Bicycle Mechanics - b-screw ineffective on Sora RD-3300
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12-26-07, 11:00 AM
I hope someone can help me with this problem. A wheel change necessitated a new cassette because my old one refused to come off (cassette lock sheared off! - it was old). The old cassette was 6 gears and the new one is 8 gears Shimano.
Since only 6 gears were being indexed I put new shifters and a rear derailleur on my christmas list. I put them on yesterday but have run into a problem with the rear derailleur (Shimano Sora RD-3300 8 speed).
The pulley is rubbing against the cog. I've adjusted the b-screw from one extreme to the other but it has made no difference whatsoever. It is positioned correctly (against that bit on the frame - sorry I don't know the technical word for it). I've tried shifting up and down as well as observing as I adjust and there's absolutely no difference.
Temporarily I've 'solved' the problem by pulling the cable housing that goes into the derailleur up over a bolt that holds the mud guards in place. This has the effect of moving the derailleur so that the pulley positions a bit further away and no longer rubs. However, this is obviously a bit of a bodge.
This is the first derailleur I've installed so I assume I've done something wrong. Could chain length be an issue?
Thanks for any help you can give,
Chain sounds like it's too short.
Chain length sizing:
Then the "B" screw will work for proper adjustment/clearance on the largest cog:
12-26-07, 01:16 PM
Thanks for the chain length link.
I have the values C=17.5", F=48, R=26, L=54.5" (or 54.6748 if using complex formular).
I've measured the chain I've got at 56.6". It's quite old and I think its stretched a bit - hence the 0.6"
So if anything my chain is too long. Does this make sense?
12-26-07, 02:41 PM
It is not very helpful but the B-screw can be a bit like that- you adjust it and not much seems to happen-it is not a good idea though to leave the pulley rubbing against the cog as I found from painful experience that it can cause your chain to be torn apart if you try and shift under extreme load at the wrong time and also it will eventually damage the derailleur.
Is the biggest ring of the new cassette bigger than the old one - could it be that the new cassette is too big for the derailleur-not every cassette works with every derailleur.
12-26-07, 04:54 PM
The cassette is also new - HG50 8 speed (if I remember correctly). Its the right cassette/derailleur combo according to Shimano. The old components are the chain and the whole crankset.
12-26-07, 08:35 PM
You can move the wheel forward in the dropout (assuming horizontal) to help alleviate this a little, too.
Chain length might be an issue as noted. Instead of all that fancy figurin', use Sheldon's (http://www.sheldonbrown.com/derailer-adjustment.html#chain) method
12-26-07, 08:58 PM
+1 0n Sheldon's method, which is also the method Shimano specifies.
12-27-07, 05:25 AM
Thanks for the feedback everyone. However, now I'm more confused than ever.
I spotted a diagram for recommended chain length in the instructions for the rear derailleur. In that they have the chain feeding through both derailleurs and around the largest sprocket at the front and the smallest at the rear. The diagram suggests the bottom side of the chain should slant slightly down towards the crank but mine is either horizontal or slightly upwards. This to me would suggest the chain is too short like everyone is saying.
However, I've done the Sheldon method (also re-iterated on the Park Tool site) and by that method my chain is far too long. It hangs very loose. The only thing is, if it isn't fed through the rear derailleur then surely its going to hang very loose as it doesn't have to feed round the two extra cogs?
Am I misunderstanding the Sheldon method or is my chain really way too long?
12-27-07, 07:24 AM
bearing in mind I am just a wally on the internet and not a qualified bike mechanic- my understanding of the chain length thing is that the chain should be long enough so that you can shift to the largest sprocket at the front and the largest at the back (not that this a recommended combination) with a little bit of extra chain length spare but this should be a minimum. The big/big combination is what ultimately determines the chain length. If you shift to this combination and the chain is not long enough then the drive train will sieze up with possibly disastrous consequences. Thus, when you shift to other smaller sprockets (front or back) and the chain doesn´t need to be as long, the derailleur takes up the slack with its spring action and effectivley shortens the chain. You shouldn´t have too much slack or the derailluer won´t be able to take up the slack. So you can basically see how much chain you need just by wrapping the chain round the big/big sprockets (edit: i meant while the chain is routed normally through the derailleur of course) and adding a little bit.
12-27-07, 07:39 AM
I can't really tell in the picture, but is the chain routed through the derailler correctly? For the chain to that much difference in length from the old set up, there would have to be a major change in the size of the cassette.
12-27-07, 08:17 AM
In your picture it appears that your chain is NOT routed through the rear derailleur, that is proper for the Sheldon/Shimano method to determine chain length. Also, the chain looks to be going around the biggest rear cog and the biggest front chainring- that is also correct for this method. Now, figure out how long the chain would need to be to be to be TIGHT in this position, then add 1 inch (two links/two pins) to the length. Break the chain there, thread the chain properly through the rear derailleur and the front derailleur cage and join the chain. Make sure that the place where you cut (break) the chain for length is the right inner or outer set of side plates to connect properly to the other end of the chain- if it is not, just add 1/2 inch (one link/one pin) to get the right combo.
Go back to Sheldon and Park to double check.
Hope this helps.
12-27-07, 10:20 AM
I concur with ken that you're doing it right so far, just finish it up, and I also agree with you that it appears that this means your current chain is too long.
I hope you will buy a new chain for this for 2 reasons:
1. Your chain has stretched and will not shift well with a new cassette, plus
2. You may have had a 6s Uniglide cassette, and you now have an 8s Hyperglide. The Uniglide teeth were wider and had their own Uniglide chain to work with these.
plus a stretched chain will wear out your new cassette and your chainwheels more quickly
12-29-07, 04:12 AM
Very strange thing: I reconnected the derailleur and it was happy! No more rubbing - good position. Its now running pretty smoothly.
The only thing I can think of is that in partially dismantling the derailleur to get it round the chain I mucked something up first time round. Anyone - the original problem is solved (although I haven't tried the b-screw but since its happy I'm not going to touch it).
Thanks for the chain recommendation - that's my next upgrade.
Thanks to everyone for your help, Joe
12-29-07, 07:15 AM
Going from a 6 speed cassette to an 8 will make your old chain to short.
Why? Please explain. If the overall tooth range, particularly the largest cog, is the same why would the number of cogs make any difference?
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