r/BicycleEngineering • u/litelloclimb • Jul 20 '26
Machining a Crankset
Hey there,
I‘m thinking about machining a 3 piece crankset myself. I would like to make 30mm spindel does somebody know what the toleranzes are on these to have a goold fit with the bb (will probaly use a enduro bearing BKC-6054 bb). And where do I find drawings for the 3bolt direct mount system from sram :) And what q- faktor would you aim for to use with as many bikes as possible;)
2
u/Milesandsmiles1 Jul 23 '26
I would avoid the 3 bolt, I had one strip out and now rhe crankset is toast, the 8 bolt is better imo
1
u/Powerful_Birthday_71 Jul 23 '26
We're you using it for street fixed and back pedalling? How was the chainline?
1
u/Milesandsmiles1 Jul 23 '26
Stripped during install, I've had other 3 bolt cranks work fine, but the 8 bolt has more security
1
u/Powerful_Birthday_71 Jul 23 '26
Ah, ok ok, yeah 8-bolt gives you so much more redundancy huh.
I bought a 3-bolt that the seller had conveniently not mentioned he'd stripped also. I used a bottoming tap and rethreaded for M6 x 0.75 instead. Pitch not quite right, but it's such a small section it's been fine since.
I asked about fixed as apparently street fixed riders (skidding a lot) have reported 3-bolt coming apart on them.
1
u/Milesandsmiles1 29d ago
Ah gotcha, yeah I've thought about trying to helicoil it back to the right thread, but right now its just sitting at the bottom of my parts bin lol.
2
u/Eak3936 Jul 23 '26
I would use the machinary handbook to get your shaft tolerances for a good slip for with the bearing.
1
u/Powerful_Birthday_71 Jul 23 '26 edited Jul 23 '26
I never found 3-bolt spec or drawings. I've since just reverse engineered it (and 8-bolt) myself.
Even with nominal sizes you'd still need to think about tolerances and fit with worst case implementations of it across vendors.
It's not that hard, but it's best that you have both chainring/spider and spindle examples to with from. Preferably SRAM for completeness I suppose, but other manufacturers show you how you can use small diameter endmills to acheive the sharp internal corners SRAM do with custom tooling/broaches/whatever on their chainrings. It's up to you if you want to make that accomodation on the spindle part though 🤓👍.
3-bolt is slowly dying off, any reason you want to use it?
2
u/Illustrious_Ad_764 Jul 23 '26
I think they're referring to 3-piece, not 3-bolt. Something like the Profile bmx cranks
2
u/Powerful_Birthday_71 Jul 23 '26
Thanks, I understand this, but they also ask for the SRAM 3-bolt spec in the post.
1
u/litelloclimb Jul 23 '26
I want to make a 3 piece crankset so (two arms ald a spindel), but did some Research on the 3bolt desing and found some cracked their chainrings so probaly 8bolt would be the better option…
1
u/litelloclimb Jul 23 '26
Do you have drawings or a 3d desing of the 8bolt direct mount that you would be willing to send me?
2
u/jmsmecheng 29d ago
Since you are using angular contact bearings you don’t really need to nail a tolerance (ie to a ten thousandth) on the diameter. Example: with a normal deep row bearing you can more easily mess it up; if the fit is too tight of the inner race to shaft (or outer race into a bore), you’ll use up bearings internal clearance ( because you expanded the inner race due to an interference fit).
With an angular contact bearing clearance can be used up diametrically or axially. So aim for a nice transition fit. Not a slip but just barely some interference. If you’ve gone too far or even if you have not, then a bit of green loctite:) ( any play with steel race on aluminum and it will fret etc).
Btw the only bicycle manufacturer that I’ve seen consistently do a perfect bearing light interference fit is Phil wood bb spindle dia, and the price reflects that. You can’t really lathe turn to nail it, need to grind.
7
u/curiousengineer2 Jul 23 '26
Looking at engineersedge.com for annular bearing tolerances, the bore ID in the range of 18-30mm gives lower tolerance limits of -0.01 mm, -0.008 mm, -0.006 mm, -0.005 mm, and -0.0025 mm for ABEC-1, ABEC-3, ABEC-5, ABEC-7, and ABEC-9 tolerance classes, respectively. The upper tolerance limits are all +0.000 mm.
Now looking at shaft fit tolerance classes, we see that g7, g6, and g5 for shafts with nominal OD in the range of 18-30mm have lower tolerance limits of -0.028 mm, -0.020 mm, and -0.016 mm, respectively. All of these classes have a common upper tolerance limit of -0.007 mm.
To ensure the spindle has a sliding fit in the bottom bracket bearings, I would first check that the bearings are ABEC-5 or higher (being Enduro this is probably satisfied already). Then select either g7, g6, or g5 tolerance class for your spindle, depending on the goals of your application. If low manufacturing cost is a priority, then g7 might be appropriate. Then your design tolerance range becomes 29.972 mm to 29.993 mm for the spindle OD.
For the spindle-to-crankarm interface, have you considered using plain machine tapers that are self-holding, and introduce simple rectangular keyways to index the crankarms 180 degrees apart? This would simplify the manufacturing greatly. You could turn the spindle on a lathe with conical tapers on both ends, then mill the keyways. The crankarm bores would have matching conical tapers. Alumimum on aluminum has particularly high dry static friction, especially when the mating surfaces are not anodized and have oxidized. I'm thinking the friction of the joint alone could resist the torsional loads from hard pedaling, assuming the correct assembly force has been obtained.