You occasionally see people wondering why Shimano’s DURA-ACE track crankset (FC-7710) still uses the now obsolete Octalink standard. Haven’t we moved on to external bottom brackets and 2-piece cranksets as found in the current Hollowtech II road cranksets? Octalink isn’t even found on Claris anymore, it got upgraded to Hollowtech II. Octalink has pretty much disappeared from Shimano’s lineup.
Some people speculate it’s about NJS certifications or simply that Shimano doesn’t want to retool. SRAM Omnium is a favorite of track and fixie riders looking for maximum stiffness and a “modern” crankset, so it’s obviously doable.
The big missing part of the equation is U-factor, and the fact that Q-factor must be greater than or equal to U-factor. Q is the better known of the two, and the width of the crankset at the pedal eyes which dictates pedal stance. U is the width of the crankset at the spindle and allows for heel and shoe rub clearance.
Track cranks like FC-7710 use bottom bracket spindles that are less than 110mm long. This is not an insanely short length, many low-end road and MTB cranks use these sorts of bottom brackets. When accounting for the additional room required for extractor threads, it puts the U-factor right around 125mm and a narrow Q-factor of 136mm.
The problem with the SRAM Omnium is it has a 145mm Q-factor, essentially the Q-factor of a road crankset. The problem with Hollowtech II is that the the U-factor is about 140mm at a minimum, wider than FC-7710’s Q-factor. Consider the standard 68mm bottom bracket shell. Add in 11mm per side for bearing cups. Then 25+mm per side for the crankarms. 140+mm.
Now bearing cups could actually be cheated down to 9.5-10mm. Shimano chose 11mm cups for 68mm BSA bottom brackets so they could make 10mm cups for Italian 70mm shells. These 10mm cups are also used for MTB 73mm BSA bottom brackets. There’s a misconception that Shimano external bearings are 86mm on the outside faces because of the BB86 standard, but the figure is actually 90mm.
Shimano’s road Hollowtech II design requires a NDS crank arm width of about 25mm in order to have enough room for two M5 pinch bolts. Other designs that use splines and self-extracting cap can be a bit narrower.
Now Campagnolo could make a track crankset using Ultra-Torque and suffer next to no U or Q penalty. Their older Ultra-Torque cranks have a 128mm U factor despite the external bearing bottom brackets. This is achieved by not having pinch bolts or extractor threads, and why Campagnolo went with the costly hirth joint to join the spindle inside the bottom bracket shell.
However Shimano’s newer MTB Hollowtech II cranks have a couple of changes. They are open on the drive side, meaning they can shave off a couple of millimeters of U on the drive-side, and they scrapped the pinch bolts on the non-drive-side so it can be a little bit narrower, but not as narrow as Campagnolo due to the need for extractor threads for the self extractor. With MTB style cups, it could also be reduced a couple more millimeters, but that creates a standards problem for every other bottom bracket shell. So now Shimano could release a Hollowtech II crank with a 136mm Q-factor, but they wouldn’t really be able to achieve the same low U-factor without using a solution like Campagnolo’s hirth joint.
So the real question is why doesn’t Campagnolo make an Ultra-Torque Pista crankset?
Kuromori 