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Chainline/BB Question - Triple to 1x, 7spd to 9spd

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Chainline/BB Question - Triple to 1x, 7spd to 9spd

Old 04-12-21, 02:33 PM
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Chainline/BB Question - Triple to 1x, 7spd to 9spd

So I'm updating a '94 Specialized Hardrock from it's stock triple crank and 7 speed cassette to a 1x with a 9 speed cassette (on a new wheel/hub). The current bottom bracket spindle is 140mm, and the drive side of it longer than the non-drive side (i assume to account for the 3x crank and position for the cassette).

Question is, can I assume putting on a single chainring crank with the current BB spindle will still set the chainline up to be mostly the same because it would in theory just be replacing the location of the middle chainring of the triple? I realize the construction of the new and old will be different so it may not be exact, but close enough to work with the 9 speed and maintain roughly correct chainline? Or will I need to get a new spindle depending on what crank I get? And how does one find a spindle that is longer on one side than the other?

Bonus Q: Currently the BB is a cup & cone. Can this be replaced with something like a Shimano UN55? It's just threads, after all, right?

Appreciate any help!
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Old 04-12-21, 03:10 PM
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We need to know if you're changing the whole crank or just rings. If new crank, what make & model.
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Old 04-12-21, 06:01 PM
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Yes, you can replace your old loose-ball bottom bracket with a Shimano sealed unit. To determine the chainline, measure the distance from the centerline of the frame to the center cog (#5) of your cassette. The bottom bracket spindle length, either total or drive side, to duplicate that chainline up front should be listed in the crank installation instructions.
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Old 04-12-21, 07:40 PM
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Originally Posted by KCT1986 View Post
We need to know if you're changing the whole crank or just rings. If new crank, what make & model.
Complete new crank - not picked out yet though.
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Old 04-12-21, 08:06 PM
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Just use the recommended BB for the new crank.

John

Last edited by 70sSanO; 04-12-21 at 08:09 PM.
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Old 04-12-21, 09:29 PM
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Originally Posted by KCT1986 View Post
We need to know if you're changing the whole crank or just rings. If new crank, what make & model.
Going to go with a Suntour XCM: https://www.srsuntour.us/collections...-t424-44-32-22
But take off the triple chainrings and put on a single RaceFace narrow wide 38t ring.
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Old 04-12-21, 10:08 PM
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The Suntour specifies a 68-113 BB.

Your current BB is probably too long. How are you measuring the 140mm on your current BB. Measurement is usually done to the end of the square portion, not including the exposed threaded portion if this is nut type.

Also need to verify your frame width (68 or 73) before ordering a new BB.

The Suntour specifies a 48 chainline, which should be to the middle ring position. With the ring in this position, you should be within the normal range, (usually, 47-50 for MTB).

The other possible concern is crankarm clearance to the chainstay.
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Old 04-14-21, 05:41 PM
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Originally Posted by KCT1986 View Post
The Suntour specifies a 68-113 BB.

Your current BB is probably too long. How are you measuring the 140mm on your current BB. Measurement is usually done to the end of the square portion, not including the exposed threaded portion if this is nut type.

Also need to verify your frame width (68 or 73) before ordering a new BB.

The Suntour specifies a 48 chainline, which should be to the middle ring position. With the ring in this position, you should be within the normal range, (usually, 47-50 for MTB).

The other possible concern is crankarm clearance to the chainstay.
You are right -I was measuring from the end of the threaded portions. I remeasured and itís 120mm. Still longer on the drive side.
Hereís a pic


Drive side is pointing down in the pic.

So, stay clearance aside, you think using this BB & spindle would be ok? I didnít measure chainline before I disassembled everything. And I obviously havenít put it back together and put the old crank on to measure either.
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Old 04-14-21, 08:52 PM
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Based on a guess of the limited info available, the chainline would probably be aprox. 5mm further out. So, if the Suntour info is correct giving a 48mm chainline with a 113 BB then you'll be at aprox. 53mm with the long side of your current spindle on the DS.

This is based on common 113 BB that has symmetrical or nearly symmetrical sides (don't know what BB Suntour is basing their measurements on), and an 120mm older spindle which is 4-5mm longer on the DS (info pulled from Sheldon Brown's BB data). The link is in the 'sticky' to Sheldon's site and go to BB length article.

The longer chainline by 5mm wouldn't be much of an issue if your riding style/terrain would keep you on the smaller cogs end of the cassette. A 5mm longer chainline would be about 1.5 cogs to the outside.

Don't know the specifics of the Suntour, so can't guess of the effect on the Q distance (thread distance between the pedals).
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Old 04-15-21, 05:01 AM
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I highly recommend changing the bottom bracket to the recommended length. I have a bike I changed to a 1x9 and the chain kept popping off because the bottom bracket was too long.
Also I prefer to have the chainring on the outside of the crankset.
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Old 04-15-21, 09:01 AM
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I really doubt your 120mm spindle will work especially if it is asymetrical and may actually be closer to using 123mm, as far as the DS.

