schnurrp

Doesn't 74mm bcd five bolt establish a minimum diameter for a chain ring? How is it possible for there to be a 74mm bcd five bolt 42 t chain ring and a 74mm bcd five bolt 28 t?

I want to replace one or two of the chain rings on my Shimano Sora triple chainset but when I search for maybe a 74mm bcd five bolt 36 t I can't find one but I can find a 74mm bcd five bolt 28 t.

I must not understand the measuring system.

shelbyfv

Don't think I've ever seen that, 30t is the largest commonly available. https://www.sheldonbrown.com/harris/chainrings/74.html

JohnDThompson 

74mm BCD rings are typically the innermost ring on a triple crank with a dedicated mounting point. As such, they're offered in small tooth counts, 24 to 30. Larger tooth counts would be mounted as the middle or outer rings using a larger bolt circle.

hokiefyd 

Yes -- there is a physical minimum chain ring size (in tooth count) for any bolt circle diameter. Go down to the 5-bolt section on Sheldon's page here:

"Sheldon Brown's Bicycle Crank/Chainring Bolt Circle Diameter Crib Sheet"

There's not a maximum size for any one style -- theoretically, you could make a chain ring as large as you wanted. There are practical limitations, however. The more your chain ring diameter outpaces its bolt circle mounting pattern, it becomes more flexible. This is why we typically see triple cranksets with two bolt circle mounting patterns -- a smaller one for the inner ring and a larger one for the middle and outer ring.

Because of these practical limitations, the market has accepted a certain "tooth size range" for certain bolt circle groups and you are generally limited to this selection when purchasing new. You can find oddball sizes every now and again. Given enough money, you could have anything custom made. But the configurations available today generally represent combinations that are known to work well in practice.

schnurrp

Yes, but my Shimano SORA crank set has both the outer 52t and middle 42t rings attached together at the end of the five spoke crank with bcd 74mm. The inner 30t has its own attachment point on the spokes at about 43.5 mm bcd. Sorry I can't send a picture or provide a link.

Appearing on the outer ring: "SHIMANO SG A-52" with an "8SPEED" sticker.
middle ring: "SHIMANO SG 8S A - 52-42-30"
inner ring: "SHIMANO SG A-30 9S/8S YDI"

hokiefyd 

I think you must be misinterpreting the measurements. I'd suggest looking at Sheldon's page I linked above. Are you measuring the distance between two adjacent holes, and getting about 74mm for the large set and about 43.5mm for the smaller set? That corresponds to a pretty standard road triple crankset where the smaller set has a 74mm BCD (bolt circle diameter) and the larger set has a 130mm BCD. It sounds like you're measuring between two adjacent holes, and not the bolt circle itself.

schnurrp

That helps. I don't know why, though, you can find for sale a ring described as 24t five bolt 74mm bcd with bolt holes actually about 43mm apart.

schnurrp

The two outside rings are attached together at the end of the spokes. The distance between each adjacent hole of this attachment is 74mm. The inner ring has its own attachment on each spoke with holes 43mm apart. This must not be a very common connection and it only appears once in Sheldon's list.

shelbyfv

BCD is not what you think. Your Sora crank is likely 130 BCD for the two larger rings, 74 for the small ring. You should read the links provided.

rmacdona

You are measuring the distance between bolt holes. BCD is "Bolt Circle DIAMETER" which is the diameter of a circle that would pass through the centers of all five bolt holes. For the large rings, a distance between holes of 76.4mm corresponds to a BCD of 130mm. For the smaller ring, a distance between holes of 43.5mm corresponds to a BCD of 74mm. These dimensions are typical of a road triple crank. For a mountain triple, the larger rings would be a 110mm BCD with a distance between holes of 64.7mm.

hokiefyd 

Those are two different measurement points that describe the same thing.

BCD is Bolt Circle Diameter. So if you drew a circle through each of the bolt holes, it would be 74mm in diameter.

You're measuring the distance between two adjacent holes. Geometrically, a circle that's 74mm in diameter, with 5 holes equally spaced around the circumference, will have a 43.5mm center-to-center measurement between each of those holes.

Both measurements describe the same circle. Sheldon lists the "distance between adjacent holes" measurement because that's the easiest way to measure a 5-bolt crankset or chain ring You can directly measure the BCD of a traditional 4-bolt crankset...it's the distance between two opposite holes. That's the bolt circle diameter. But you can't do that easily with a 5-bolt crankset. The "distance between adjacent holes" measurement is quick way to gauge the diameter of the circle (the BCD) in this case.

This is why any chain ring sold with a 74mm BCD will have adjacent holes 43.5mm apart. Geometry dictates that.

schnurrp

Oh, okay, "BCD" stands for "bolt circle diameter". Of course that changes everything and the knowledge of which is why I decided to join the forum. Thanks!

hokiefyd 

I think another point of confusion is 74mm BCD is one standard and you're also measuring ~about~ 74mm between two adjacent holes on your Sora crankset. Because those two measurements are close, it may be easy to conflate them. The distance between two adjacent holes on a 130mm BCD crankset is actually 76.4mm. Your Sora chain rings should have a distance between two adjacent holes of exactly 76.4mm.