Have I got a H screw headache ?

JimmyK

after yesterdays mess that I made of my gears and having them reset by LBS guy, I was pedalling home from work at teatime tonight. Coming up to my home there is an incline, I was in inner front ring (34 teeth)and 3rd largest cog on the rear . I started to feel the incline in my legs so I shifted from inner front ring to outer front ring (50 teeth)and the chain fell off.

when i refitted the chain and tried the same shift about 6 times, the shift was spot on. this intermittency is driving me nuts and its getting to the point where im waiting for the chain to fall off when shifting from inner to outer front ring

the chain never comes off when shifting from outer front to inner front, just the other way round with frustrating intermittency. i absolutely do not want to make a botch of my gears like i did yesterday, would turning the H screw on the front mech in 1/8th turn increments until the shifts from inner to outer become velvet smooth be a practical fix ? if so, should i be turning the H screw clockwise or anticlockwise to fix my problem ? if there is any cable retensioning needed, im not gonna touch it.

I will, of course, mark where i started from with a tiny bit of tape. this chain coming off problem is really ruining my fun in cycling i gotta say.

Jimmy

iain_j

JimmyK wrote:

shifted from outer front ring to inner front ring and the chain fell off.

JimmyK wrote:

the chain never comes off when shifting from outer front to inner front

Sorry you lost me.

iain_j

As someone said in the other thread, front mechs are a pain in the ar$e... which way is it falling off and when?

Is the chain coming off completely? If it's falling off the inner ring, adjust the L screw, if it's falling off the outer ring, adjust the H screw.

If it's falling from the outer ring onto the inner one...... I haven't a clue sorry :P I guess the cable needs adjusting.

JimmyK

the situation which has been driving me nuts is that sometimes i will change from my 34 front ring up to my 50 front ring and the chain falls off , it never does this if if change from the 50 down to the 34 . sometimes i have to change from the 34 to the 50 and this falling off happens intermittently , it falls straight off and when i try to replicate it without changing any gears, it works perfectly.

is the adjustment im describing counter clockwise or clockwise turning of H screw ?
is turning the H screw gonna sort this infuriating situation out ?

not sure if this is relevant, but, in order to make sure the chain stays on, i have to whip the gear shifter quickly from left to right, if i just pull it across with a gentle pull, the odds of the chain coming off are greatly increased :?

Jimmy

iain_j

Gotcha. When you're shifting up to the 50 ring is the chain going "past" the ring and falling off onto the crankarm? If so you need to turn the H screw clockwise.

Bhima

JimmyK wrote:

is the adjustment im describing counter clockwise or clockwise turning of H screw ?

Tightening the H screw restricts the outward movement. Loosening it, allows more outward movement.

I used to have mine set up perfectly, but the chain would come off on the very rare occasion that I used the 11t sprocket on the back because the slight bend in the chain line was pulling it outward, if you get me. It was actually happening when switching from the 12t to the 11t, which is how I worked out that it was happening. So the rear gear you're on can affect weather or not the chain will come off. Get on your smallest sprocket and try to change up - the chain will probably fall off if you've got a real problem. Similarly, it will probably never come off when on your largest sprocket.

JimmyK

iain_j wrote:

Gotcha. When you're shifting up to the 50 ring is the chain going "past" the ring and falling off onto the crankarm? If so you need to turn the H screw clockwise.

yeah, sometimes i make the change and next thing i know, the chain is 3/4 off the 50 front ring and wrapped around the right pedal.........its driving me nuts with frustration. problem is that its so damned intermittent, it would be easier if it did it every time....but it doesnt :roll:

you reckon clockwise H screw adjust then, how much of a turn ?

Jimmy

JimmyK

Bhima wrote:

JimmyK wrote:

is the adjustment im describing counter clockwise or clockwise turning of H screw ?

Tightening the H screw restricts the outward movement. Loosening it, allows more outward movement.

I used to have mine set up perfectly, but the chain would come off on the very rare occasion that I used the 11t sprocket on the back because the slight bend in the chain line was pulling it outward, if you get me. It was actually happening when switching from the 12t to the 11t, which is how I worked out that it was happening. So the rear gear you're on can affect weather or not the chain will come off. Get on your smallest sprocket and try to change up - the chain will probably fall off if you've got a real problem. Similarly, it will probably never come off when on your largest sprocket.

exact opposite, chain never falls off down in the small rear cogs. , how did you get your problem fixed ?

Jimmy

iain_j

JimmyK wrote:

you reckon clockwise H screw adjust then, how much of a turn ?

Jimmy

That's as far as my expertise goes - the rest is trial and error for me. Turn it too far and the mech won't be able to move far enough to shift the chain from the 34 to the 50 ring.

I don't think the front mech adjustments are as sensitive as the rear mech, but I'd still go by, say, quarter or half turns. Play it by ear.

JimmyK

if i make any adjustments to the H screw, will i have to touch the barrel shifters at the handlebars ? not something i want to do.

iain_j

Don't think so.

iain_j

Just discovered that when you're adjusting the front mech's H screw, you should have the chain on the smallest (rear) sprocket, presumably to check the front mech is far over enough not to rub against the chain.

GavH

^^^ What Ian said. When adjusting the H screw, you need only do so enough to give clearance between chain and cage when in your highest gear (Biggest front, smallest back rings).

Likewise, adjusting the L screw the opposite applies.

Have a look at your mech from above and make sure the outer cage plate is perfectly straight. I had a problem once where it had bent (don't know how as it was brand new!) and this gave erratic shifting until I straightened it with a pair of pliers.

