Chainrings and Guides

If you plan to replace or upgrade your bike's chainrings, you must match the new chainrings to your existing cranks. Chainrings come in a variety of sizes and are measured by their bolt-center diameter (BCD). Chainrings which have different BCDs are not compatible, even if the chainring has the same number of teeth!

Additionally, chainrings come in both 4 arm or 5 arm configurations to match various crankarms. Your cranks and chainrings must match. See Fig. 1 for an example of a 4 arm chainring.

Determining your bolt center diameter

BCD is equal to the diameter of an imaginary circle drawn through the center of the bolts which secure the chainring to the cranks, measured in millimeters. Regardless of whether the crank has 4 arms or 5 arms, the BCD is measured the same way. Note: the BCD is NOT simply the distance between two bolt holes!

Fig. 1 - Chainring Bolt Center Diameter (BCD)

When measuring a double crankset, the same BCD is used for both the inner and outer chainrings. If you are measuring a triple crankset, be sure to measure the BCD of both the inner chainring and the middle/outer chainrings, as they are different.

Common size pairings

Mountain bikes and other bikes with triple cranks have two different BCDs per bike. The inner (smallest) chainring has it's own BCD, while the middle and outer chainrings share the same BCD.

Two different sets of BCDs are common on mountain bikes: first, a 58mm (inner) and 94mm (middle, outer) setup. Other mountain bikes use a 74mm (inner) / 110mm (middle, outer) configuration.

For road bikes and other bikes with double cranks, the inner and outer chainring share a common BCD. The most common BCD found on double cranks is 130mm.

Once you've determined the correct BCD for your cranks, simply make sure to order chainrings with the same dimensions for proper fit.

Sizing and compatibility guide for chainguides

Chainguides are becoming increasingly popular for use on downhill and freeride bikes. Properly installed, a chainguide reduces chainslap and keeps your chain from being thrown on bumpy sections. A variety of chainguides exist, it is important to choose one that is compatible with your frame and drivetrain.

There are two primary mechanisms to secure the chainguide to your bike: traditional and international standard. To determine which style of frame you have, refer to Figure 1.

Fig. 1 - International Standard for chainguide mounting (left), conventional mounting (right)

Traditional Mounting

In a traditional mounted setup, the "boomerang" section is sandwiched between the bottom bracket shell and the bottom bracket fixed cup. To prevent the chainguide from rotating out of position, small set screws are turned into the bottom of the bottom bracket shell. Such a chainguide can typically used on any frame EXCEPT for frames with International Standard mounting tabs, barring any compatiblity issues with your specific frame (see below).

International Standard Mounting

The more modern way to mount the chainguide is the international standard mount. In order to use an international standard chainguide, your frame must have built in tabs to mount the chainguide. This method eliminates the need for set screws. Refer to Figure 1 to identify your frame - if your bike has mounting tabs around the bottom bracket as shown, you will need to order a chainguide specifically made for International Standard frames (a traditional mounted chainguide will not work).

Fig. 2 - "MiniMe" Chainguide from MRP

Compatibility

Many models (such as the MRP LRP) replace the outer chainring on your bike, allowing the use of the middle ring when the guide is installed. Please note that these guides are specific to a single chainring pattern - you must match both the number of arms on the crank, as well as the bolt circle diameter of your chainrings. Chainguides designed for 4 arm cranks will not fit 5 arm cranks and vice versa.

Additionally, due to the complex designs of many full suspension bikes (including swingarms and pivots) there are clearance problems with some chainguides that prevent them from being used. Mountain Speed provides a handy guide that can be used to determine if your frame will have a problem, we highly recommend reviewing it prior to ordering.

Some info sourced from our affiliate partner: Jensonusa