truflip

I have a bike with Shimano 105 compact crankset and the big chainring seems to be out of true only in one spot. It moves out of true maybe 1mm but with an 11 speed system, theres very little room for even 1mm of movement. Otherwise, it'll rub the FD when cross chained (big chain ring, large cog or smallest cog). I have STI shifters with trim shifting or whatever it is called and have adjusted for that already. The smaller chainring doesnt have issues.

I brought the bike back to the LBS I purchased the bike from and they said could not replicate issue. I got the opinion of another LBS who detected the issue but said it's not out of "tolerance". They said the new Shimano chain rings are hard to true since theyre so stiff. Only choice is to replace them it'd be an entire crankset, not just chainring.

Any of you have experience with this? Should I call Shimano? Should I source the big chainring online and replace it?

I get by for now by not crossing chain but more times than not, I can't help but be cross chained to keep my cadence and rhythm steady. TIA!

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.

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.

Andrew R Stewart 

At one time in the 1990s Shimano included an instructional page with new bikes which stated that rub on the rings by the chain wasn't a wrong thing. It pictured the cog/ring combos where rub was to be tolerated. Andy.

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.

truflip

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.
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.
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...

oldbobcat

Finally got one right on this forum. Thanks for the feedback, flip.

Setting up Shimano 11-speed road is a bit of a mind shift. A lot of experienced mechanics haven't bothered to read the manuals (or take the Shimano online training), and a lot of experienced mechanics can't be bothered to, either.

Andrew R Stewart 

We've seen a number of 11 speed road bikes (Shimano equipped) with the ft der's bracing screw not fully set against the additional pressure plate, or just run into the carbon frame surface. We do mention this to both the customer and the Trek rep. Some one needs to know that the new stuff isn't being set up correctly by some shops. Andy.

truflip

My pleasure! thank you for sending my in the right direction. It's my first road bike so I had no preconceptions on how these FD's work mechanically so it was easy to just simply read the manual and redo the install. I guess I'll be doing my own tune up/repairs if most mechanics can't be bothered to update themselves.
Do you mean the braze on FD's? I have a band type FD on a CF frame and there are no additional pressure plates nor was it mentioned in the manual. It was mentioned for braze on types though.

Andrew R Stewart 

truflip- Yes. the bracing screw and pressure plate are on the "braze on" (or rivet and glue ons) mounts that we have only seen. All have been on plastic frames.

I can fully understand why Shimano has added this extra feature as soooo many "braze ons" (on carbon frames) are very flexible. Even with the more common ft ders in years prior the non clamping band mounting is quite flexible. Just one more example of the how changing materials and their application without changing component designs can be a bad choice. Andy.

oldbobcat

Which is worse, ignoring the bracing screw completely or grinding it into the frame? I vote for grinding it into the frame.

Hear, hear, about flexible front derailleur tabs. Well, maybe not the tab so much as the part of the frame it attaches to. I was working on a newer Kestrel this week, that generally shifts pretty badly. The in-frame cable routing is pretty bad and you can watch the front tab flap when pulling up to the big ring. The tab's stiff, but the seat tube behind it isn't.