I love my beautiful new Aegis carbon fiber bike, but there is one annoying problem: the bottom bracket is so stiff that I can flex the crank/bottom bracket enough to make the chainrings rub in the front derailleur cage more than I'd like.
It occurs to me that a stiffer crank MIGHT help (unless it's the bottom bracket axle that's flexing). Anyone out there have experience with a carbon crank like the FSA? Would this do it?
Yes, I'm sure that dropping another $450 or so for the Dura Ace crank/bb would do the trick.
Thanks in advance for your help.
Anyone have experience with carbon fiber cranks?
Thu, 08/19/2004 - 3:06pm
Fri, 08/20/2004 - 10:05am
#1
A number of things don't add up here
>the bottom bracket is so stiff that I can flex the crank/bottom bracket enough to make the chainrings rub<
First, if the BB is too stiff how can you be flexing it?
Second, you don't say what crank and BB you're using, whether it's a double or triple, how short the chainstays are, whether you're getting chain rub in normal gears or crossover gears (big-big or small-small), how much you weigh and whether this occurs while seated, standing, or both.
Third, if you're actually flexing the crank, a CF crank isn't gonna be the answer for you (CRACK!).
And finally, you're gonna hate to hear this but it seems most likely that your frame is what's flexing - all frame materials can flex to a greater or lesser extent and CF is no exception. The way the frame is built, the design, the construction, is far more important than the actual material used.
Did you try contacting Aegis about your concerns?
Fri, 08/20/2004 - 3:31pm
#2
to be specific...
The rub is really only in cross-over gears like big-big and small-small. The crank is a double. I weigh 173 lbs and the rub occurs usually when I'm hammering. The chainstays are on the short side, but what isn't these days on a race bike?
Jeez, do people really break CF cranks? That would be scary.
Fri, 08/20/2004 - 7:50pm
#3
People break everything
A friend snapped a steel Phil Wood bottom bracket when he was sprinting down a hill at around 40 mph.
His theory is that there was a nick in the axel from dropping the chain on it and the imperfection led to a stress fracture.
Old Campy aluminum cranks were notorious for breaking– a design flaw, I think.
Carbon is strong out of the box, (if it's well designed), but it's made from glue and carbon fiber. Chip the perfect finish and who knows... But it's pretty. That's why I have FSA carbon cranks. I don't notice any functional difference between them and aluminum.
I don't worry about failure– I weigh about 150 lbs and spin. A tire blowout at high speed is much more likely, but we still ride light-weight high pressure tires.
I weighed the odds and then bought the pretty cranks. Life's a risky proposition.
Mon, 08/23/2004 - 8:20am
#4
Don't do that
"""The rub is really only in cross-over gears like big-big and small-small.""
You shouldn't be using those combinations anyway. In the small-small case, it's more common for the chain to rub against the big ring than the front der (with the typical 53/39 crankset). That has nothing to do with flex, just geometry.
""Jeez, do people really break CF cranks?""
Yep. See:
http://www.serotta.com/forum/attachment.php?s=9935a2edb6ddd7313fbc15725168c6ce&postid=6286
""Chainwheel"""
Mon, 08/23/2004 - 2:31pm
#5
Cross-chaining
I think that many people know that they shouldn't use gear combinations that put the chain at an extreme angle (especially small-small and large-large) but do it anyway. My theory is that with the shift controls at the brake levers, shifting is automatic and doesn't require any thought, and you can't tell the gear you're in by looking at the shifters. Thus people shift to small-small or large-large without knowing it. With my downtube shifters I know what gear I'm in by glancing at the shift levers.
Tue, 08/24/2004 - 4:59am
#6
It's a feature!
"""Thus people shift to small-small or large-large without knowing it.""
Good point. Maybe Shimano's marketing people should start touting the chain rub as a feature that helps folks avoid cross-chaining.
""Chainwheel"""
Tue, 08/24/2004 - 6:31am
#7
Carbon
Those $800 Colnago carbon cranks were just a marketing ploy by Ernesto to jump on the carbon bandwagon. They were known to be pieces of crap. They were manufactured poorly with no regard for safety or reliability. FSA carbon cranksets have been used on the MTB cross country circuit for quite sometime where they have been tested for reliability and strength. Only then did FSA start making carbon road cranksets.
I would also stay away from the two piece carbon cranksets, where the aluminum spider is separate and bolted to the carbon crank arms. Eventually they will start to creak. The $129 crankset from CC is two piece.
Tue, 08/24/2004 - 8:27am
#8
Are you for real?
Who wouldn't expect chain rub in big/big, small/small chainring/cassette combinations? This is a given. The chain is running at an extreme angle in these gear combinations.
Tue, 08/24/2004 - 12:14pm
#9
Well, with all this talk, the solution to your problem has been obscured...
1) Determine whether the rub is coming from the
(a) derailleur cage or
(b) the outer chainring (when you're in small-small)?
Confirm this by running the drivetrain with the bike in a workstand, or just by looking and listening carefully.
Sctchhzttccthcchth is chainring rub
Sring-sring-brnig-srsrnng is inside plate of the fd rub
kack-kkkkack-ckkakc is outside plate of fd rub
If the answer is (a), you should be able to trim your front derailleur to avoid noise in all gears, even sprinting or climbing out of the saddle.
If the answer is (b), the only real solution is to get a bike with longer chainstays. Many modern bikes are saddled with overly short chainstays, which cause severe chain angles, bumpy rides, and terrible weight distribution for taller riders. Anything under 41cm is pretty suspect.
As others have stated, avoiding big-big and small-small is a given.
- Christian
Sun, 08/22/2004 - 4:41pm
#10
Carbon Cranks
I just bought the FSA cranks because they were finally on sale at Colorado Cyclist for $129!!!
I haven't put them on the bike, so all I can say for sure is that's a lot of pretty for a little money.
Mon, 08/23/2004 - 2:13pm
#11
CF? Pretty?
To each his own, but... black plastic cranks are pretty?
Hold that FSA up next to a old Campy C Record, and tell me which one wins the beauty contest!
Mon, 08/23/2004 - 5:11pm
#12
Are they still on sale??
I usually buy from CC but I never saw that price.
