to do it that way, but that had to take into account all possible variations. So, like @tomparkin (and me as it happens), you may find that your application gets along fine with a narrower chainline.
Cool. Will experiment with the default angles before messing with the bb spacing. Cheers!