When we began designing the new 2011 R-series Cervélos as part of our Project California engineering effort, we were looking for every advantage in structural efficiency. We wanted more stiffness, more strength, and less weight. And it shouldn't come at the expense of non-standard derailleurs or the rider's biomechanics. As part of our quest for structural efficiency, we did a lot of analysis on the frame tubes of course, refining Squoval™ and tuning the cross sections and Smartwall™ carbon layups. But we also realized there was an opportunity to widen the bottom bracket shell to stiffen the frame, simultaneously increasing lateral stiffness and reducing system weight.
We realized there was a big opportunity for improving the bike if we designed the bike as a system - not as a collection of individual parts, i.e. designed together to work together. People had designed cranks that were stiffer (e.g. BB30) and they had designed wider frames (e.g. BB90 and BB86, both of which use a standard 24mm axle) but no one had looked at the crank AND frame together to optimize the complete bike as a system.
When we investigated the system we found that the biggest opportunity to increase system stiffness and reduce weight was with bigger frame tubes, but the larger diameter crank spindle as used on BB30 also improved stiffness and weight significantly.
So how did we decide on BBright™'s dimensions?
We didn't want to change things that didn't need changing:
- Q-factor (pedal stance width) is unchanged.
- The chainrings' position is unchanged (chain line).
That means no need to redesign driveline components including derailleurs or hubs.
We also wanted to maintain good compatibility:
- Compatible with crank-based power meters. (Wider bearing placement on the drive side would have protruded into that space.)
- Compatible with triple chainrings, especially important on MTB. (Again, wider bearing placement on the drive side would have protruded into that space.)
We did want to change things that improved performance. Take a look at the loads the frame tubes see, as illustrated below. Under hard riding, the frame tubes that connect to the bottom bracket shell (chain stays, seat tube and down tube) see a lot of bending load, so structural improvements there yield an especially large increase in performance. Many high-end frames' tubes have gotten bigger and bigger, to the point where some frames are at the max. In fact, Cervélo's right side chain stays are already as wide as permitted by reasonable tire and chain ring clearance. Only the left side remained as an opportunity to increase tube size. This is where the wider BB shell comes in.
BB90 and BB86 designs widen the bottom bracket shell, however, the right side chain stay can't get any wider, so the extra shell width on the right side is "wasted" structurally; there's no increase in performance available there. And with BBright's 30mm axle and 42mm OD bearings, if we had pushed the right side outboard it would have meant that it would be incompatible with power meters and triple chain rings (especially for MTB) because there would have been interference between spider and frame. So the shell's right face stayed at the same lateral location as BB30.
However on the left, external bearing bottom bracket standards (Shimano's Hollowtech II, FSA's MegaExo and SRAM's GXP cranks, among others) had already "claimed" an approximately 11mm wide space outside the left face of the standard 68mm BB shell. This is exactly the space BBright transfers to the frame. Instead of threading a bearing cup into a 68mm BB shell, BBright keeps the left bearing in the same location but houses it inside the frame. Because of this there's no change to crank arm spacing, ankle clearance or Q-factor with BBright; the main difference is that the extra 11mm width in the BB shell now permits wider frame tubes than before: 11mm wider. This extra 11mm in frame tubes yields huge increases in the frame's structural efficiency, thus system performance.
Stiffness & Moment of Inertia
To really appreciate the improvement of the 11mm wider frame tubes it helps to understand the engineering of stiffness. Stiffness of any structure depends on two things: the stiffness of the material ("E" or "modulus of elasticity") and the cross section property known as the Moment of Inertia (I). In engineering terms, stiffness is E times I or: Stiffness = E × I
The really neat thing is that Moment of Inertia, I, is calculated with the equation below. Where b and h are the height and thickness, respectively, as shown with two imaginary chain stays.
Notice that that the lateral stiffness of these tubes is related to the cube of each tube's lateral thickness, (h)! That means that if you double the thickness (i.e. 2x) the stiffness increases much more than 2x. In fact, the new tube's stiffness is "2 cubed" times the old tube's, or 8x.
When we apply this 11mm increase in width to various tubes on a bike frame we get some interesting results:
Consider the down tube. Even the theoretically widest down tube possible with a standard shell, 68mm, when increased to 68+11=79mm, gains 57% in moment of inertia in the lateral direction. That's a giant increase in stiffness and strength compared to traditional techniques like thickening the wall or increasing the height of the down tube. The seat tube gets a similar increase.
The really amazing tube is the left chain stay. Typical chain stays are between 15 and 17mm wide, limited on the inside by tire clearance, and on the right side by the chain rings. With no chainrings on the left, adding 11mm increases the moment of inertia of a typical 16mm chain stay by over 381%. Yes, that's not a typo - almost four times stiffer with BBright™!
Let's do the math:
For a regular chainstay, let b=30, h=16, then using the equation above, I=10,240.
For a BBright chainstay, let b=30, h=16+11=27, again using the equation above, I=49,208.
Clearly the bigger "h" dimension with BBright makes a huge structural improvement: 49,208 / 10,240 = 4.81 times as stiff!
The chain loads the right chain stay more than the left, so the right chainstay is the first place to make improvements. Cervélo's right chain stays are already the maximum width that anybody could make them, given the space occupied by the rear wheel and the crank (which obviously every manufacturer has to contend with). So the left side is the next place to focus. This works because in a bike, the two chain stays always act together: the rear hub axle joins them at the back end, and the frame's bottom bracket joins them at the front end, so stiffening either one stiffens the pair. (This also calls into question asymmetric frames with a "larger" right side chain stay - isn't that just another way of saying the left chain stay is too small?)
Summary
Asymmetric, stiffer, lighter: BBright™ is the only Bottom Bracket standard that uses an oversize 30mm axle and allows for oversized frame tubes (up to 16% wider than standard bottom brackets). It offers the optimal combination of stiffness and weight for the overall system (crankset and frame together).
BBright™ was designed as a better system. The crankset and frame were designed together to work together. Not only is the oversize 30mm crankset stiffer and lighter, but the frame tubes are 11mm wider, which yields lighter weight and a big increase in frame stiffness - over 4 times stiffer in the chainstay alone.
BBright™ has all of these advantages while maintaining the same biomechanics and using standard drivetrain components, including power meters and triple chainrings.