Rear chain line calculation

[Nics]

Hi Relja,

Here https://bike.bikegremlin.com/17361/reader-questions-2023/ you assumed that the right end of the cassette (the end of the cassette near the smallest sprocket) ends at the top end of the 5.85mm dimension (see bottom of this drawing: https://bike.bikegremlin.com/wp-content/uploads/2023/09/bitex-hubs-bx312r-freehub-dimensions.jpg).
How did you come to this conclusion? How much uncertainty is in this assumption?

Also, acc to https://forum.customframeforum.com/uploads/short-url/6eXvty1Bcsp8DjoOLUHgbXBzFD0.pdf the "official" chain line for 142 hub spacing is 49mm and for 148 hub spacing it is 52mm.
49mm is quite a bit away from our 41.35mm calculation

Part of the solution to this mystery might be that the "official" chain line might not be at the center of the cassette (as indicated by https://bikerumor.com/wp-content/up...-boos-142-x-12-chainline-spacing-cassette.jpg) but 44mm would still be different from 41.35mm.

 

[BikeGremlin]

Hi,

Hi Relja,

Here https://bike.bikegremlin.com/17361/reader-questions-2023/ you assumed that the right end of the cassette (the end of the cassette near the smallest sprocket) ends at the top end of the 5.85mm dimension (see bottom of this drawing: https://bike.bikegremlin.com/wp-content/uploads/2023/09/bitex-hubs-bx312r-freehub-dimensions.jpg).
How did you come to this conclusion? How much uncertainty is in this assumption?

The 5.85 mm dimension is noted in the hub chart (at the bottom, from the “locknut” to the start of the freehub body).

Also, acc to https://forum.customframeforum.com/uploads/short-url/6eXvty1Bcsp8DjoOLUHgbXBzFD0.pdf the "official" chain line for 142 hub spacing is 49mm and for 148 hub spacing it is 52mm.
49mm is quite a bit away from our 41.35mm calculation

Part of the solution to this mystery might be that the "official" chain line might not be at the center of the cassette (as indicated by https://bikerumor.com/wp-content/up...-boos-142-x-12-chainline-spacing-cassette.jpg) but 44mm would still be different from 41.35mm.

Hmm. As far as I know, the 142 mm thru-axle hubs have the same OLD and chainline as the 135 mm quick-release hubs (the extra 3.5 mm at ends is for secure mounting in the frame’s recess).

Chainline of the hub in the sketch calculation:

• 42.6 - 5.85 is the freehub mechanism’s width (36.75).
• Divided by 2 is the middle of the cassette (18.375).
• Add 5.85 to get the cassette’s mid section distance from the hub’s end (locknut) = 24.225
• OLD is 135.3
• Half of that is the hub’s centreline = 67.65 (from centre to locknut).
• 67.65-24.225 = 43.425 should be the mid-cassette distance from the hub’s centre, i.e. rear chainline.

If I hadn’t accounted for the 5.85 mm distance from the locknut to the freehub’s end, then I would have gotten a 49.275 mm chainline (67.65-18.375). But that would assume that the cassette goes all the way to the locknut (minimum clearance from the frame).
But, it would even then be more accurate to take a half of the cassette’s total width as the basis for calculation, and just subtract that from 67.65.

To be 100% certain, I would need to mount the cassette on the hub and measure.

Add any frame offset to the result, of course.

 

[Nics]

As far as I know, the 142 mm thru-axle hubs have the same OLD and chainline as the 135 mm quick-release hubs (the extra 3.5 mm at ends is for secure mounting in the frame’s recess).

Off topic because it shouldn't matter to the calculation below but to my knowledge OLD signifies where the frame clamps the hub. On a thru axle frame the clamp surface is recessed in the frame (3.5mm exactly?) and these hub clamp surfaces have a distance of 142mm. I guess that the OLD of a 142 hub is actually 142 but that the chain line is not different from a 135 quick release hub whose OLD is 135.

Divided by 2 is the middle of the cassette (18.375).

Thereby you assume that cassette center is at the freehub center. Given that the cassette is 4.15mm (40.9 - 36.75) wider than the freehub you assume that the cassette overhangs 2.075mm on the left end of the freehub and overhangs 2.075mm on the right end of the freehub. Do we know this to be true? I don't mean that it would need to be exact to a tenth of a milliliter but could it also be a 0.5mm overhang on the left and a 3.65mm overhang on the right?

