Bottom bracket size advice please? am going CRAZY!

[Malvern Rider]

Please Obi Wan, you're my only hope...

Giant XTC 2.5 (2007). Need to spec a bottom bracket! 68mm shell on bike.

When I bought this bike it was stock spec: ie 3x9 (Race Face Ride XC crankset with a 68 x 113 RF SRX (Isis) BB

but BB was knackered so decided to go hollowtech/Octalink route

Tried an FC-M572 triple crankset with a (what I had in spares box) BB-ES51 (68x11 -and of course the D/S spindle was too short so much so that the granny ring was flat against chainstay.

Tried to measure front chainline (am a newbie BTW at this) but of course I paid a LBS to remove the old RF/Isis setup and so have nothing to go by. Best I could think was to measure from centre seat-tube to centre of front derailleur in middle postion.

50mm. I think. Seems closer to 50mm than 47.5mm. Does 2.5 mm matter that much when speccing a BB?

So I consulted the Shimano docs for the FC-M572 crankset for BB-ES51 and it says:

So if I'm understanding correctly - my combination (68 shell/50mm chainline) isn't specced? Was going to push the button on a 68x126mm BB-ES51 or BB-ES30??

What do you guys think?

Also my D/S crank is stamped 'FC-M572' but has a random 'K' stamped askew just above this. Does that mean something else magical and mystical to trip me up further? :shock:

 

[cce]

a 126mm axle should give you the 50mm chainline you desire.

 

[FluffyChicken]

Re:

It's saying for a normal cramped to the frame front mech. You need the 126mm for your bike (or go shorter if you want, 47.5 is the original spec.)
It saying it is for a 73 shell, but will for 69 and 73. So you'll have spacers.

If you have a front mech that clamps around the bottom bracket you need to move to the e-type style. But it doesn't sound like you have this.

 

[FluffyChicken]

Re:

I would look for a 68 shell 121mm axle, since iirc 113 on RaceFace (Isis?) cranks gives a 47.5 setup.

Edit, just check the signature Isis docs and they aim for 48.5 to 49.5 for optimal shifting on RaceFace cranks.

Anyway, get what you can for 68 shell (it must say compatible with 68 shell just in case).

 

[Malvern Rider]

Thanks Obi-Wans! Really am a very grateful retrobiker. 126mm it is.

(Front mech is frame-clamped btw.)

 

[Malvern Rider]

I would look for a 68 shell 121mm axle, since iirc 113 on RaceFace (Isis?) cranks gives a 47.5 setup.

Oh no! Cross-posted but just saw this.

Ok - I fitted a 68/118 with the M572 and chainring was against the chainstay so tight I couldn't fully tighten the crank without bending the chainring/scoring the chainstay.

So (may be a stupid question)how would an extra 3mm on the spindle give me enough clearance/ correct line? Is it 3mm extra on the drive-side or 1.5mm both?

Appreciated

 

[FluffyChicken]

Re:

Could be anything with Isis. ;-)

Chainreactioncycles cycles has es300 with what you are looking for or es51 with 68x121.

 

[foz]

your method of measuring chainline is a bit dodgy, I wouldn't rely on it too much... If you were tightening the granny ring against the chainstay then I'd say almost certainly a 126mm BB, just make sure you get one for you 68mm shell (the correct XT model in that spec might not exist, but there will be other options)

 

[Malvern Rider]

Yeah I realised it was dodgy - but knew for sure at least that the chainrings on both Shimano and RF cranksets are seemingly identically spaced when I compared side by side. The RF set (original) that I removed was shifting perfectly with the front mech still set as is when was on the middle ring.

Anyway, tis done. Fingers crossed, just ordered BB ES300 68/126 from CRC. Bargain at half price so if it's poor fit at best I'll lose a tenner, at worst another week or so off MTBing in this nice Autumn weather. Gah. Hope it's the former. Cheers all!

Now the wait.