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Quick question about chainline and bottom bracket length

Old 08-28-21, 12:39 AM
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Quick question about chainline and bottom bracket length

I'm planning to purchase some cranks, "Dimension Cyclocross". Linky here: https://www.treefortbikes.com/Dimens...t-110-74mm-BCD

Anyways, my intention is to use it as a triple on a bike with a 68mm bb shell and 126mm OLD. If it helps, the specific bike model in question is a 1982 Miyata 1200. I'm assuming the cranks were made for a bike with a 73mm bb shell, and 130mm OLD.

According to the website linked, they recommend 113mm or 118mm as spindle lengths. I'm assuming that's for if they're set up as a triple or a double, respectively.

Given the differences between my bike and the bike that the cranks were made for...do I purchase cranks longer or shorter than 113mm? I feel like the smaller bottom bracket would mean I would want a longer spindle, but the shorter OLD would mean I would want a shorter spindle...leaving us where we started. 113mm spindle length for a triple. Am I correct? Or way off?

Thanks.



P.S. Any verdict on the Shimano un300 as a replacement to the un55? Or should I look elsewhere?
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Old 08-28-21, 05:10 AM
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Originally Posted by Piff View Post
I'm assuming the cranks were made for a bike with a 73mm bb shell....
I didn't see anything on the website to indicate this. They describe intended use as "cyclocross/road."
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Old 08-28-21, 08:15 AM
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Originally Posted by Piff View Post
I'm planning to purchase some cranks, "Dimension Cyclocross". Linky here: https://www.treefortbikes.com/Dimens...t-110-74mm-BCD

Anyways, my intention is to use it as a triple on a bike with a 68mm bb shell and 126mm OLD. If it helps, the specific bike model in question is a 1982 Miyata 1200. I'm assuming the cranks were made for a bike with a 73mm bb shell, and 130mm OLD.

According to the website linked, they recommend 113mm or 118mm as spindle lengths. I'm assuming that's for if they're set up as a triple or a double, respectively.

Given the differences between my bike and the bike that the cranks were made for...do I purchase cranks longer or shorter than 113mm? I feel like the smaller bottom bracket would mean I would want a longer spindle, but the shorter OLD would mean I would want a shorter spindle...leaving us where we started. 113mm spindle length for a triple. Am I correct? Or way off?

Thanks.



P.S. Any verdict on the Shimano un300 as a replacement to the un55? Or should I look elsewhere?
What a coincidence! I just went through almost the same analysis, except I was trying to figure out the bottom bracket spindle length for using these cranks on a hybrid bike with a 135 mm OLD for a 1X conversion.

Converting Trek FX2 to 1x - Bike Forums

The crank arms seem to be designed for the current standard road bike having a 130 mm OLD:

110 Crankset | Dimension Bike Products

So we are working on essentially the same problem but going in opposite directions. From what I have learned online regarding bottom bracket spindle length vs. chain line, the consensus seems to be: (a) the actual results may not work out exactly as calculated, so expect some trial and error, and (b) you have about a +/- 2mm margin of error, maybe less in your case if you are aiming for a perfectly setup triple crank. You have a tougher task than I do because you have to account for both (1) the movement range of the front derailleur, and (2) spindle length vs. chain line, whereas I only have to account for the latter.

I have also researched whether the readily available BB-UN300 is the same quality wise as the no longer available in many sizes BB-UN55, and only found one article theorizing that the latter is better:

BB-UN55 vs BB-UN300; which is the better square taper bottom bracket? - restoration.bike

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Old 08-28-21, 08:20 AM
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Originally Posted by shelbyfv View Post
I didn't see anything on the website to indicate this. They describe intended use as "cyclocross/road."
The Dimension Cyclocross crank arms are actually Andel RSC6 triple crank arms; the inside of each arm is stamped as such. I have read about others installing these crank arms on both 68 mm and 73 mm JIS square taper bottom brackets.
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Old 08-28-21, 08:22 AM
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Other than a response to a question on the Treefort site, I don’t see anything about being a triple. The Dimension site shows it as a double; it looks like the same one.

John
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Old 08-28-21, 08:27 AM
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Originally Posted by 70sSanO View Post
Other than a response to a question on the Treefort site, I donít see anything about being a triple. The Dimension site shows it as a double; it looks like the same one.
That is true. Dimension is apparently marketing an Andel RSC6 triple crankset (110/74 BCD) as a cyclocross double, presumably by omitting the 74 BCD granny chain ring.
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Old 08-28-21, 08:36 AM
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The dimension site shows a CR2330 and CR2331 as a 110/74 double.

I’d contact Treefort and find out if it has a middle ring position.

John
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Old 08-28-21, 08:45 AM
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Originally Posted by 70sSanO View Post
The dimension site shows a CR2330 and CR2331 as a 110/74 double.

