Weight Weenies

SvenNijs wrote:Interested to see what 6870 will look like.

davidalone wrote:

havana wrote:I fail to understand Shimano's crank strategy. No BB30, weird 4-arm design, assymetric blades, no 110mm 39T blades, etc, etc.

shimano does not do BB30 because they do not believe that BB30 is a superior standard. they have argued this before, and as a mechanical engineer, their argument does have merits. BB30 gives you a tiny and, to a human, imperceptible increase in stiffness for much much higher complexity in maintaining the system. you get smaller Q factor, yes, but that is not a good enough reason IMO. it is, technically more robust to the elements than outboard bearings, but they are a PAIN to replace if something goes wrong. outboard cransk are much easier to replace, more versatile, and less likely to damage your frame on installation. same goes for PF30, and to an extent, BB86 and BBright as well. shimano will also make less money. so I DO see why they would not support BB30.

BB30 is popular among frame manufacturers because it saves them money. it's much faster and cheaper to purchase an OE spec bearing and press it into the fram than having to machine out steel threaded inserts and bond them into frames. it's a simple case of frame manufacturers versus shimano here. but shimano is sticking to their guns and as the major industry player, they have evrey right and clout to do so.

the 4 arm design may LOOK weird, but it offers tangible engineering benefits. the 4 arm design has been on shimano MTB cranks for awhile now. 4 arms are lighter than 5 arms, and shimano claim that the crank still retains the same level of stiffness. in addition, the new 4 arm design makes 110 or 130mm BCD pointless. you can put a 50-34 on that crank. you can also put a 53-39 on the very same crank. this is a game changer. cuts down inventory that stores need to hold ( you don't neeed to carry both compact and standard cranks anymore.) it also streamlines manufacturing. these savings may eventuaklly be passed down to the consumer if enough people follow their 4 arm standard and make chainrings for 4 arm cranks. ( shimano chainrigns are great but exoensive.) the problem here is that most other crank or chainring manufacturers are 5 arm, so they are trying to revolutionize the game here. My feeling is that this is a real game changer. once you get used to the 4 arm design, it actually looks pretty cool.

the new crank is certainly an impressive bit of techy kit and I'll be trying to get my hands on one as soon as ultegra 6800 comes out.

Rush wrote:I hope they make a glossy silver option.

Kurt wrote:Ok, now I think I'm ready to upgrade from Ultegra 9-speed. I never thought 10-speed would catch on, so I'm glad I waited

davidalone wrote:why is steel for 24 mm 'the best'? how are you defining the best. most stiff for lightest weight? there is no 'best'. basically you'd select a chosen level of stiffness/weight compromise you want and go from there.

the stiffness ( ability to resist deflection ) of a circular shaft are dependent on the elasticity of the material, length, and area moment of inertia of the beam.

http://en.wikipedia.org/wiki/Deflection_(engineering" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;) .

I for a circular hollow shaft is dependent on the difference between the inner and outer radius.
I'm no shimano engineer, and I don't have the calculations with me, but it should be possible to make a crank axle made of aluminium as stiff as the steel one. you'd just have to use a larger diameter ( hence the 30mm axles.) would it be lighter? Not necessarily. it would depend on the wall thickness and length of your crank. besides. your weight savings is taken back up by the bigger bearings, so it's basically a wash. by the same note, you could certianly make a 30mm axle out of steel. it wouldnt necessarily be heavier. you'd just have to play around with the other variables.

do note that the sacrifice for this would be that the crank would be less bombproof. aluminium is far softer than steel and more susceptible to stress cracking and shear failure. Titanium... slightly better, but still not as good. steel is used here because it is STIFF and BOMBPROOF in a place that you really, cannot afford to fail. and also relatively cheap.

bigger bearings better? slightly true... to an extent. you could actually mitigate this by designing a better bearing system ( e.g. using angular contact rather than radial contact.) shimano bearings already last a really long time, so I don't really see an advantage here. the fact remains that PF30/BB30 BBs still make maintainence a PITA. with the risk of damaging your frame when replacing them. stiffer frame with pf30? very, very, very, marginal. you're right that shimano goes with bombproof stuff. outboard bearings are bombproof and easily replaceable. AND also much more easy to find. if you are a pro with a mechanic who wrenches for you and who can afford a damaged frame by all means, BB 30 has advantages for you. for regular joes like us, regular outboard bearings are still much more practical.

TrekUk wrote:

Kurt wrote:Ok, now I think I'm ready to upgrade from Ultegra 9-speed. I never thought 10-speed would catch on, so I'm glad I waited

what on earth are you talking about? 10 speed would never catch on?