Problems with changing to compact chainset - chain slips

mcav79
mcav79 Posts: 17
edited January 2010 in Workshop
I have just replaced my chainset 53/39 for a compact 50/34 I have replaced both the chainset and bottom bracket but when I am in the larger sprockets at the back, the chain comes off the the big ring at the front? The teeth actually pick up the chain mid link and drop it down onto the smaller ring. Its not the front mech causing the slip do I need to remove chain links to compensate for moving to a compact? I know the chain is a bit worn but it worked fine on the old chainset?

Comments

  • Pokerface
    Pokerface Posts: 7,960
    If you replaced your chainset with a brand new one, then the front chainrings will have nice, new, sharp teeth.

    If you're still running your old (and possibly worn out) chain with the new chainrings, the slipping COULD be caused by this. The wear on the old chainrings and the old chain would be 'matched' with each other. Or some such nonesense.

    Just one theory anyway.

    Yes - you may need to take link or two out as well.

    I would suggest a new chain. Doesn't cost much and it's good practice to replace them on a regular basis. When chaining chainsets is the ideal excuse to get a new chain!
  • Meds1962
    Meds1962 Posts: 391
    I took out two links going from 39/52 to 34/50.

    If you know the chain is worn it would make sense to change it but it may cause slipping at the cassette if that is also worn.
    O na bawn i fel LA
  • Repace the chain...for the cost of a new one..... its not worth the bother
  • mcav79
    mcav79 Posts: 17
    So you think it could be solved with just replacing the chain its nothing more serious like the Chainset not being compatible?
  • John.T
    John.T Posts: 3,698
    If the front mech is adjusted correctly this will not happen. Did you lower the mech and check that the new chainset and BB had not altered the chain line.
    If you fit a new chain you may need to change the cassette as well. New chains on old cassettes give far more problems than old chains on new chainrings.
  • Pokerface
    Pokerface Posts: 7,960
    John.T wrote:
    New chains on old cassettes give far more problems than old chains on new chainrings.

    My experience has been the other way around. But that is JUST me I guess.
  • John.T
    John.T Posts: 3,698
    Pokerface wrote:
    John.T wrote:
    New chains on old cassettes give far more problems than old chains on new chainrings.

    My experience has been the other way around. But that is JUST me I guess.
    Not saying an old chain on new rings would not give trouble. It may well. Just if he fitted a new chain he should also change the cassette as a new chain on a worn cassette will almost always give trouble. If things are not so worn then the chain should be OK on the rings as cassettes wear faster than rings (normally).
  • mcav79
    mcav79 Posts: 17
    I dropped the front mech to compensate for the compact, and it is not the front mech that is the problem, the problem arises from the teeth picking the chain up on the big ring without any touching or rubbing on the front mech? How would I measure/adjust chain line?
  • John.T
    John.T Posts: 3,698
    Did you have to move the mech top limit screw when you fitted the chainset. If not has the gap between the cage inner plate and the chain got bigger. If not then the chain line is probably much the same. It may well be that the teeth have got rather sharp edges and need to bed in a bit. Use the bike but avoid using the gears that are affected. Give the ones next to them a fair bit of use and see if it improves. I can not be much more help without seeing the bike.
  • keef66
    keef66 Posts: 13,123
    Does the new BB have a longer axle than the old one? It sounds like the big chainring is too far outboard for the biggest sprockets.

    You can't really adjust chainline.
  • ajb72
    ajb72 Posts: 1,178
    Before you buy a new chain, it is probaly worth checking the length first. As one or two guys have mentioned it is very likely to need shortening by a couple of links in order to fit the change to compact.

    If this is sorted, it is most likey to be a front mech issue and hopefully a 2 minute job.

    As for getting the correct chain length, there are a few different methods including one very scientific calculation.

