What are Torque Caps?

The question about Torque Caps and the mysterious letter combination TC are asked about frequently. Therefore, the subject matter deserves to be covered in more detail.

Same, But Different

 The Torque Cap or TC in nomenclature used in conjunction with Onyx Racing Products is an interface standard used in mountain bike forks and hubs. Introduced by Rock Shox back in 2016, the TC standard hares the same axle dimensions with standard Boost spacing. The difference is found in the interface between the hub and the dropouts of the fork.


The goal of the TC standard is to improve torsional stiffness. It is achieved by increasing the contact surface between the fork and the hub. With regular Boost standard the diameter of the hub end caps is 21mm. With TC standard the diameter is increased up to 31mm – a noticeable difference! The relevant dimensions are listed in the table below.



Axle spacing (mm)

Axle diameter (mm)

Hub end cap diameter (mm)






A TC-compatible fork can be run with a standard Boost-sized hub.

Torque Cap / TC




A TC hub can only be run with a fork that’s TC-compatible.



 A glance towards the dimensions reveals that regular Boost-sized hubs can be used with TC-compatible forks. However, the same does not work the other way around. When running a hub equipped with a TC axle, it needs to be paired with a TC-compatible fork. 


Because the TC standard was introduced by Rock Shox, it is only natural to find it in all of its high-end models. In addition, other manufacturers – like EXT – also employ the TC standard on their forks.

Classic, and Vesper

Just like the rear hubs with their ingenious MFU axle design, Onyx Racing Products employs a similar modular design. Both hub models, the Classic and the Vesper, can be ordered or later converted from Boost to TC standard or the way around simply by replacing the end caps and/or the front axle.


With the Classic model, a dedicated front axle with associate end caps is required. The wide flanges found on the end caps use a threaded interface with the axle – a cunning design which makes installation a breeze.


Onyx Classic TC Boost ISO Axle


Things are even easier with the Vesper hub. Converting the front hub from Boost to TC or vice-versa only requires swapping the end caps! No need to touch the axle itself! 


Onyx Vesper Boost ISO TC End Caps

If you’re in the market for a front hub that employs a Torque Cap axle, look for the TC abbreviation in the product name.


Some parting words to summarize the matter.


  • Boost and TC standards share the same axle dimensions: 110x15mm.
  • The goal of the TC standard is to increase torsional stiffness and steering precision.
  • The difference between Boost and TC standards is found in the hub/fork interface. The former uses end caps with a diameter of 21mm while the latter employs end caps with a diameter of 31mm.
  • A fork with TC standard will work with Boost or TC hub. However, a TC hub requires a matching TC fork.
  • If some additional torsional stiffness is desired, running a front hub and a fork equipped with the TC standard might offer some benefits.