front mech shifting problems

2wheelmotion

Hi all, first post, I'll be short.

I have been building/modifying a 90's british eagle bike. It was a nice lightweight frame so thought i would use as i quite like to get into building/modifying.

I have changed various bits that were either broken or i didnt like. One of those things was the brake levers and gear shifters. The bike had gripshift which i do not like and a broken brake lever so opted to replace with these

They are 18 spd shimano rapid fire. The brakes work fine, the rear mech works fine, but the front mech will not work.
I cant get adjustment correct. I have set the derailer umpteen times adjusting the height, alignment and limit screws but when I try shift up to the middle cog it wont go. The cable tension is either not enough and doesnt move the derailer enough, or too much. When the tension is what I assume to be just right, when I shift up, the cable gets pretty tight (but doesnt feel overly tight compared to my other bike) but doesnt shift cogs and the shifter jumps back down.

Im kinda stuck as to what the problem is. Im thinking the shifter maybe faulty and thats why it jumps down, or maybe there is a compatability issue between shifter and mech (if there can be such a thing)? I dont know.

Any help much appreciated.

bikaholic

The good thing about most front gripshifts is that they have small intermediate clicks for trim and work almost like a friction shifter, meaning that they can be made to work with most front derailleurs irrespective of cable pull ratios.

What front derailleur have you got?

2wheelmotion

I suspect i wouldnt have this issue if i had stayed with gripshift but i cant get on with them. Im not 100% sure on FD. It's shimano I know that. The cable feeds in from the bottom and is a top pull. I will check when home to see if I see any numbers etc.

Thanks for the reply :-)

ride_whenever

Just replace the front mech, if you know what you're doing and can't get it to work then blame the mech

Ouija

Should always set the cable tension for the middle ring first. When it sits exactly where you need it to get and keep the chain in the middle ring you then leave cable tension alone and use the limit screws for correct shifting into the outer and inner granny ring.

Also worth mentioning that a lot of older Shimano 8 speed shifters and below sometimes came with a trim position between the granny ring and middle. In other words, you'd have to push the lever until you felt two clicks (one smaller, one larger) before the mech would jump up to the middle.

cooldad

This but remember to have the shifter in the middle position.

2wheelmotion

aah I didnt know it was best to set it on middle cog. I will try it when I am home then let you know how I get on.

thanks

2wheelmotion

UPDATE

I set the mech into position for the middle cog with the shifter in middle position, set cable tension to hold. It then wouldnt shift up or shift down. I adjusted the limit screws to try aid the movement and it wouldnt adjust far enough to move it. I then re-set the mech and cable in a slightly differnent positon so the mech was already nearly touching the chain on the inside, still wouldnt shift up.

I am cheesed off with it now :x

I have set in onto the middle cog and have just left it there and i wont shift out of it

bikaholic

What front derailleur have you got?

The Rookie

2wheelmotion wrote:

I adjusted the limit screws to try aid the movement and it wouldnt adjust far enough to move it.

This says you don't understand the limit screws, wind them both right OUT so the cable is doing the work, once it's shifting then set them to 'limit' (get it) travel to that which the cable is already using.

2wheelmotion

bikaholic - All I know is it is a shimano derailleur, no numbers visible anywhere.

The Rookie - you maybe right I probably dont fully understand the limit screws (although I think I do but the quote you found does make it look like I dont).

The reason I mention the limit screws is I have seen various videos saying to adjust the limit screws so that the derailleur is nearly touching the chain so it is closer to it for when you are shifting. I fully understand that the limit screws are to stop the chain from popping off the inside or outside of the crank. Feel free to correct me if I am wrong.

Ouija

As mentioned above, first set the derailleur so that it's correct in the middle ring. Make sure the derailleur cage is completely parallel with the chainrings. In other words, the cage isn't angling in towards the frame at the back. Also make sure all limiter screws are screwed most of the way out.

Then, with it set on the smallest ring on the back (second smallest on some systems) use the cable adjusters to get the outside edge of the cage almost kissing the chain. Some systems even expect you to have slight rub on this combination in order to get a better setup on the outer chainring (which is the last one you setup).

Next up, shift up the gears on the back until your two or three teeth up the cassette and then shift the front derailleur down to the granny ring. The reason for this is that your never supposed to be in the small/small combination of gears as it messes up your chain which will then procede to wear down your cassette and drivetrain quite quickly. Also, you'll find that the derailleur often doesn't want to move the chain down to the granny ring when your in the small/small combination but will do so if the chain is part way up the cassette (at least three rings). This doesn't mean the derailleur is set wrong because it does this, it's simply that the manufacturers don't design the derailleur to function in a gear combination they specifically tell you not to use.

Once your on the granny ring, keep turning the cranks and the chain should be rubbing against the derailleur, even though your 3 teeth up the cassette. This is where the limit screws come in. Tighten the screw for the inner limit (inside screw on a SRAM system, outside screw on a Shimano system) until the derailleur as moved far enought away from the frame to just stop rubbing against the chain (tugging on the cable can make screwing the screw in a little easier as it takes the pressure off the bottom of the screw as your making the adjustment).

As well as being the correct setup for the granny ring, it also acts as a reminder of when to shift gears. So, when your out riding the trails on the granny ring and your shifting towards the smaller rings on the cassette, the second you hear the chain rubbing against the derailleur (3rd ring up, remember) is the second you should shift your front chainring up into the middle ring instead of heading for that forbidden Small/Small combination.

Next up is to set the outer ring. This is where you can sometimes encounter problems depending on the chainline of your frame and crankset and bottom bracket combo. With the limiter screw all the way out you shouldn't have problems going into the big ring, but you may find that the cage moves too far and takes the chain right off the outer ring when changeing at speed (or severely limit the number of gears you can go up at the back before it's rubbing against the inside of the cage....... which shouldn't be more than halfway up the cassette anyways, to prevent cross chaining). Screwing in the limiter screw (outside one for SRAM, inside one for Shimano) will move the cage toward the frame but then you may find that you really have to press hard on the shifter to get to the outside ring and may not be able to do it at all.

This is why both Shimano and SRAM sometimes mention setting up the derailleur for the middle ring so that it DOESN'T rub in the second smallest ring on your cassette but DOES on the smallest (another forbidden gear combination on some frames). Losing that gear combination and having the derailleur sit a tad closer to the frame means that it wont over extend when going up into the big ring.


Lastly. Another common mistake i've seen (and experienced first hand) is to correctly thread the gear cable into the derailler. Especially with derailleurs designed to take a cable coming straight down the seatpost or able to take both top and bottom pull cabling. In some cases, it's possible to thread the cable into the screw from two slightly different directions. One straight in from above (which causes the derailleur to swing farther with each press of the lever), the other via first angling round the outside of the screw and clamping along the bottom edge, which causes the derailleur to do smaller incremental jumps on each lever press as the cable is partially unravelling like the string of a YoYo (indirectly pulling across a cambered surface). Some derailleurs don't always make it clear as to which way the cable should be orientated before clamping (look for depressions and guides on the derailleur).