What is 'offset' in a chain ring specification?
The Garbaruk 34T, 96 mm BCD 4-bolt chain ring specification, as shown in the first 2 attachments, appears to be a suitable upgrade to the existing 32T Shimano , 96 m, 4-bolt chain ring on my 2021 Cube Reaction Pro (as shown the 3rd and 4th attachments). The area of doubt I have is the 'offset' parameter, which the Garbaruk specification mentions. I have no idea what this offset is or where it is measured from, so I have no way of knowing if this 34T chain ring would fit on my bike. Can somebody please clarify?
Comments
-
The offset refers the the fact the chainring is slightly dished, meaning the centre line of the teeth are offset slightly from the centre line of the mounting flange to the crank (bolt holes in this case).
It serves two purposes. One is chainline matching, although only significant if your rear hub is Boost and your cranks aren’t for example.
The other is frame clearance. A lot of current frame designs are trying to accommodate wider tyres, which means bigger spacing needed between the chain stays at the bottom bracket. In order for cranks to be still set in (roughly) the same position on all bikes, the chain ring size in teeth can start to interfere with the drive side chain stay.
Using a single ring crank means the optimum position of the chain ring corresponds to a point approximately in the middle of the cassette for a straight chain line. That’s best for wear and noise etc on the drivetrain. With a wider spaced chain stay at the front to fit fat tyres, it’s not uncommon for there to be a practical limit of about 36 teeth as the maximum before the ring hits the frame. “Dropped” chain stay designs address this to a fair extent to give more clearance for the chainring from the chain stay.
The offset on the bigger sizes in the Garbaruk spec therefore adds some “dish” to the ring to move the teeth a bit further out from the frame to help with frame clearance.
Open One+ BMC TE29 Seven 622SL On One Scandal Cervelo RS1 -
I’m in agreement with this, but I find the negative sign confusing. In my mind negative offset suggests moving inwards (clearly it doesn’t and would not make sense for frame clearance).
0