    The one I use is very easy though; put the chain on the largest chainring at the front and the smallest sprocket at the back - in otherwords the largets gear on the bike. Pass the chain though the front and rear mech as normal and join the chain so that there is a straight vertical line passing through the two jockey wheels on the rear mech when you put the two ends of the chain together. Hope that makes sense.
  • mcav79
    mcav79 Posts: 17
    Axle length is the same, I have had to adjust the limit screws, but I think I will try removing a couple of links first as this to me sounds like the most likely culpret, if that seems to solve the prob I will then purchase a new chain and fit that, cheers for the advice guys!!
  • John.T
    John.T Posts: 3,698
    If it is coming off when pedaling forward then it is not likely to be chain length as the length at the top is un-altered and the tension is provided by you. If the small tongue on the lower part of the rear mech cage is clear of the chain when you are on small/small then it is OK.
    You say you had to adjust the limit screw. Was this the top limit one and did you have to move the cage out. If you needed to move it by more than a couple of mm then your chain line may be poor. You fitted the same length BB but is this the correct one specified for the new chainset?. Is it an outboard bearing BB and did you fit any spacers on the drive side?.
    When I fitted 10sp on my race bike but kept the 9sp Dura-ace chainset my chain would come off when pedaling backwards on big ring and the 2 big cogs. As the chain bedded in this went away. As I said earlier give it a bit of time and see if it improves before throwing money at it.
  • mcav79
    mcav79 Posts: 17
    If it is coming off when pedaling forward then it is not likely to be chain length as the length at the top is un-altered and the tension is provided by you. If the small tongue on the lower part of the rear mech cage is clear of the chain when you are on small/small then it is OK.
    You say you had to adjust the limit screw. Was this the top limit one and did you have to move the cage out. If you needed to move it by more than a couple of mm then your chain line may be poor. You fitted the same length BB but is this the correct one specified for the new chainset?. Is it an outboard bearing BB and did you fit any spacers on the drive side?.
    When I fitted 10sp on my race bike but kept the 9sp Dura-ace chainset my chain would come off when pedaling backwards on big ring and the 2 big cogs. As the chain bedded in this went away. As I said earlier give it a bit of time and see if it improves before throwing money at it.

    I fitted the BB that came with the chainset and is appropriate for the job, its an external cup type, no spacers were added, I haven't had chance to even pedal yet :-( this is happening on the stand after I have just changed it over, I have tested the chain and know it is beyond its limit for wear, and think it kind of makes sense to remove a couple of links if I am changing to a compact/change the chain and set it to the appropriate length. Also the wear on the chain could be causing sideways movement hence it getting picked up at a mid point along one of the plates. Unfortunately I cant just keep riding it until it beds in as I feel this might cause unnecessary wear on my chainset, and it might cause problems when I am out and about if this happens at a point of strenuous effort and the chain breaks i'll be up some kind of a creek without a paddle.
  • John.T
    John.T Posts: 3,698
    mcav79 wrote:
    I fitted the BB that came with the chainset and is appropriate for the job, its an external cup type, no spacers were added, I haven't had chance to even pedal yet :-( this is happening on the stand after I have just changed it over, I have tested the chain and know it is beyond its limit for wear, and think it kind of makes sense to remove a couple of links if I am changing to a compact/change the chain and set it to the appropriate length. Also the wear on the chain could be causing sideways movement hence it getting picked up at a mid point along one of the plates. Unfortunately I cant just keep riding it until it beds in as I feel this might cause unnecessary wear on my chainset, and it might cause problems when I am out and about if this happens at a point of strenuous effort and the chain breaks i'll be up some kind of a creek without a paddle.
    With an external BB the chainline should be correct so no worry there. Changing from a 53 ring to a 50 is 1.5 links. ajb72's advice for the correct chain length is good. It is how Shimano say to do it. Ere on the long side if you have to. 50/34 with 12/27 uses almost all of the chain slack take up.
    If your chain is at or beyond the wear limit you should change it but be prepared to change the cassette as well. The new chain may well jump on the most used cogs. Faults that show up on the stand often do not when riding (and vice versa).
    It is difficult to give really good advice on here as we often only have a small part of the picture and never all of it. Hence a certain amount of guessing happens.
  • mcav79
    mcav79 Posts: 17
    No probs you guys have been very helpful!