End-stop screws limit the movement of the front derailleur, so that it cannot drop the chain off the inside or the outside of the chainring.
There are two separate screws, each controlling one end of the derailleur range. The ’low’ adjustment screw prevents the derailleur swinging too far towards the frame, dropping the chain into the gap between chainset and frame. The ’high’ adjustment screw prevents the derailleur from throwing the chain off the outside of the chainring.
Before you start adjusting, work out which screw does what. They’ll have H and L marked on them, or a long line next to the H screw (representing the big chainring) and a short line next to the L screw. These indicators will helpfully be printed in black on a black background.
Usually, on ’conventional’ front derailleurs (where the band that clamps the derailleur to the frame is higher than the derailleur cage), the H screw is the one furthest from the frame. For ’topswing’ derailleurs (where the band is lower than the cage, as shown below), this is reverse – the H screw is nearest the frame.
Choosing the right type of derailleur
There are three types of derailleur: ’conventional’, ’topswing’ and ’e-type’. The conventional type is the older design, where the clamp that attaches the front derailleur to the frame seat tube is higher than the derailleur cage. Topswing derailleurs also bolt to the seat tube, but the clamp is lower than the derailleur cage. On many bikes, these two types are completely interchangeable, but some configurations of suspension frame will force you to use one or the other. If in doubt, replace like with like.
E-type derailleurs don’t clamp onto the seat tube – they’re usually mounted to the bottom bracket shell, via a separate plate which is sandwiched between the bottom bracket cup and the frame. On long travel full suspension bikes, there’s been a movement towards mounting e-type front derailleurs directly to the swingarm. This means that the front derailleur moves with the suspension action.
Your frame will also dictate whether the front derailleur is ’top pull’ or ’down pull’. As the name suggests, down-pull derailleurs are activated by a cable that runs down towards the bottom bracket, whereas with top-pull derailleurs the cable will head straight upwards then along the top tube. Most modern derailleurs are dual pull, so can be converted to accept either top- or down- pull routings.
The third piece of information you need to know for a band on derailleur is your frame diameter. The three sizes are 28.6mm 31.8mm and 34.9mm. (1 1/8”, 1 1/4” and 1 3/8”. Shimano derailleurs avoid this issue altogether, as they’re supplied in a 34.9mm size with shims to convert them to either of the other diameters.
Setting end-stop screws
Step 1: Start with the chain in the middle ring. Check the shifter is in the middle of the three positions. Turn the pedals and shift into high gear. The chain should lift onto the big chainring as you turn the pedals. If it won’t go, you need to unwind the ’high’ (A) end-stop screw (marked ’H’ on the derailleur, or with a wider line that is often hard to see since it’s usually printed in little letters). Unscrew the ’H’ screw a couple of turns and retest.
Step 2: Once you’ve got the chain onto the big chainring, you need to make sure it won’t go too far. With the chain still in the big chainring, gently roll in the ’H’ screw until you feel it touching the body of the derailleur – it will roll in fairly easily, then you will encounter resistance. At this point, back it off half a turn and test again.
Step 3: Now try shifting into the smallest ring. Again, it should drop in first time. If not, you need to back off the low adjusting screw (B). It will be marked with an ’L’ or with a narrower line. Test, adjust, then test again. Once the chain is in the small chainring, you need to set the ’L’ screw so it won’t move too far. Wind it in, but watch the derailleur cage as you turn the screw and stop as soon as you see that turning the screw is moving the cage. Test again.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
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