I’d contact Treefort and find out if it has a middle ring position.

John
I have ordered and received the same crank arms from another vendor. The drive side crank arm has a 110 BCD 5 arm carrier for installing the outer and middle chain rings on either side thereof, and additional "nubs" on the back of the carrier onto which a 74 BCD chain ring can be bolted thereafter. I would take a picture to settle this debate, but OP is not even concerned about this debate.
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Old 08-28-21, 09:29 AM
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There is no debate. I only brought up what was on Dimension’s website. I honestly don’t care.

If you already have the same crank, why not just measure it to get the chainline? It doesn’t matter if it is 126mm or 130mm dropouts, you just have to make the adjustment for the 68 road/73 “traditional” mtb.

John
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Old 08-28-21, 09:38 AM
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It really doesn't matter what the OLD of the bike is, the chain rings just have to clear the chain stays. The crank arms don't determine this, you have to get the right bb. A bike shop should be able to help you with that.
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Old 08-28-21, 09:38 AM
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Originally Posted by 70sSanO View Post
There is no debate. I only brought up what was on Dimensionís website. I honestly donít care.
I was merely using the word "debate" as a shorthand to describe our back-and-forth above; I am not saying that there is anything wrong with having such a "debate."

Originally Posted by 70sSanO View Post
If you already have the same crank, why not just measure it to get the chainline? It doesnít matter if it is 126mm or 130mm dropouts, you just have to make the adjustment for the 68 road/73 ďtraditionalĒ mtb.
Because the chain line does not only depend on the crank arm, but also on bottom bracket width and spindle length. So far, the only things Piff (the OP) and I have in common are the bottom bracket width and these crank arms.
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Old 08-28-21, 09:43 AM
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Originally Posted by unterhausen View Post
It really doesn't matter what the OLD of the bike is, the chain rings just have to clear the chain stays. The crank arms don't determine this, you have to get the right bb. A bike shop should be able to help you with that.
But in addition to just clearing the chain stays, the middle chain ring of the crank set should ideally line up with the center of the cassette, for smoother shifting overall and to minimize the chain angle in any cross-chaining gear combinations.
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Old 08-28-21, 10:04 AM
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If you have set up a 1x with 68mm BB and 135mm dropouts, what BB spindle length did you use?

Just take 5mm off that and it should work for the OP.

John
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Old 08-28-21, 10:27 AM
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Originally Posted by 70sSanO View Post
If you have set up a 1x with 68mm BB and 135mm dropouts, what BB spindle length did you use?

Just take 5mm off that and it should work for the OP.
1. I don't know the spindle length on the BB of my bike. I am about to install the right crank arm, and if the chain line looks bad, remove the BB to look for and/or measure the spindle length, and order a new BB accordingly. (Depending on chain stay clearance, I may even order one with a shorter spindle so I can use the outer chain ring position rather than the middle chain ring position.)

2. Even if I figure out all of the above, I am not sure this is applicable to the OP. He has different OLD and potentially different cassette width (I am on a Shimano 8 speed). But I will report back so he can hopefully use the information.
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Old 08-28-21, 11:16 AM
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I’m sure your findings will be helpful.

John
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Old 08-28-21, 11:24 AM
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Originally Posted by shelbyfv View Post
I didn't see anything on the website to indicate this. They describe intended use as "cyclocross/road."
Perhaps I'm wrong, but I was under the assumption no one was mass producing frames with 68mm bb shells anymore, since the 90s or so? I thought most everything had shifted to 73mm for square taper crank-destined frames these days.

edit...I am wrong, 68mm is still the norm for road. But more for external bearing bottom brackets and other newer standards? Rather than bikes outfitted with internal bearing bottom brackets + square taper.

Originally Posted by SoSmellyAir View Post

I have also researched whether the readily available BB-UN300 is the same quality wise as the no longer available in many sizes BB-UN55, and only found one article theorizing that the latter is better

BB-UN55 vs BB-UN300; which is the better square taper bottom bracket? - restoration.bike
Hmm, ty. I've seen others also recommend the Tange Sekei bb.

Originally Posted by SoSmellyAir View Post
The Dimension Cyclocross crank arms are actually Andel RSC6 triple crank arms.
Fantastic, this should help researching the spindle length!

Originally Posted by SoSmellyAir View Post
but OP is not even concerned about this debate.
Yes, I did some googling prior to posting after getting momentarily confused by Dimensions' own specs.

Originally Posted by SoSmellyAir View Post
1. I don't know the spindle length on the BB of my bike. I am about to install the right crank arm, and if the chain line looks bad, remove the BB to look for and/or measure the spindle length, and order a new BB accordingly. (Depending on chain stay clearance, I may even order one with a shorter spindle so I can use the outer chain ring position rather than the middle chain ring position.)

