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Hi there I have just bought a Trek Madone SLR Di2 disc road bike. All the components are Dura Ace except for the rear derailler which is Ultegra. I was told that it could not be Dura Ace as when changing gear it would travel into the rear cassette. Would this be true? I am riding a 50 34 front chain ring. Hope this makes sense.

  • Thankyou very much for your answer and compatibly chart . – Anthony Raymond Smith Oct 5 at 15:49
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    I don't think DuraAce has a medium- or long-cage rear derailleur that works with cassettes larger than 28t (or maybe 30t?) If you have a cassette that's larger than that, you need a medium- or long-cage derailleur. Ultegra Di2 does have a medium-cage derailleur that supports cassettes up to 34t. – Andrew Henle Oct 5 at 16:03
  • What is the tooth count of the largest cog in your rear cassette (ie the largest gear in your block) – Criggie Oct 6 at 0:08
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Possibly the bike has a cassette size that the Dura-Ace Di2 rear derailleur does not support.

The current DA RD-R9150 (short cage) rear derailleur has a max low sprocket size of 30 teeth whereas the Ultegra RD-R8050-GS (medium cage) will take up to a 34. If you have an 11-32 or 11-34 cassette, you do in fact need the Ultegra GS derailleur.

SS and SG versions of derailleurs don't just have different cage lengths and ability to take up chain slack, the angle of the parallelogram is different also - so the cage of a short cage Dura Ace derailleur may in fact interfere with the largest sprockets of a 11-32 or 11-34 cassette.

Di2 derailleur specs here: https://productinfo.shimano.com/#/spec/ROAD%20Di2%20spec/Rear%20Derailleur

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Whoever told you that is a liar. Shimano Di2 parts have excellent compatibility between component groups. Here's a compatibility chart. Basically, as long as you're using parts of the same "generation" (10-speed vs 11-speed), you should be good.

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    Whoever told you that is a liar. Not necessarily true. DuraAce doesn't have a medium-cage rear derailleur. Ultegra does. – Andrew Henle Oct 5 at 15:51

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