Originally Posted by
cny-bikeman
It is not necessarily the chainring itself. the large chainring will show more deflection due to its diameter. Make sure first that it is only the large chainring. Rotate the chainrings without the chain mounted and with something as a reference point over the next smallest chainring. If it varies at the same point, and does not vary anywhere else than the problem is the spider being out of true. Within tolerance is of course a matter of opinion. It's simply not true that you would have to replace the entire crankset. Please confirm 1st whether it's the chainring or the spider.
i assumed it's the big chainring since the part that portrudes is in between two of the widest part of the BCD. Opposite end of the crank arm to be exact. It comes out like sine wave so to speak only at that section.
Originally Posted by
Jamminatrix
1mm? You could grab practically any new bike off a showroom floor and the big ring will have a slight runout on true.
I have the same crank as you - 5800 (53/39) - and my big ring wasn't perfect either. I didnt measure, but I'd guess maybe 1mm of variation to either extreme. That said, 1mm shouldn't be enough to cause issue.
Well, you shouldn't run crosschain to begin with, but that's another issue.
If you are getting rub, even with trimming the front derailleur, your front derailleur probably isn't setup as good as it could be (shops tend to be in a rush and aren't always perfectionists).
The reason the shop can't replicate it is because there's no force on a bike stand to flex the rings/crank, as if someone was riding it.
There are some resources online that may help you determine if the setup of your FD is correct, or you could always get a second opinion/adjustment from someone you think is more competent.
Yea, i did that... went to shops, checked out the bikes and of the close to a dozen bikes I checked, mix of FSA Gossamer Pro and Shimano 105, they were straight.
1mm may not be much but on an 11 speed, tolerances seem to be so tight.
I've resolved the issue I think but I will try not to ride cross-chained still.
Originally Posted by
oldbobcat
I've seen that on new 105 cranks. The manufacturing tolerances on the rings are definitely looser than on Ultegra, but I haven't met one yet that I couldn't tune properly. That means no rub at all on the big ring using cogs 2 or 3 through 11 in back, and trim for 1 and maybe 2.
By the way, low trim is lower than default low with 11-speed fronts. After you drop to the inner ring, there's one more click to eliminate rub when you're using cogs 1 and 2. I've known some shop mechanics to be unfamiliar with this. I'd see what another shop can do.
BINGO! If anyone is searching this thread having a similar issue or just having chain rub in general, check the shimano dealer manuals. There's a note in there that says line up the outer edge of the FD cage and the teeth of the chain ring WITHOUT tension on the cable. To do this you tighten the low limit screw which pushes the FD outward. Once you have that set, you attach the shift cable. What this does is give proper tension which allows the L trim and T trim on the shifter to work better. My issue was whoever assembled the bike did not seem to do this step. They simply attached the shift cable as tight as possible in the small chain ring setting. This cause the travel for the T trim to be very small thus not clearing the chain.
They also routed the cable wrong on the FD. it was on the right of the converted pin. It's supposed to be on the left side of it. Why the shop didnt notice this just means they're likely unfamiliar, didnt care enough or just plain lazy. Irks me...