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Help with converting a 3x8 to 1x8: chainlines & bottom brackets

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Help with converting a 3x8 to 1x8: chainlines & bottom brackets

Old 09-10-19, 12:30 PM
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Help with converting a 3x8 to 1x8: chainlines & bottom brackets

I'm definitely overthinking this, so if you have a minute can you please read and talk me down off the ledge? Cool, thanks.
I've done a lot of googling over the last few days and haven't been able to arrive at a solution - even with some older threads on here. So I'm hoping some fresh thoughts can help me out.

I've removed the Shimano RSX triple crank because I want to make it a 1x8 (keeping the 8 speed freewheel that came on it, along with the RSX rear derailleur).

I've picked out some cranks, but I don't know if it will fit on the new BB I will get - and what length spindle I will need.

Here's what I know so far from the existing original setup:
-BB shell is 68mm
-Current BB is a Shimano LB27 original to the bike (square taper) - needs replacing
-I measured the spindle and I think it was 118mm
-Chainline 45mm (according to the ‘99 C’dale catalog & Sheldon Brown)
-Original triple was 52t-42t-30t

So, my quandary is what size spindle do I need for the crank I've picked out so that chainline is good for the 8-speed, and also so the chainring clears the chainstays. (also the chain the 1x crank takes will also work with an 8 speed freewheel?)

Now, I'm definitely open to suggestions for different cranks, but the 2 I have my eye on are a Sugino RD2 https://www.retro-gression.com/colle...o-rd2-crankset
and the SRAM S300 Courier https://www.retro-gression.com/colle...-s300-crankset
(These are the top of my budgeted range, really, but am always open to suggestions)

They both look about the same, but the Sugino is silver which matches all the rest of the components so it's my first choice - plus it fits a square taper spindle (ANY? square taper as far as I could tell??) while the SRAM one fits its own matched BB. The Sugino fits a 3/32" chain - will that work on an 8-speed? Google told me yes, but I trust real people here more lol.

Sugino says the crank needs/matches with a Sugino BB with a 103mm spindle for proper chainline - but is it safe to assume this is a typical single speed measurement? The chainline per that is only 42mm (i read this somewhere but now can't find it, so...who knows?)

I was looking at what info these other online shops had for the Sugino
https://www.treefortbikes.com/Sugino...30mm-alloy-RD2
andhttps://www.modernbike.com/sugino-rd...peed-cranksets
and it says 103mm spindle for track, & 110mm for road, which leads me to believe the crank will fit a regular Shimano UN55 square taper BB purchased with whatever spindle length I need. True?

So if the crank will supposedly fit whatever size I need, how do I determine what spindle length I need so that the chainline is correct & the chainring clears the frame? I don't know how to do the math.

Clear as mud?

Thanks!
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Old 09-10-19, 12:46 PM
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Good luck with that. Whenever i do single crank conversions i mix and match existing parts to make it work.. BBs and cranks. The chainring close as possible to the frame, not all square taper cranks are compatible to square taper BB spindles, might rub.

Last edited by le mans; 09-10-19 at 12:51 PM.
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Old 09-10-19, 12:54 PM
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Seems it'd be a lot less expensive to just not shift the front.
IF the bike actually has a Free Wheel, that would infer it's lower end and less likely to justify alleged "upgrades".
If it's a cassette and you don't know the difference, I'd suspect you're over your head.
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Old 09-10-19, 01:31 PM
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I'm doing a single speed conversion atm, using an old Sugino No.3 single ring crank, the BB is a Shimano UN51, spindle length - 114mm, sized it up and will fit perfectly

If that helps.
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Old 09-10-19, 01:37 PM
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BTW, I used this same crank converting an old Indi 500 road bike, made it a 5X1..index thumb shifter, the 48 tooth crank was a good compromise, i thought








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Last edited by le mans; 09-10-19 at 01:41 PM.
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Old 09-10-19, 01:44 PM
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Originally Posted by the sci guy View Post
So if the crank will supposedly fit whatever size I need, how do I determine what spindle length I need so that the chainline is correct & the chainring clears the frame? I don't know how to do the math.
This is difficult to get precise and, like others have said above, it often involves trial and error and parts mixing. Cranksets are typically matched to spindle lengths, as you found with Sugino.

It sounds like your current chainline is 45mm (at the middle ring, right?) on a 118mm spindle. You're considering a crank designed for a 42mm chainline on a 103mm spindle. In theory, if those numbers are right, you'd want to move your crank 3mm outboard, and would need a 6mm longer-than-recommended spindle to do so (109mm total), assuming it's a typical centered spindle and not an offset design.

