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ISIS to square BB conversion size?

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ISIS to square BB conversion size?

Old 04-25-20, 10:37 AM
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Yandex
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ISIS to square BB conversion size?

Long story short, I have an old Kona Cinder Cone "07. The bottom bracket has been shot for a while but finally got around to changing it.

Bought an ISIS BB, but thankfully managed to realise that the w***s at FSA made a proprietary BB for their crankset before it was dispatched. Powerdrive uses the same number of splines but the diameter is slightly smaller. A completely unnecessary complication that was used to create a proprietary BB style, on top of that only they make it. It is still available but at 27 for a sealed bracket it is steep.

Now, I could just buy a new one and call it a day but I am so annoyed at FSA that I will do a conversion to a normal no-nonsense Shimano crankset. Something of the likes of Altus or Acera. Cheap and cheerful.

The question - do I order the exact same size BB as I have now i.e. 68x113mm or are the square splines a different design (e.g. they are 2mm longer each so I actually need a 109mm BB to achieve the same spacing)? I hope it is clear what I am asking.

Also, any point splashing the cash for an Octalink crankset and therefore an Octalink BB?

Thanks.
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Old 04-25-20, 10:55 AM
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I always ordered the same size BB, ISIS to square, but the 2 mm per side wouldn't have been an issue (to me) if it's a consideration. 68 X 113 is a size that I use frequently. Also, always had good luck with Truvative BB's, but used them with "their" cranks.
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Old 04-25-20, 11:04 AM
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Originally Posted by 2old View Post
I always ordered the same size BB, ISIS to square, but the 2 mm per side wouldn't have been an issue (to me) if it's a consideration. 68 X 113 is a size that I use frequently. Also, always had good luck with Truvative BB's, but used them with "their" cranks.
Thanks, that helps a lot. I don't actually know that they are 2mm longer, I was just giving an example if they are different and how/if that would relate to the position of the arms. In that case, I will go with the same size.

Truvative Powerspline and FSA Powerdrive might also be different from each other, no clear answer on the web and I am definitely not giving them my money to find out...

Also, is it best advised that I go for 1:1 on the chain tooth numbers (22-32-44) as to avoid buying a new chain?
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Old 04-25-20, 11:23 AM
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Well the reason the w***s at FSA made a proprietary BB is because their spindle on their crank is a different size than the w***s at other mfrs, such as shimano and others that also make proprietary designs. If you don't like proprietary stuff, then you need to go back to the older cup and cone BB's. Most everything else has proprietary pieces to it.

All you need to know is what size your bb shell is, width and inside diameter. If threaded, then what TPI. Then you find out if the crank set you want has a bb that fits both it and the shell of your bike. It's that simple. If you only want to go by trademark names like ISIS and Octalink and such, then you'll likely remain confused about what you need.

If you are wanting a fairly easy BB and crankset to install, then look at the two piece cranksets. Most all the decent mfrs have them. And they fit all sorts of bikes. I've put them on bikes from the seventies that used to be cup and cone bb'.
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Old 04-25-20, 11:26 AM
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You buy the BB that the crankset chosen requires. The crankset drives the BB type & spindle length.

Be aware there are OCTALINK V1 and V2 versions.

Altus and Acera requires 123mm
https://www.jensonusa.com/Shimano-Al...Square-Crankst

https://www.universalcycles.com/shop...BoCLnQQAvD_BwE
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Old 04-25-20, 11:48 AM
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I thought it was commonly agreed that the state of BB's in bicycles is a complete and utter mess, I don't see any reason to defend the decision to make a proprietary "Powedrive" BB when there is minimal difference between it and the ISIS standard. The only purpose for this, if you ask me, is so that you force the consumers to buy your own BB's too.

Also, as I mentioned I want to go to a non-nonsense square taper BB. To me, that is not proprietary but a timeless standard.
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Old 04-25-20, 11:58 AM
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dedhed Never thought of it this way, that makes sense.

Finding the spindle length required is quite difficult. It is not mentioned on Shimano's own website.

Still not sure about my last question - can I mismatch the teeth number?
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Old 04-25-20, 12:53 PM
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As long as your chain is long enough for the new number of teeth, no problem. However if you're procuring a new crank, you should consider a new chain and cassette unless your chain still has "life" (determine by measurement) or the old chain will wear out the new chainrings prematurely. Is the old square taper crank toast?
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Old 04-25-20, 01:02 PM
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Originally Posted by 2old View Post
As long as your chain is long enough for the new number of teeth, no problem. However if you're procuring a new crank, you should consider a new chain and cassette unless your chain still has "life" (determine by measurement) or the old chain will wear out the new chainrings prematurely. Is the old square taper crank toast?
The chain has life, confirmed with tool.

