1x11 conversion and chain line

Phil Mountain

I'm converting my 3x9 speed to a 1x11 speed. I'm swapping my current middle chainring for a narrow wide chainring and my 9 speed cassette for a 10-42t 11 speed cassette. My Shimano BB has one spacer between the frame and the cup on the drive side. Should I keep the BB spacer, remove or add more to give a good chain line and minimise the back pedalling problem on the 42t sprocket?

The Rookie

Assuming it was correct as a 3x9, leave it well alone!

LimitedGarry

Check chainline by sight. You want it to be slightly over the middle of the cassette (as the smaller gears are less problematic).

Generally, you adjust chainline by adding spacers between your crankset and chainring.
The alternative and sometimes preferable method is swapping the BB spacer from drivetrain side to the non-drivetrain side. This will, of course, move your whole crankset a bit to the left. On flat pedals, it's likely you will never even notice. If you have clipless, just adjust your cleats accordingly and again, you'll never notice.
Swapping the BB spacer side alone gets me good enough chainline on 11-36 cassette. On 10-42, I'd probably go for additional spacing on the chainring.

cooldad

The chainring will be in exactly the same place. If it was right before, it still will be.

LimitedGarry

cooldad wrote:

If it was right before, it still will be.

Not necessarily true. Most of the time, the middle chainring on 3x setups is either dead centre or favours the smaller sprockets slightly. While it is functional in it's original state, it's often not ideal. And when you put in a 40T or larger sprocket, you'll often introduce the backpedaling problem and will need to move the chainline closer to the frame.

cooldad

The 'backpedalling problem'?

LimitedGarry

cooldad wrote:

The 'backpedalling problem'?

If your chainline is too far from optimal, after 1-3 backpedals (peddling the cranks backwards, obviously) while on the largest sprocket, the chain will slip and often get stuck.
Some bikes have it worse than others. Worst case scenario, the frame can actually obstruct the chainring from getting into a position where this problem won't occur.

cooldad

Never had that, ever. But why would you back pedal more than half a rev or so?

LimitedGarry

cooldad wrote:

Never had that, ever. But why would you back pedal more than half a rev or so?

I have no idea. Especially on the lowest gear, where you need to pedal constantly in order to not stop. But apparently, some people do that. That said, If I knew my bike had that issue, it would bother me.