Fortunately, chainline doesn't have to be exact (especially with a rear derailleur), and bottom bracket spindles are available in a variety of sizes. My best advice would be to visit your local bike shop, tell them what you're doing, and buy two or three bottom brackets with lengths that you think might work and do a very light dry fitting to your frame. Clean your bottom bracket shell threads of all grease, and install the drive side just until snug, and fit the crank arm to it, just until it stops on the taper. Ensure the crank arm is plumb to the spindle and measure chainline; subtract 3-4mm, as that's probably how far the crank arm will tighten up on the spindle, and that should be your finished chainline. Try the various bottom bracket sizes until you find the one that will work best and return the ones that won't work. My local bike shop would be supportive of this, and I'm sure yours would, too, if you explained what you were doing.

Take-away of the story (at least in my opinion): buy the crank you like, and then choose a bottom bracket length that will work with it. It will likely involve some trial and error, but you'll be pleased with the results in the end.
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Old 09-10-19, 01:44 PM
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Originally Posted by the sci guy View Post
I'm definitely overthinking this, so if you have a minute can you please read and talk me down off the ledge? Cool, thanks.
No worries about overthinking. I do it all the time. My recommendation is to just initially use your existing bottom bracket. The two reasons for this recommendation are 1) since it was sized for a triple, with the center ring the optimal one for chain line, it may be right on the money for a single (which can be considered a triple with out the inner and outer rings, and 2) it's always easier for me to figure out spindle lengths by mounting a new crank on a bottom bracket, measure the chain line, and then use the difference between actual and target to determine the spindle length of a new bottom bracket (if necessary). Different cranks will fit differently on the same tapered spindle but it won't be much.

I like the looks of the Sugino crank, but if you want to consider used cranks look for Shimano 600 cranks on ebay. They are nice quality 130 BCD cranks that I've used to set up a couple of 1x bikes. You'll probably want a narrow-wide chain ring to avoid chain drop - I like the WolfTooth rings.

I would not trust the bottom bracket recommendations for the Sugino. It may be that they are assuming a certain cog configuration like a track hub that has a 120mm OCD. Best to try the crank on your existing spindle and go from there. I know you need to replace it, but remember that it's for testing.

What size ring will you be using? I wouldn't think that any ring smaller than 48t would hit the chain stay, but again using your current BB will provide insight. And yes, you will need a 3/32" chain.

Good luck
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Old 09-10-19, 01:50 PM
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Originally Posted by Moe Zhoost View Post
My recommendation is to just initially use your existing bottom bracket.
Even better advice than mine! Start with a known quantity. Even if it's trashed -- if chainline is correct, OP can just buy a new one of the same length.
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Old 09-10-19, 01:57 PM
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Originally Posted by hokiefyd View Post
This is difficult to get precise and, like others have said above, it often involves trial and error and parts mixing. Cranksets are typically matched to spindle lengths, as you found with Sugino.

It sounds like your current chainline is 45mm (at the middle ring, right?) on a 118mm spindle. You're considering a crank designed for a 42mm chainline on a 103mm spindle. In theory, if those numbers are right, you'd want to move your crank 3mm outboard, and would need a 6mm longer-than-recommended spindle to do so (109mm total), assuming it's a typical centered spindle and not an offset design.

Fortunately, chainline doesn't have to be exact (especially with a rear derailleur), and bottom bracket spindles are available in a variety of sizes. My best advice would be to visit your local bike shop, tell them what you're doing, and buy two or three bottom brackets with lengths that you think might work and do a very light dry fitting to your frame. Clean your bottom bracket shell threads of all grease, and install the drive side just until snug, and fit the crank arm to it, just until it stops on the taper. Ensure the crank arm is plumb to the spindle and measure chainline; subtract 3-4mm, as that's probably how far the crank arm will tighten up on the spindle, and that should be your finished chainline. Try the various bottom bracket sizes until you find the one that will work best and return the ones that won't work. My local bike shop would be supportive of this, and I'm sure yours would, too, if you explained what you were doing.

Take-away of the story (at least in my opinion): buy the crank you like, and then choose a bottom bracket length that will work with it. It will likely involve some trial and error, but you'll be pleased with the results in the end.
Originally Posted by Moe Zhoost View Post
No worries about overthinking. I do it all the time. My recommendation is to just initially use your existing bottom bracket. The two reasons for this recommendation are 1) since it was sized for a triple, with the center ring the optimal one for chain line, it may be right on the money for a single (which can be considered a triple with out the inner and outer rings, and 2) it's always easier for me to figure out spindle lengths by mounting a new crank on a bottom bracket, measure the chain line, and then use the difference between actual and target to determine the spindle length of a new bottom bracket (if necessary). Different cranks will fit differently on the same tapered spindle but it won't be much.

