Can't get onto the big ring

HowardG

Hi folks,

Merry Christmas to you all. Just about had enough eating for now so to distract myself I thought I'd seek some advice.

My new TT bike, a Cannondale Slice, had the chainrings changed at the shop prior to delivery to suit my requirements. The original equipment rings and chainset are trimax pro compact (105 FD) and I'd requested standard size rings 53/39. They put TA rings on (they had some in the shop) and all seemed fine.

Now the bike has been ridden a bit I've been having issues getting the chain to climb onto the big ring. The FD pushes the chain over but it doesn't catch the teeth and engage, just rub and chatter alongside. The limit screws are not preventing the FD moving over. The temptation is to pull the gear changer a bit harder, this having the effect of pulling the FD round and out of parallel with the chainrings.

On my road bikes, compact and standard gearing, I only have to nudge the lever and the chain is on the big ring regardless of the pressure on the pedals. On this bike I have to be completely off the power to even have a chance of shifting.

Although I usually do all my own maintenance, I took the bike back to the shop for them to tinker. Slight adjustment of the FD and limit screws later, they said it was fine.

Same problem again. I've straightened the FD again, adjusted the limit screws so it doesn't throw the chain but it still doesn't pick up on the big ring as I believe it should.

So what is it? FD slipping round under excessive pressure (wouldn't need so much pressure if the chain picked up immediately)? Poor quality chainrings? Chainrings the wrong way round(not sure if this is possible, but they seem slightly assymetric when viewed edge on)? Chainring and chain mismatch?

Any ideas/ comments would be gratefully received!

Semantik

Front mech should not be 'slipping round' under any sort of pressure-it mustn't be clamped on tight enough.This can't be helping your situation.
Otherwise, would say front mech cable tension is likely the cause -try adjusting the tension with the barrel adjuster(s) for starters.
Or take it back to the bike shop and politely tell them to sort it out.

HowardG

Hi,

Thanks for responding. The front mech is as tight as can be without stripping the thread. Could be that the paint finish on the mech and that on the bracket are a bit too smooth and need roughing up a bit. The cable is adjusted to pull the mech the full range of its movement, so adjustment won't help.

Thanks again.

Semantik

The shop must have had to move the front mech further up the seat tube to accommodate the larger big chainring. It should NOT be spinning round. Was it doing this before the change of chainset?

bigflangesmallsprocket

Just a few thoughts.

You've changed the ring sizes up, so, has the FD actually been adjusted height-wise? You may need to lift the FD up a bit more to make it easier for the chain to slip on.

The gear cable may be a little on the slack side, maybe there's simply not enough tension to pull it over. After faffing about with gear and cable settings I've found that re-adjustments are often necessary, as after the wire has had a chance to settle down it may slacken after initial use.

You say the outer limit screw is okay, but it does need just enough play to pull a little further over than the centre of the ring. One thing I've found to watch out for is exactly what sprockets you're on when changing rings, maybe obvious but I'll mention it anyway: in higher gear (rear sprocket) the front end changeovers seem easier, but if you often change rings in a lower gear and the front end won't change, this is also an indicator that you may need more 'pullover' clearance for want of a better word.

I've had all these issues myself when messing about with the gears and rings, it may just need a little patient fettling; I tend to have a little play with the gears after any work in these areas, setting the bike upside down and going through the gears a few times, closely observing exactly whats going on. I might experiment by putting extra finger pressure on the cable when it doesn't quite make the change, or gently forcing the FD out with my fingers to see if that helps the change; then I'll adjust accordingly and try again.

EDIT: I've just noticed you mention you use Shimano. I'm not familiar with Shimano, but with reference to my note on being able to pull the FD past dead-centre on the front ring, my Campag lever has a trim action, so that when you pull onto the big ring, if the FD is scraping on the chain, I can simply tap it back a little. I have no idea whether Shimano has this capability.

HowardG

To pick it up: the front mech rotates a few degrees, slightly out of alignment, certainly not 'slipping round'. I'm more sure now that it's to do with the painted surfaces creating a slip plane.

As for adjusting the cable and limit screws, they both allow full movement. The thing with bar end shifters, particularly the left (FD) control is the shifter has far more movement than the mech needs, so when the lever is pushed down to move the mech to the small ring, the limit screw is reached before the end of the lever travel, so the cable goes slack. When the lever is lifted, the chain is on the big ring and the limit screw reached, the lever is yet to reach the end of its possible travel, allowing the rider to pull harder than is best, rotating the mech perhaps.

