Balancing cable tension for FD limits

Chunky101

Need some advice tweaking my gears please folks. I've been reading up on the Park Tool repair website and learned a fair bit but still not grasping something and I'd love to get this resolved by the weekend to get out for a ride.

The issue is one of chain rubbing on the front mech cage depending on how I've adjusted it (see below) on Ultegra 11 speed.

I've set front H and L screws such that there is around 1 mm of clearance at the furthest range of cage motion to the chain and shifting in general works ok.

I believe the problem is the cable tension. When I set the cable at the correct tension to move the cage to meet the H screw it's then too tight to allow travel to the limit of the L screw. This means it rubs on one of the chain rings.

If I loosen the cable tension to allow the L limit screw to be reached there is no longer enough tension for the H limit. If I tightened it up the H is fine but not the L. No matter how I try and balance the tension between the cable connector, barrel adjuster and the two limits I can't get a happy compromise.

Any suggestions on what I'm doing wrong?

fleshtuxedo

Worth checking for any slop in the cable run around the bb - I set up a new complete groupset on a giant defy recently and had this problem because the plastic bb cable guide bent more as the cable got tighter on the big ring. I managed to get a happy compromise in the end, but I nearly had to get into reinforcing or even shortening the bb guide.

rolf_f

If you mean that there is 1mm clearance between the edge of the chain and the derailleur cage then that sounds like quite a lot. I'd cut that by half and see where it gets you.

Look at it this way - unless there is some dirt restricting movement, the shifter must be able to move enough cable for the front mech to reach both extremes. If it isn't, then forget about the tension - the mech must be moving from a point beyond optimum which means adjusting the limit screws.

Chunky101

Rolf F wrote:

If you mean that there is 1mm clearance between the edge of the chain and the derailleur cage then that sounds like quite a lot. I'd cut that by half and see where it gets you.

Look at it this way - unless there is some dirt restricting movement, the shifter must be able to move enough cable for the front mech to reach both extremes. If it isn't, then forget about the tension - the mech must be moving from a point beyond optimum which means adjusting the limit screws.

I'd adjusted the limit screws such that there was just enough clearance to not rub the chain at either extreme but without excessive spacing. The challenge being getting the cable tension set to balance between them both. I will double check this though tonight as it sounds like my understanding of how it should work isn't too far off track.

Bozman

The 6800 FD is an absolute pain in the a55 to set up. Good luck.

trailflow

Here's a set up guide i posted on another thread. So ive copied and pasted it. This guide also applies to the 105 5800 and Dura Ace 9000 front derailleurs.

Here's the Shimano 6800 set up pdf aswell
http://www.google.co.uk/url?sa=t&rct=j& ... 24&cad=rja

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The ultegra 6800 front derailleur has a trim function on both the big AND the small ring. So big lever and little lever. (if you dont know what the trims functions are - they are lever clicks that allow you to move the cage slightly to eliminate chain rub)


To set up the 6800 front derailleur correctly an inline barrel adjuster should be used. It will make set up much easier. if you dont have one, install one. Make sure it is screwed all the way in too, before starting.

Start off by making sure the outer limit screw is screwed outwards but not all the way that it pops outs (the limit screw closest to the chain rings)

Start over. undo the cable

When clamping the front derailleur to the frame make sure the gap between the big chainring teeth and front derailleur cage is around 2 to 4mm.

Toe the rear of the cage in slightly (around 5mm) when clamping. Then use the support bolt to align the cage with the big chainring. The support bolt is a small screw that pushes up against the seat tube to add stiffness to the front derailleur when shifting. It also moves the front cage outwards as you screw it in. See page 8 of the pdf below
http://www.google.co.uk/url?sa=t&rct=j& ... 24&cad=rja

So first , with the cable de-attached. Put the chain on the small front ring and 25t (or biggest) back cog. -

Adjust the inner limit screw (closest to the frame on the front derailleur) so the gap is 0.5mm from the chain and the front derailleur's plastic plate,and not rubbing on the chain.

