BB92 reamer sizing

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  • 11-21-2012
    febikes
    BB92 reamer sizing
    I am in discussion with Park Tools about having a 752 reamer set reground to work with the BB92 standard.

    I know PVD and some others are using this standard and want to be sure that I have the reamer ground to the correct size.

    My understanding is that the standard is 41mm press fit cups. Shimano has not to my knowledge published specifications for the standard. Calvin at Park Tools measured some bottom brackets they had on hand for showing that a SRAM GXP was 41.04, a Shimano was 41.06 and a Race Face was 41.08.

    Calvin thinks 40.95mm with tolerances of +0, -.02 will work for a nice tight fit.

    I would hate to go through all the expense associated with this and end up with something too loose or too tight.

    Before I pull the trigger and place my order can anyone let me know exactly what size they have used for custom ground reamers that support the BB92 standard?

    The BB92 standard seems to be a good thing to avoid crimping and bending the hell out of the chainstays to fit wide tires on short chainstay 29ers. It seems the bike will be stiffer and stronger with less crimping and a shell that is supported at a wider point.
  • 11-21-2012
    smudge
    Quote:

    Originally Posted by febikes View Post
    I am in discussion with Park Tools about having a 752 reamer set reground to work with the BB92 standard.

    I know PVD and some others are using this standard and want to be sure that I have the reamer ground to the correct size.

    My understanding is that the standard is 41mm press fit cups. Shimano has not to my knowledge published specifications for the standard. Calvin at Park Tools measured some bottom brackets they had on hand for showing that a SRAM GXP was 41.04, a Shimano was 41.06 and a Race Face was 41.08.

    Calvin thinks 40.95mm with tolerances of +0, -.02 will work for a nice tight fit.

    I would hate to go through all the expense associated with this and end up with something too loose or too tight.

    Before I pull the trigger and place my order can anyone let me know exactly what size they have used for custom ground reamers that support the BB92 standard?

    The BB92 standard seems to be a good thing to avoid crimping and bending the hell out of the chainstays to fit wide tires on short chainstay 29ers. It seems the bike will be stiffer and stronger with less crimping and a shell that is supported at a wider point.

    Mark, PM me with your email adress and I'll send you what I have. Shimano sent me a bunch of .tif files years ago with the dimensions. At that time, they wrote that they don't have a recommendation on the ID of the shell. I'll look through my 2013 tech manual to see if they've changed anything.

    IMO, the shell won't solve the biggest problem with short stay 29ers, that is chainring/tire/stay interference on the drive side stay. It will make it easier to take the severe bend out of the non-drive stay though.
  • 11-21-2012
    Walt
    Not gonna solve your problem
    Chainring/chainstay/tire is the problem area. The shell won't help you there.

    -w
  • 11-21-2012
    febikes
    Quote:

    Originally Posted by Walt View Post
    Chainring/chainstay/tire is the problem area. The shell won't help you there.

    -w

    I am not saying that there will be no dimple and bend but there will be less with a BB92 and it should be both stiffer and stronger. Many bikes are built for 2x10 and as such the chainstay can be pushed outward a little bit more with BB92. For single speed and 1x10 bikes the build can be pushed outward even more when only one ring is run up front.

    I am not saying it will open up a whole new world but it will give just a little bit more space with just a little bit less bending and crimping of the stays for a stiffer and stronger overall build.
  • 11-21-2012
    Walt
    No, it won't. Reduce bend, yes. Give "more space", no. The limiting factor is and will always be fitting in the 3 items I mentioned before. You can mess with chain line or do elevated stays or any number of other things but the shell itself will never give you any extra room.

    I mean, feel free to get set up for bb92, it won't hurt anything. I'm just trying to save you some $ if you are thinking this will be a short stay panacea.

    IMO you are not going to get a useful stiffness difference either.

    But it's your money, not mine.

    Walt
  • 11-22-2012
    febikes
    My attraction to BB92 is that it simply seems to be a fundamentally better design.

    My current bike has sliders that can be set as short as 425mm. The bike rides very well and I don't think I will go shorter but I may want to have the option of running wider tires.
    http://febikes.files.wordpress.com/2...0031.jpg?w=574

    I can run 2.1 tires in the super short position. If I slide back I can go up in tire size but there is not lots of room for mud or to deal with broken spokes and the such.
    http://febikes.files.wordpress.com/2...0231.jpg?w=574

    When I look at other brands it seems they squash the stays and put in a much sharper bend. Clearly the squashed and bent stays work fine but they require a thicker wall tube and/or a loss in strength and stiffness. From a structural standpoint a round or nearly round stay with a larger cross section should be both stronger and stiffer.

    For a single speed, it is possible to get a bit more space then a geared bike. When running the ring in the outer position there is room to move the chainstays out a good bit even with fairly large and thick rings. A small dimple can be made if needed but even without a dimple it should be possible to run some very wide tires.
    http://febikes.files.wordpress.com/2...0241.jpg?w=574

    For single speed mountain bikes it makes more sense then any of the other specifications that are floating around for BB standards and seems to be an improvement over the threaded/outboard bearing approach.
  • 11-23-2012
    smudge
    Mark, try it and see what it gets you. What I/Walt mentioned...you can't get around it without getting tricky with chainline. The BB gets you nowhere.

