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Had a look online and seen different methods described. I have a set of truativ double 39/26 cranks. The outside chainring BCD is stated on crank as 120, the inner is not given.
I am looking to put a narrow wide ring on, I think the outer ring is too big to fit a 32 tooth NW ring. So can I fit a NW ring to the inner chain ring mounts or will this adversely effect chainline or foul the chain?
When I measure the 26 ring bolt holes it measures 80mm from the 2 o'clock to 5 o'clock holes, is this this right??? If so i can buy a blackspire ring.
Thanks
How many bolts?
If 4, just measure the distance between opposite bolts.
If 5, you need to do some sums (google says multiply bu 1.7)
Its a 4 arm crank, what do you mean by opposite? If a,b,c and d are bolt holes
a.....b
c.....d
a to b is 49mm a to d is 80mm
sorry if i am being thick.
12 o'clock to six, or 3 to 9.
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Sheldon Brown has a handy chart: http://www.sheldonbrown.com/cribsheet-bcd.html
But it might not include some of the more modern sizes.
Also it seems some cranks have the bolts at uneven spacings around the circle. So it depends on just which holes you measure the distance between.
Tomvet - your a to d (80mm) is what you need.
we have this at work because:
-you never get the BCD measurement wrong which saves buying the wrong chainring for a customer
-more tools = 🙂
-it looked cool!
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a to b is 49mm a to d is 80mm
Do you mean a to c is 80mm ?
If not, then surely you've got asymmetrically spaced boltholes and the 80mm may not be right either
If a-b = b-c = c-d = d-a, and a-c (and b-d) = 80mm, then crack on
(or if thye line between any two crosses the cetre of the ring then that measurement should be OK too)
It is 80, SRAM double chain sets are, so however you measure that it's right 😉
Just measure the centre of the axle to one chainring bolt to get the radius the multiply by 2 to find the diameter.
Njee, - does your knowledge go so far as FSA K-Force Light 38T/22T double chainset?
mjsmke - Member
Just measure the centre of the axle to one chainring bolt to get the radius the multiply by 2 to find the diameter.
I see no reason why this wouldn't work, no matter what the bolt configuration is
mjsmke - Member
Just measure the centre of the axle to one chainring bolt to get the radius the multiply by 2 to find the diameter.
I see no reason why this wouldn't work, no matter what the bolt configuration is
Njee, - does your knowledge go so far as FSA K-Force Light 38T/22T double chainset?
You sure about those ring sizes, that's a big jump? It doesn't, but it's rare to find a 22 on anything other than a 64mm BCD, with 104mm for the outer ring. That's guessing though, don't know for sure I'm afraid.
Pretty certain it's 22T, either that or a 24T but yes I'm pretty sure the outer is 104mm without cheekily checking.
I see no reason why this wouldn't work, no matter what the bolt configuration is
Correct. It's sometimes not easy to estimate the centre of the axle. It should be easier to measure the distance between chain bolts.
But, given that rings only come in fixed sizes, you'd have to measure it pretty badly (which ever method) to make such an error you buy the wrong rings.
Thanks for all the replies, I think it is 80 then, will buy a blackspire NW ring (that seems to be the only one in an 80 bcd that i can find).
Any thoughts on if I will have any issues with chainline by fitting it into the inner ring?
So an to update this the blackspire ring i bought fits, I am now left with a crank with 4 ugly tabs poking up. The only bash gaurd with a BCD of 120 I can find is a blackspire defender at £53!!!!!
So will I die if I take a metal file to the outer chain ring tabs and do a little bit of agricultural engineering?
No, but I'd make sure the chainline is OK before committing to this plan.