Cannondale's BB30 system
Recently, Velonews published an article about the BB30 system. Although the article was positive overall, we did not agree with some of the conclusions that they reached.
Let’s break down our argument for you:
The BB30 system is LIGHTER
The BB30 system is NARROWER
The BB30 system is STIFFER
The super-stiff, super-efficient SuperSix frame is the perfect complement to BB30, which first debuted during the 2000 Tour de France.
The industry-changing BB30 has revolutionized frame design. Even Specialized has built frames that accept our standard.
The article correctly stated:
“A standard BB30 bottom bracket is comprised of two sealed-cartridge caps, two dust caps, a handful of spacers and a wave washer for bearing preload; this package weighs about 60 grams. By comparison, FSA’s MegaExo K-Force weighs 100 grams, and SRAM’s BlackBox GXP bottom bracket weighs 113 grams.”
But it failed to note that the lightest production crankset in the world is achieved only when Cannondale’s own Hollowgram Si SL cranks are mated with the BB30.
Compared to the lightest crank tested by Velonews (FSA K-Force Light BB30), the Hollowgram SL is 75 grams lighter.
Compared to a standard Dura-Ace crank (745 grams), the Hollowgram SL is 160 grams lighter.
The article states:
…BB30 isn’t really narrower in terms of Q-factor (the width between the pedals), which is determined more by frame design, chainring size and chainstay dimension. However, BB30 does offer a narrower U-factor (the width between drive and non-drive BB spindle attachment, which determines heel clearance).”
At least the editors got the part about U-factor correct.
Q-factor is determined by frame design in combination with BB/crank design.
Q-factor of the BB30 and Hollowgram Si SL is 143mm – narrower than any of the BB30 cranksets that Velonews compared, and several millimeters narrower than Trek’s BB90.
A narrow Q-factor helps most riders maintain a more efficient pedaling motion and put more power to the pedals.
The article conceded that:
“The BB30 cranks proved stiffer, but not by as much as expected.”
Based on Cannondale’s testing:
Hollowgram SL cranks have a measured stiffness of 450N/mm
This measurement is stiffer by 20N/mm than the heaviest crankset in the Velonews test, and approximately 40N/mm stiffer than the lightest crankset tested
Only 8% of deflection comes from axle bending. Trek claims that with the BB90, putting the bearings far apart gives dramatic increases in crank stiffness since the BB spindle is supported closer to the ends. But the bending of the axle only has a small influence on overall crank stiffness, so the wider width of the bearings is not critical.
Approximately 70% of LH crank deflection comes from axle torsion (a twisting motion), so having a narrow BB shell and a short, fat axle gives a lighter, stiffer system.
BB90 only allows for a traditional small diameter steel spindle which is inferior in resistance to torsion loads.
BB30 allows for a large diameter aluminum spindle which is far superior in resisting torsion forces, and is much lighter in weight. Think about it…do today’s top racers win on steel frames with small diameter heavy tubes?
But Velonews did not test the Cannondale Hollowgram SL BB30 crank….
to be continued....