    Keep in mind that because the shell is longer, likely larger and possibly thinner wall, you're likely to get more heat distortion that you're used to getting with a normal shell. Watch your process/sequence and report back once you've done it.
  • 11-26-2012
    Blaster1200
    I can see where febikes is going with this. On one of my short-stayed SS 29ers, I set it up so that the chainring was in the outer position to get as much clearance as possible. However, the furthest out I would join the chainstay was 1/8" from the end of the BB to give me some area to braze. I even positioned the stays as far to the outside of the dropouts as possible. While I had plenty of tire clearance, I wanted more. I had plenty of clearance between the chainring and the chainstay, but I was limited that I couldn't push the stay any further out on the BB. If I had a BB92, I could have pushed the stay outward more to get more tire clearance.

    Based on this, my next short-stayed SS 29er will have a BB92 shell. I will use straight gauge 4130, so the shell will be fairly thick, and ream it as needed.
  • 11-26-2012
    Walt
    3 words
    Deda. S-bend. 29er.

    Call Joe Bringheli and utter those 3 magic words, I think you'll like the result. FWIW I've done bikes with 73mm shells and 70mm of tire clearance, 41cm chainstays and nothing unconventional other than a weird chainline. You'll run out of ring/tire clearance before you run out of real estate on the BB with those stays.

    Or bend your own, or use a plate/yoke setup. IMO all easier and better solutions, press fitting bearings directly into a steel frame is a disaster waiting to happen.

    -Walt

    Quote:

    Originally Posted by Blaster1200 View Post
    I can see where febikes is going with this. On one of my short-stayed SS 29ers, I set it up so that the chainring was in the outer position to get as much clearance as possible. However, the furthest out I would join the chainstay was 1/8" from the end of the BB to give me some area to braze. I even positioned the stays as far to the outside of the dropouts as possible. While I had plenty of tire clearance, I wanted more. I had plenty of clearance between the chainring and the chainstay, but I was limited that I couldn't push the stay any further out on the BB. If I had a BB92, I could have pushed the stay outward more to get more tire clearance.

    Based on this, my next short-stayed SS 29er will have a BB92 shell. I will use straight gauge 4130, so the shell will be fairly thick, and ream it as needed.

  • 11-26-2012
    febikes
    Thanks for the tip about the Deda s-bend stays!
  • 11-26-2012
    Blaster1200
    Yes, I use the Deda stays, and the bends still don't always work out right, so I have used a yoke type setup. But I agree, they are unquestionably the best off-the-shelf chainstay out there for most MTB applications.
  • 11-26-2012
    Walt
    To be clear
    These are the very new version - they are not the old ones, I'll get a pic in the morning. Most aggro bend I have ever seen.

    -Walt

    Quote:

    Originally Posted by Blaster1200 View Post
    Yes, I use the Deda stays, and the bends still don't always work out right, so I have used a yoke type setup. But I agree, they are unquestionably the best off-the-shelf chainstay out there for most MTB applications.

  • 11-26-2012
    Blaster1200
    Quote:

    Originally Posted by Walt View Post
    These are the very new version - they are not the old ones, I'll get a pic in the morning. Most aggro bend I have ever seen.

    -Walt

    Interesting! :thumbsup: The last box of them I bought was around September 2011.
  • 11-28-2012
    smdubovsky
    1 Attachment(s)
    Quote:

    Originally Posted by Walt View Post
    press fitting bearings directly into a steel frame is a disaster waiting to happen.

    Eh? I thought the shimano press-in bearings had a thin plastic outer cup? Haven't seen one in person but I did NOT think it was steel on steel.

    BB92:
  • 11-28-2012
    smudge
    Quote:

    Originally Posted by smdubovsky View Post
    Eh? I thought the shimano press-in bearings had a thin plastic outer cup? Haven't seen one in person but I did NOT think it was steel on steel.

    BB92:

    This is the way it's always been. There are some companies that use their own version with direct press-in bearings though. It's still not going to solve any problems for people looking to gain space for short stays/big tires/chainrings.

    Mark, you have an email.
  • 11-29-2012
    jay_ntwr
    Quote:

    Originally Posted by smdubovsky View Post
    Eh? I thought the shimano press-in bearings had a thin plastic outer cup? Haven't seen one in person but I did NOT think it was steel on steel.

    BB92:

    Yep, they do have the plastic cup around them for sure. I really wanted to go the BB92 route but that was before CK started making outboard BBs. Now I have no desire to do it and I'll 2nd or 3rd or whatever the Deddicai stays from Joe.

    I have a 34t SS ring, 2.4" Racing Ralphs, and 437mm stays and everything fits perfectly without a bit of rub on a 73mm shell. There was no margin for error during building, but it all fits.
  • 12-01-2012
    Walt
    1 Attachment(s)
    New Dedas
    New on on the left.

    I have a frame in progress with these with 75mm of tire clearance, 41cm stays. You could probably go crazier if you wanted to.

    -Walt