Bb30 crank spline standard (NOT BB STANDARD)

[crowe-molybdenum]

I’m in hot pursuit of a drawing/specification for the quasi-standard that is the 8 lobed tapered spline that we see on the end of a bb30 spindle; something like this

I want this because I want to draw and have made a pair of cranks which can interface with existing available spindles.

I am not interested in information about the bb or frame standards, that is readily available with a quick google search.

the best/closest resource i’ve found is this thread on Vital MTB

https://www.vitalmtb.com/forums/hub/bb30-standard

most of the thread is spent explaining that the frame/bb standard people keep posting isn’t what is being looked for… but there is a post at the bottom with the beginnings of a drawing.

I’m not near anyone I could recruit to machine this, and shipping to/from my part of the world is a total joke (6 months corrupt customs delay not unheard of)… so I’m trying to avoid a test cut here; ideally a drawing with SOME verification would go some way towards minimising the ever growing amount of scrap I see to produce

willing to share finished designs if my incredibly esoteric wants are something anyone is interested in too… ( yes, these are for my adjustable bb height track bike)

[Daniel_Y]

I would reach out to Sturdy Cycles, they make a printed crank. No idea what spindle standard they use, but I’m sure they have a better idea.

Titanium 3D printed crankset - Made to order — Sturdy Cycles

[crowe-molybdenum]

thanks for the reply,

I have reached out to Tom, he replied: “Unfortunately it is not an open standard that I could find either and so I developed my own which is not likely to be of very much help to you”

so while that looks like the splines we know, it just may not be.

[stevenshand]

I don’t think that you’ll find what you’re looking for. I did similar work when looking into crank interfaces for the Intra-Drive (www.intra-drive.com) drive unit. I’m not saying drawings don’t exist but they are likely to be proprietary. I was unable to find anything open other than ISIS.

[floppywonka]

if you have access to a 3d printer, you could start from scratch by taking measurements from the spindle in question and start drawing. once you think you’re close you can print a donut with the spindle hole in the center and see how it fits, make adjustments and print again, etc until you have something useable, and then you can get it machined from metal.

i did this with raceface’s cinch interface and it worked well for me

[floppywonka]

also, the post you linked is less than a year old, you could probably make an account over there and hit up the digit bikes person with the model and see if you can get the info from them

[crowe-molybdenum]

thanks for the thoughts team.

I have reached out to digit bikes; he has said that he is now using that drawing to produce spindles which work with existing cranks with no problems. so thats certainly something.

Ive been doing some thorough measuring of the cranksets I have on hand, and will have another measured up by a trusted third party within a few days all going well,

(here’s the dangerous assumptions part)

IT APPEARS that the crank splines are “the same”, but they extend between 1.5 and 1.6mm further outboard (following the same form & taper) before ending with a surface square to the spindle centreline.

IT APPEARS this surface is bottomed out on the end of the spindle at or before the recommended torque ( the cranks I have are all at least lightly used )

I don’t know enough about designing splined interfaces to apply occams razor and say this makes sense… but I don’t know nothing and this is feeling like an arrangement which I think i’m willing to try. ill get those other cranks and spindles measured and make a call.

as for the 3d printer idea, thats a nice approach for the application you’ve shown, because unless your riding that crank arm fixed gear (in which case, you’re rad, and further power to you) those splines only have to resist torque in one direction (the crank arm trying to turn when you pedal, and the chainring resisting that turning); any slop or slack is quickly taken up and then remains “rotationally bottomed out” which means it doesn’t cause any issues.

in contrast, the connection between a crank-arm and spindle has to resist (among other things) torque which alternates back and forth in clockwise, and then counterclockwise directions every pedal stroke. which means any freeplay or induced play (by the deformation of the crank) rocks back and forth and wears the splines out.

thus crank splines are far more precisely made, and almost always tensioned radially (via a taper or a pinch bolt or two).

Ive ruined a bunch of cranks and chainrings by trying to get away with riding splined chainrings or spiders fixed gear, and without fail, that little bit of available play slops out real bad, real fast.

[earle.b]

The Cannondale Hollowgram cranks were I think the first to use this interface.

Someone has posted a cad of Hollowgram cranks on GrabCad years ago.

Free CAD Designs, Files & 3D Models | The GrabCAD Community Library

You could 3d print this to verify it’s accuracy.