Front Mech Adjustment Help

dry664

Been fiddling with my gf's bike for a while trying to get her front gears to shift correctly. Also followed the Parktools guide to the letter but still cant get things right.
Basically, I get everything looking good and the bike will shift up 1-2-3 perfectly. But on the way down it wont quite get to 2 cleanly and but will get to 1. If I adjust the tension so it handles the downshifts perfectly the it misbehaves on the upshifts. It seems almost as if the deraillieur is a fraction too wide.
The mech is the standard Suntour xcm on a Sunn Seasons S2. Any suggestions? Thx

cooldad

Sounds like sticky cables, I'd replace them.
When all else fails though, slacken everything off, check the limit screws and start again.

dry664

the bike has barely done 100 miles since new. Could it really be the cables? I tried starting from scratch a few times but I just keep getting the same thing. Am also concerned that the actual mech is available on amazon for £7.49:-)

cooldad

Unlikely but even cheap bits should shift, especially if it's that new.
Doesn't the seller offer a 6 week check up?

dry664

Got it from Chainreaction. Actually had to replace the chainset already because it bent and had it done at LBS at CR's expense. The shifting was dodgy on the old chainset and dodgy on the new one even after LBS set it up. Still think its worth replacing cables?

cooldad

Probably not, maybe just components made of cheese. But even so they should work.

cooldad

Might still be worth stripping them off and having a look. If it was on sale probably been in storage for a while. Just needs a bit of moisture and time to rust a cable - if the components are cheap you can assume the cables are not stainless.

dry664

Had a look at the cables and they look fine to me. Think the mech is made of cheese. The stock chainset was also a suntour xcm and bent on it first outing.
Im going to replace it. The current mech is top mounted, bottom pull. Should I stick with a top mount or could I use a bottom mount? Thx.

cooldad

You have to use the same swing and pull. Many mechs are dual pull.

bikaholic

For square taper crankset installations, especially where there is a temperamental front derailleur, a good thing to know is that the chainline can be adjusted depending on how tight you bolt the drive-side cranks on. So, between hand tight and super tight, there will be approximately 2mm of inboard adjustment. And, the drive-side crank rarely falls off even if the bolt is slightly loose so we know that it would be safe to do.

Cable issues aside, most clueless people will just bolt on the drive-side crank and then attempt to adjust the FD.

Clued-in people will adjust the FD with the crank at the loose position, then one step at a time, gradually tighten down the crank bolt in order to find the optimum chainline position for FD shifting and for chain position on the extreme combos (ie biggest chainring/smallest cog, smallest chainring/biggest cog, smallest chainring/smallest cog, biggest chainring/biggest cog).

For the Suntour XCM FD (max capacity 42T), you have to ensure that the lower edge of the cage runs as close as possible to the biggest chainring without touching either the teeth of the outer ring or the teeth of the middle ring. So, the height of the FD cage may need to be as close as 1mm; and don't assume that chaingrings are perfectly circular - so you will need to spin the crank to check as you adjust the height of the FD.

cooldad

I have never heard of leaving crank bolts loose to adjust the chainline, and would certainly not recommend it. Sounds like a recipe for problems to me. The is enough adjustment in mechs to cater for different chainlines.
And I have Biopace on a couple of old bikes, certainly no problems with shifting, even though they are not round.

dry664

Im a noob when it comes to bike maintenance but i think looseing the cranck arm seems a little sketchy. I just had a thought. As I said above it seems as though the gate is a fraction too wide and to clarify the bike is running 3x9 gearing. Is it possible that its been mistakenly fitted with a 10 speed chain? Could this cause the problem? If so is there any way to check on the chain or best just to measure? Thx

cooldad

Doubt it but the chain will have a model no stamped on the links. What is it (and what make of chain).

bikaholic

cooldad wrote:

I have never heard of leaving crank bolts loose to adjust the chainline, and would certainly not recommend it. Sounds like a recipe for problems to me. The is enough adjustment in mechs to cater for different chainlines.
And I have Biopace on a couple of old bikes, certainly no problems with shifting, even though they are not round.

I know that it sounds crazy but you won't know any better unless you try it. Btw, I did advise torquing the crank bolt hand tight to begin with.

Super tight would be somewhere near bottoming out so hand tight would be a few steps before that.

I believe Cycling Guru, the late, great Sheldon Brown, wrote an article about the drive side crank phenomenon and its reluctance to fall off in use.

dry664

The chain appears to be a KMC Z9000 which is a 9sp chain. There goes that theory!

dry664

@bikaholic, I'm not sure I understand how adjusting the chainline is going to help me here. Can you explain?

cooldad

bikaholic wrote:

cooldad wrote:

I have never heard of leaving crank bolts loose to adjust the chainline, and would certainly not recommend it. Sounds like a recipe for problems to me. The is enough adjustment in mechs to cater for different chainlines.
And I have Biopace on a couple of old bikes, certainly no problems with shifting, even though they are not round.

I know that it sounds crazy but you won't know any better unless you try it. Btw, I did advise torquing the crank bolt hand tight to begin with.

Super tight would be somewhere near bottoming out so hand tight would be a few steps before that.

