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68mm bb/chainset compatible with 73mm frame?

fiendoidelfiendoidel Posts: 28
edited September 2016 in Workshop
Sorry if this has been done to death in the past, but can anyone tell me whether there is a simple solution to this:

I want to swap a Tiagra 4600 groupset from one bike onto another. The chainset is a FC-4650 and the spec sheet for that calls for a BB shell width (thread dimensions) of 68mm (1.37x24 TPI) or 70mm (M36 x 24 TPI). The frame I want to fit it to has a 73mm shell width.

Is there any way of fudging the chainset into the above frame? I would be quite happy to buy a new BB, but I don't really want to buy a new BB, new chainset, new shifter and new front mech...

Or will it be better simply to look for a different frame?

Any tips welcome.

Thanks

Posts

  • pinnopinno Posts: 42,794
    I think you can, you have to use spacers.
    seanoconn - gruagach craic!
  • Thanks

    I think that page refers to the inverse problem to mine though - fitting a BB designed to 73 into a 68 frame. My problem is fitting a chainset designed for a 68mm BB into a 73 mm frame, so I need the opposite of spacers unless the spindle on the drive side of the chainset is long enough to cope with having to pass through an extra 5mm of frame.

    I'm not sure that the above make sense, but hopefully you can see the problem I'm trying to describe...

    Thanks again
  • pinnopinno Posts: 42,794
    It does make sense.

    Maybe SJS cycles could help - ring them up. 01278 441500
    seanoconn - gruagach craic!
  • Hmm, I'd be surprised if it works that well, if at all. Two problems - leaving 5 mm less thread engagement on the NDS crank arm is probably less than ideal from a strength and longevity point of view, and not sure I'd trust it under that arrangement. Second issue is that the chainline will possibly be poor and you'll end up with shifting problems, plus there's an outside chance the crank arms Q factor will be sufficiently different that they foul the chainstays on the new frame. 73mm shell suggests you're using an MTB frame, which have substantially more space between the stays to accommodate knobby tires, meaning the cranks must be wider to fit.

    One way to check that is at least fit the drive side crank to the new frame with the BB cup installed, and spin it slowly - see if it clears the stays. You should have at least a 5 mm gap between crank arm and chain stay to allow for some flex.

    The BB will be compatible between frames if they are both BSA thread- only difference is the length of the plastic tube between the two bearing sets, and you can probably scrounge a longer and suitable one from a friendly LBS. You will be better off with a new chainset though IMO.

    You may - repeat, may, not will - find the front mech doesn't work optimally anyway. They are designed to work within specific chainlines give or take a bit, so a road mech on an MTB frame with an extra 5 mm shell width may not swing out far enough to shift to the big ring. Should do, but will be trial an error, not guaranteed.
    Open O-1.0 Open One+ BMC TE29 Titus Racer X Ti Giant MCM One Cannondale Prophet Lefty Cannondale Super V SL Cove Handjob Cervelo RS
  • pinnopinno Posts: 42,794
    You've well and truly piddled on his chips haven't you WS? :roll:
    seanoconn - gruagach craic!
  • Pinno wrote:
    You've well and truly piddled on his chips haven't you WS? :roll:
    Always helpful, you know me.. lol
    Open O-1.0 Open One+ BMC TE29 Titus Racer X Ti Giant MCM One Cannondale Prophet Lefty Cannondale Super V SL Cove Handjob Cervelo RS
  • No, that was a helpful response. Thanks.

    I've not bought the frame yet, so I'm still weighing up options - doesn't sound like fudging it is a great idea.

    The problem I have is that I need a disc-compatible, women's (or very small men's) frame with a 31.8mm seat tube, a 1 1/8 head tube and a 68mm BB. Oh, and it's got to be cheap too. This looked perfect:

    https://www.evanscycles.com/pinnacle-li ... e-EV214315

    But the BB shell seems like it's going to be a sticking point....
  • The RookieThe Rookie Posts: 27,748
    You could get the shell faced back by 2.5mm each side, looks like plenty of room for it.

