I am thinking about replacing the cranks on my Dual. I am getting a bit of flex (I think) and it is affecting the shifting on the front deraillur. The stock cranks are FSA something or another. Was thinking of going to an Ultegra crank. Any thoughts?
I have a hard time with “crank flex” are you sure that it is not frame flex?
No I’m not sure, but the LBS says that a new crank would eliminate the problem. I trust these guys and they have worked on the bike three times at no charge trying to alleviate the problem in another manner. Including changing the front chainrings out for free.
try an integrated BB for addl stiffness, such as the new Dura Ace has, and does Rotor Cranks: http://forum.slowtwitch.com/gforum.cgi?post=132763
.
Things I would check before changing the crank, because it seems to me it’s a expensive part and that it’s unlikely to solve the problem …
bottom bracket : misalignment, “trueness” of bearings, etc.
chainrings “trueness”
front der setup …
Because the chainrings are not bolt to the arm of the crank but to the spider, I think the flex of the crankarm doesn’t have much effect on shifting … or you are a pure monster near 1500 W ;-)) and able to flex the crank spider …
HTH
or you are a pure monster near 1500 W ;-))
Don’t I wish. I may be misrepresenting the information given to me by the bike wrench since I am fairly illiterate when it comes to bike mechanics. I will try and restate this in as close to the original wording as I can. “We have it as close as we can, it is possible you are putting too much force on the pedal as you shift up, try to lighten up as you shift, a upgraded crankest would eliminate 95% of this problem” I am lightening up as I shift but still have an occasional problem, would love to not have to think about this as I try to pass the guy ahead of me.
I think your mechanic isn’t wrong when he says that you can lighten up your pedal while shifting but I doubt a upgraded crank is necessary for different reasons :
first, it’s perhaps the quality of the chainrings that is questionable, then no need to upgrade the crank, just the rings, but you wrote it has already been done
second, I guess the stiffness of the crank is not very well correlated with the price and level so …
Check front der (height, parallel to chainrings, cable tension, etc.), chain length … and at the end, live with it ;-))
first make sure the chainring bolts are tightened and the derailleur is properly adjusted. that’s free.
second, check for chainring and chain wear. replace if necessary. pinned and ramped rings might shift better, but i never had problems with regular non-pinned 7 and 8sp rings either. still, pinned Ultegra or DA rings will pick up quicker than a plain 8sp ring on an 80’s crankset.
if you’re still having shifting problems, check that the bb cups are not loose, or that the spindle is not cracked. yes, that does happen.
as far as crank stiffness… all that’s pretty much hooey.
the crank gives a pretty large lever acting on a rather skinny bottom bracket spindle. some people test crank arms alone to ascertain stiffness among various brands, but few check the spindles. if you work out the mechanics, most of the perceived flex is in fact in the bottom bracket, where the spindle bends between the two bearings.
furthermore, the non-drive side is responsible for the majority of this flex, because the torque on the spindle on the drive side is countered by the chain tension acting on the rings/spider. it can still affect shifting, because the bending of the spindle in the middle will deflect the drive-side shoulder / arm / rings.
Ritchey used to make a lovely square-taper bb with a spindle that was fattened up in the middle, and had a shorter shoulder. this shortened the effective lever and stiffened up the portion which flexed the most - the section of the spindle between the bearings.
anyhow, bottom line is get yourself a stiffer bottom bracket spindle. that usually means steel and none of that titanium BS.
******
okay, i re-read your posts, and if a competent shop person tried swapping rings and adjusting the fd, there’ something screwy. they would’ve also noticed a cracked bb, but that’s not a certainty.
when you shift into the small ring, is the lower edge of the der. cage no more than 2mm above the teeth of the big ring?
is the limit screw tightened too much? back off half a turn and try again.
are you running out of cable pull?.. make sure the cable is just a little taut when you’re in the small ring. trim if it isn’t.
how about chain wear?.. with the chain on the big ring, pull on a link in the 3 o’clock position and try to expose a link. if you can, you should replace it.
I’ve ridden plenty of 70’s and 80’s era thin, flexy-flexy cranks and never had a shifting problem that couldn’t be corrected by proper derailleur adjustment…
a $120 for a new crank and maybe another $30 for a new bb is a bit too much to fix what is in all likelihood a derailleur adjustment problem, imo…