Viewing 32 posts - 1 through 32 (of 32 total)
  • Can you explain this?
  • Premier Icon Blackflag
    Full Member

    I had a non boost codeine and swapped all the parts to a boost Pace 29r. This included the BB which worked perfectly.

    fast forward 2 years and the BB breaks so i buy a new one (Shimano XT)

    Pace BB shell is 73mm (measured and on spec sheet) so only 1 spacer needed. Old BB had one spacer on the drive side. I fit new one exactly the same.

    Crank wont fully tighten and wobbles.

    So ive added 1 more spacer drive side and 1 non drive side (as if the BB shell was smaller) and it works perfectly.

    Whats the hell has happened? Its all good but ive clearly missed something.

    Premier Icon jam bo
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    new XT BB’s are smaller in diameter, are they thinner too?

    Premier Icon Pyro
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    Old BB was a 73mm specific, new one is a 68/73mm?

    Premier Icon kayak23
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    Brexit

    Premier Icon monkeyboyjc
    Full Member

    Magic?

    Premier Icon dangeourbrain
    Full Member

    Can’t be that kayak. If it were it would work just fine now but come with a warning that on 31st of December the bottom bracket will change and you should be prepared for that, to make it easy for you to prepare we’ll give you a clue “the bottom bracket won’t be the same as it is now. Probably.”

    Is it some strange “boostable” bottom bracket OP?

    Premier Icon Tim
    Free Member

    Old BB was a 73mm specific, new one is a 68/73mm?

    This

    But if it works, it works

    Premier Icon kerley
    Free Member

    Old BB was a 73mm specific, new one is a 68/73mm?

    Are the cups wider on a 73mm specific BB and what makes it 73mm specific?

    Premier Icon twisty
    Full Member

    An explanation is that the new BB cups are narrower than the old ones.
    Could be verified using a ruler.

    Are the cups wider on a 73mm specific BB

    No

    and what makes it 73mm specific?

    They don’t really exist, but if they did it would be a short thread so can’t be used with spacers.

    68mm specific would be wider cups, 73mm compatible can be used with 68mm using spacers.

    Premier Icon munrobiker
    Free Member

    Have you used a road bottom bracket by mistake? The cups are about 1mm narrower.

    Premier Icon Blackflag
    Full Member

    Have you used a road bottom bracket by mistake? The cups are about 1mm narrower.

    Nope its this….

    Shimano XT MT800 MTB Bottom Bracket, Black, 68/73mm – English Thread – 24mm Spindle

    As my BB is a 73mm then it should be 1 spacer as the 68mm requires 3. I’m totally lost.

    Premier Icon twisty
    Full Member

    Only other thing I can think of is either spacers are different width, or something was snagging when trying to preload the crank the first time. If you undo the preload bolt then you can check how much clearance there is between the end of the spindle and the face of the crank

    Premier Icon Blackflag
    Full Member

    something was snagging when trying to preload the crank the first time

    Thats one of the things i was thinking it could be. Somehow some crap may have gone into the NDS crank (Sram GX) thats stopping it fully winding on.

    Premier Icon tomnavman
    Full Member

    There have been lots of reports of SRAM cranks being loose – if I remember correctly the official guidance is add grease and tighten? I think the torque spec is crazy high?

    Premier Icon joebristol
    Full Member

    Hang on – shimano bb with sram gx crank? Is the crank gxp then and you’re using some kind of top hat thing in the non driveside to bodge it? Perhaps you haven’t moved the extra bodge thing over from the old bb to the new?

    Sram non driveside on gxp is 22mm rather than 24mm axle….

    Premier Icon lovewookie
    Full Member

    I had similar.
    old cannondale from 2002. 68mm BB, new XT BB in. 2 spacers drive side. 1 non drive. crank in. not enough axle showing to fit the crankarm safely.

    one spacer off driveside. so now have one spacer either side. fits fine.

    the BB came with 3 spacers, so I don’t think they’ve designed it to not work with BB mounted gubbins, like chain guide/e type mech.

    still, it works…

    Premier Icon Tim
    Free Member

    If it’s a Shimano bb and a gxp crank it won’t work without a top hat as GXP uses a smaller bearing than Shimano. The axle length is fixed so you may need spacers to preload the bearing. Is the Wave washer in place?

    Premier Icon Blackflag
    Full Member

    Wave washer? Top hat?

    All i have is the Sram Crank going through a shimano BB….

    I think i’m about to get STW schooled.

    Premier Icon joebristol
    Full Member

    Look in the left hand side of the old bb and there’s probably a spacer thing inserted into the cup. Either that or it’s stuck on the non-Drive crank side.

