Petachhi's Dogma frame has a new BB shell size it looks Pinarello will be pushing, I wonder how easy it will be to sourse spares in 5 years?

http://www.cyclingnews.com/photos/2004/ ... F04-Tech15

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Success is how far you you bounce back up after being knocked down

To me, it's just a matter of time before shells get bigger with hollow spindles pushing the diameter out.
Design wise, is there a downside to larger dia. shells?

I have a CAAD 7 with si cranks, for me the only downside for larger diameter BBS is the fact that (at least in Cannondales case) the bearings are press fit into the frame like headset cups, so the potential for f-ing up your BB if you are not careful is greater. IN pettachi's case, the larger BB probably makes up for the square taper BB spindle Campy still uses. I'll bet that guy flexes spindles like they're nothing. I love campy and it is by far the best components in the world, but they need to re-evaluate their BB spindle design.

Excuse me for my bluntness but spindle flex is hogwash. The Record BB/carbon crank combo is one of the stiffest available. The difference between mid-range up to high-end is a matter of fractions of millimeters. There's no actual performance increase to be gained in that area. Your frame will flex an order of magnitude greater. On the other hand if they're lighter without loosing out on stiffness, similar to Cannondale SI, that's something credible. Otherwise, it's just stiffer for the sake of being stiffer.

well, bluntness aside, spindle flex isn't hogwash. Why would shimano have gone to lager spindles, just for the hell of it? Yes, it is lighter, but its also a lot stiffer, and not just stiffness for stiffness sake. When you flex a spindle, you also flex the frame, but larger bb shell will also decrease frame flex.

This subject bring up questions of how and why designs evolve over time.
With the new ISIS hollow spindles increased dia. you'd think they would have increased the BB shell at the same time. A complete new intigrated system would make more sense than doing it a piece at a time. Why do they hold back like that?

Simple marketing. They have to come up with some reason for why you should buy theirs instead of Brand X. Like "29% stiffer"

They neglect to mention that any bending, twisting etc is going to be elastic. Meaning all the energy used to wind up that spring is returned to you. Springs only store energy they don't dissipate it. Hysteresis losses are incredibly small in metallic structures stressed below the elastic limit. Like a bottom bracket or frame.

It's more about feel. Most people don't like to feel flex.

FSA has tried to do this with its mega tech (or something like that) system. But I guess they had a hard time getting manufactures to adopt the system because I have never seen it anywhere...like most things, change does not come with out struggle.

They neglect to mention that any bending, twisting etc is going to be elastic. Meaning all the energy used to wind up that spring is returned to you. Springs only store energy they don't dissipate it. Hysteresis losses are incredibly small in metallic structures stressed below the elastic limit. Like a bottom bracket or frame.


If the wind up is returned, would it not be returned in the reverse direction?
Imagine a bike with only a crank on one side and this becomes obvious. Could this affect forward motion and if so by how much?????

Think of it this way. Let's say you are cranking out of the saddle and are the the non-drive side down stroke, from 9 o'clock to 6 o'clock. Moving the crank through 90 degrees of rotation, 60 degrees of this rotation is transferred to the rear wheel, and 30 degrees is "wind up" in the bottom bracket (so now the crank arms aren't parallel anymore !) . At the bottom of the stroke, at 6 o'clock) you aren't exerting anymore torque so the bottom bracket springs back and moves the crank arm and your foot back to the 7 o'clock position. You now have gained the spring energy back in the form of potential energy in the lifting of your leg and body center of gravity and can now let your weight fall the extra 30 degrees to gain back what you put into stored spring energy.

The point is that energy is not lost in this system. The work you do winding up a spring is "done back" on you during "unwinding".

PS just for grins I calculated the following.

A rider exerting 750 Watts of power (huge !) at a cadence of 80 RPM is exerting an average torque of 66 foot-lbs on the crankarms. This torque will twist a 110 mm long, 0.625 inch diameter solid Cro-Moly cylinder (like a campy bottom bracket) through an angle of 2.4 degrees. I seriously doubt you can even feel 2 degrees of crank non parallelism.

But for the sake of comparison a 1 inch OD, 0.875 inch ID hollow cro-moly spindle of the same length (like an ISIS BB) will twist 0.89 degrees under the same load.

So I guess you could say an ISIS BB is 3 times stiffer. But are those kind of angles significant? We're talking about one third of the angle that the second hand on your watch ticks through during one second.

Now on the other hand I think frame twist will be much more significant. But again this energy is returned to you.

Want a stiffer bottom bracket? Buy a frame with the shortest most monstrously thick chainstays you can find.

I have a feeling that you are right, that frame/bb flex is almost insignificant in real world power. But I also think that stiffer stuff can sometimes FEEL faster, and if you feel faster you will go faster. Even if then reason you feel faster is because some marketer told you it was stiff, however shallow that is. So is it faster? Depends on how you feel and what you think.

I think that if you take a bare frame with a hub clamped in the rear dropouts, then clamp the hub in a vise, and put a long pipe through the bottom bracket shell and start twisting (like you were pushing down on one pedal and pulling up on the other) you'll find the chainstays to be quite flexy in torsion. Part of the problem is that a twin beam structure like that just isn't that great for transmitting torque. And if you make the chainstays really beefy then ride quality suffers.

The solution would be to isolate the seat from the effects of increasing chainstay stiffness, like in a Softride (sp?) cantilever beam type frame.

750Ws is not that high, my VO2Max used to be a 685W, with a max aroud 1450W. 80rpm is a good starting speed for a road sprint, but a standing start can produce massive torque. Not sure of exact values but somewhere aroud 3x what can be produced at 50rpm+.

_________________
Success is how far you you bounce back up after being knocked down

Craig MacLean and Chris Hoy can put out almost 2300W in their standing starts for kilo and team sprint. That's high!

Actually I made a mistake in calculating the polar moment of inertia of the axle. It's actually half what I calculated. So using the 2400 Watts at a starting pace of 60 RPM that the best guys in the world can put out, they twist a Campy cro-moly bottom bracket axle through 5.1 degrees. And everyone else on this board might be able to do half that. Again I doubt you can even feel that small of an angle of crank non-parallelism and the energy is returned to you since it's elastic deformation.

But if we were talking about titanium campy axles, then we'd be looking at about 9.3 degrees of twist from the best sprinters in the world. That might be a problem in terms of very limited fatigue life.