Cycling UK Forum

saps · Post by **saps** » 20 Jan 2009, 5:27pm

hi
can anybody tell me the bcd of stx rc chainrings. And anywhere to get them. Thanks for any reply
brian

random37 · Post by **random37** » 20 Jan 2009, 5:52pm

They're standard sizes. Just measure the diameter of the circle formed by the bolts, and note the number of bolts, that will be either 4 or 5.
Spa Cycles do them cheap.

hubgearfreak · Post by **hubgearfreak** » 20 Jan 2009, 7:24pm

AASHTA

rogerzilla · Post by **rogerzilla** » 20 Jan 2009, 8:16pm

chris667 wrote:They're standard sizes. Just measure the diameter of the circle formed by the bolts.

That's incredibly hard to do for a 5-bolt ring. You can, however, easily measure the distance between two adjacent bolt centres; knowing the angle subtended is 72 degrees (360/5), the diameter can be found using trigonometry:

sin = O/H

sin 36 = (half the distance between holes)/(half the BCD)

or, simplifying, BCD = (distance between holes)/sin 36

Use sin 60 instead if you come across a 3-bolt ring; with a four-bolt ring you can meaure the diameter directly between opposite hole centres.

Edwards · Post by **Edwards** » 20 Jan 2009, 8:55pm

This BCD table from SJS might be useful.

one-eyed_jim · Post by **one-eyed_jim** » 20 Jan 2009, 9:51pm

STX and STX RC came in a few different flavours over the years. Some versions used unusual mounting arrangements where one chainring mounted to another, which in turn attached to the spider. In one version, the cranks had a 58mm bolt circle to which the two smaller chainrings attached, and the outer chainring bolted to the middle. Another version used a 94mm main bolt circle, with the granny ring attached to the middle.

Replacements for the odd carrier-rings have always been hard to get - I don't know if they're available from Shimano at all now.

If you have the late 4-arm model, you're in luck: the chainrings are standard 104mm bcd.

You'll find a model code on the rear of the arm (of the form FC-M***) that will help narrow things down, and a set of exploded diagrams at:

http://www.paul-lange.de/produkte/shima ... _archiv/FC

2Tubs · Post by **2Tubs** » 20 Jan 2009, 11:41pm

BCD?

I thought I'd gone to my "other" forum where a BCD is a buoyancy control device.

>;o)

Gazza

cycleruk · Post by **cycleruk** » 23 Jan 2009, 11:48am

Middle and outer, 5 bolt = 94mm
Inner (granny) , 5 bolt = 58mm

As far as I know Stx.RC only came in 5 bolt arrangement.
The inner is usually steel but middles & outers can be either steel or ali'.

I would have expected any MTB shop to sell them but Spa & Merlin advertise them.

http://www.spacycles.co.uk/products.php?plid=m2b0s149p0
http://www.merlincycles.co.uk/?fn=categ ... goryId=118

one-eyed_jim · Post by **one-eyed_jim** » 23 Jan 2009, 3:13pm

cycleruk wrote:Middle and outer, 5 bolt = 94mm
Inner (granny) , 5 bolt = 58mm

As far as I know Stx.RC only came in 5 bolt arrangement.
The inner is usually steel but middles & outers can be either steel or ali'.

I would have expected any MTB shop to sell them but Spa & Merlin advertise them.

You might have done a little fact-checking.

Here's a 1995 STX RC showing the first arrangement I described, with a single 58mm primary bolt circle, and a special middle chainring that carries the outer:

http://bernd.sluka.de/Fahrrad/Shimano/T ... 3_1995.gif

Here's a 1997 STX RC showing the second arrangement, with a single 94mm primary bolt circle, and a special middle chainring that carries the inner:

http://bernd.sluka.de/Fahrrad/Shimano/T ... S_1997.gif

Here's a 1998 STX RC four-arm crank:

http://www.paul-lange.de/produkte/shima ... C40-98.pdf

The '97 model can be run as a double with conventional 94mm bcd chainrings, but the '95 model needs the special, scarce middle ring to carry the outer.

PACE used to make an adapter to allow conventional chainrings to be mounted on the first version.

Geoff Dale · Post by **Geoff Dale** » 23 Jan 2009, 5:22pm

Hi. I have just had the same problem ? The online Co. Dotbike supplied me the answer. Measure the distance between two adjacent holes (mm) and Multiply this measurement by 1.7 (mm) this will give the BCD for your required chainring. When ordering/purchasing simply give this with the No. of holes.

Post by Si » 23 Jan 2009, 5:34pm

Geoff Dale wrote:Hi. I have just had the same problem ? The online Co. Dotbike supplied me the answer. Measure the distance between two adjacent holes (mm) and Multiply this measurement by 1.7 (mm) this will give the BCD for your required chainring. When ordering/purchasing simply give this with the No. of holes.

If you have a hunt around Sheldon Brown's site there is a conversion table somewhere.

stewartpratt · Post by **stewartpratt** » 23 Jan 2009, 9:04pm

rogerzilla wrote:You can, however, easily measure the distance between two adjacent bolt centres; knowing the angle subtended is 72 degrees (360/5), the diameter can be found using trigonometry:

sin = O/H

sin 36 = (half the distance between holes)/(half the BCD)

or, simplifying, BCD = (distance between holes)/sin 36

Use sin 60 instead if you come across a 3-bolt ring; with a four-bolt ring you can meaure the diameter directly between opposite hole centres.

Or if you prefer the simple life... Measure the chainring bolt to the crank bolt, centre-to-centre. Multiply by two.

thirdcrank · Post by **thirdcrank** » 23 Jan 2009, 9:32pm

Si wrote:If you have a hunt around Sheldon Brown's site there is a conversion table somewhere.

Cunningly filed under Bolt circle diameter (BCD)

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

(A link already posted by hubgearfreak above.)