indigophox

So, I first discovered this after I blew up the internals of my right/rear shifter, and was forced (until I adjusted the high-stop screw to force me over to my 12t cog) to ride around in 50-11 on my 20sp/compact drivetrain. Under sufficient power (read: I'm 180lbs, and singlespeeding in 50-11 required significant restraint to avoid problems) I can throw the chain off the outside of the big ring. More recently I've found that I can torque hard enough at speed to do this, too.

I'm running a -reasonably-, but not insanely, stiff ~3.5lb (@61cm) lugged unidirectional carbon frame, with Rival cranks+ring/cassette, and had this problem with both the original PC-1070 chain and a KMC DX10SC.

I have my own thoughts on things that are contributing to this, but I'd like some clean-slate input first.

Retro Grouch 

With the limited information available and without seeing your bike, my guess is that your front derailleur angle may be off a tweak. That's a pretty common cause for a lot of front derailleur woes.

Mr. Underbridge

Bad chainline? Clamp-on FD at wrong height? Flexy chainrings (you're not running a goofy small BCD crankset, are you?)

Those are my first three guesses.

indigophox

I'm running a 110BCD crankset, yes, but my suspicion is that if anything chainring flex would be helping this, given what I've seen happening and the direction the ring should be flexing under chain tension.

Front der is adjusted so it just -barely- doesn't drag when running 50-11. If it was height/alignment I'd expect a collision with the outside of the cage, and ejecting towards the inside of the ring.

Now, my suspicions...

I do think chainline is part of the issue here: This -only- happens in my 11t cog, and my impression (not that I ever expect it, so I'm not paying an incredible amount of attention) is that it finishes ejecting on the right pedalstroke, so it's likely happened with weight on the left crank. There is some visible deflection of the chainring when force is exerted -- not only about the fore-aft axis, but also about the vertical axis, both of which would contribute to misalignment of the chain and ring. Combine this with a not-brand-new chain which I -SUSPECT- (this is impossible to observe) may be capable of "walking" up tooth edges to help the chain eject.

So, the questions from that:

-Where on my cassette should the big ring be lining up with so I know my chainline isn't out by too much?

-What else can I do to work around this? Right now I'm considering a stiffer frame as a solution (which of course has a plethora of other benefits, too!)

Ex Pres 

As in all cases - Campy is your solution.

indigophox

Sorry, Campy Camper, I like my OG cassettes and stiff-as-hell cranks, and being able to... buy chainrings Of course, if this turns out to be an issue of chainline and/or frame flex I'd be back in the same boat with Campag. I guess having more flexy crankarms might save me flexing my frame as much though!

Al1943

Have you checked for cracks in the frame in the bottom bracket area?

indigophox

I've looked before, but it's near-brand-new, and also flexes completely symmetrically, suggesting there are no cracks. I'll double-check tonight, though. FWIW, although the flex axis was a bit different, I found I could bend a brand-new Trek Madone about as much. If I can't find a workaround for this, my solution is likely to be "buy a Cervélo".

DieselDan

The object of cycling is efficient use of power, not break everything and complain about it.

indigophox

True. It's hard to efficiently use power when my chain is no longer on my chainring.

Seriously, though... anyone run into this before? Anyone able to answer my chainline question in #4 above?

bikinfool

I have a button on my handlebars when I want to eject my chain, sort of James Bond like.

Maybe it's your unidirectional carbon frame, maybe it's your weenie crankset or weenie chains but more likely you're the weenie. Hard to tell from here.

Mr. Underbridge

Let's look at some other issues. What's your cadence when you're driving that 50/11?

It could just be a confluence of issues. If your a bigger guy by cycling standards, and you've got legs built for mashing, and you have a flexy frame, and you have flexy chainrings, and you're riding at a relatively low RPM...then I wouldn't exactly be surprised if you're able to create enough stress that things get a little out of line.

My suggested solution would be to spin more. Higher RPM is probably your answer here. The goal is not to use the highest gear ratio possible, but to use the optimal one.

Regarding your chainline issues - I believe the goal is to have the middle of your chainrings overlap the middle of your cluster. It's easier on a 9-speed triple, since you'd overlap the middle ring with the 5th cog. For a 10-speed double, the midpoint between the rings should fall between the 5th and 6th cogs. But I'd be surprised if the manufacturer screwed that up *unless* your rear wheel isn't centered correctly.

BarracksSi

I have had this problem with a Truvativ Elita crank. Tall gear, stand to accelerate, and it drops, even with the FD adjusted within a hair of the chain.

I took it in, and was told that the crank spider was coming loose from the axle. I hadn't even put 500 miles on the bike (CAAD8), so they warrantied it. I chose a Shimano 105 compact as a replacement, and it hasn't dropped a chain in 1500+ miles.

Your Rival crank appears to have the same construction, so I'd consider using a different crank. I'd even keep my Shimano crank if I changed to SRAM shifters.

AEO

crankset spider arms, or between the two chainrings, to the middle of the cassette should be pretty much a straight line.
https://www.sheldonbrown.com/chainline.html

AEO

how new is your chain?

indigophox

I'll doublecheck the chainline. There is definitely -some- variance between freehub positioning on different hub models. I'm definitely not mashing -- with the exception of my accidental singlespeeding experience, which was a super-low cadence, I've been seated and spinning at least 90+ (and probably a fair bit higher than that) when this has happened.

I've been wondering recently if this is related. I can't say it's ever happened on a brand-new chain -- the only relation to a worn chain I can think of is that the increased link spacing allows links to bind against tooth faces and "walk" up towards the tops of the teeth under constant torque.