I like the looks of the Sugino crank, but if you want to consider used cranks look for Shimano 600 cranks on ebay. They are nice quality 130 BCD cranks that I've used to set up a couple of 1x bikes. You'll probably want a narrow-wide chain ring to avoid chain drop - I like the WolfTooth rings.

I would not trust the bottom bracket recommendations for the Sugino. It may be that they are assuming a certain cog configuration like a track hub that has a 120mm OCD. Best to try the crank on your existing spindle and go from there. I know you need to replace it, but remember that it's for testing.

What size ring will you be using? I wouldn't think that any ring smaller than 48t would hit the chain stay, but again using your current BB will provide insight. And yes, you will need a 3/32" chain.

Good luck
Thanks to the both of you - some good starting info. I was thinking of keeping the BB in there for testing but was also getting caught up in trying to put all the pieces together at once.
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Old 09-10-19, 02:02 PM
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Originally Posted by Bill Kapaun View Post
Seems it'd be a lot less expensive to just not shift the front.
IF the bike actually has a Free Wheel, that would infer it's lower end and less likely to justify alleged "upgrades".
If it's a cassette and you don't know the difference, I'd suspect you're over your head.
Thanks for that. =)
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Old 09-10-19, 09:42 PM
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Originally Posted by Bill Kapaun View Post
Seems it'd be a lot less expensive to just not shift the front.
IF the bike actually has a Free Wheel, that would infer it's lower end and less likely to justify alleged "upgrades".
If it's a cassette and you don't know the difference, I'd suspect you're over your head.
A) I like the puzzle of building up & customizing a bike
B) I don't want to just run a clunky ugly triple and "not shift" - it's as much about making the bike look awesome as functional. I already have new bars, stems, & levers to give it the look I want
C) "Lower end" bikes are ripe for upgrades
D) my original cursory glance had me thinking it was a freewheel. I took the wheel off and inspected it today and it is a cassette - not that it matters at all to my questions

Bu thanks for being condescending instead of helpful.
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Old 09-11-19, 08:58 AM
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Originally Posted by Bill Kapaun View Post
Seems it'd be a lot less expensive to just not shift the front.
IF the bike actually has a Free Wheel, that would infer it's lower end and less likely to justify alleged "upgrades".
If it's a cassette and you don't know the difference, I'd suspect you're over your head.
Now that's a novel idea.....
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Old 09-11-19, 10:00 AM
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I don't know why you want to make this change. It ain't broke.
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Old 09-11-19, 12:55 PM
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Originally Posted by trailangel View Post
Now that's a novel idea.....
Originally Posted by noglider View Post
I don't know why you want to make this change. It ain't broke.
You know how you go and buy that classic 1956 Ford pickup truck because even though it still runs, it's a little beat down and has some older parts, but with a facelift and some TLC it could be a badass hotrod machine? I mean, it certainly would be cheaper to leave it rusty, with the old bubbly white-walls, and that clunky engine that knocks, and dashboard gauges that kinda work, but are also just kinda ugly.
So you put some new wheels on it, replace the steering wheel, cherry out the dash, wire in new LED gauges & a stereo, drop a new engine in to make it roar like a beast. It's still the same, old, low-end truck in it's bones, but you've changed some stuff to make it even better and increased it's attractiveness by 150%.

It's kinda like how i went and bought a 1999 entry level bike from a renowned brand for $100 with the intention of making modest updates & upgrades to remove old or ugly parts that serve no function for my taste or riding, and replace them with newer shiny parts to help it become a bad ass road ripper.
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Old 09-11-19, 12:56 PM
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Just use the original crank......take of the small ring, put the ring size you want in the middle, and if you don't like that look...... get a big ring for the outside and get rid of the teeth.....or find a matching bash guard
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Old 09-11-19, 01:30 PM
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Originally Posted by the sci guy View Post
A) I like the puzzle of building up & customizing a bike
B) I don't want to just run a clunky ugly triple and "not shift" - it's as much about making the bike look awesome as functional. I already have new bars, stems, & levers to give it the look I want
C) "Lower end" bikes are ripe for upgrades
D) my original cursory glance had me thinking it was a freewheel. I took the wheel off and inspected it today and it is a cassette - not that it matters at all to my questions

Bu thanks for being condescending instead of helpful.
A- Well, you do seem puzzled.
B- Yeah, right.
C-They're bigger money pits too.
D- In which the ignorance you displayed resulted in me being "condescending " as already stated.

