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Need help with chainline/bottom bracket calculations

Old 07-03-22, 04:53 AM
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alexk_il
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Need help with chainline/bottom bracket calculations

I'm currently riding a 10yo Boardman hybrid bike with 10 speed 11-32 cassette. Will replace the cassette with Shimano M8000 11-42t 11speed cassette.

As this forum pointed out to me, the new cassette will fit my 10 speed hub because the lower gear has a recessed wide opening on the back:
​​​​​​
10 speed to 11 speed hub upgrade

The bike currently has 44mm chainline before the swap, I assume the center of the new cassette would be closer to the rim. Is this correct? If yes, by how much?

I'm replacing the bottom bracket too, might be able to compensate by getting the BB in a different size.
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Old 07-03-22, 06:51 AM
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Bicycle cassette (rear chainrings) standards | BikeGremlin

You plan to go from a 10 speed cassette (37.2 mm wide) to an 11 speed Shimano MTB cassette (40.6 mm wide), a difference of 3.4 mm in width.

In theory, if the outer edges of both cassettes (as installed) are in the same place, the center of the new cassette is 1.7 mm closer to the center of the bike.

In practice, this is a very small difference, and you may not need to move the chain ring(s) inward at all, especially if the 42T cog is just a bail out gear which sees only occasional use. What rear derailleur, bottom bracket, crank, and chain ring(s) are on this bike?
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Old 07-03-22, 09:46 AM
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Bill Kapaun
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Originally Posted by alexk_il View Post
I'm currently riding a 10yo Boardman hybrid bike with 10 speed 11-32 cassette. Will replace the cassette with Shimano M8000 11-42t 11speed cassette.

As this forum pointed out to me, the new cassette will fit my 10 speed hub because the lower gear has a recessed wide opening on the back:
​​​​​​
10 speed to 11 speed hub upgrade

The bike currently has 44mm chainline before the swap, I assume the center of the new cassette would be closer to the rim. Is this correct? If yes, by how much?

I'm replacing the bottom bracket too, might be able to compensate by getting the BB in a different size.
Let's face it. Unless you ride on only 1 cog per ring, your chain line is always going to be off on the other cogs. Sometimes way off.
Don't fret a couple mm.
I intentionally set my chain line a bit wide because I tend to use the outer cogs more.
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Old 07-03-22, 10:21 AM
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SoSmellyAir
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Originally Posted by Bill Kapaun View Post
Let's face it. Unless you ride on only 1 cog per ring, your chain line is always going to be off on the other cogs. Sometimes way off.
Don't fret a couple mm.
I intentionally set my chain line a bit wide because I tend to use the outer cogs more.
One thing I learned after doing a 3x to 1x conversion -- if I were to ever do another -- is to have the chain line slightly inward of the center of the cassette, even if the biggest and smallest cogs are both used the same amount. As you point out, one's chain is almost always at an angle. Since chain tension, and thus noise, increases with cog size, it makes sense (at least to me) to move the chain line slightly inward to reduce the angle to the larger cogs.
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Old 07-03-22, 03:09 PM
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alexk_il
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Originally Posted by SoSmellyAir View Post
Bicycle cassette (rear chainrings) standards | BikeGremlin

You plan to go from a 10 speed cassette (37.2 mm wide) to an 11 speed Shimano MTB cassette (40.6 mm wide), a difference of 3.4 mm in width.

In theory, if the outer edges of both cassettes (as installed) are in the same place, the center of the new cassette is 1.7 mm closer to the center of the bike.

In practice, this is a very small difference, and you may not need to move the chain ring(s) inward at all, especially if the 42T cog is just a bail out gear which sees only occasional use. What rear derailleur, bottom bracket, crank, and chain ring(s) are on this bike?
I have new rebranded Sugino 42/26t cranks to replace my old BB30 FSA 50/34t cranks.

I am planning to install FSA BB30 to English threaded adapter. Then I will add a 68mm square JIS BB for these cranks. Since BBs come in different axle lengths I can just get the correct size.

Here is my math.

​My bike has chainline of 44mm, with a new cassette I will need 44-1.7=~42mm.

According to the shop, my new cranks have 43.5mm chainline with 68x116mm bottom bracket. A standard Shimano UN300 can be bought in 113mm. This should bring the chainline down to my target of 42mm. Plus/minus.

Did I get this right?

Last edited by alexk_il; 07-03-22 at 04:50 PM.
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Old 07-03-22, 04:53 PM
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Originally Posted by alexk_il View Post
I have new rebranded Sugino 42/26t cranks. My bike has BB30, I am planning to install FSA BB30 to standard threaded adapter. I will need a 68mm square JIS BB for these cranks, these come in different lengths, so I am in position to just chose the right size.

Here is my math.

​My bike has chainline of 44mm, with a new cassette I will need 44-1.7=~42mm.

According to the shop, my new cranks have 43.5mm chainline with 68x116mm bottom bracket. A standard Shimano UN300 can be bought in 113mm. This should bring the chainline down to required 42mm.

Did I get this right?
Yes, your math checks out.

I highly recommend you spend $15 more for a Made in Japan Tange Seiki LN-3922 bottom bracket:

Tange LN-3922 Bottom Bracket | (benscycle.com)

Tange LN-3922 Square Taper Bottom Bracket The Bikesmiths
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Old 07-04-22, 05:24 AM
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alexk_il
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Originally Posted by SoSmellyAir View Post
Yes, your math checks out.

I highly recommend you spend $15 more for a Made in Japan Tange Seiki LN-3922 bottom bracket:

Tange LN-3922 Bottom Bracket | (benscycle.com)

Tange LN-3922 Square Taper Bottom Bracket – The Bikesmiths

Thanks, already discovered that Tange is behind Shimano, IRD and others, definitely going to get Tange instead. We also have Tange LN-7922 in the UK for a few extra pounds, what's the difference? I don't care about the weight, Is 7922 it worth to upgrade to?

Last edited by alexk_il; 07-04-22 at 05:32 AM.
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Old 07-04-22, 11:08 AM
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Originally Posted by alexk_il View Post
Thanks, already discovered that Tange is behind Shimano, IRD and others, definitely going to get Tange instead.
Great!

Originally Posted by alexk_il View Post
We also have Tange LN-7922 in the UK for a few extra pounds, what's the difference? I don't care about the weight, Is 7922 it worth to upgrade to?
Here the USA, the LN-7922 is more than double of the LN-3922 and was not available with the spindle length for my 1x conversion. The main differences seem to be:

LN-7922: ~$70, hollow chromoly steel spindle, aluminum cups.
LN-3922: ~$30, solid boron steel spindle, stainless steel cups.

But you don't care about the slight weight difference. I suppose the metal alloys of the LN-7922 might be slightly more resistant to rust?
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