Why wont bike drop into highest gear ?

Raffles

Feel like such a tool asking this, but my chain came off and since then I cant get into top gear. :oops:

Mine is an 8 speed rear cassette and I usually have no trouble getting into my highest gear. My chainset is 105 compact 34/50 and the cassette is 12-25. I can get into all the other cogs without issue, but if I try to drop into 50/12 ( my biggest gear ) it just makes that clack-a-clack-a clack that sounds like it is thinking about it. If I lift the back wheel off the ground and spin the pedals harder with my hand , after a while the chain finally drops in.

Ive flipped the bike over and put wd40 on the cable that goes to the 105 rear deraillure, ive turned the rear barrel adjuster up to 1/2 turn anti-clockwise , ive loosened the H screw on the rear deraillure up to 1/2 turn and none of this makes any difference , the chain just sounds like its thinking about dropping in.

Got any ideas as to what I should do to return my bike again to dropping into 50/12 without issues ?

mcp73

Perhaps you could click your shifters all the way down to the highest gear (least amount of tension in the cable), undo the cable at the derailleur and then pull it through until it's tight and then re-index using the barrel adjuster (after re-clamping the cable).
Sometimes if there's slack in the cable it won't shift.
Just my thoughts.

k-dog

You need to work out what is causing it not to go into that gear. That's easiest on a stand or at least hanging so the wheels are off the ground.

It does sound like it's just an adjustment issue if it goes eventually - and 1/2 a turn isn't too much (although slightly strange that it happened after the chain came off - maybe just a coincidence or a not quite right setup).

Click the shifter all the way down and the cable under your downtube should be slack - noticeably. If it's not then it's just an adjustment issue and either turning the barrel adjuster (might not be enough if it's a whole gear out) or loosening the cable clamp and letting a bit through should do it - and then you can use the barrel adjuster to fine tune it.

If the cable is slack but it doesn't drop down then it is the H stop that is the problem - you need to turn that until it is enough to let the chain drop to that cog - but make sure you don't make it go too far as then your chain can drop off when you do that shift. It's a balancing act between letting it go far enough and not too far.

Plenty of good videos on youtube - just a basic derailleur adjustment. Take your time and think about what you're doing - it makes sense if you do it methodically.

Raffles

Now its sitting on the smallest cog but is making the chatterring sound , whereas before it was silent, how do I silence that annoying sound ?

vorsprung

road gear rear derailluers work so that tension in the cable pulls the arm across to the "biggest" sprocket with the most teeth, nearest the wheel. As tension is released, the spring in the derailluer moves the arm down towards the "smallest" sprocket with the least teeth, that gives the biggest gear.

So the reason the bike isn't happy about getting into the smallest sprocket is that the tension from the cable is too high or is not being released properly

Cables will sometimes loose a little tension due to wear. Often this is a precursor to the cable snapping. This isn't happening to you. There is too much tension in the cable. The increase in cable tension could be from knocking the derailluer, the barrel adjuster on the down tube or the shifters. Alternatively, there could be dirt in the spring or the derailuer partially jamming the releasing action

lef

best off to start the set up from scratch if you can't get it working. Apologies if you know all this but if not...
change gears so chain is (or at least 'should be' in your case) the smallest cog.
release the cable at the rear mech, tighten the finger adjuster (clockwise) on the mech so all the way in.
change the gear up, it probably won't do anything or the chain chatters, so turn the rear mech finger adjuster anti clockwise until it changes gear and stop as soon as there is no chattering. go up and down the gears, if it struggles to go up to the larger cogs at any point then quarter turn anti clockwise to increase cable tension. If it struggles to move down the cassette quarter turn clockwise to lower the tension.

if still not working the limiter screw may need adjusting. check the pulley wheel is below the last cogs (should actually be slightly offset at least on shimano - see tech docs)

mikeabanks

BTW - don't use WD40 as a lube :-)

John.T

First make sure cable tension is not the reason. Disconnect the cable and manually move the mech to change to the next sprocket. Then while still turning the pedals let go of the mech. If the chain drops straight on to the top sprocket then the problem is with the cable setting. If it does not then either the limit screw is not set right or the mech hanger has been bent. Adjust or straighten/replace as necessary.
If all OK then fit the cable back and adjust as shown on the Shimano tech doc. This is for 105 but they are all the same.
http://techdocs.shimano.com/media/techdocs/content/cycle/SI/105/SI_5VN0B_001/SI-5VN0B-001-ENG_v1_m56577569830683908.pdf

Dafo

This one is really clear:

http://www.youtube.com/watch?v=SkzvfCaIbyQ

cycleclinic

It could also be the limit screws.

John.T

thecycleclinic wrote:

It could also be the limit screws.

I think I mentioned them while giving a logical way of troubleshooting the problem.

MGmech

This guide will show you how to properly index your rear derailer

http://www.madegood.org/bikes/library/i ... -derailer/