F- For being a whiner.
Ignore list achieved.
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Old 09-11-19, 02:43 PM
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Spindle length on square-taper BBs can be difficult to figure. You might have to try a few to see how narrow you can go. I really like narrow Q-factor. Narrower seems to help with power transfer and comfort but the cranks need to clear the chainstays by two or three millimeters. If you're running 1x you want the narrowest possible IMHO. You can buy a bunch of those inexpensive blue-colored alloy BBs from China and its worthwhile to have 103, 107. 110, 113, 115 on hand to experiment with. They seem to have the same spindle dimensions as Tange-Seiki (maker of Shimano BBs).

Bear in mind that you don't screw the drive side in all the way. You only screw it enough to create equal spindle length on both sides of the bike, then you tighten the non-drive side. You can temporarily set your cranks to point the same direction on the spindle (instead of normally opposed) and gauge them against the chainstays for even distance and to measure Q-factor. I've been able to use 110 with Sugino triple cranksets on a thick aluminum frame, and typically 107 for triples on steel frames. Even if you only run one ring on a triple crank the Q-factor doesn't change because the chainstays are the constraint. It's also important to not over-tighten the BB cups which will create pressure on the bearings increasing their wear.

I like Sugino cranks and can get the Q-factor down around 165mm, generally.

Last edited by Clem von Jones; 09-11-19 at 03:09 PM.
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Old 09-11-19, 03:06 PM
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Originally Posted by Clem von Jones View Post
Bear in mind that you don't screw the drive side in all the way. You only screw it enough to create equal spindle length on both sides of the bike, then you tighten the non-drive side.
I've never heard this before. Are you saying you would get the spacing where you want it (and say there are 4mm left before the flange on the drive side of the cartridge bearing contacts the bottom bracket shell), and then you'd pull it back out and add spacers on the drive side and torque it down? Or are you saying that you'd just torque it down against the non-drive side cup, leaving bare threads exposed on the drive side?
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Old 09-11-19, 03:12 PM
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Originally Posted by hokiefyd View Post
I've never heard this before. Are you saying you would get the spacing where you want it (and say there are 4mm left before the flange on the drive side of the cartridge bearing contacts the bottom bracket shell), and then you'd pull it back out and add spacers on the drive side and torque it down? Or are you saying that you'd just torque it down against the non-drive side cup, leaving bare threads exposed on the drive side?
You could add spacers but I wouldn't bother. Just leave space on the drive-side threads. Every bike is a little bit different so equal spindle lengths will vary..
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Old 09-11-19, 03:37 PM
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What's a Shimano LB27 BB?
maybe swap out the 3 chainrings for a 48T, bolt that on to the crank... service the BB and you're done.
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Old 09-11-19, 03:49 PM
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Originally Posted by Clem von Jones View Post
You could add spacers but I wouldn't bother. Just leave space on the drive-side threads. Every bike is a little bit different so equal spindle lengths will vary..
Interesting. I wouldn't think that would get the cartridge installed tight enough, or keep it stable enough, especially if it has a plastic non-drive side "cup".
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Old 09-11-19, 05:12 PM
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Originally Posted by le mans View Post
What's a Shimano LB27 BB?
maybe swap out the 3 chainrings for a 48T, bolt that on to the crank... service the BB and you're done.
That’s the original bottom bracket from 1999 according to the Cannondale catalog
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Old 09-11-19, 05:34 PM
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Sealed unit?

What model is the Cannondale?
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Old 09-11-19, 06:34 PM
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Originally Posted by le mans View Post
Sealed unit?

What model is the Cannondale?
I'm not 100% sure. It's a R300 triple. page 53 here: https://vintagecannondale.com/year/1999/1999V2.pdf
specs are page 91.
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Old 09-11-19, 06:39 PM
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As said, the easiest is to check to see if you can use the existing crankset. The middle chainring should have the best chainline as you can usually run up and down the cassette without cross chaining.

In this thread (starting in post 6) I did this very thing by grinding the teeth off the large one to make a bash guard. I left the small ring back on as a manual shift granny bailout but that's just optional.

26" Frankenbike gravel build... just because I could.



Last edited by Happy Feet; 09-11-19 at 11:30 PM.
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