Putting together a winter HT.

Frame is a new Commencal META HT (boost rear).

Groupset is (supposedly) a SRAM Boost NX, bought as OE from Alpkit.

All set up nicely as per SRAM recommendations. BAS 68 so I put x1 2.5mm spacer either side of the BB shell.

And then the 4.5mm spacer on the drive side.

Put the chain on and it jumps off the ring on the inside as soon as I shift up to the largest but one cog on the cassette. It shifts perfectly from 1 to 10, 10 to 11 is where the issue starts.

I then tried all manners of spacers configurations but the only way it will shift into those last two gears without dropping the chain is for the chain ring to almost foul the frame. And that’s using a weird configuration of x1 2.5mm and the 4.5mm spacer on the non drive side. And even then I can’t put any pre load or it does foul the frame.

What am I doing wrong ?

The only thing I can think of is that the cranks are super Boost rather than Boost. Not that I know if that even exists.

Am I right in thinking there is only ONE type of DUB bottom bracket for BSA and that using the spacers is correct, not that I can use both anyway…

Is it possible that there is a damage to the chain ring

It’s very unlikely in a new set up but my bike suddenly started doing something like this. Eventually solved after much head scratching with a new ring

Oh and I think my bike has the same spacers both sides. That’s in 73mm I think but I can’t see why 68mm wouldn’t be symmetrical too. My bike is boost

Cheers. I have now got the 2.5mm spacers back on either side, as they should be.

I have had to put the 4.5mm spacer on the non drive side though but it seems to be OK now. Not sure it is viable long term solution though? I probably have 2 to 3 mm clearance to the chain stay now.

I don’t know about damage to chain ring.

Or could they have sent the wrong chain ring with the wrong offset.

I assume they cranks are the same and the only difference between boost and non boost is the offset of the chainring.

What is the chainring offset? (should hopefully be written on the inside of the chainring)

6mm, 3mm and 0 available for Sram from what I remember.

Good point. Deffo not 0. Will check.

I checked and the ring is a 3mm offset which is correct for boost. At least it rules out one area of confusion…

I had to check on Commencal’s website to see if in fact the BB shell is 68mm wide. I thought that had died out circa 1998. How bizarre!

Yep for the sake of argument there is only one BSA DUB BB (ignoring fat bikes etc). Wrong chain? Connecting link not 100% connected? That’ll throw a chain off the chain ring.

Good point. Will check if the link is properly seated. I upgraded the chain from the NX chain delivered with the group to a GX chain. The thing is it was only thrown out on the 2 largest cogs.
I found another guy on YT who had to put the 4.5mm spacer on the “wrong” side so I am not alone, safety in numbers lol.

The only downside now is that the left crank arm is probably 5mm further away from the chain stay than the right arm. Not sure if that’s bad on my knees or anything.

What if I get a 6mm offset ring? They’re only £15? Any counter indications ? It should balance both sides without moving the ring again if I swap the 2.5mm to the other side?

Sorted. I took everything off and put everything back together again reverting to the SRAM instructions of a 4.5mm on the drive side. And the chain didn’t come off the ring this time through the cassette.

Absolutely no idea what I did differently as there isn’t that much to do wrong but it was clearly user error and all is well.

10mm clearance between chain and tyre, probably 6mm between ring and frame. Perfectly positioned crank arms. Chain line is around 52mm.

Thx for the help and sorry for being a plonker.