I’ve been having many sleepless nights because of my dilemma. I came upon a Zipp Vumaquad crankset for quite cheap so I couldn’t pass it up. It also happens to be something I’ve wanted to try and Its almost as light as it gets. Problem is it has a 30mm spindle. BB90 bottom brackets are designed for 24mm spindles. So to put it simply, its going to be a tight fit.

BB90 bottom bracket shells are 90mm wide. They take 37mm OD bearings and the shell is either cut to fit 7 or 6mm wide bearings.

The Zipp Vuma quad spindles are 30mm wide and just long enough for a 90mm bottom bracket, but the fit is quite close. BB30 SPECIFIC cranksets will not work with BB90 for this reason, their spindles are too short.

The only bearings I’ve found that fits the bill are 6706 bearings. These are quite rare, quite thin, and there aren’t too many options. Their dimensions are 30x37x4mm. That is, 30mm ID, 37mm OD and 4mm width.
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Because they are so thin the bearings have a very low load rating. This means they’ll either get crushed by the crank forces or just wear out very quickly. I hoped to counter that by using 2 bearings on each side of the cup to strengthen the system.

Even with a set of 4 bearings the weight comes out to be less than the set of FSA bearings.
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Here they are juxtaposed.
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I let them chill in the freezer to help with the install. Even though the BB90 system is advertised as a slip fit system, it never is! The fit is quite tight so plenty of grease and some chill time will help.
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With the bearings being 4mm wide, I had hoped that the BB shell was cut to 7mm so 2 bearings can be slipped in with only 1mm protruding on each side. That wasn’t the case, however, the BB was cut to 6mm so the 2nd bearing would stick out 2mm.
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Here’s one bearing pressed in.
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This is two bearings in. Exactly 2mm of bearing sticking out.
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I pressed on (no pun intended) and installed all 4 bearings and tried to install the crankset. With 2mm sticking out on both ends the spindle had to clear 94mm in order to work. As seen in the picture though, it came short.
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So why am I going through the trouble?

Most high end super light weight cranksets nowadays run 30mm spindles. So if you have a BB90 or BB86 you’re missing out unless you figure out a way to fit those cranks.

These thin bearings, given they last a decent amount of time, will make for a SUPER light weight system, likely 550 grams total.

Lastly, with smaller bearings comes less drag.

The next step is to machine out 2mm spacers so that I can try to use 1 bearing on each side of the BB cup. The only worry with this is how long the bearings can last.

If having only one bearing on each side fails the next step is to increase the seating width to 8mm in order to fit 2 bearings. Given the bearing load ratings, this should work.

If that also fails… Then either go with Lightning BB90 cranks (which allegedly also use 4 6706 bearings) or sadly go back to standard cranks and call it a fail.