Make sure to down click the left shift lever into its lowest position. Press it 4 times

Clamp the cable (pull the cable fairly tight with pliers and clamp). Then shift the chain onto the 11t cog (or smallest) at the back, and shift the front onto the big ring. (if it doesnt shift up - add more tension by loosening the bolt and pulling more cable ,then re-tighten the bolt). Now when in the big ring there should now be some chain rubbing on the cage if you spin the cranks.

Now wind the inline barrel adjuster (adding more cable tension) until there is a 0.5mm gap between the outer cage and chain, and no chain rub. Look down at the front derailleur cage as you do it, and spin the crank until the rubbing stops.

Then wind the outer limit screw in until it touches the stop. (If the outer limit screw is in too much it wont shift up - this is why you screw it out at the beginning.)

Next shift the front down onto the small ring and keep the rear on the 11t (or smallest) cog. Then click into the front small ring trim position. now whilst in the trim position,add a little bit more tension with the barrel adjuster, until there is also no chain rub. This will eliminate any slop in the trim and make it feel more positive to click.

Now it should all be set. Check all gears and trim settings. Make sure the chain isnt being thrown over off the big ring.

If you find the small ring trim click isnt very positive. Start over and try the second cable routing tab position. (remember to wind the barrel adjuster in again before starting.)

There will always be rub in the big/big combo. This is cross chaining.

Chunky101

Thanks for the post, much appreciated. I'll run through it is tonight and let you know how I get on.

rafletcher

Chunky101 wrote:

Rolf F wrote:

I'd adjusted the limit screws such that there was just enough clearance to not rub the chain at either extreme .

Therein lies your mistake. Correctly adjust your mech and your chain WILL rub, probably in the largest 2 sprockets and the smallest two sprockets when cross-chaining (ie big-big and small-small). Only the SRAM mech with their "yaw" technology prevent this (on mechanical groupsets, I have no idea about electric ones). As above, 0.5mm is plenty of space between chain and mech cage (when following the Shimano instructions on how to set it up)

And the 6800 is no worse than any other front mech in this respect.

Bozman

rafletcher wrote:

Chunky101 wrote:

Rolf F wrote:

I'd adjusted the limit screws such that there was just enough clearance to not rub the chain at either extreme .

Therein lies your mistake. Correctly adjust your mech and your chain WILL rub, probably in the largest 2 sprockets and the smallest two sprockets when cross-chaining (ie big-big and small-small). Only the SRAM mech with their "yaw" technology prevent this (on mechanical groupsets, I have no idea about electric ones). As above, 0.5mm is plenty of space between chain and mech cage (when following the Shimano instructions on how to set it up)

And the 6800 is no worse than any other front mech in this respect.

6800 has trim and this eliminates any rub that you may have, but there's very little if any when you've set the FD up.

Chunky101

So the latest update is it appears my left shifter is faulty and a spring has broken internally. Not convinced this was entirely causing my setup issues but I've booked the bike into a quality LBS and we're trying to get Shimano to resolve as a warranty replacement. I feel slightly defeated at having to resort to this but it became obvious something was wrong with the shifter. Thank you everyone that took the time to contribute, all advice gratefully received.

trailflow

i had a similar problem with my left shifter at first. i thought it was broke too. It would miss shift and jump erratically every now and then. I think the internal mechanism didnt reset as its supposed to. After spraying the internals with lube it started working again. I would have a look into that and keep persevering with it. Make sure the cable is seated in the shifter and frame correctly.

Also maybe try the other cable routing tab position.

rolf_f

rafletcher wrote:

Chunky101 wrote:

Rolf F wrote:

I'd adjusted the limit screws such that there was just enough clearance to not rub the chain at either extreme .

Therein lies your mistake. Correctly adjust your mech and your chain WILL rub, probably in the largest 2 sprockets and the smallest two sprockets when cross-chaining (ie big-big and small-small). Only the SRAM mech with their "yaw" technology prevent this (on mechanical groupsets, I have no idea about electric ones). As above, 0.5mm is plenty of space between chain and mech cage (when following the Shimano instructions on how to set it up)

And the 6800 is no worse than any other front mech in this respect.

More careful quoting please! I didn't say this...........

However, my comment that Chunky was responding to made no reference to the cassette. I was talking about the front mech in isolation. I'm not sure if Chunky was though.

Mind you, my Campagnolo setup doesn't rub at either extreme despite not having any yaw!