There are a few things to consider. I’m far from an expert on all the ins-n-outs, but you do need to consider the rear dropout width and BB shell width.

Generally, in the 90’s, 68mm is a road width with 130mm dropouts. 73mm is mtb width with 135mm dropouts. In overly simplistic terms, if your dropout with is 135mm the first position cog is out further from the centerline than with a 130mm.

Using a cartridge BB for a 73mm shell should adjust for this and the specified 48mm chainline for the crank. Using a 68x113 BB might move it out more on the DS and give you a 50.5mm chainline. This does make the BB asymetrical in a sense. It is such a small amount it might be moot. Keep in mind the overall 9 speed cassette is wider and pushing the crank out too far does increase the angle on the low cogs.

John

Edit added: Remember, the 113mm spindle has no clue how wide the BB shell is. A cartridge BB is symmetrical and the spindle just sits there. Using the wrong BB for the shell just centers it too far in or too far out.

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Old 04-15-21, 04:25 PM
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Originally Posted by 70sSanO View Post
I really doubt your 120mm spindle will work especially if it is asymetrical and may actually be closer to using 123mm, as far as the DS.

There are a few things to consider. Iím far from an expert on all the ins-n-outs, but you do need to consider the rear dropout width and BB shell width.

Generally, in the 90ís, 68mm is a road width with 130mm dropouts. 73mm is mtb width with 135mm dropouts. In overly simplistic terms, if your dropout with is 135mm the first position cog is out further from the centerline than with a 130mm.

Using a cartridge BB for a 73mm shell should adjust for this and the specified 48mm chainline for the crank. Using a 68x113 BB might move it out more on the DS and give you a 50.5mm chainline. This does make the BB asymetrical in a sense. It is such a small amount it might be moot. Keep in mind the overall 9 speed cassette is wider and pushing the crank out too far does increase the angle on the low cogs.

John

Edit added: Remember, the 113mm spindle has no clue how wide the BB shell is. A cartridge BB is symmetrical and the spindle just sits there. Using the wrong BB for the shell just centers it too far in or too far out.
Thanks for this.

The BB is 68 and the rear width is 135 (see pics). Since my plan is to remove the triple chainrings and put in a single one, itís safe to assume the recommended spindle width might be a tad short since going single chainring makes the whole apparatus smaller so Iíd need maybe 2 more mm of spindle to account for it? Am I thinking about that correctly?



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Old 04-15-21, 05:33 PM
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A lot will depend of which end of the cassette you think you'll use most and if you have a preferred position of the front ring.

With a single ring, using the extreme ends of the cassette will be like cross-chaining and presents the problems that comes with it.

The Suntour info of a 48mm chainline, if stated correct, is to the middle position. If you plan to put the ring in the outer position, add about 5mm.

The freehub/cassette position in relation to the dropout can also slightly effect the acceptable chainline.
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Old 04-15-21, 06:11 PM
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Originally Posted by KCT1986 View Post
A lot will depend of which end of the cassette you think you'll use most and if you have a preferred position of the front ring.

With a single ring, using the extreme ends of the cassette will be like cross-chaining and presents the problems that comes with it.

The Suntour info of a 48mm chainline, if stated correct, is to the middle position. If you plan to put the ring in the outer position, add about 5mm.

The freehub/cassette position in relation to the dropout can also slightly effect the acceptable chainline.
I'd like the chain to hit right in the middle of the cassette, and most likely it will trend toward the smaller more but probably not a lot. I'm building it to be a gravel/trail bike so it's not going for speed but the trails aren't super hilly/mountains, etc. As long as I can get into the bigger cogs without throwing the chain i'll be happy.
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Old 04-15-21, 08:01 PM
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Originally Posted by the sci guy View Post
Thanks for this.

The BB is 68 and the rear width is 135 (see pics). Since my plan is to remove the triple chainrings and put in a single one, itís safe to assume the recommended spindle width might be a tad short since going single chainring makes the whole apparatus smaller so Iíd need maybe 2 more mm of spindle to account for it? Am I thinking about that correctly?



You should be fine with the middle position. Triples are set to hit the middle of the cassette, or very close to it from the middle ring. I donít see any problem running the whole cassette with a 1x.

Donít worry about chain angle with a 9 speed. People are running 1x11 and 1x12, as designed with more severe angles, without issues. Itís funny, I heard about the perils of cross-chaining with a 2x6 decades ago.

John
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Old 04-17-21, 06:11 PM
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for all of you waiting on baited breathe, i ordered a new BB with a 113 spindle. we'll see what happens!
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