I’m trying to figure out what “normal” is for chainline offset to avoid chainrub on wide tires and a 135 OLD.
Imagine a 135 OLD rear hub with an 11-32 cassette. If you center a 3x so that the chainline is straight to the center of the cassette, from what I can tell you end up with chain rub if the tire gets to be 2” wide.
Looking at SRAM specifications (see pg. 22 here), they mention only problems with cross-chaining going from chainrings to the small sprocket (dropping from the big chainring to the outside or getting caught by the ramps on the big chainring when crossing from the small chainring to the small sprocket) but they don’t discuss problems going the other way, i.e. the chain coming off the small chainring when it runs to the large sprocket.
Given that old MTB rear hubs were 135 OLD, did they only use wheels less than 2”, or did they purposely push the cranks out and offset the chainline to help the chain clear wider tires? If so, how far could they push it without the chain falling off the front chainring (assuming a front derailleur is present). If people did do this, were they just accepting that the front derailleur would make a racket when in the granny gear?
One of the things about old MTBs that’s very different to modern bikes is they used 26” wheels. That gives you masses more tyre clearance for a reasonable chainstay length. We now have the triple whammy of wanting really short chainstays, fat tyres, and large diameters.
On most MTBs, which ran about a 2” tyre (47mm I think was normal), it was nowhere near the chain in any gear.
Older MTB chainlines were usuually in the 47.5-50mm range. Shimano’s most common 1x chainline for 135 OLD / 142 Thru was/is 49mm. A lot of modern 1x gravel setups are running chainlines almost that wide. I believe Shimano GRX is 47mm and SRAM Road Wide is 47.5mm.
Also manufacturers are generally quite conservative with their spec. Like how SRAM said everyone should run a 55mm chainline with transmission (and buy their new cranks) but in reality it works fine with standard boost/52mm chainline.
Sheldon brown gives 45mm for the chainline on a Shimano road triple and 47.5-50mm for a MTB triple here. (Same area says that SRAM would use 49mm for a MTB triple, but assumes 142 OLD which is about the same as 135 OLD and a 45mm.)
Using the Shimano numbers, assuming both go back to a 135 OLD, does this imply that the front chainrings can actually be shifted 2.5-5mm offset to the outside in a road or fatter tire application (assuming the forks are wide enough) and derailleur shifting will still work fine or is Shimano assuming you’re only going back to a 142 OLD?
In short, this is a question about how much I can offset the chainrings of a 3x to the outside and still have confidence that the small chainring will be able to reach the large sprocket on the cassette.