Apologies for what I suspect is a somewhat tired subject, but I wondered if I could engage the hive mind on the marvellous subject of 1×11 chainlines!

I recently got back into mountain biking after about 15 years off and bought by first full suspension bike. Its lovely (new Boardman FS Pro). However, it has a 1×11 drivetrain which has the classic “pedal back and the chain drops down the cassette” problem. Not a massive issue, but the way the bike is setup makes me think its fixable. Here are the facts I have gleaned as a novice so far:

– it’s a SRAM GX derailleur and cassette (10-42)
– it’s a FSA single ring comet crankset (32t) and threaded BB, 73mm
– The chainline appears straight somewhere around the 8th gear (i.e. 4th smallest)
– therefore the chain looks pretty crazy when in first (42 cog), but it works
– if I backpedal at all in first, it changes gear. This is a pain on hard ascents, just setting the pedals in position causes it to drop down, meaning a gear change as soon as you start off
– there is a single ~3mm spacer on the drive side of the BB
– I measured the chainline to be ~52mm

So, given the above it appears to my novice eyes that

– the chainline would be better around 49mm
– I have heard mention of changing the BB spacer to the non-drive side. But I have also heard caution as that can cause issues with thread depth on the BB cup.
– I *think* a SRAM GX crankset and BB would result in a 49mm chainline.

So I have a few specific questions:

1. What *should* the chainline be for 1×11?
2. If it should be 49mm, but is setup as 52mm have Boardman messed up here, or am I misunderstanding something?
3. Can I change the spacer to the other side to fix it?
4. Or do I need replace the BB etc? (or just ignore it for now)

I did ask in Halfords about this, and was told it was just a feature of 1×11, but I felt I was being fobbed off tbh.

Any help much appreciated.

1. What *should* the chainline be for 1×11?

46ish would be about right, but 49 is as good as you can find ‘stock’

2. If it should be 49mm, but is setup as 52mm have Boardman messed up here, or am I misunderstanding something?

Unless its a boost crank on a non boost frame, then its just the way FSA made it.

3. Can I change the spacer to the other side to fix it?

yes.

4. Or do I need replace the BB etc? (or just ignore it for now)

Changing the BB will do nothing. Ignore it for now. If its still really annoying you, consider a more expensive chain possibly as they seem to do this less. Also, don’t pedal backwards.

Move the BB spacer, fit chainring spacer washers between the chainring and crank spider to move the chainring inwards. you can get 3-4mm movement all in that way.

Never run into to a thread depth issue on the BB.

in all honesty, it should have been set up properly before being handed over to you!

Had to do the spacer move on my Bronson when i changed chain ring. it still drops but the chain line is better.

Thanks all, I’ll probably swap the spacer and leave it at that. Still slightly annoying that Boardman spec the bike that way. It seems so easily avoidable…

That BB/crank is set up exactly the way the manufacturers recommend. The drive side spacer is there to “fill in” the gap that an E-type mech would take up.

Sure, but why spec that crank/BB on the bike in the first place? Just seems a weird choice.

Cheers for starting this thread Memoryleak, I’ve got one to test at the moment and it never ocurred to me that chainline issues could be causing the ghost shifts.

I’ve just tried the spacer swap and it does improve things. However it means the frame is no longer central relative to the cranks.

As to why it’s been specced that way, it’s already a good £2-300 cheaper than similarly-equipped bikes, and I guess something had to give.

I’ve had similar issues with my 1×11, I put 1.5mm washers between the chainring and Crank to push it out a little (1.5mm was all I dared since I’m robbing length from thread engagement and couldn’t find longer bolts). This helped although not cured it. Also I think a few days of riding has helped free up the chain a bit. While it’s not 100% cured, I now don’t notice it when riding. HTH

I’ve found this article on t’interweb about chainline, other than the problem of dropping chains when you back pedal the actual chainline so long as it’s within 5mm of ideal doesn’t seem to actually effect anything.

A bigger issue is chain tension, which appears to be the lower it is the less efficient you are.

The amazing unimportance of chainline and chain lubrication

Ben – whats the optimum chainline for M8000 with a 135mm rear ❓

Ta

If you move the spacer to the non-drive side on an sram crank, do you need to move the wavy washer as well? No idea what this does tbh.

Just put 1×11 on an anthem and the chainline does look poor, but it worked pretty smooth first time out. A bit of chatter makes me reluctant to really stand on the pedals on a hard climb, so needs a bit of fine tuning.

I might be mistaken, but iirc the spacer and wavy washer on GXP bottom brackets aren’t negotiable, they’re there for bearing preload reasons. GXP and HT2 have some fairly fundamental differences in terms of design expectations regards loading and installs.