So what's happening? Having adjusted it all yesterday I went out on the bike today. Firstly it did change up easier, but there is still a point at which the chain won't catch on the big ring and it's here that the temptation is to pull harder on the lever. Unlike my other bikes, there seems to be a need to ease right off on the pedaling pressure when changing up. I might have to live with this even though I reckon its a chain and sprocket mis-match.

I would note that the front mech was moved when the rings were changed. There is about 2mm of clearance between it and the big ring.

A final thought is that much of my use on the new bike has been on the turbo. Is it the case that due to the resistance of the turbo the ability to reduce pedalling pressure is hampered, ie you can't really reduce the pressure without almost stopping?

Was bloody cold and damp out this morning trying to sort this. So cold I didn't stop for coffee on my way home!

bigflangesmallsprocket

EDIT: jump to bottom of post, which is an edit, and read the rest of my waffle later, as THIS just occurred to me, I've had cause to adjust FD cage_reach when I've experimented with different BB widths for Q-factor.

you seem to have covered all the usual bases then from what I can see. I noted the usual issues I have hoping they'd help.

as mentioned, the front mech really shouldn't be slipping, I think you'd have to use a lot of pressure to get it to move sideways, especially when the wire is mostly pulling in a downward motion, though it may be set at a slight angle. If it is, then that may be a faulty front mech, being over-polished or badly finished on the inside, opening it up to more than the required internal diameter for a good grip, especially if it's fully tightened and still doing it; if anything, I'd imagine it may bite into the paint if you really tightened it up and leave noticeable clamp marks, possibly even improving the grip. Or, maybe someone made a booboo and the wrong diameter mech was fitted in the first place; is there a stamp stating internal diameter on the mech, and do you have available the external diameter for the tubing? There's lots of different sizes of both, and like seatposts, I imagine sometimes they appear as if they should fit, but not actually!

I think 2mm is about the usual recommended height for FD's (don't quote me, I'm just going from memory), but, if there's a recommended height and the limit screws are already fully opened, another thought occurs to me. Could it be the opposite to the more common problem (for me) of too low a height, in this case being too high instead? I'm just throwing it out there as an idea. If it's sitting too high above the mech, then this also may have the effect of not pulling the chain over far enough. if it was lowered very slightly, with the cage being closer this would also bring the chain closer. Obviously you'd have to be careful of not overdoing it.

No experience with bar end shifters personally I'm afraid, though when I was younger I seem to recall a general consensus of them being more fidgety in aligning that down-tube shifters. Don't know if technology has caught up with bar end shifters.

I believe that when changing gear, you're supposed to spin up and then ease off the pressure a little when changing, something I sometimes forget to do. it doesn't stop me changing however, but i did notice on my cheap rear cassette that this can cause premature wear and tear, with some slivers of metal from the sprocket attaching to the oily chain (no failure though, spotted in time and works fine when I change properly). I'm not sure what to say about the turbo, seems like it's just the way it is! A decent sprocket would presumably be more robust and stand up to the strain more easily?

The SIMPLEST solution I can think of: just get the shop to stick a spare correct fitting mech on, and see what happens. Your bike is new, so any problems you shouldn't really need to do anything drastic to it, it's their responsibility, plus, you don't want try any drastic unusual solutions that would get them off the hook and using the 'warranty invalidated' card.

If it's none of the above, then I'm stumped! If it's still not working after looking at all possible solutions, then the bike or parts fitted simply aren't up to standard, and I'd ask for a replacement. Interesting problem.

OOPS, EDIT! I forgot a couple of things. Actually one was something you already considered but possibly not checked. Chainring on the wrong way round, though this shouldn't happen. Are your chainring bolts flush or standing out? If they're standing out then the chainring is on the wrong way round, and this also MAY make your ring stand further proud and away from the mech. Easier to see what I mean if you've got spare rings hanging around; the teeth aren't always central on the ring, sometimes they'll sit slightly to one side. Obviously it's not much, but it might be just enough to make a difference.

MAYBE THIS, WHICH HAS JUST OCCURRED TO ME, AND SHOULD HAVE SOONER! The other thing, and possibly the most obvious now, thinking about it: BB width. It's a new bike but you said you had the chainset changed, which could also effect how far out the chainset sits. Did you get a new matching BB fitted with the chainset or use the old one? If not, and there's a specific BB recommended for said chainset, then something to look at. Different BB's obviously change pedal Q-factor, but also how far out your chainset sits, or your chainline (most obvious on a fixed gear). Maybe you just need a slightly smaller width BB? If you've got lots of play for the inner ring, but problems reaching the outer, then this may be IT! Remember, changing your BB width, that each side will only come in half of the difference between the two sizes...112mm down to 108 for example, will bring your chainset in 2mm.