Hi,
I'm going to modify
this 3x8 MTB to 1x10speed system. For further modification by adding a rear hub motor in future, I would like now to put as much larger narrow-wide chainring. Currently BB 68mm tapered will be replaced by BB-MT501 Hollowtech II. No front derailleur. I will do some tweaks with the BB spacers and choose the right size of the chainring to keep ideal chain line.
Can you please advise what is the required minimum clearance between chainring and chain stay? On some bikes I've noted a scratched chain stay thus factory mounted parts with about 10mm of space between chain stay and chainring. Are those scratches made by chain only?
Thank you and all the best,
Metod
Hi Metod,
That's a great, reasonable question that I don't remember having answered online before. It's good that you're already also thinking about chainline (not just chainring clearance).
To help others and myself find this answer more easily in the future, I will add this heading (at the cost of making my comment seem "a little bit weird"):
Recommended chainring to chainstay clearance
Brief answer: 5 mm or more, depending on the chainring size (diameter), frame stiffness, and drivetrain setup. 10+ mm is a safe bet, while some users and bikes do get away with only 3mm or so.
Yes, that's a big difference (from 3 to 10+ mm). Explanation, with all the nuances (important to understand):
Factors that affect the optimal chainring clearance
I'll briefly and clearly explain the crucial factors (with how-s and why-s):
1. Frame flex
The stiffer a frame, the less likely it will be to flex and get caught by the chainring when put under load from pedalling or terrain irregularities.
Frames with
rear suspension have an extra point of flex when the suspension linkage develops play (or comes less-than-100%-flex-free from the factory).
Note:
Frame material does not directly affect flex, it boils down to design (for details see "
Bicycle frame materials – explained").
With a rear wheel hub motor planned, I would suggest getting a strong and stiff frame.
2. Bottom bracket flex
Some bottom bracket standards are a bit more flexy than the others. For example,
Hollowtech II is pretty stiff compared to
square taper.
In those terms, your choice of Hollowtech II bb will help reduce any flex.
3. Crank and chainring flex
Cranks (including the spider where the chainring is mounted) and even chainrings themselves are not 100% stiff. There is some flex there too, depending on the model.
4. Chainring size (diameter) - and final "formula"
We know that the
total flex is a result of:
frame_flex +
bb_flex +
crank&chainring_flex
For a
given flex in degrees of turning from the centre,
chainrings with a larger diameter will move more at their edge. Here is a table overview:
Chainring
size | Outer
diameter | Movement for
1º of flex |
| 28 T | 122 mm | 1.06 mm |
| 38 T | 166 mm | 1.45 mm |
| 48 T | 210 mm | 1.83 mm |
1º is 0.0174533 radians;
movement of 1º flex angle = 2 * (half_diameter * sin(0.5º) )
To reiterate:
Apart from any extra flex of a larger chainring, diameter itself introduces extra movement at the end - the larger the diameter, the more movement with the same amount of flex!
Yes, chainstays move outwards as you go towards the rear end of the bike, so a larger chainring's end will be near the wider section of the chainstays - but, it's the very end where we measure the clearance, so we leave it out of consideration in this case.
Conclusion?
I hope you now understand why I was reluctant to give an absolute answer. There are too many variables (and I haven't even mentioned loads, rider strength, weight, riding conditions etc.).
So, to give some rough guide, at least for the start, 5 mm or more is good, 10 mm is great.
To protect the frame, chain, and chainring(s) in case of a chain drop, I would recommend mounting chain guards/guides and similar stuff, because it makes no sense to create some huge clearance for such a scenario. Why?
Because we also have the
chainline to consider, as you've rightfully mentioned in your post!
Relja
WhiteAndNerdy Novović
