Bob Dopolina 

I'd like to do an informal survey of anyone using road disc (with 135OLD) who is experiencing chain rub on the front der, chain drops - esp in the small ring - or who has experienced their chain catching on the back of the large chainring when using the smaller cogs on the rear.

Another symptom is having a chain fall off while pedaling backwards.

I've seen threads about this on other forums and I am wondering how wide spread it really is.

spectastic

I built up a cross frame with a racy geometry. It's got a 3x8 drive train with bar end shifters, and mountain bike derailleurs. not exactly orthodox, but I'm finding myself having to trim the FD every 2 (definitely 3) clicks on the RD. it's kind of a pain in the ass

shifting characteristics also change as I progress through the cassette. Say I'm in the big ring. shifting is smooth for the smaller cogs, but gets more sloppy going from say 5th to 6th in the back. but again, this is a weird setup. 8 speed chains have less lateral flexibility, and the triple offsets the big ring by a couple of mm's.

only used the granny gear a few times. no problem so far. not expecting trouble, considering it's a triple.

Bob Dopolina 

Which rear hub did you use?

spectastic

sram x7.. it's a 29er. why, is that important?

Bob Dopolina 

I wanted to know about the chainline on the hub itself. I'm assuming it's an MTB chainline, hence the issues.

spectastic

didn't know they were different from existing disc road chain lines.

Bob Dopolina 

Yeah, that's kinda the point of the thread.

Road chainline is 41.5mm while MTB is 44mm. It doesn't seem like much but that 2.5mm difference can cause some of the problems I mention as well as the ones you describe.

I see this as an possible issue going forward and I'm trying to get a real sense of how widespread it is. As I mentioned, I do know the problem exists with specific bikes/models and now I'm fishing...

spectastic

Bob Dopolina 

spectastic

well yea, but i didn't know that actually had implications for the rear hub. i know the BB is different, but I thought the cassette position were the same.

Bob Dopolina 

Actually, it's the distance from the center line to the middle of the cassette that is the issue. If you use a MTB hub (with 44mm chain line) on a road bike or many cross bikes - which are designed for road chain line (41.5mm) - it places the cassette too far outboard. This can cause the problems I described.

spectastic

the shifting problem I was describing was a little different though. it runs into a little trouble shifting into the bigger cogs on the big ring, which suggests that the more outboard cassette on the mtb hub actually probably helps the problem. But yes, I do get some drag on the big chain ring when I go middle ring with the smallest or second smallest cog in the back.

Dean V

I have been using DA9000 cranks (53/39) with 135OLD 11 speed. Haven't had any issues. Works great.

FBinNY 

OP, you're linking two unrelated things. The disc has nothing to do with chainline, since it's on the left and doesn't affect the placement of the cassette.

However you are right that widening the tight triangle by 5mm moves the cassette out 2.5mm. That's easily compensated by moving the crank outboard by the same amount to compensate.

All the problems you list are matters of adjustments, except for the "shadowing" of small chainring by the large one, and that's been a minor issue as a result of the rings being closer together, which happened before road disc.

One thing we are seeing is conflict between the objectives of widening the rear triangle to reduce dish or make room for a disc, and keeping the cranks inboard because of Q-factor concerns. You don't have both.

Bob Dopolina 

I am not talking about the rotor, at all. It has nothing to do with the point I'm making.

Moving the crankset 2.5mm outbound to line up with the cassette is not a viable solution for a few reasons: First, it changes the Q-factor. Second, it would require a complete retooling for entire crankset ranges, which is pretty spendy, and then it leaves us with a whole new set of specs for cranksets. No thanks. Lastly, it moves the crankset out of the sweetspot in terms of the arc the fr der travels. This could make shifting even worse.

The least expensive and most correct answer is to put the chainline of the hub back where it is supposed to be. This solves all the problems I mentioned and requires a minimal amount of tooling.

GeoKrpan

I have two 135 OLD bikes, both with 50/39/30 triple cranks, one with 68mm BB shell, one with 73mm BB shell.

The one with the 73mm BB shell has some chainring rub but I'm certain that I could eliminate it with some tweaking. The other has none.

I have found that the chain coming off when pedaling backwards can be lessened by alignment of the derailleur hanger.

I also have a 2x10 with 170mm OLD and a 100mm BB shell. No chain rub.

andr0id

I understand, the rotor just forces you to use a 135OLD rear hub.

First, you are correct about crank sets. When you use the correct BB Bearings and crank on the correct type of BB, that should be it. No spacers, shims or other nuttiness should be required or ever used.

At the rear, they basically are going to put the small cog as close to dropout as they can get away with then work inward from there. If you have a short chain stay, the chain angle of small chain ring to small end of the cassette is going to be more severe and you're more likely to get FD rub. We already get this a lot on 130mm rear hubs, so going to 135 OLD is only going to make it worse.

popeye

This is probably more likely/worse on smaller sized bikes.

thermalattorney

Hey bob. I built up a custom steel disc road 6mo ago. Di2 hydro, 135mm rear, Shimano RS500 cranks w/ Praxis compact rings. Chainstay clearance issues on this particular frame require wider CX-style cranks w/ compact rings.

There's FD cage rub in the two smallest cogs while in the small front. My frame happens to have short 410mm chainstays, which is the minimum length Shimano recommends on 130mm hubs. Throw in an extra 2.5mm and boom: rub. I feel like this could be fixed if there was a way to move the FD outboard slightly.

Pretty sure there's no chain/ring contact while in small-small. I've had that before on 130mm spaced bikes with semi-compact rings but didn't cause any issues then (beyond the noise).

I have not experienced any issues with chain drop while pedaling backwards, but did have a single instance of bad chain suck a couple of months ago.

Spoonrobot 

I've got about 4000 road miles and 400 gravel miles on a Shimano FH-M615 Deore Centerlock Disc Rear Hub and have one of the issues you mentioned. I get chain rub on the front derailleur in the smallest two cogs that I cannot fix. I'm running Sora 3500 and have not had this issue on my regular road bike.

My front chainring is 46t so that may prevent the large chainring rubbing. This is a Soma Double Cross Disc frame.

Racing Dan

CL is supposed to be 43.5 mm for road double and 45 mm for road triple. However a 135 mm hub does move the cassette further out and can give rub on the big ring when using small ring / small cog combinations.

Bob Dopolina 

I'm sorry, but those are not the current dimensions.

Regardless, you are describing the same issues this informal survey is about.

Racing Dan

Maybe we are not talking about the same thing then, but the the numbers I referenced are from the Sheldon Brown page.

Can you please provide som kind of reference for the "new" 41.5 mm chain line spec? I have not heard of that before.

All About Bicycle Chainline

Bob Dopolina 

Yes, that page is out of date.

I can upload a hub drawing, if you want, but the spec is taken directly from the Shimano Factory Manual.

Racing Dan

The "hub chain line" (middle sprocket) is, to my knowledge, not necessarily the same as the "crank chain line". IE the middle sprocket is not in the same place on a 9 sp cassette as on a 11 sp. I would like to see the new spec for cranks if possible?