Shimano Road Rear Mech - Capacity Question

Total capacity is the difference between front rings plus the difference between small and large sprocket on the cassette.

I have a 50/34 chainset (16T difference) and an 11-28 cassette (17T difference) so I need a capacity of 33 or more.

Largest sprocket is just that, the largest sprocket on the cassette that the mech will clear. These are usually conservative though. The 7800 rear mech was a max of 27 but a 28 should be fine.

Largest sprocket = biggest sprocket on cassette (eg 28)

Total Capacity = Maximum value for (largest cog – smallest cog) ＋ (largest chainring – smallest chainring)

Eg (50-34) + (28-12) = 32t capacity.

Capacity is representing, iirc, the maximum the chain will have to stretch to fit the entire drivetrain.

From reading about this, largest sprocket will be your largest toothed sprocket on your cassette (28) and capacity would be difference between largest/smallest cassette sprockets and difference in chainring teeth ((28-12) + (52-39)) = 16 + 13 = 29. What cassette did you have previously?

Have you checked the rubbing isn’t the chain on the front derailleur cage?

Hi all - many thanks for the responses, I'm actually really touched at the enthusiasm for helping me out here. Thankyou!

For awareness, this is part of my journey back in to cycling and my bike is becoming a mash-up of old and new.

The bike has a 52-39 chainset.

Previously the bike was fitted with an 12-25 cassette (not fully worn out, cassette and chain are happily packed away in a little bag).

I wanted to make things a little easier for me as at the moment I'm picking routes on the basis of where hills are and whether or not I think I'll survive riding up them so decided to fit a larger cassette. Foolishly I concentrated on the upper size limit and not the lower so bought an 11-28.

So to take into account what has been kindly written above

My original set up:

(52-39) + (25-12) = 26 Capacity

My revised set up:

(52-39) + (28-11) = 30 Capacity

So I conclude that with reference to the tech sheet referenced in my original post - I have exceeded both the largest sprocket and total capacity by a value of 1.

I guess the question now is - how big a deal is this?

@shedbrewed - good thought. The rubbing I mention does sound alot like that however I spent a bit of time experimenting by deliberately making the front mech rub - the noise I mention was present when the front mech was rubbing and when it wasn't so I don't think it's that.

Which rear mech is it? With 27t max sprocket i'm guessing a 7700 or 6600 rear mech?
And FWIW, rubbing in the 39 chainring is incredibly unlikely to be capacity related, especially if it occurs over the full range of the cassette.

Shimano capacities tend to be stated based on the worst possible geometry (mostly drop out, but chainstay has an impact), which occur very infrequently, and have been getting less frequent for ~20 years, people simply don't buy road bikes with tiny non-standard gear hangers or sub 390mm chainstays.

I've happily exceeded max sprocket and capacity by 3 or 4 teeth on most generations of shimano rear mech (including the 7700 and 6600 mechs?), without having to make any concessions.

Shimano are pretty conservative in terms of their capacity/largest sprocket ratings. I don't think +1 would make any difference. I've previously jumped up from an 11-28 to an 11-30 on a 105 short cage mech which was +2 over the limit but it made no difference and performed the same as the 11-28. I don't think you will have a problem with +1 over the limit. On your rubbing issue, if it's not the front mech could you need the b screw adjusting to push the mech further out from the sprockets? Although would probably only be an issue in the larger sprockets but maybe worth a look.

Hi @mert, you're not far off, it's a 7800 series rear mech. I am aware that Shimano can be a bit conservative with what they say will work and what won't I'm just really racking by brains at what the issue could be.

Hi @alanf - yup, I've been giving my b-tension screw a good hard staring at this morning! I've made some adjustments but it doesn't seem to be affecting the issue.

Check chainline and re-index? If you've changed cassette, it might not be in quite the same place as the previous one, and if the indexing is a splash out, shifting might be fine but the chain might be rubbing against larger sprockets? That's my only thought.

Hi again - so the latest update, I've checked b-tension at the highest and lowest settings and it has had no effect.

The suggestion by @infovore above seems credible as the 'noise' sounds like the noise I used to get on budget 7speed systems about 25 years ago (I feel old just writing that) when a shift wouldn't quite get all the way there...

Anyway - it sounds like a complete re-indexing is the way ahead despite the fact that it looks fine in the stand just now.

Thankyou once again for all the responses...I'm really touched by the enthusiasm for helping me diagnose this.

If you’ve changed cassette, it might not be in quite the same place as the previous one

Unlikely if they are both shimano (even if they are different brands TBH). Not much room for error on 10 speed road cassettes!

One thing to try, is the hanger straight.

If the hanger is twisted, the rotation of the cage and mech round their respective pivots moves the cage across slightly. Causing rubbing. Have had one bike where you could index perfectly in the big ring or the little ring, but not both, that was a twisted hanger. Can get similar issues from worn out mech pivots too.

