PCD = Pitch Circle Diameter. It's the diameter of the circle that the chainwheel bolts are on.
It's also commonly called the Pin Circle Diameter (as in chainwheel pin, as it fits the abbreviation).
Also known as BCD (Bolt Circle Diameter)
If you get a chainring with the wrong PCD, it won't fit.
As well as different PCDs, chanwheels can use 4 bolts (mountain bikes) or 5 bolts (most road bikes).
This table lists the common PCDs, with rough conversions to a bolt-to-bolt measurement to make it easy to work out what PCD you're dealing with.
Bolt to Bolt
|64mm (4-bolt)||45mm||Current standard MTB inner chainring|
|74mm (5-bolt)||43mm||Old triple crankset inner ring|
|80mm (4-bolt)||This is used by SRAM/Truvativ on some of their double MTB cranksets (inner chainring)|
|Used on some SRAM cranks/chainrings.|
|94mm (5-bolt)||55mm||Short-lived triple middle & outer. 8-speed Deore LX (also in 110mm)|
Current standard MTB middle and outer ring. An important variable within this standard is the inner diameter of the chainring (the support-ramp diameter of the spider). This is either 88mm or 90mm and both diameters can appear on the same crankset (different for middle or outer ring). This applies to Shimano, SRAM/Truvativ and RaceFace cranksets. Very difficult to predict - you'll just have to measure, even though this is difficult for the middle chainring.
Some middle chainrings from Shimano are made of carbon fibre and steel and as the carbon gives them great rigidity they don't use a support ramp on the spider, so the inner diameter of these is not important (though it measures up at about 82mm).
|110mm (5-bolt)||65mm||Shimano compact road and Campagnolo compact road. Also was used on old MTB cranksets before the 104mm standard took over.|
|120mm (4-bolt)||This is used by SRAM/Truvativ on some of their single and double MTB cranksets (outer chainring)|
|130mm (5-bolt)||76mm||Shimano road but the 110mm standard is taking over. Minimum chainring size of 39T.|
|135mm (5-bolt)||79mm||Campagnolo Road|
|144mm (5-bolt)||84mm||Old Campagnolo Road and (if I'm not mistaken) current Campagnolo Track.|
Chainring Inner Diameters
There is a variable in mountain chainrings apart from number of teeth, bolt circle diameter, number of gears and (sometimes relevant) brand. The inner edge of the chainring will be either 88mm or 90mm in diameter (matches the support ramp diameter of the crankset). This is easily measured when the chainring is off the crank, and is also easily measured for the outer chainring while the ring is still on the crank. But be aware that Shimano did at one stage make a crankset that had one ramp diameter for the outer chainring and another for the middle chainring. So, to ensure compatibility, measure carefully.
|Crank Model||Middle Chainring||Outer Chainring|
|SRAM X-5 Triple||90mm|
|SRAM X-O Triple||90mm|