It's an FSA "Powerdrive" ISIS-lookalike BB that is completely toast and I'm replacing with square taper.

The tricky bit now is deciding the correct length of the spindle that the cranks I'm interested in require. Chainline also seems to be important but still wrapping my head around it.
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Old 04-25-20, 01:25 PM
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Originally Posted by Yandex View Post
Finding the spindle length required is quite difficult. It is not mentioned on Shimano's own website.
If you look in the right places it does. as well as chainline

https://si.shimano.com/pdfs/si/SI-6NMFA-004-ENG.pdf
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Old 04-25-20, 02:35 PM
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Originally Posted by dedhed View Post
If you look in the right places it does. as well as chainline
I managed to find it on the website. It says it in the description of every crankset.

One question is left then - I measured my chainline at 47.5 - are the numbers given for 50mm or 47.5? Or does it matter at all?

On the instructions that you have linked they seem to show that for 47.5mm they use a 122.5mm spindle which I don't think exists. Even if it does they use 123mm for 50mm but the difference is only 0.5mm whilst the difference between the chainlines is 2.5mm.

I can't quite understand the chainline relation to BB spindle length.

EDIT: To add it seems that the only current cranksets that support 47.5mm are Tourney. Wondering if a 50mm crankset will work fine...

Last edited by Yandex; 04-25-20 at 03:01 PM.
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Old 04-25-20, 03:06 PM
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Some of that has to do with oversize tubes, 68mm vs 73mm BB shell width. Honestly chainline within a few mm isn't a big deal depending on which gears you ride in most. The more important thing is that the FD can reach all the gears.

You'll also notice the letter after those BB (E, K) which I believe to not be symmetrical as they are for "E" type derailleurs and chain cases which shift the BB outward on the drive side.

Chainline is measured from the center line of the frame. A longer spindle moves the chainrings farther from the frame resulting in a "bigger" chainline. Most cranks & FD are able to accommodate different chainlines. Typically I'd use whatever the crank recommends and rarely has it been an issue.

https://www.parktool.com/blog/repair...nline-concepts

47.5 versus 50mm chainline





https://www.amazon.com/Shimano-BB-UN.../dp/B00653OEL2
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Old 04-25-20, 06:04 PM
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Don't be too focused on chainline. Remember that on a multi-speed bike the chainline is only "ideal" in one gear combination.
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Old 04-26-20, 05:11 AM
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Old 04-26-20, 05:12 AM
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After hours of reading, I think I have figured it out, bottom brackets are more complex than I thought! That being said here is what I have found - in case someone finds my thread in the future with a similar question.

Most higher-end cranksets from Shimano have an axle length for a 50mm chainline supplied on the website and there is no way to make a mistake. If you want to make them work on 47.5mm line you can risk by buying a 5mm shorter BB than the one suggested and hope that the cranks will fit on the shorter spindle or just buy the one that Shimano advised and accept that the chainline will be 2.5mm off (not an issue it seems on MTBs). When it comes to the cheaper cranks, namely Tourney(TX), they have a couple of sizes listed which can make it hard to understand.

For a select number of BB sizes, Shimano uses a series of letters to describe the axle length instead of the actual size being the name of the BB (e.g. LL123). These being:
- D-NL - 122mm
- D-EL - 127mm


Further, those types of brackets can be made offset to accommodate a chaincase or E-type derailleur. (thanks dedhed)
- BB-UN26 (-K) (no K in the name) are non-chain case compatible.
- BB-UN26-K are chain case compatible. Chain line at 47.5mm + offset (usually at 0.5mm).

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It seems that all of that might be wrong... There is a serious mishmash regarding the length. It seems that LL123 (what I thought meant 123mm is actually a variation of 122.5mm but not an offset variation e.g. K). If anyone has any idea please let me know. At this point, it's not about what will fit the bike but trying to understand the numbers themselves.


Regarding my application, I have decided to go ultra-cheap and get a SHIMANO FC-TY501 with a DN-L bracket (for 47.5mm chainline). The risk being whether shifting will be satisfactory as that is a 6/7/8-speed crank but it seems that it will probably work with a 9-speed chain based on a thread in this forum.

Last edited by Yandex; 04-26-20 at 06:12 AM.
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