    The Lithium is actually a Hybrid (not a road bike like the frame add claims) so comes with an MTB crankset as standard, hence the 73mm shell. The frame looks rather MTB orientated with the downtube to headtube gusset and generous stays.
    https://www.evanscycles.com/en-in/pinna ... e-EV244092
  • The Rookie wrote:
    You could get the shell faced back by 2.5mm each side, looks like plenty of room for it.

    The Lithium is actually a Hybrid (not a road bike like the frame add claims) so comes with an MTB crankset as standard, hence the 73mm shell. The frame looks rather MTB orientated with the downtube to headtube gusset and generous stays.
    https://www.evanscycles.com/en-in/pinna ... e-EV244092
    True, could do, but that might still make the chain set foul the stays, which I suspect is an issue. Road cranks are roughly 150mm Q factor, MTB are typically around 170. That means roadie cranks on an MTB will sit 10 mm closer *each side* to the chain stay.. cutting it fine I reckon...

    See this article:

    http://www.cyclingweekly.co.uk/news/latest-news/what-is-q-factor-and-does-it-make-a-difference-187403
    Open O-1.0 Open One+ BMC TE29 Titus Racer X Ti Giant MCM One Cannondale Prophet Lefty Cannondale Super V SL Cove Handjob Cervelo RS
  • Thanks all

    I knew it was towards the MTB side of hybrid, rather than more road oriented, but I thought it would be a good frame for a fastish flat bar road hybrid that I am trying to build for the wife using a tiagra gpset and disco brakes I have aquired - back to the drawing board I think.

    Thanks again
  • The RookieThe Rookie Posts: 27,748
    edited September 2016
    Have a look at the PX London road, still some offers, check the quality though before building!
    1/ Seat tube - isnt 31.6, I was using a 27.2 seatpost with shim anyway so am using a perfectly fitting 31.8 shim.
    2/ Headset bores - the upper comes in a generous.
    3/ Rear brake calliper mount can be very (too) close to the disc on some wheels.

    Otherwise it does look nice, is fairly light and certainly aerodynamically is a lot faster than my old bike, on downslopes I'm much more often in top than before.
  • Everyone loves a frankenbike!
  • Thanks

    I looked at Planet X stuff (London Road and Kaffenback), but even their small frames are still quite large (standover height 74cm, wife's inseam = 75cm).

    I think my problem is that my desired specs are a bit too niche - maybe I'll ditch the disc brakes in order to expand my range of possible frames.

    I sure it wasn't this hard the last time I built up a bike, although that was about 15/20 years ago and I probably didn't give compatibility issues too much thought as a teenager.
  • Everyone loves a frankenbike!


    I saw some good monsters on another thread on here - maybe I should just build her one of those instead. Bike with a sidecar and a windshield, perhaps?
  • The RookieThe Rookie Posts: 27,748
    I'm on a Large PXLR with an inseam the same as your wifes (well actually a smidgen shorter) as my short leg/long torso build needs a long top tube, not a problem to ride.

    Or look for a good Carrera Gryphon or Marin Point Reyes on ebay.
  • The Rookie wrote:
    I'm on a Large PXLR with an inseam the same as your wifes (well actually a smidgen shorter) as my short leg/long torso build needs a long top tube, not a problem to ride.

    That's interesting..... and you don't need a step ladder to get on and off it? I might need to look at the numbers again.

    Thanks
  • The RookieThe Rookie Posts: 27,748
    Not at all. Being short of leg I'm used to having little or no (or negative) standover, I stop by moving my posterior in front of the saddle, one foot on the pedal and one on the floor with the bike lent slightly. When all your bikes are like that and have been for a while its second nature.
  • pinnopinno Posts: 42,794
    Fiendoidel wrote:
    The Rookie wrote:
    I'm on a Large PXLR with an inseam the same as your wifes (well actually a smidgen shorter) as my short leg/long torso build needs a long top tube, not a problem to ride.

    ... and you don't need a step ladder to get on and off it?

    He never said that he didn't.
    seanoconn - gruagach craic!
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