Concentric BB

...the Datum’s concentric Bottom Bracket / Lower Link Pivot assembly is an exercise in simplicity.

It's really quite simple, sturdy and like everything on the Datum, designed to deliver exceptional reliability.

The most outboard bearings, shown below in grey with blue seals, are the BB bearings. These are the bearings in which the included 30mm crank spindle spins relative to the frame. We havesourced the best sealed, longest lasting double-row bearing BB's available.

Working our way inboard, shown in green are machined spacers to keep the lower link centered between the BB bearings. The channeled shape of the spacers helps to keep them in place during assembly (i.e. to make assembly less fiddly).

Next, shown in yellow are two rows of bearings each side on which the lower link spins relative to the crank spindle. Each of these bearings is rubber sealed, and they are well shielded from the elements by being hidden inside the assembly. The friction in these bearings is negligible and the lifespan good because they ride on a sturdy axle inside a sturdy bore,  and because they do not suffer cantilever loading.

It’s not necessary to use two rows of bearings here, but I’d rather design things to be excessively reliable than just reliable enough.

In the center of the assembly, again shown in green, is a wider spacer which maintains the separation between the inner races of the lower link's bearings. 

If you prefer to use a Shimano Hollowtech II (24mm) or Sram DUB (28.99mm) crank, spindle sleeve parts may be installed as shown below.

UPDATE 17/Apr/2023: A spindle sleeve for Sram DUB cranks is now available.

The 30mm spindle is known to be compatible with the following cranks: Race Face Cinch Next SL/Next R/Turbine/Era, FSA 30mm, Cannondale, Ignite, 5Dev, Garbaruk.

It is NOT compatible with 30mm cranks from Hope, Cane Creek, E13, White Industries, because they use proprietary spline shapes. Please ask if you’re interested in using a 30mm spindle whose manufacturere is not mentioned above, it may be compatible.

It is NOT yet compatible with 28.99mm DUB system cranks from Sram because they cannot be removed from their spindle. UPDATE 17/Apr/2023: A spindle sleeve for Sram DUB cranks is now available.