jcpups608

I'm getting a fairly heavy vibration when I'm in the two small cogs(DA 7800 group w/ about 2500 mi.). I don't use them all that often but when I do it's really annoying. The RD L-limit, H-limit and B screws are all adjusted to the best of my ability and the chain is not crossed.

I think these cogs are ti if that would make any difference. It also looks like the the outside of the chain is rubbing on the cog next to it but I don't know if that would matter either.

Any helpful info would be greatly appreciated.

Thanks.

DeadSailor

if the rd and fd are adjusted right you shouldnt get any noise/vibrations

MAY be the b screw, it pulls the upper pulley farther away from the cogs....

may just be bad upper pulley

jcpups608

It's weird, it's kind of a low rumble vibration, or reverberation might be a better word. The B screw is good. The pulleys are good, I just overhauled the pulleys and the rumble was there before I did that.

I'm wondering if this is just normal for the 7800...

...I hope not, I would think not.

LWaB

Do some research on "chordal action", relating to small diameter cogs.

Spinz

May be one of those gear combinations that just make noise. That's alot of stress of lateral stress on the chain ----- look at the angle from above. Lp

Trakhak

The roughness occurs only when you're in the small chainring in front, correct? If so, see the "Chain rub" thread posted recently by Grizwold.

cbchess

sound like cross chaining OR..

it could be the rest of your set up is worn in a bit - the rear cogs and chain have worn each other and still mate up - the other two cogs are not worn and now don't quite mate up with the chain.

jcpups608

No, big ring front, two smaller rings in back.

jcpups608

The chain is not crossed.

It might be the difference in wear seeing as the two small cogs are the gears I probably use least.

Is 2500 mi. of wear enough to cause that though?

meb

I had this same problem on one of my recumbents. It eventually went away-I had wondered if the cog wearing is what solved it.

meb

The chessmaster may have hit the nail on the head for your situation-you have a lot of miles, two lightly used Ti cogs (and Ti cogs will wear slower than other cogs). How about chainging the cassette and chain -you sound overdue?

Sixty Fiver

I have found that many small cogs run rougher during the initial break in... they are subjected to a great amount of stress and have minimal tooth engagement and in general, can wear out very quickly.

The Ti cogs are a good solution to this issue of wear.

Depending on your riding conditions and level of maintainence you could have much greater wear on your other cogs and could very well be in need of a new chain and if the chain is stretched and running on virgin cogs it could also makes things rough.

jcpups608

Overdue?

The chain is all good, I've got a chain wear indicator and the cassette seems fine. When in the small(12) cog, about four times a crank rotation, a tooth of the cog above it will engage just a little bit (sort of catches) on the chain pin as it goes by(It's not the same tooth each time). I think that is only part of the problem, the wear difference between the cogs is the issue. The ti wears more slowly and I use them less so they might not be as "broken in".

All other gears feel as good or better than they did 2000 mi. ago.

And I like my bike clean. I don't think maintenance could be a concern

Maybe I'll do some, how fast I can ride on rollers, rides and wear out those ti cogs a bit, otherwise they'll never catch up.

I don't know if I've figured it out completely but all the info has been very helpful.

Thanks all.

Metzinger

Titanium wears way faster than steel. If the Ti cogs feel different than the steel ones, then they're probably worn more. The chain is sitting to deeply on them, causing the vibration.

Titanium was probably chosen by Shimano for these cogs because they thought people wouldn't really use them.

Metzinger

One other thing. Dura Ace Ti cogsets use Ti for the larger cogs, not the smaller ones. You can confirm this with a fridge magnet. If the small cogs are a problem and you don't use them much, I'd say that it's the chain that's stretched.

Madsnail

Is this a constant or irregular vibration?
I have sort of similar symptoms on my bike, 105 with Ultegra chain. I feel it only on the big front ring and smaller cassette cogs because more pressure is applied on the pedals (I guess). The vibration is not constant, it's got a regular pattern though, quicker on bigger cogs and slower on smaller cogs.
If I put the bike on a stand, turn the cranks and watch the rear derailleur, I can see it rocking slightly back and forth in rhythm with the vibration. I suspect the chain, but I don't really know. Definitely not wear anyway, the whole thing is brand new.

smovlov

I get the same thing but in small/big combo. I was going into a headwind and when I applied more force there was a vibration running through the frame. No crossing/rubbing chain. I do have a new chain on an old drivetrain but the cogs don't appear to be worn. The old Sedis chain was not too bad either.

meb

2500 miles is a long way for a chain to last. But since your indicator says ok, I'll accept that.

Were these small cogs giving this same problem when the cogs and chain were new?

jcpups608

Really. The larger cogs? If what you're saying about the rate of wear w/ ti is true it makes engineering sense that those would be made of ti. Well that changes the thought process a bit.

It might make sense that the chain is sitting a little deep. It seems logical the way the vibration feels.

Is this a constant or irregular vibration?

Constant.

I'll try a new chain, then cassette. We'll see. It's just frustrating when everything looks so nice and rides... almost as nice.

Thanks for all the help.