    Premier Icon Blackflag
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    Ahhh tits, thats probably in the bin then….

    Premier Icon dovebiker
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    SRAM GXP cranks have a stepped spindle – 24mm chainring side and 22mm non-drive. They are designed to be tightened firmly against the NDS bearing – any axial play is taken up by a wavey washer between the bearing and crank. Your XT BB is 24mm dia both sides, so you need a top-hat spacer over the NDS crank spindle to stop it waggling about. Hope make a suitable spacer.

    Premier Icon Blackflag
    Full Member

    Cheers. Could i trouble you to point me at the correct hope spacer mate? it looks like they do a few.

    Premier Icon reeksy
    Free Member

    That looks like it.

    This got me in reverse. I bought a 24mm Hope BB off a guy online and when it arrived I nearly had a stroke trying to get the XT crank through it. Had a bit of a disagreement with the vendor about it obviously not being a 24mm – him claiming that Shimano don’t even make 24mm cranks!!!

    Bloody top hat was blocking it. Knocked it out and it’s sitting on the bench in case i ever need it. You can have it for postage … except i’m in Oz and would probably be cheaper and quicker to buy it.

    Premier Icon philjunior
    Free Member

    Will the Hope top hats fit on a Shimano BB without clashing? I got a Token one on the GF’s bike and it had to be “gently modified” to fit.

    Premier Icon joebristol
    Full Member

    Or another option here is to buy the best quality gxp bb you can get instead of bodging it……

    Premier Icon Blackflag
    Full Member

    Well i bought the hope thing last night so will see how I get on.

    Premier Icon twisty
    Full Member

    Ah, sorry I assumed a Shimano crank. Not only do you need a hat thing but you need a wave washer and/or use spacers that precisely match the gap between the bearing faces and spindle length.

    The GXP bearing interface is one that leaves me scratching my head why any Engineer would use it for a crank system. By design the bearings preload is limited to the tension on the wave washer, and then in use the wave washer will be expanding/compressing with the spindle correspondingly rubbing against the bearing race wasting energy and wearing itself out of true until everything is misaligned and crappy.

    Premier Icon joebristol
    Full Member

    Do all the gxp cranks have a wavy washer? I’ve got a bsa screw in bb with X1 crank and I’m pretty sure I didn’t use a wavy washer. The below guide doesn’t suggest the use of one either – I thought the idea was meant to be if you have the bb spaced correctly and you tightened the crank so the specified torque the crank would tighten to a stop with the other arm and that’s just meant to be it. Clearly this is fine if tolerance are perfect, but perhaps Sram aren’t brilliant at this. Guide below:

    https://www.sram.com/globalassets/document-hierarchy/user-manuals/sram-mtb/drivetrain/95-6118-016-000-rev-c-mtb-cranksets-and-bottom-brackets.pdf

    My other bike has Dub which is different and has the preload collar built into the no drive side crank arm like the bb30 version of gxp seems to have.

    Premier Icon twisty
    Full Member

    Wow those instructions are horrible.
    The GXP crank is intended to be used with the wavy washer (part 5 in yellow), but as I mentioned if you can space it *precisely* then it wouldn’t be required. The tolerance would include the width of the frame. The powerspline or bb30 with preload adjuster systems obviously won’t require one.

    Angry engineer Hambini explains the GXP thing in detail in the below video, WARNING – he swears a lot.
    sweary explanation of GXP

    Premier Icon Blackflag
    Full Member

    Cheers guys. I ordered a wavey washer at the same time. It does annoy me how much stuff is incompatible with other stuff these days. I miss the old “plug and play” aspect of yesteryear.

    Premier Icon joebristol
    Full Member

    @twisty

    Wow those instructions are horrible.
    The GXP crank is intended to be used with the wavy washer (part 5 in yellow), but as I mentioned if you can space it *precisely* then it wouldn’t be required. The tolerance would include the width of the frame. The powerspline or bb30 with preload adjuster systems obviously won’t require one.

    Angry engineer Hambini explains the GXP thing in detail in the below video, WARNING – he swears a lot.
    sweary explanation of GXP

    Part 5 is on a press fit gxp bb though isn’t it – the instructions aren’t entirely clear but it doesn’t show a wavy washer on the screw in gxp bb?

    Rockshox instructions for forks are brill – don’t know why Sram have made the gxp bb ones so s***e!

    All I know is I have no wavy washer on my gxp one – the bb and cranks didn’t have a wavy washer included with either part. It’s done up to the 40 something NM of torque and it spins ok and has no play in it. 73mm bb she’ll so no spacers if I remember correctly.

    Fingers crossed that’s right – 3rd gxp set of cranks / bb I’ve had setup like this.

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