Hi,
I am new to all this crankset talk, but been trying to understand how it all fits together.
I'd like to replace the existing Stout 1x, forged alloy (which is broken), to a Shimano FC-M5100-1 (EFCM51001EXA0) Deore, 30t, 175mm and BB-MT501 Bottom Bracket. Rockhopper Elite BB is 73mm - so good with Bottom Bracket part.
All was looking fairly straight forward, until I read about chainline. Unfortunately I cannot find any information about what the 2021 Rockhopper Elite chainline is with existing configuration? Shimano clear state that the FC-M5100-1 chainline is 52mm,
The SHIMANO DEORE FC-M5100-1 mountain bike crank delivers precise and reliable shifting for 1x10 or 1x11-speed drivetrains.
bike.shimano.com
Also states that it will support 135 mm, 142 mm and 148 mm O.L.D. Frames.
My question is, can I simply replace my crankset without stuffing around with spaces on the chainring?
Hi and welcome to the BikeGremlin forum,
Short answer:
It should work fine.
Explanation:
There are several things to consider here:
- Does the Shimano FC-M5100-1 crank fit bicycles with 73 mm wide bottom bracket shell?
As far as I know, it does - and that is OK.
- Does the Shimano BB-MT501 fit 73 mm BB shell?
Yes, that is good to.
With that out of the way, let's see about the chainline.
Modern shimano MTB Shimano chainlines are usually:
- 50 mm for triple cranks.
- 52 mm for double and single speed cranks.
The front chainline is how far out the mid-section of all the chainrings is from the bicycle's centre-line.
With double and triple cranks, chainrings are spaced at about 5 mm from each other (perhaps about 6 mm on 7 and 8 speed cranks). So:
- With triple cranks, when the middle chainring is at 50 mm, the outer (largest) as at 55 mm, and the inner (smallest) at 45 mm.
- With double cranks, when the chainline is 52 mm (that is the mid section between the two chainrings on a double), the inner chainring is about 2.5 mm inward at 49.5 mm, and the outer chainring is 2.5 mm outward, at 54.5 mm.
Now, for the 1x cranks, there is no chainline adjustment via shifting (shifting to the smaller front chainring when using the inner, larger cassette sprockets, and shifting to the larger chainring when using the outer, smaller cassette sprockets). So, it is a compromise, and you get a pretty awkward (bad) chain angle when using either largest two or smallest two cassette sprockets.
Another peculiarity is that 1x systems are usually paired with pretty large cassettes (40+ teeth on the largest sprocket) to provide both low and high enough gearing with only one front chainring. This results in chain rubbing against the largest rear sprocket when on the second largest rear sprocket if the front chainring doesn't sit far enough out.
That is why 1x drivetrains usually use a bit larger chainline for their only chainring (~ 52 mm), instead of aiming for the 50 mm.
Is it bad for the chain and chainrings when climbing on the largest rear sprocket and putting out a lot of torque? Yes it is. Increased component wear (and cost of the huge cassettes) is a downside of 1x drivetrains (
1x drivetrain pros and cons). That is why people invented front derailleurs and multiple front chainrings over half a century ago.
Having said that, modern chains are quite flexible sideways, so they can take that without breaking. Just increased cost and wear. That's good for sales. Cyclists, on the other hand, are happy for not having to figure out
how to shift gears with multiple front chainrings, while the extra cost is not that problematic for many.
The cynic in me says the whole modern 1x (as 1x did exist decades ago before getting replaced with doubles and triples) was started by the marketing team of the company that just couldn't make a decent front derailleur, and was desperate to gain an edge over Shimano, but nonetheless, 1x took off, people like it, it is modern, trendy, and is here to stay (until companies "re-invent" the good old triple when they go looking for something new, and not compatible with the existing stuff
).
Edit:
I've updated my
bicycle chain-line article with a list of common chain line values and the above-given 1x chainline explanation.
Relja
