Chinese Carbon MTB > 29er
Deciding between 057, 256, or 041
Jerryno:
I know you have reasons for every gram put off from your bike, but sometimes it can grow into obsession.
I can't feel 200g difference on my bike frame at all, but I can feel every 10gram on my rims and tires for example (and their stiffness too like you said).
Let's put that aside, we have different opinions here. What bothers me are the bolts.
--- Quote from: cmh on January 19, 2016, 11:36:25 AM ---If you think "yeah but still, 6 bolts are safer", I'll ask why you don't safety wire every bolt on your bike, because that's clearly even more saferer. Maybe apply red Loctite 271 before assembling, then safety wire for the saferest! It's only a little more weight. ;D
--- End quote ---
It's not about making it safererer than designed but about not messing with the engineering of the guy that designed it. If the rotor would be designed for 3 bolts (there are discs like that - even designed for 4 bolts) I would put 3 bolts in. The argument shouldn't be "I am half the weight of a heavy rider so I can cut half the carbon from my frame, use half the spokes in wheels and half the bolts in brakes." The result of less weight is truly less stress but that means just less component wear. I've seen plenty bolts to wear out and snap (not on bikes though).
Forces in brakes are enormous even for light riders - the kinetic energy to be stopped depends on half the mass but quadrature of velocity! And I would ques that lightweight rotors are not exactly over-engineered. Even using 6 Ti bolts should be monitored with care because titan is not that strong as steel. I hope you are not using 3 Ti bolts, that would be just stupid even for light rider.
There are spokes in the brake disc and there are usually 6 of them (or more, never less) and they push against the bolts. If some bolts are not there and anchorage points are missing then that creates bending inside the disc that can add to the bending from thermal expansion and the forces on the remaining bolts can be in different directions than designed for.
cmh:
--- Quote from: Jerryno on January 20, 2016, 03:04:07 AM ---I know you have reasons for every gram put off from your bike, but sometimes it can grow into obsession.
--- End quote ---
Heh, sometimes? I was protected from my WW tendencies in the past by simple physics. A guy my size should not be doing WW stuff. Then when my wife got into it, well, just about anything is completely over engineered, so at that point it's a matter of how much you wanna spend. :D
--- Quote from: Jerryno on January 20, 2016, 03:04:07 AM ---Let's put that aside, we have different opinions here. What bothers me are the bolts.
--- Quote from: cmh on January 19, 2016, 11:36:25 AM ---If you think "yeah but still, 6 bolts are safer", I'll ask why you don't safety wire every bolt on your bike, because that's clearly even more saferer. Maybe apply red Loctite 271 before assembling, then safety wire for the saferest! It's only a little more weight. ;D
--- End quote ---
It's not about making it safererer than designed but about not messing with the engineering of the guy that designed it. If the rotor would be designed for 3 bolts (there are discs like that - even designed for 4 bolts) I would put 3 bolts in. The argument shouldn't be "I am half the weight of a heavy rider so I can cut half the carbon from my frame, use half the spokes in wheels and half the bolts in brakes." The result of less weight is truly less stress but that means just less component wear. I've seen plenty bolts to wear out and snap (not on bikes though).
Forces in brakes are enormous even for light riders - the kinetic energy to be stopped depends on half the mass but quadrature of velocity! And I would ques that lightweight rotors are not exactly over-engineered. Even using 6 Ti bolts should be monitored with care because titan is not that strong as steel. I hope you are not using 3 Ti bolts, that would be just stupid even for light rider.
There are spokes in the brake disc and there are usually 6 of them (or more, never less) and they push against the bolts. If some bolts are not there and anchorage points are missing then that creates bending inside the disc that can add to the bending from thermal expansion and the forces on the remaining bolts can be in different directions than designed for.
--- End quote ---
None of this is surprising to me, I've got a degree in Mechanical Engineering. I'm not just some guy pulling parts off a bike and thinking "Sure, that'll work, why not!" You think it's stupid, that's fine, but I've got loads of empirical evidence to the contrary and always keep a close eye on ALL of the parts on her bike.
There was one time that I had an issue with a brake rotor not staying tight, okay, I admit it. Know what happened? I put a new disk on my own bike and gave the bolts a quick hand tightening - and forgot to torque them down. Even at that, I heard the strange rattling and discovered my fuckup when I stopped to investigate. So, which is more dangerous, a rush job, or running less than the designed number of parts? Based on my empirical evidence, the rush job was far more dangerous.
Pretty sure we won't be agreeing on this one, so I'll just share this: http://forums.mtbr.com/weight-weenies/3-bolts-disc-brake-safe-427825.html - actually saved me the work of doing the math.
cmh:
Trying to get the thread back on-topic, I just ordered up the CS-041, which Peter said was in stock. Hoping it comes pretty soon, looking forward to the build!
xcbarny:
Interesting link to the 3 bolt analysis.
As a side note. I got to ride with a very good UK sponsored downhiller and Enduro rider several years ago. He used to run 3 bolts in his discs without issue.
Wasn't a weight weenie thing for him obviously, he just said that he was travelling so much with his bike, that it was quicker packing his bike up with just the 3 bolts to remove the disc.
cmh:
--- Quote from: xcbarny on January 20, 2016, 07:52:41 PM ---Wasn't a weight weenie thing for him obviously, he just said that he was travelling so much with his bike, that it was quicker packing his bike up with just the 3 bolts to remove the disc.
--- End quote ---
I've heard the same thing said for XC racers. The mechanics run three bolts to make swapping disks faster. Seems like a strange explanation to me - pro XC racers have spare wheels with disks already attached, waiting in the pits. They're not going to swap disks unless things have already gone completely sideways, and at that point, the time taken to swap three bolts vs six isn't going to make a podium difference... that race is sunk.
The traveling thing, I can kinda understand, but the bike box we have I can just leave the disks in place and they're fine.
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