JimmyK

so if its intermittently throwing the chain on a 34 to 50 shift, would i be right in surmising there should be fair amount of chain rub if select highest gear at rear and 50 ring at front ?

although, guy at LBS told me any usage of highest gear at rear imparts some rub to varying degrees, similar situation for lowest gear at rear also. (according to him )

would inserting a feeler gauge set to 1mm and slipping it between outer edge of cage and chain after adjusting H screw be of any worth ?

Jimmy

GavH

JimmyK wrote:

so if its intermittently throwing the chain on a 34 to 50 shift, would i be right in surmising there should be fair amount of chain rub if select highest gear at rear and 50 ring at front ?

although, guy at LBS told me any usage of highest gear at rear imparts some rub to varying degrees, similar situation for lowest gear at rear also. (according to him )

would inserting a feeler gauge set to 1mm and slipping it between outer edge of cage and chain after adjusting H screw be of any worth ?

Jimmy

(IME) Guy at LBS is talking bollox. You should be able to set the front mech up such that it neither rubs on lowest OR highest gears. It may well rub if you use big front/big rear but then there are a number of reasons why that isn't considered as good practice, chain rub on front mech being but just one. No need for gauges - just look with the good ole' Mk1 eyeball. If it looks like a gaping chasm - too much. If it looks like a baw hair would struggle to fit between chain and cage - too little. You should see a definite air gap, but not much and certainly no more than 2mm.

JimmyK

gav, just say i check it out tomorrow and there is a gap that looks just right.........but chain still getting randomly slung off whilst doing a 34 to 50 change.

what then ? (im a pessimist by nature )

Jimmy

GavH

Then take a pic of the mech from the top down. Post it here. Let us all have a look and see if anything looks out of the ordinary. Believe it or not, that's how I cured my bent mech problem and it had been into my LBS (who obviouslt failed to notice!).

John.T

When the chain is on the big ring and smallest sprocket there should be between 0.5 and 1mm clearance between the cage and chain. This is set by the cable tension and you will have to use the barrel adjuster for this. Tightening the cable will increase the gap, slackening will close it. When you push the shifter lever to the right the cage should move a little further, no more than 1mm. This is set by the H screw. Turn it clockwise to close the gap and anti to open it. It is important to have this extra movement as not having it will lead to shifter lock up or failure.
Your problem sounds like the Cage is over shifting and the H screw needs tightening. Turn it a 1/4 turn at a time and get the settings as above.

JimmyK

the front derailleur is a shimano 2203 type and it doesnt have a L or H marker on it to show which screw is which :roll: , before i take a look at it, which screw on a shimano road front derailleur is usually the H screw, they are in a vertical line , is it top or bottom ?

I told ya I dont have a clue , ive done a bit of reading and it seems a generally held notion that front mech setup is a right pain in the hoop.

Jimmy

John.T

Just take a screwdriver to one of them and move it a little but remember where it was. If the cage moves then that is the L one. Now put it back to where it was.

JimmyK

bizarre thing is , a shimano tech doc has a troubleshoot section which matches my problem and says turn L screw 1/2 turn clockwise.

John.T

The L screw controls how far the cage goes towards the little ring position. The cable should be slightly slack and the cage held against the L screw by the spring tension. The cage position on the big ring is controled by the cable tension and limited by the H screw.
You are dumping the chain past the big ring ?. The L screw will only affect whether the chain drops off the little ring. It has no affect on the big one at all.

rake

try and look at the bottom of the adjustment screw while moving the mechanism with your hand. you may see the lever that it stops against indicating which screw does what.
there is a slight chance you will have to loosen the front cable slightly if it wont ratchet properly. just turn the barrel adjuster in a bit if need be.

JimmyK

LOL, i still dont know which is the H screw, top or bottom.

skyd0g

Have you looked at the guide on the Park Tools website: http://www.parktool.com/repair/readhowto.asp?id=75

The H screw is normally on the outer side of the bike.

rake

ok well put the chain on the big ring with the gear lever to it maximum position(ie no trim) and turn the screw in, you should see the cage move slightly is you have the right screw.

JimmyK

skyd0g wrote:

Have you looked at the guide on the Park Tools website: http://www.parktool.com/repair/readhowto.asp?id=75

The H screw is normally on the outer side of the bike.

so in my case, would that be the top one or the bottom ?

John.T

http://www.chainreactioncycles.com/Models.aspx?PartnerID=79&ModelID=42301
If it looks like this but with a different cage then the H screw is the top one.

No Sweat

Before you move any adjusters, I'd suggest checking to make sure that the mech is correctly aligned with the chain rings - the inner face of the outer plate of the front mech should be parallel with the outer chain ring (or just a little bit further out at the rear, but only a small amount). If this is not the case it could explain the occasional derailing of the chain and rubbing when on the smallest sprocket when you think the front mech is at the right distance from the chain when on the outer chain ring. Oh and also check that the mech is properly cleaned and lubricated - it might be sticking and then ocasionally moving a bit further and throwing the chain....

oldnslowcp

The 2203 tech doc is here:- http://techdocs.shimano.com/media/techd ... 625446.pdf

In the troubleshoot if the chain falls to the crank side then adjust the top screw clockwise about 1/4 turn

JimmyK

John.T wrote:

http://www.chainreactioncycles.com/Models.aspx?PartnerID=79&ModelID=42301
If it looks like this but with a different cage then the H screw is the top one.

thats the one...........so top it is.