Shimano Square Taper Bottom Bracket Question
05-14-22, 11:16 AM
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Shimano Square Taper Bottom Bracket Question
I noticed that some of my spare Shimano (square taper) bottom brackets have a nonremoveable ring on the drive side (see photo). Does anyone know why this is?
05-14-22, 11:25 AM
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It forced me to buy a different BB tool.
This is a UN-26, but the ring kept my Shimano TL-UN72 tool from reaching deep enough.
PARK BBT-22 does clear. I just hated to have to buy another tool that does the same thing.
This is a UN-26, but the ring kept my Shimano TL-UN72 tool from reaching deep enough.
PARK BBT-22 does clear. I just hated to have to buy another tool that does the same thing.
Last edited by Bill Kapaun; 05-14-22 at 03:44 PM.
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05-14-22, 05:00 PM
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These press fitted rings on the drive side BB axle are locating shoulders for the crank to butt up against. This locating insures the chain line specs are maintained over the years and prevents people from over torquing the retaining bolts resulting in a crank arm that ends up bottomed out on the axle end (or more to the point, the retaining bolt's being bottomed out against the axle end). Sometimes these rings will come loose. As mentioned these rings are not a must. Shimano does take into account the factory assembly issues when they design their parts, sometimes having a detail that many don't consider. Andy
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05-14-22, 08:49 PM
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That's to ensure consistent chainring clearance on the drive side. When you install the cartridge, you thread it in until the flange on the cartridge seats against the frame bottom bracket shell. This establishes a known clearance for the appropriate crank to mount on the bottom bracket.
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05-15-22, 04:54 AM
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Nothing is ever "non removable"
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05-15-22, 11:09 AM
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That's to ensure consistent chainring clearance on the drive side. When you install the cartridge, you thread it in until the flange on the cartridge seats against the frame bottom bracket shell. This establishes a known clearance for the appropriate crank to mount on the bottom bracket.
05-17-22, 05:29 AM
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My concern with the ring as a stop for a square taper crank is that it would seem impossible to take into account the depth of every new and used JIS crankset that is in use today.
If it is used as a stop the ring could allow the crank to be properly torqued without being completely seated and effectively ruin the crank arm.
If there is no ring, what’s going to happen if my 40 year old non-Shimano crank needs and extra .1mm of depth to seat?
I guess the answer is to buy a new Shimano square taper crank.
John
If it is used as a stop the ring could allow the crank to be properly torqued without being completely seated and effectively ruin the crank arm.
If there is no ring, what’s going to happen if my 40 year old non-Shimano crank needs and extra .1mm of depth to seat?
I guess the answer is to buy a new Shimano square taper crank.
John
05-17-22, 05:54 AM
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This is what I was thinking when I read the replies above. I guess I could see that ring as something that would prevent a gross "over torque" where the arm is pushed way too far up on the spindle...and the ring is a last-ditch effort to prevent damage to the whole system. But I'm surprised to hear that it's intended to set chainline, and designed so that the crank arm butts up against it. I could see that type of thing working well on a non-tapered spindle, but it seems like there's precious little tolerance for any parts variation in the field with something like this on a tapered spindle design.
