What Is Shimano Shadow RD? Features and Benefits
What is Shimano’s Shadow RD used in their latest rear derailleurs? This article explains its features and the benefits of choosing Shadow RD.
- English
- 日本語
What Is a BOOST Crank? Differences from Standard MTB Cranks and How to Choose
Posted at: Mar 7, 2024
This article explains what a BOOST crank is, how it differs from conventional MTB cranks, and provides guidance on selection across different manufacturers.
Editorial Note: We earn a commission from partner links on cyclabo. Commissions do not affect our editors' opinions or evaluations.
Table of contents
Sponsored Link
What Is a BOOST Crank on an MTB?
In the world of mountain bikes, frames and hubs with a 148mm rear-end spacing are referred to as using the “BOOST” standard, while those with a 157mm spacing are known as “SUPER BOOST.”
MTB End Standards: What to Consider When Buying
Detailed explanation of MTB end standards! What is the boost standard? What is the O.L.D. that fits your MTB, points to keep in mind when buying an MTB, etc.
At the same time, you’ll also find what’s called a “BOOST crank” for MTBs.
Strictly speaking, a BOOST crank is not a new standard for cranks or bottom brackets. Rather, it’s a crank designed to be compatible with rear ends using the BOOST standard.
How It Differs from Traditional Cranks
A BOOST crank is primarily designed to correct the altered chainline caused by the BOOST rear-end spacing. So, instead of thinking of it as a “BOOST crank,” it’s more accurate to describe it as “a crank compatible with BOOST rear ends.”
With a BOOST rear end, the spacing is wider than the traditional 142mm thru-axle standard, which shifts the chainline outward toward the rear. This happens because, even though the rear end is wider, the distance from the derailleur to the largest cog (low gear) remains the same.
As a result, the chainline becomes more angled, which can negatively affect shifting performance. To fix this issue, the front chainring is shifted outward, realigning the chainline properly. This is the role of a BOOST-compatible crank.
How to Choose a BOOST Crank for Your MTB
MTB cranks should be selected according to the rear-end spacing of your bike. Manufacturers typically list the compatible rear-end widths in the spec sheets, so be sure to match the crank with your bike’s configuration.
BOOST Crank Lineup
Shimano
Shimano offers MTB cranks with different crank axle lengths based on rear-end spacing. The difference can be found in the last two digits of the model number.
- Rear end with 142mm thru-axle:
FC-MXX00 - Rear end with 142mm/148mm thru-axle:
FC-MXX20/FC-MXX25 - Rear end with 157mm thru-axle:
FC-MXX30 - For front double setups, the BOOST-compatible model is
FC-MXX20-B2
These are the general rules. The differences come down to the axle length and the presence of spacers. The non-drive (left) crank arm is the same across all models.
BOOST-compatible cranks come with spacers that go on both sides of the spindle, so your existing bottom bracket can usually still be used.
The following is the model lineup of XTR M9100 series cranksets along with their corresponding compatible rear-end widths.
| Model Name | Front Gearing | Q-Factor | Chainline | Recommended Rear End (Thru) | Recommended Rear End (QR) |
|---|---|---|---|---|---|
| FC-M9100-2 | 2 | 162 | 48.8 | 142 | 135 |
| FC-M9120-B2 | 2 | 168 | 51.8 | 148 | 141 |
| FC-M9100-1 | 1 | 162 | 52 | 142, 148 | 135, 141 |
| FC-M9120-1 | 1 | 168 | 52 | 142, 148 | 135, 141 |
| FC-M9125-1 | 1 | 174 | 55 | 148 | - |
| FC-M9130-1 | 1 | 181 | 56.5 | 157 | - |
The Deore XT M8100 series and SLX M7100 series share the same model naming conventions and specifications as the XTR series. While the specs are almost identical to XTR, the main difference is a wider Q-factor.
| Model Name | Front Gearing | Q-Factor | Chainline | Recommended Rear End (Thru) | Recommended Rear End (QR) |
|---|---|---|---|---|---|
| FC-M8100-2 | 2 | 172 | 48.8 | 142 | 135 |
| FC-M8120-B2 | 2 | 178 | 51.8 | 148 | 141 |
| FC-M8100-1 | 1 | 172 | 52 | 142, 148 | 135, 141 |
| FC-M8120-1 | 1 | 178 | 55 | 148 | 141 |
| FC-M8130-1 | 1 | 181 | 56.5 | 157 | - |
| FC-M7100-2 | 2 | 172 | 48.8 | 142 | 135 |
| FC-M7120-B2 | 2 | 178 | 51.8 | 148 | - |
| FC-M7100-1 | 1 | 172 | 52 | 142, 148 | 135, 141 |
| FC-M7120-1 | 1 | 178 | 55 | 148 | 141 |
| FC-M7130-1 | 1 | 181 | 56.5 | 157 | - |
SRAM
SRAM also offers three types of cranksets to match different rear-end widths in MTB, all compatible with the BOOST standard. Similar to Shimano, the crank spindle length increases depending on the standard.
- Standard: DUB
- For Boost spacing: DUB Boost
- For Super Boost spacing: DUB SuperBoost+
SRAM’s DUB standard uses a combination of inner and outer spacers on both sides depending on the bottom bracket and crank spindle length.
DUB MTB Bottom Bracket Spacer Chart | SRAM
In other words, unlike Shimano, you match the appropriate DUB bottom bracket with the compatible BB shell and crankset for correct installation.