Thanks for the input so far. I'm pretty sure this boils down to a combination of 4-ish things: frame flex, chainline (I'll also have to see how well I can check BB alignment, but it looked good), worn chain, and possibly stupid tooth design (Rival rings make some interesting noises when they're brand-new, which I haven't experienced with Shimano or FSA, so I don't think they're exactly on-the-ball here).

indigophox

So, given where analysis of this has led, is there a straightforward solution other than another frame? I'll still be riding this around as my (relatively) "beater" bike, so if there are any suggestions that would be awesome.

Has anyone experienced this with these rings and not another brand/model, or with some rings and not others? Once I get around to buying a new cassette I have an FSA Super Road 50t I'll be dropping on instead, and I'll see how that behaves.

Thanks for all the help and input!

BarracksSi

I had that happen not with a Rival crank, but with a Truativ crank (same manufacturer, and at least the same basic construction) and its GXP BB; same crank model, two different bikes, both exhibiting the same problem. I have not had it happen with either of my Shimano Hollowgram cranks that were later installed on the same bikes as the Truvativs originally came on (105 on one, R600 on another).

Let us know how it goes.

indigophox

I'm guessing there was a corresponding switch in rings from SRAM/TV to Shimano?

I'm unimpressed with the tooth profiling on the Rival rings (specifically the side-profiling, so I also doubt their competence in terms of shift profiling that might be causing this problem), but compared to anything else (including 2.5lb welded cr-mo setups) the cranks/BB seem rock-solid, while the frame moves a lot, so I instantly suspect the rings. Thanks for the feedback on the SRAM/Truvativ setup coinciding with this problem for you as well! That's exactly the sort of info I've been hoping to track down on here.

FlatMaster

Bent chainring?

BarracksSi

Certainly a change in rings, but I can't say for sure if that was all that mattered. Sometimes the chain, if it was on the big ring, would rub the FD under heavy load in a tall gear; I could even see it move back & forth. Even though I had it adjusted that close laterally and about as low as would go, it would still dump the chain.

With the Shimano cranks, I can't see the ring wobble at all, and even though the FD is as close to the chain as I had it on the Truvativ, I can't make it rub. I think their rings help even more with shifting, especially from the small ring to big.

FWIW, the first bike on which I made the switch is a CAAD8, and the other bike is a Schwinn DBX (disc brakes & fenders make it my rain bike ).

indigophox

Nope, definitely not.

Very interesting. I definitely get this too (not rubbing, but I can make it get damn close to both sides of the cage when climbing/sprinting/etc.), so I'll be borrowing some Ultegra cranks soon just to see how much movement I get there. Given that the frame is definitely moving though, I'm inclined to think that the movement of ring relative to cage is a product of where and how much the frame flexes, as I can't picture how a crankset would affect this when pressing on the left crank. However, I don't doubt your observations, so I'm VERY curious what's going on on our bikes.

indigophox

I -finally- got around to checking this out (i.e. testing an Ultegra crankset in my frame). It is, beyond any reasonable doubt, the fault of the crankset and/or bottom bracket.

This raises two questions:

-The GXP (Gimpy-X-Pipe?) bottom brackets are designed with a fair bit of -angular- bearing play, to accommodate crap-ass frames that aren't manufactured properly. Their road stuff, consisting of Rival and then even-more-high-end stuff, IS NOT DESIGNED TO BE PUT INTO CRAPPY-ASS FRAMES. So, what the hell are they doing with this hokey technology in top-notch race groupsets? Bloody hell, I got away from Shimano because their mountain bike stuff is frequently this stupid... now what?

-As to "now what?" -- Chris King/KCG has confirmed to me that their bottom brackets should be appearing in GXP versions around the end of the summer. I'd -LOVE- to confirm that this is purely a bottom bracket issue. I don't see how the axles or cranks could be causing this deflection -- force on the left crankarm translates to exactly the same movement on the right side, suggesting that the crank/spindle assembly is moving as one rigid unit (standing on both cranks suggests the thing is bloody stiff, too). If that's the case, this suggests that a -PROPER- bottom bracket, i.e. a King or maybe one of their Blackbox models, would fix the issue. If the Blackbox bottom brackets don't pull this $#!+, that'd explain why Cancellara breaks chains instead of having the stupid problems that I'm having with SRAM's $23 bottom bracket offering.

So, all of that said, I'm going to get in touch with KCG and SRAM tomorrow (ok, today now, 'cause I didn't finish writing this post last night) to determine whether I should be building up another bike with Rival on it. Maybe I can twist SRAM's arm into giving me -their- cost on a couple of Blackbox BBs if that'll solve it, as sort-of an apology for all the effort I've put into tracing down their incompetence.

Thanks again, BarracksSi, for tipping me off to this likely being a fault of the SRAM setup. You've saved me more time than anyone else who's been involved in this, of course with the exception of the friend who loaned me the Ultegra cranks I just mounted tonight (I wouldn't know squat without those, now...).

I am fairly certain that this is purely an issue with a beyond-******** bottom bracket "design", so the next step for me is going to be to try to find an appropriate replacement part for this semi-proprietary disaster of a bottom bracket.

BarracksSi

Whew... luckily I was right this time..

Wasn't it nicer with the Ultegra crank? I remember when I put the different cranks on my bikes -- simply the lack of "grrring-grrring" noises made the whole bike seem like it was at a higher price bracket.

indigophox

The cranks weren't noticeably better, and my Rival rings at this point are worn down so they don't make evil Rival noises. It would seem that the major advantage is in the BB -- makes sense, considering that you can barely go out for dinner for the price of that piece of junk. I'm going to put the Ultegras back on for a longer run and make sure I cannot move my chain around as much as with the Rivals, and then I'll be looking at replacing the bearings on my Rival setup ASAP.