I believe Cycling Guru, the late, great Sheldon Brown, wrote an article about the drive side crank phenomenon and its reluctance to fall off in use.

Maybe so if you want to try it on your own gear, but to suggest to someone struggling to index a mech that loosening their crank to fiddle with the chainline seems a bit of a recipe for disaster to me.
It shouldn't be necessary.

dry664

Where does this leave me Cooldad? Try a new front mech???

cooldad

TBH I'd either try again or get someone else to have a go first. If it indexes cleanly one way but not the other it's generally cruddy cables or the tension is wrong.
Front mechs look simple but they are really inaccurate clunky things and can be a bugger to index properly. (Rear mechs look more complicated but are actually much easier to get perfect)
If you look you will see the cage is quite a bit wider than the chain, and it needs to connect with the chain properly on both up and down shifts. If the tension is wrong, it might be a bit out, so will connect the chain one way, but not quite the other.
Start again with no tension and get the limit screws perfect, shifting the mech by hand until it just shifts cleanly into the big and small rings. Then reconnect the cable with the barrel adjuster mostly in , tight but without moving the mech. Then try fine tune if necessary. Shouldn't need much, if any if if the limits are right, maybe half a turn or so.
Difficult to say but if the mech is that cheap, the shifter may not be up to much.

bikaholic

cooldad wrote:

bikaholic wrote:

cooldad wrote:

I have never heard of leaving crank bolts loose to adjust the chainline, and would certainly not recommend it. Sounds like a recipe for problems to me. The is enough adjustment in mechs to cater for different chainlines.
And I have Biopace on a couple of old bikes, certainly no problems with shifting, even though they are not round.

I know that it sounds crazy but you won't know any better unless you try it. Btw, I did advise torquing the crank bolt hand tight to begin with.

Super tight would be somewhere near bottoming out so hand tight would be a few steps before that.

I believe Cycling Guru, the late, great Sheldon Brown, wrote an article about the drive side crank phenomenon and its reluctance to fall off in use.

Maybe so if you want to try it on your own gear, but to suggest to someone struggling to index a mech that loosening their crank to fiddle with the chainline seems a bit of a recipe for disaster to me.
It shouldn't be necessary.

Have been building and fixing thousands of bikes using this method since 1990 and none have come back with issues relating to drive side cranks falling off or damaged as a result of my method.

Optimising chainline in this manner is just simply minimising the effects of the assumption made by the majority of component manufacturers: that your bike frame will be perfectly straight and aligned.

Now, regarding the OP's problem, there are two other "fixes" that are available:

1. Tweak the FD cage itself by grinding and bending.
2. Let the human make up for the FD's shortcomings.

I will assume OP is not confident enough to do fix (1), so will apply fix (2) instead.

Fix (2) is nothing more than adapting your shifting technique. If you have ever used a friction shifter, gripshifts or a road shift lever with adjustments for trim, then you will understand the logic.

Basically, you overshift to get the chain to derail onto the next chainring and then "apply" trim to bring the cage back in order to reduce chain rub. Well, trigger shifters can be made to work in a quasi-similar way.

Perfectly setup triggers will move the chain onto the next biggest chainring on or immediately after the click, leaving plenty of thumb lever throw. This excess lever throw is your overshift bit.

So now, setup the FD so that shifting is biased towards the downshift (ie perfect the downshift whilst aiming to get a roughly good upshift).

To perform an upshift whilst riding, simply push the thumb lever beyond the click (and into the excess throw position) and hold it there until the chain is derailed onto the next chainring. Release the lever when you feel a step up in resistance in your legs (ie when a shift occurs).

The shifting will be slower (typically taking up to three quarters of a turn of the crank as opposed to about a half for a really fast shift), but you know that it will work perfectly.

dry664

thanks bikaholic. The problem with option 2 is that when I get the downshift perfect, the bike doesn't want to get back from first to second. I can get the outer gear easy by adjusting the 'High' screw. I've been thinking of taking some mole grips to the cage to make it narrower but I think that will be a last resort. I'm off to get a new shifter cable now and then I will have another go setting it up. Thanks for taking the time to address this post, much appreciated.

Chunkers1980

Don't bend the cage. Double check it's at the correct height and angle.

cooldad

@ bikeaholic - Seriously? You are talking about his girlfriends bike. I'm sure all she wants it to do is shift properly when she pushes the lever. As it should.

dry664

Changed the cable, followed cooldads guide and it seems to be much better now. The H and L screws were already set perfectly but the initial tension was much looser before (following the Parktools guide!?!). Only took half a turn of the barrel adjuster to get it right although I'm still very surprised by how sensitive it seems to be. Half a turn either way seems to screw (lol) it all up again. Will try and get the gf back out on it this week and report back. Now she doesn't have an excuse for walking up all the hills anymore! Thanks all for the advice.

cooldad

Good to hear. They are sensitive - bearing in mind you are only moving the chain a few mm's on a shift, it doesn't take much to screw it up again.
The cable will bed in after a while (outers compress a tiny bit) so it will probably need a tiny tweak soon - work on quarter turns at a time.