3sheets

Hi,
I'm used to maintaining my bikes, but relatively new to digging into them. I'm building up a kid bike, and am changing the gearing around so that he'll have a better time of it. I'm using Sheldon Brown's gain ratio calculator and trying to match the range that I pedal around town. I've acquired a Sinz octalink (BMX) crank on the cheap and need to get a bottom bracket, but can't find any info about the crank's chainline offset. I'm not sure if this is the proper term, but I'm imagining something like:
(axle length - shell length) / 2 - Sinz crank offset = chainline
It occurs to me that this is probably a measurable feature of the crank, but without the BB already in hand, I can't figure it out. If anyone has experience with these cranks, and could give me a quick number, I'd be thankful.

Here's the stats so far:
Fuji Ace 20
Sinz 140mm crank
36t chainring
11-34 8 speed freewheel
Combo trigger shift: TBD (used xt if I can find on the cheap)

HillRider

If the crank is truly an Octalink (not the similar appearing ISIS) you have several choices. Octalink V-1 (road and XTR) bottom brackets were only available in 109 mm (double crank) and 118 mm (triple crank) lengths and for 68 mm wide bottom bracket shells. The V-2 version (most MTB cranks) were available in 113, 118, 121 and 126 mm lengths and for both 68 and 73 mm bb shells. Here is Amazon't listing of all of the road and MTB Octalinks:

https://www.amazon.com/s/?ie=UTF8&key...l_7k9tidhypq_b

At this point, taking the frame and crank to a competent bike shop is the best way to get a firm idea of the bottom bracket type and length you need.

fietsbob

Just a single Chainring?, You want the chainline between #4 & 5 on the rear cassette,

that is where the middle chainring should be, on a triple crank.

so inside face of the outer bolt circle spider.. nothing on the outside.

3sheets

Yes, single chainring. I know where I want the chainline to be, just don't know the length of axle I need to make it work. I've found shimano octalink at 103, 109.5, 113, and 118. Not sure about the isis; what's the difference?

fietsbob

ISIS is incompatible with Octalink.
obviously spindle length.. but how the profile design of the crank you bought interfaces with that ..
It's is a situation where
maybe
you have to go with bike and crank to your LBS and ask. they may have stock or not.
Octalink came and went and Shimano made another 'new and Improved' ..

or get all of them from someplace that will let you return stuff. and send the other 3 back

this is just a blind guess , since in the Shop they can see things , 1st hand.

onespeedbiker

I'm guessing your crank is ISIS not Octalink https://www.jensonusa.com/Cranksets/S...t-Isis-Arm-Set. I would also think that if Sinz manufactures ISIS bottom Brackets https://www.jensonusa.com/CrankBB-Int...Bottom-Bracket, it unlikely they market Octalink cranksets. BTW this Sinz BB says it's for their BMX cranks, however BMX usually has a 110 rear dropouts (mtb are 135mm), so if a 113mm spindle is for a 110mm dropouts, you may not be able to get a good chainline are on your son's bike, or even clear the chainstays.

fietsbob

BMX crank?
.. might be better off getting a 110/74 MTB crank and the proper BB at the same time..
then build it up as your 1x8..

3sheets

Yup, thanks. The crank is ISIS, not octalink. (I was asking "what's the difference" between ISIS and octalink: 10 vs. 8 splines.) FWIW, I sent my "offset" question to Sinz, as they should know, but to no avail. Perhaps they're tets up?
onespeedbiker , BMX is usually set up for a 50mm chainline, is this right?
Looks like a better way of expressing what I'm looking for is:

1/2 axle length - chainline = crank offset

So, if a BMX set up uses a 113mm to acheive a 50mm chainline, then the offset for the crank would be 6.5mm. Still speculation though; someone's gotta have installed one of these cranks.

3sheets

140mm crank arm for the kido, 68mm shell.

onespeedbiker

No, a BMX chainline is 40-42mm and a single chainring on a mtb is about 48-50mm; since ISIS spindles are symmetrical, I'm guessing a 125mm spindle; I've seen 118 and 128mm in ISIS.

3sheets

If anyone else is interested:
The sinz isis crank gives a 12.5mm offset. For example, 113mm axle will yield a 44mm chainline; offset being: 1/2 axle length - chainline = crank offset.
I had the crazy idea of calling the folks that I bought the crank from! So, not sure if this against any forum rules, but bmxguru.com rocks, as he didn't know, but helpfully measured the crank and bb assembly to figure it out. Thanks, all.

3sheets

Good guess, onespeedbiker. 125mm would, in fact dial out to 50mm.
Thanks again.