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Ultegra front ring shifting under pressure

50x1150x11 Posts: 315
While out today I was doing some intervals, and twice the front ring shifted from large to small ring when I was out the saddle sprinting.

Never happened before and everything is indexed properly on both sides of the drivetrain. This doesn't happen when riding normally and it shifts perfectly every other time.

Anyone got any advice?

Posts

  • slowbikeslowbike Posts: 8,498
    Something is flexing
    At a rough guess - it's mis-shifting as you stomp on the right pedal.
    The front Mech should hold it in place - can you tweak it out a smidge?
  • 50x1150x11 Posts: 315
    slowbike said:

    Something is flexing
    At a rough guess - it's mis-shifting as you stomp on the right pedal.
    The front Mech should hold it in place - can you tweak it out a smidge?

    Thanks I'll have a look and give it a little tweak.
  • mtb-idlemtb-idle Posts: 2,177
    is it simply that the chain is worn? it's looser on the chainring and therefore more likely to slip under pressure.
    FCN = 4
  • 50x1150x11 Posts: 315
    mtb-idle said:

    is it simply that the chain is worn? it's looser on the chainring and therefore more likely to slip under pressure.

    It's done about 500 miles, so shouldn't be that worn
  • mtb-idlemtb-idle Posts: 2,177
    fair enough, twisted link then maybe? It's stating the obvious but something is causing it to shift off the large chainring so my assumption is that the chainline isn't straight and it's a case of working your way through the reasons.

    Another obvious one; my mate had similar when he replaced the chain on his SS. He called me around to his place. It took me a while but I realised his back wheel wasn't in straight therefore chain dropping off due to chainline issues.

    Be thankful it's going to the small ring. if it went the other way you may be OTB
    FCN = 4
  • 50x11 said:

    mtb-idle said:

    is it simply that the chain is worn? it's looser on the chainring and therefore more likely to slip under pressure.

    It's done about 500 miles, so shouldn't be that worn
    I've just replaced a Shimano chain that had stretched by one full link in less than 1000 miles.
    It had stretched so much that my Park Tool chain gauge was telling me that there was plenty life left in it. One full link is approx one inch. Took me some time to find the problem, which was similar to OP, because like him I couldn't believe that a chain could stretch so much in so short a time. Chain wore out, and knackered cassette and big ring.
    I was having trouble keeping chain in big ring just like OP.
    From my experience the chain will be sh*gged.
  • thistle_thistle_ Posts: 5,163
    Obvious ones you've probably already checked:
    Is the chainring bent?
    Is a tooth on the chainring bent?
    Are your cranks bolted on tight?

    A weird one that I had a while back was the spider that connected the crank to the chainrings had come loose (SiSl crank). Should have had loctite on when it left the factory but didn't.
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