2. Even if I figure out all of the above, I am not sure this is applicable to the OP. He has different OLD and potentially different cassette width (I am on a Shimano 8 speed). But I will report back so he can hopefully use the information.
It's a 6 speed Shimano 600 freewheel on a Shimano 6207 hub, 126 OLD. Thank you for offering to tell me how your experimentation goes, greatly appreciated I'm not too fussed on getting a perfect chainline given how flexible modern chains are, but it would be good to be in the ballpark. And, I think I may have found our answer. Surly spec'd their 2014 disc trucker with this crank set up as a triple, and they used a 118mm Shimano un55 bottom bracket. The 2014 Disc Trucker has 73mm bottom bracket shell, and 135mm OLD, and had a 9speed shimano cassette.

SO, since my bike has a 126mm OLD, and a 68mm shell we have: 118 - 9 + 5 = 114mm. Or, in other words, pretty damn close to the 113mm spindle length recommended by Dimensions if using the cranks as a triple on a 73mm bb shell/130mm OLD bike frame. Now, if only some vendor sold the silver version in the states...Oh Surly and their ridiculous fascination with black components.

Last edited by Piff; 08-28-21 at 11:38 AM.
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Old 08-28-21, 11:35 AM
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The Treefort site listed two spindle lengths: 113 and 118. These are for the 68 and 73mm BB shells respectively. Wider spindle for wider BB. Narrower spindle for narrower BB. Itís that simple.
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Old 08-28-21, 11:49 AM
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Originally Posted by Piff View Post
Now, if only some vendor sold the silver version in the states...Oh Surly and their ridiculous fascination with black components.
I am not following; you are looking for the silver version of what?
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Old 08-28-21, 11:50 AM
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Originally Posted by icemilkcoffee View Post
The Treefort site listed two spindle lengths: 113 and 118. These are for the 68 and 73mm BB shells respectively. Wider spindle for wider BB. Narrower spindle for narrower BB. Itís that simple.
I see; so nothing to do with using the cranks as double or triple.
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Old 08-28-21, 11:54 AM
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Originally Posted by Piff View Post
Perhaps I'm wrong, but I was under the assumption no one was mass producing frames with 68mm bb shells anymore, since the 90s or so? I thought most everything had shifted to 73mm for square taper crank-destined frames these days.
edit...I am wrong, 68mm is still the norm for road. But more for external bearing bottom brackets and other newer standards? Rather than bikes outfitted with internal bearing bottom brackets + square taper.
the vast majority of aftermarket cranks sold nowadays are external bearings and not square taper, but square taper cartridge bb are still available. It's pretty common for low-end hybrids to have a square taper with a cartridge bb.

I you were really worried about the chainline, you could get something like a Phil Wood bb with an adjustable chainline.
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Old 08-28-21, 11:56 AM
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Originally Posted by icemilkcoffee View Post
The Treefort site listed two spindle lengths: 113 and 118. These are for the 68 and 73mm BB shells respectively. Wider spindle for wider BB. Narrower spindle for narrower BB. Itís that simple.
If that's the case, shouldn't I use a 109mm bottom bracket since my bike has a 126 OLD rather than a 130 OLD?
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Old 08-28-21, 11:58 AM
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Originally Posted by SoSmellyAir View Post
I am not following; you are looking for the silver version of what?
A silver version of the Dimension crank. On Andel's website they show a silver version as well.

Originally Posted by unterhausen View Post

I you were really worried about the chainline, you could get something like a Phil Wood bb with an adjustable chainline.
If I was going to buy a Phil Wood bb I don't think I'd bother with some cheapy $60 crank

Sorry for the double post!
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Old 08-28-21, 01:38 PM
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Originally Posted by Piff View Post
A silver version of the Dimension crank. On Andel's website they show a silver version as well.
Got it. I am partial to silver rings on black crank arms:

Bike Forums - View Single Post - All Silver Chain Rings on Black Crank Arms
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Old 08-28-21, 02:03 PM
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While it may seem hard to believe, it is possible to buy a square taper BB in the correct shell width, so you don’t have to buy a 118mm and have more spindle or 113mm and have less spindle out the NDS just to fudge a mis-matched BB.

While this might be a SWAG, I would think a 113mm spindle might work for a 126mm/130mm and the 118mm for a 135mm with either a 68mm or 73mm shell.

John
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Old 08-28-21, 02:43 PM
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Originally Posted by Piff View Post
If that's the case, shouldn't I use a 109mm bottom bracket since my bike has a 126 OLD rather than a 130 OLD?
The 126 or 130 OLD rear hub, has nothing to do with the chainline.
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