Gear problems..

[chesterpete]

Asking for a friend.......

Ok so, gear problems and something I’ve never had before.
I can’t get my bike to ride in “big ring front, big ring rear”. Yeah ok I know you “shouldn’t”, but if I do it falls off the front chainring down to the little front chainring.

It does this on mechanical and on di2.
Also tried two different sets of wheels incase dishing was an issue. Same result.
Tried two different chainsets. Same result.
Two different BBs. Same result.
Chain isn’t worn, and neither front or rear mechs are bent.
If I angle the front Mech it is better but not as it should be.

Any ideas?

[Pebble]

if you put a straight edge on the big chain ring, whereabouts on the cassette does it align up with?

[Marcus Aurelius]

As far as the mechanical set is concerned It’s possible that the big ring is bent. It doesn’t have to be particularly bent for the problem you describe to occur. With the Di2, the same thing could be occurring, or the synchro shift is set to automatically prevent big-big. Another thing to check on the mechanical set, is that the derailleur wire is moving freely in the outer. If in doubt replace the front wire and outer cable.

[peetee]

Don’t do it. It’s asking too much of the drivetrain. The chances are that the gear ratio is replicated with the smaller ring on the cassette somewhere.

[iandusud]

You say you've tried two different BBs. Were they the same length? It could be a chainline problem that could be cured using a shorter BB assuming there is enough clearance on your frame. If this is the case it would be good to sort it out. However as already stated there is no need to use that combination and the inability to do so is not a problem IMO.

[Valbrona]

Some wheels place the cassette further inwards than others.

[Brucey]

supposing that it is a 2x11 'road compact double' then the chainlines (measured from the centreline of the bike) might be as follows

Chainset; normally ~43.5mm to the centre of the spider. However the big ring in a compact double is heavily dished so can be offset from there by ~5mm, so a chainline of ~48mm. It can be more than this is the frame has oversized tubes and/or is meant for fatter tyres.

Sprockets; assuming 130mm OLN rear hub, 11s road spacing. The largest sprocket is inset ~42mm from the inside of the RH dropout, i.e. a chainline of ~23mm.

This means that, in the big-big combination, the chainlines typically differ by ~25mm (or more if the front chainline is wider than normal). Especially with short chainstays it is a big ask for this to work at all; as others have mentioned you shouldn't be looking to actually use this gear. Normally you should (at most) aim to use sprocket 2 or 3 on the big ring. A reasonable expectation is that the big-big combination can be selected without catastrophic damage ensuing.

cheers

[Pneumant]

I've a road bike with full 105 5600 10-speed. On the 52 big ring I won't take it lower than 4th as the chain angle is quite angled. On the 39 small ring I wont take it higher than 6th. This bike is getting on a bit now and the decent 105 cassettes are no longer available so I do nurse the transmission.
Often wondered why these Shimano gruppos have dished chainwheels, what is the point

[Brucey]

Often wondered why these Shimano gruppos have dished chainwheels, what is the point

I think the point is to allow larger intervals between the chainrings, without the chain catching on the big ring when running small-small. Simply spacing the chainrings further apart does this too, but leaves a skinny chain prone to sitting on top of the small chainring following a front downshift, whereas a dished chainring is less likely to cause this.

cheers

[Pneumant]

Ok thanks, bike has just been cleaned so will have run through the gears on the stand to see that. Learn something new everyday!

[Pneumant]

Often wondered why these Shimano gruppos have dished chainwheels, what is the point

I think the point is to allow larger intervals between the chainrings, without the chain catching on the big ring when running small-small. Simply spacing the chainrings further apart does this too, but leaves a skinny chain prone to sitting on top of the small chainring following a front downshift, whereas a dished chainring is less likely to cause this.
cheers

I see your point now re the dished chainrings. I tried it and small to small is noisy but could be quietened, I suppose, with some judicious f mech adjustment.

Big to big is quieter but my oh my the friction can be felt .

Bike has short chainstays which won't help here. Whatever in either scenerio the chainline is horrendous at the extremes and a sure recipe for transmission fatigue.

[gregoryoftours]

Probably best not to adjust the front mech to not rub in small/small - the noise will be a notification that you're in the bad gear! Plus the chain will also likely catch on the lifting pins of the inner surface of the big ring. As someone else has said Di2 will actually not let you shift into some bad gear combinations.

[iandusud]

I know it sounds like I'm stating the obvious but all this talk about chain rub on small-small or problems on big-big is somewhat academic as the use of these combinations is totally unnecessary and ill-advised.

[Brucey]

yes it is, but then again it isn't; should the chain start to pick up on the big ring when (foolishly) running small-small, you can easily smash the FD to bits. Likewise if the chain isn't long enough to engage big-big, the transmission can suffer severe damage.

So in an ideal world you should be able to select and use the extreme gears in most systems, even if it isn't a good idea per se, with rubbing etc. If this isn't possible then it should at least not cause the system immediate damage if you try.

cheers

[Marcus Aurelius]

I know it sounds like I'm stating the obvious but all this talk about chain rub on small-small or problems on big-big is somewhat academic as the use of these combinations is totally unnecessary and ill-advised.

Personally, I’ve got Campagnolo super record EPS, it’s a cracking good system. You can set custom shifts on a synchro setting which prevents big-big / small-small, and also automatically finds the closest ratio to where you were, when the ‘saver shift’ occurred, and moves the rear to the correct sprocket. It’s very clever.