Note that in this calculation https://bike.bikegremlin.com/17361/reader-questions-2023/ you made a different assumption: that the cassette has zero overhang on the right, i.e. the cassette ends where the freehub ends (i.e. at the top end of the 5.85 dimension in the Bitex hub drawing).

Note that with my descriptions above (...left and right end of the hub...) I always assumed we look at the hub from above.

So the crucial question is: where on the freehub roughly does the cassette sit?

 

[BikeGremlin]

I’ve always measured that with the cassette mounted. Don’t know exactly otherwise. It’s usually about 1 mm.

 

[BikeGremlin]

Forgot:
The “locknut” width of 135.3 as noted in the graph is what sits against the inner side of the dropouts. The rest is within the frame recess.

 

[BikeGremlin]

Using a smartphone with bad Net connection. Another 10 days before I’m back to civilization.

 

[Nics]

I’ve always measured that with the cassette mounted. Don’t know exactly otherwise. It’s usually about 1 mm.

What dimension is typically 1mm?

 

[BikeGremlin]

What dimension is typically 1mm?

Cassette’s end overhang from the freehub body’s end.

 

[Nics]

Cassette’s end overhang from the freehub body’s end.

The following clarification is regarding your 1mm overhang dimension. Assume we are looking onto the hub from above: are you referring to the overhang of the cassette over the end of the freehub on the right or on the left end of the freehub?

I.e. in the attached picture: overhang of the cassette over the end of the freehub at the green or the blue arrow?

 

[BikeGremlin]

The following clarification is regarding your 1mm overhang dimension. Assume we are looking onto the hub from above: are you referring to the overhang of the cassette over the end of the freehub on the right or on the left end of the freehub?

I.e. in the attached picture: overhang of the cassette over the end of the freehub at the green or the blue arrow?

The green arrow. Outer part.

 

[Nics]

The green arrow. Outer part.

That would bring the chain line to 42.35 = (43.425 - (2.075-1))

2.075 is the overhang of the right end of the cassette beyond the right end of the freehub body which you had assumed before here https://www.bikegremlin.net/threads/rear-chain-line-calculation.346/post-1495

 

[BikeGremlin]

That would bring the chain line to 42.35 = (43.425 - (2.075-1))

2.075 is the overhang of the right end of the cassette beyond the right end of the freehub body which you had assumed before here https://www.bikegremlin.net/threads/rear-chain-line-calculation.346/post-1495

43.425 is the calculation in this thread, assuming zero overhang. With a 2.075 overhang towards the outside (tight side, away from the centreline), the chainline would be 2.075 larger.

That is 43.425 + 2.075 = 45.5
If my math is correct.

You can see my article explaining what chainline is and how it is measured and calculated for more details:

https://bike.bikegremlin.com/1755/bicycle-chainline/

 

[Nics]

43.425 is the calculation in this thread, assuming zero overhang. With a 2.075 overhang towards the outside (tight side, away from the centreline), the chainline would be 2.075 larger.

That is 43.425 + 2.075 = 45.5
If my math is correct.

You can see my article explaining what chainline is and how it is measured and calculated for more details:

https://bike.bikegremlin.com/1755/bicycle-chainline/

Let's try to convince each other and thereby find the truth

I'm referring to post number 2 in this thread:

• 42.6 - 5.85 is the freehub mechanism’s width (36.75).
• Divided by 2 is the middle of the cassette (18.375).

Given these two bullet points you assume that the center of the cassette is at the center of the freehub, correct?

Given that the cassette is 4.15mm wider than the freehub (40.9-36.75) and given your assumption that the center of the cassette is at the center of the freehub it follows that the cassette overhangs the freehub by 4.15/2 = 2.075 mm on each side of the freehub.

 

[BikeGremlin]

Cassettes that are wider (like 11-speed MTB cassettes) will overhang the freehub towards the inner side (towards the spokes).

Outer overhang of Hyperglide cassettes should be just enough for the locknut to compress them without (the locknut) bottoming out against the freehub.

Too much overhang (like 2 mm or more) can result withstripped locknut (or freehub) threads because of only one thread engaging.