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Ask the Engineers: Birth of BBright

March 30, 2011 - Filed under: Ask the Engineers

Comments (16)

Have you ever had a burning question of how or why Cervélo design and built your frame the way it is? Well, now you can ask it in our new feature, Ask the Engineers. We will select questions you send in and get one or more of our engineers to provide the answer.

Question Why did you come up with BBright instead of using one of the existing bottom bracket standards?
Aaron S., Melbourne, Australia.

Answer 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 SquovalTM and tuning the cross sections and SmartwallTM 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. 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 as:

Where b and h are the height and thickness, respectively, as shown with two imaginary chain stays below:

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
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.

Cervélo models with BBright
Cervélo R3
Cervélo R5
Cervélo R5ca

Answered by Damon Rinard and Richard Matthews

(March 15: Small edits to the above text were made for clarification)

Do you have a question for our Engineers? Click here to ask it.

16 responses for this Entry

Damon Rinard, Cervelo Engineer says:

October 20, 2011

Hi Tim, Great question. Wider is not always better; when we engineered BBright of course we considered going wider on both sides, but didn't. BBright is wider on the left, not both sides, because wider on the right has no performance benefit. There are three reasons: 1. Widening the BB shell (on either side) has structural benefits only because it permits wider frame tubes (down tube, seat tube, chainstay). In a well-designed bike these tubes cannot be made any wider on the right; the chain, front derailleur and chain rings are already in the way. So widening the shell on the right has no structural benefit. 2. The space on the right side is needed to work with existing crank-based power meters (Quarq, SRM, Power2Max). So widening the shell on the right has no power meter compatibility benefit. 3. A triple chain ring would interfere with the bearing. So widening the shell on the right has no triple benefit. So there you go: BBright is the right width, not too narrow, not too wide. Cheers, -Damon

Tim says:

October 19, 2011

The 'new' FSA 386Evo bottom bracket found in some bikes for 2012 seems to have heavily copied design characteristics from BBright. The main difference seems to be that BBright is asymmetric while the FSA is not. Can you please quickly address the benefits BBright may still have over 386Evo? Thanks Damon.

Glen says:

May 02, 2011

Hello Angel,

You can contact Jorge at D'Bikes jorge@dbike.com.ve. They should be able to get you a bike.

Damon Rinard, Cervelo Engineer says:

May 02, 2011

Hi Samuel,

You ask "why the 2011 RS does not feature any of the advances from Project California".

The answer is timing. The 2011 RS is structurally unchanged from last year's model, so doesn’t have the features incorporated into the newly redesigned 2011 R-series models. Despite having more engineers than bike models, Cervélo still has to limit the number of projects we can tackle at any given time.

Damon Rinard, Cervelo Engineer says:

May 02, 2011

Hi Steve,

The Garmin-Cervelo pros are riding some R3s in the classics because the R5s weigh too little. Last year the R5ca was tested in the Tour of California (see the video documentary here: http://www.youtube.com/watch?v=ipjuWSTMDkA&feature=player_embedded

At that race, we filled up the frame with chains and still it weighed too little. We had to make a 565 gram bar in solid steel to insert into the seat tube of the frame to bring it up to the UCI rules minimum weight.

Damon Rinard, Cervelo Engineer says:

May 02, 2011

Hi Mario,

You can purchase BBright Campy bottom bracket cups from any Cervelo retailer. You can locate one near you by going to cervelo.com and clicking "Find Your Nearest Retailer" on the left side of any page. Cervelo part number BB-P-CAM-BBRI is listed with other technical details on the Compatibility Table on BBright.net web site here: http://bbright.net/supportingbrands.aspx

Damon Rinard, Cervelo Engineer says:

May 02, 2011

Hi Gracie, Why sloping top tubes? As you point out, Cervelo S-series bikes are already the most aero bikes out there (as VeloNews recently showed in their wind tunnel testing http://velonews.competitor.com/2011/02/news/vn-bike-lab-tests-four-aero-bikes_159639 ). On road bikes, sloping top tubes are a convenient way to save weight, improve ride quality (seatpost flex) give more fitting options (Cervelos still come in six sizes), increase stand over clearance and improve rear triangle stiffness.

karl says:

April 15, 2011

Great explanation. Thank you for taking the time to explain these points. Details/service/communication like this set Cervelo apart from the rest.

Samuel Lai says:

April 13, 2011

May I ask why the 2011 RS does not feature any of the advances from Project California, such as the BBright, R5ca tapered headtube, headset and fork, reverse thread rear derailleur hanger, R5ca bottom bracket cable guide, reverse seattube collar?

Damon Rinard says:

April 13, 2011

Hi Grayson,

Why no BBright on the S-series Cervelos? The S-series frames were designed before BBright was born. The S-series frames already have very stiff bottom brackets, due to Cervelo's unique Smartwall carbon layup and oversize bottom bracket area. These features are described on cervelo.com, under Bikes, then S2 or S3.

Also, Cervelo S-series bikes have long proven themselves speedy in the sprints: Thor Hushovd won the pro world championship in a sprint on his Cervelo S3, among other sprint wins on his different S-series Cervelos. Even this year, so far Team Garmin-Cervelo riders have won sprints on their Cervelo S3s. Just click on "News" at the top of this page to see Race Reports describing Tyler Farrar's sprint win in Tirreno-Adriatico, Heinrich Haussler's two sprint wins at the Tour of Qatar, etc.

Grayson R. says:

April 13, 2011

Why does the S-series not utilized BBright technology? I seems it would be very useful in the sprints.

steve says:

April 13, 2011

Why are the Garmin-Cervelo riders using the R3 instead of the R5 in some of the classic races?

Mario Traversi says:

April 08, 2011

How practical will it be to integrate the BBright design into your aero bikes, especially the S3, and what sort of tiem-frame do you envisage for that sort of development? If such a development were on the cards, what sort of difference would it make in terms of frame stiffness, and especially frame weight?

gracie says:

April 04, 2011

Why don't you make the S and R series bikes with horizontal top tubes to make them more aerodynamic (especially the S series although I understand it's already the most aerodynamic road bike out there)? Is it to do with stiffness?

Ángel Leonardo Pescara says:

March 30, 2011

Necesito saber si Ustedes pueden mandarme una bicicleta a Ecuador, pero, montada con componentes Campagñolo Super Record Carbón, completa, como tiene que ser el pago a travez de que banco número de swift y todo lo demás.

Muchas gracias.

mario miglio says:

March 17, 2011

Good morning looking for BB right Campy adaptors cups for my new Cervelo R3 (2011) could you please tell me who has them in stock? so I can purchase them

Thank you Mario