Middleburn question

[nic]

I need a middle chainring for a set of Middleburn RS 7 cranks.

When I look on the web, there are two kinds: ATB and CD - could someone educate me about the difference between the two please!

Many thanks

Nic

 

[gibbleking]

i think the cd is for compact drive and the other is pre compact...

 

[one-eyed_jim]

ATB: 110mm and 74mm bolt circles

CD: 94mm and 58mm (sometimes, but rarely 56mm) bolt circles

To measure the bolt circle diameter, measure between the centres of two adjacent holes and multiply by 1.7

 

[nic]

Thanks guys

 

[DM]

To measure the bolt circle diameter, measure between the centres of two adjacent holes and multiply by 1.7

That only works for 5-bolt chainrings... but then you shouldn't be discussing 4-bolt cranks on Retrobike. 4-arm cranks are nonsense. :x

Modern road "compact" cranks also use 110BCD, as did many older BMX cranks. So there are lots of cranks & chainrings still available in 110mm size, even though it's not currently used for mountain bikes.

http://www.sheldonbrown.com/gloss_bo-z.html#bcd

 

[one-eyed_jim]

To measure the bolt circle diameter, measure between the centres of two adjacent holes and multiply by 1.7

That only works for 5-bolt chainrings... but then you shouldn't be discussing 4-bolt cranks on Retrobike

Okay, pedant.

bcd=x/sin(pi/n) where x is the measured distance between adjacent bolt centres, n is the number of arms, and pi/n is an angle in radians (180/n if you prefer degrees).

I figure anyone with 4-arm cranks is just going to measure the diameter directly though...

 

[nic]

Thanks

Thanks Jim.... from your formula, and the fact that these are RS7s, I reckon Cd is the right one.

 

[Xesh]

......(sometimes, but rarely 56mm)......

Like on Coda M900 cranks. :evil: