In addition to questions via the website comments, I get a lot of questions via the consulting pages, email, social network messengers etc.
To enable the answers to help everyone with a similar problem/dilemma/question, I’ve decided to copy/paste the questions here (anonymously, without any private information), along with my answers.
Table Of Contents (T.O.C.) – questions:
Chainline questions for a custom-designed frame
via the Priority support
I’m a Patreon supporter but I like this system better because it solves the incentive problem. But I still hope that that my question and your answer will show in public so everyone can benefit.
My primary question, which is a little urgent, is: what is the cause for the discrepancy in chainline between 11 speed road and mtb cassettes (discrepancy = 52-43=9mm) given that the cassette width difference is only 1.6mm?
Here is the quote from https://bike.bikegremlin.com/3573/bicycle-cassette-rear-chainrings-standards/ :
“Shimano, SRAM 11-speed road ** 3.74 ** 1.6 2.14 39
Shimano, SRAM 11-speed MTB ** 3.9 ** 1.6 2.3 40.6″
Context: I am building/designing a custom bike and would like to use the “Cannondale AI concept” which moves the right chainstay 3mm or 6mm to the right which allows the following benefit: less assymetrically spoked wheels which should be more durable. Illustration of the Cannondale AI concept: https://www.lightbicycle.com/light-bicycle/purchase/cannondale-ai-offset-system-vs-traditional-wheel.jpg
The rest of my component choices are:
- Shimano XT 11 speed 11-40 casette.
- Any suitable 2x front derailleur (e.g. 105 or GRX).
- A power2max, Rotor based, power meter and crankset (https://www.power2max.com/en/product/ngeco-gravel-rotor-aldhu3d-powermeter-set/) which allows to either use a gravel (46mm chainline) or MTB (152mm chainline) specific axle. I know that this is a bit of an oddball but has been confirmed to my by Power2max.
- A Bitex Bx312R rear hub (142mm wide): https://bitexhubs.co.uk/product/bx312r-straight-pull-centerlock-rear/
- Light bicycle WR45 wheelset which allows Standard (assymetrical) or Cannondale AI specific (3mm or 6mm offset from the standard) spoke lacing: https://www.lightbicycle.com/light-bicycle/purchase/cannondale-ai-offset-system-vs-traditional-wheel.jpg
My goals are:
- Roughly even chainline between the rear and the front.
- Front chainline: 152mm and 46mm both have their pros and cons. I don’t have much of a preference.
- Rear wheel that is as durable as possible. Therefore I prefer 6mm offset compared to 3mm offset and I prefer 3mm offset to standard/assymetrical spoke lacing.
- The component choices above are constraints.
- Because it’s a custom frame there are no constraints regarding the frame, i.e. I can make the chainstays assymetrical, i.e. move the right chainstay to the right.
Do you think it’s possible to have a 6mm offset, a 152mm chainline front and rear, with the Power2max/Rotor crankset?
Let me rephrase my last question: Do you think it’s possible to have a 6mm offset, a roughly even chainline between front and rear, with the Power2max/Rotor crankset?
Thank you very much for your support.
I think that a public answer is a great idea. 🙂
Yes, I think 6 mm will work. 🙂
The full answer:
There are two questions here:
1. The Shimano MTB vs Road cassette width
(I’ll discuss that briefly before answering your questions in the “part 2” below)
2. Chainline dilemma
1. The cassette width
11-speed MTB cassettes are designed to “overhang” the hub’s right flange – because they have very large largest few sprockets. That lets them take less space on the hub. 11-speed road cassettes often have the largest sprocket of “only” 28 teeth, so it needs to fit before the hub’s flange starts.
That is why the 11-speed road cassettes need a wider freehub (and why you need to add a 1.85 mm wide spacer to mount an MTB cassette on an 11-speed road hub).
I’ve also measured some 0.9 mm difference in total cassette width when comparing Shimano 11-speed road, and MTB cassettes. However, that does not seem to affect sprocket spacing or shifting, so the cassettes are compatible (though most 11-speed road derailleurs won’t be able to tackle sprockets larger than 34 teeth without adding a RD hanger adapter for more clearance).
See this question and my follow-up comments for more details:
2. The chainline
Discrepancies between the road (and “old”), gravel, and MTB chainlines are mostly due to the super-wide rear hubs (for more details, see the section on boost hubs).
You opted for a 142mm rear hub. That is effectively a standard, old 135mm MTB freehub, with a thru-axle mounting system (OLD is still set at 135 mm so it doesn’t provide a wider rear chainline).
The safest way to be sure about the (rear) chainline is to measure it. Ideally with a cassette mounted (to measure it exactly as it sits on the hub) – because some cassettes will sit a mm “higher” or lower (depends on the cassette model/type).
- What is a bicycle chainline and how to measure it?
- Video demonstration of chainline measuring
With that possible source of error in mind, I will calculate the chainline using the available info on the manufacturer’s website for your freehub of choice (copying the picture here for easier understanding):
- OLD is 135.3 mm, and half of that is 67.65 mm
- The distance from where the hub’s right end to where the cassette starts is 5.85 mm
(the bottom section of the picture above)
- Half of a Shimano MTB cassette’s total width is (based on my measurements) 40.9 / 2 which is 20.45 mm
- That means that the middle of the cassette (i.e. the rear chainline) sits at:
67.65 – (5.85 + 20.45) =
from the frame’s centre-line
Of course, that goes for a frame that is symmetrical. For an offset frame, we need to add the frame’s offset to that result.
With a 6 mm offset frame, the rear chainline would be about 47,35 mm (or a bit less – if the cassette sits a bit deeper on the freehub when mounted). That should work fine with a 46 mm front chainline. I’m not so sure about a 52 mm front chainline though. Especially if you like riding in the big ring on flats.
My idea with Patreon was to just let people support my work, without any conditions or strings (like having to pay in order to get information, advice or help).
I grew up in relative poverty and know very well what it’s like to not have one single dollar, literally.
However, it doesn’t feel right for you to pay, while you are already supporting me via Patreon.
So, I am considering adding the “Priority support” benefit for all my Patreon supporters. It seems more fair that way.
Forgot to answer this one:
Cannondale AI concept looks like a “marketing-friendly” and fancy way to say “a 6 mm offset frame.” 🙂