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? |
Originally Posted by Piff
(Post 22204211)
I'm assuming the cranks were made for a bike with a 73mm bb shell....
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Originally Posted by Piff
(Post 22204211)
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? 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 |
Originally Posted by shelbyfv
(Post 22204302)
I didn't see anything on the website to indicate this. They describe intended use as "cyclocross/road."
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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 |
Originally Posted by 70sSanO
(Post 22204453)
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.
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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 |
Originally Posted by 70sSanO
(Post 22204471)
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 |
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 |
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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Originally Posted by 70sSanO
(Post 22204531)
There is no debate. I only brought up what was on Dimension’s website. I honestly don’t care.
Originally Posted by 70sSanO
(Post 22204531)
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.
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Originally Posted by unterhausen
(Post 22204540)
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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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 |
Originally Posted by 70sSanO
(Post 22204574)
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. 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. |
I’m sure your findings will be helpful.
John |
Originally Posted by shelbyfv
(Post 22204302)
I didn't see anything on the website to indicate this. They describe intended use as "cyclocross/road."
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
(Post 22204444)
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
Originally Posted by SoSmellyAir
(Post 22204451)
The Dimension Cyclocross crank arms are actually Andel RSC6 triple crank arms.
Originally Posted by SoSmellyAir
(Post 22204477)
but OP is not even concerned about this debate.
Originally Posted by SoSmellyAir
(Post 22204611)
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. 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. |
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. It’s that simple.
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Originally Posted by Piff
(Post 22204687)
Now, if only some vendor sold the silver version in the states...Oh Surly and their ridiculous fascination with black components.
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Originally Posted by icemilkcoffee
(Post 22204711)
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. It’s that simple.
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Originally Posted by Piff
(Post 22204687)
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. I you were really worried about the chainline, you could get something like a Phil Wood bb with an adjustable chainline. |
Originally Posted by icemilkcoffee
(Post 22204711)
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. It’s that simple.
|
Originally Posted by SoSmellyAir
(Post 22204735)
I am not following; you are looking for the silver version of what?
Originally Posted by unterhausen
(Post 22204743)
I you were really worried about the chainline, you could get something like a Phil Wood bb with an adjustable chainline. Sorry for the double post! |
Originally Posted by Piff
(Post 22204748)
A silver version of the Dimension crank. On Andel's website they show a silver version as well.
Bike Forums - View Single Post - All Silver Chain Rings on Black Crank Arms |
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 |
Originally Posted by Piff
(Post 22204746)
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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