You can use one of these to allow you to extend the range of your rear derailleur...have used on a 9 speed 53/39 to 32/11 with no probs, and now with a 50/34 & 32/11

@roadie_in_denial fair enough. If it’s any help I have a self-assembled 12-32 10 speed cassette with 53/39 front rings on da7900 with standard (short cage) rd. I used to get a little front dr rub but gently adjusted cable pull and low limit screw to stop it.

New chain, new cassette...old chainrings? What state is the 39 chainring in, given that seems to be the common factor?

Evening all. Thank you for the suggestions, I will provide a quick update.

I have borrowed a a hanger alignment tool from a mate and identified that the hanger was bent. It is now straightened and I will test ride in the morning.

@montgomery - good shout about the chain ring. You're correct that it's old but visual inspection suggests it's ok and at no point in my test rides has it slipped or given any chain-suck so I'm inclined to think that it is OK for now. However, it's not discounted so thankyou for the reminder.

Ok a little update for anyone tracking this

The deraileur hanger was straightened last night and test ridden this morning to no change. I have re-indexed the gears and once again they work perfectly in the stand but not when there's a load on the pedals.

I've checked torque on the QR - it's a shimano QR so I reckon is unlikely to be slipping in the frame.

But still - I have that annoying rubbing noise...

I hate to state the obvious here but... If 'something' is rubbing then what you need to do is erm, find out what it is!

I know that sounds like I'm being facetious but if you can get the bike into a stand and then soft pedal it (or get someone else to), you *should* be able to locate the noise with a mix of visual checks, sticking your ear next to things, and even using your fingers on things to feel where there is vibration from something making contact.
Once you've actually worked out what is rubbing then we might be able to advise better on how to resolve it.

So, the list of normal culprits for drivetrain rubbing are:

- chain on front mech cage
- chain on rear mech cage (twisted cage or worn jockey wheels)
- chain on the inside of the big ring (when it's on the small ring)
- top jockey wheel on cassette sprocket (when b-tension insufficient or max-sprocket exceeded, or when chain too long)
- bad indexing causing chain to 'chatter' or rub on adjacent sprocket
- very VERY badly worn jockey wheels that have a lot of side-to-side play

The last two may be exacerbated in specific gears due to chain angle.
I may have missed some things, the list is not exhaustive!

Another thing worth checking if you haven't already, is that there is no side to side play or wobble in either the rear hub axle, freewheel, or bottom bracket, as you may find that things work fine in the stand, but under load if there's any movement or play there it can cause things to move just enough for something to rub. Again this can be exacerbated in some gears, especially the smaller chainring and bigger sprockets.

Also, FWIW, Shimano specs are *massively* conservative, and with careful tuning you can operate successfully way outside them. For example a 6600 rear mech has a documented max sprocket of 27t and 29t capacity, On my my wife's bike we have one running successfully with a 34t max sprocket and 36t total capacity. There is *one* gear she cannnot use and that's the inner chainring with the 11t sprocket, not a gear you should ever use anyway.
Getting out of spec setups to work is a mix of playing with chain length, b-tension, hanger length, front mech positioning and luck. Sometimes it'll go as good as stock, sometimes it might be a little sub-optimal in some gears, and sometimes it'll flat out not work, but normally 1 or 2 teeth outside spec won't cause issues.

Interesting. Just thinking of other things; not brake pad rub with wheel flexing? Not fitted the chain the wrong side of the cage guide between the jockey wheels? Front derailleur cage not catching on outer chain ring? Jockey wheel bearing’s spinning freely?
All these are things that happened to me.

Hey everyone - apologies for the delay to the updates. Life has gotten in the way of bike maintenance for a few days!

There's many salient points above and I'm really grateful to @amedias for the really thorough diagnostic suggestions.

I've been through everything I can think of (including all that has been listed above) all to no avail for the following reasons:

Not front mech rubbing because - I removed the front mech and the noise was still present.

Unlikely to be rubbing against the outer chainring because - with front mech removed, I could observe the chain as I used different sprockets and the noise was present on all sprockets.

Not to do with the rear mech because - I removed the rear mech, replaced it with the Ultegra 6600 mech from my CX bike and the rubbing was still present. The jockey wheel position for both the 6600 and 7800 mechs was set to each extreme and the rubbing was still present.

My current thoughts are that despite it being ok in the stand and the 39t chainring 'looking' ok - as @montgomery has said, the chainring is the one common factor so it is increasingly likely to be the culprit. I have a 38t on order (because why not take the opportunity to really make sure I can get my fat MAMIL ass up more climbs in this locality?) so will replace, ride and report back.

Thanks once again for the tips and advice.

Hi all, I just thought I'd close this one off in case anyone wondered.

I just received a 38 tooth TA chainring and fitted it. The noise has gone.

So - @montgomery called it right! Brew on me for you should we ever meet.

Thanks to everyone for the support and suggestions and for furthering my knowledge of Shimano documentation.

See you down the road.

Thanks, closure is always good and might help others in the future. Your symptoms didn't really match the other diagnoses, so when you've eliminated the impossible, the unlikely, however improbable and all that.