Usable Gears/Trimming

cassettequestion Posts: 41
edited May 2016 in Workshop
Hi. My bike has a 10 speed on the back and compact double on the front (shimano 4700.

Took it to the bike shop to set up the gears because I couldn't do it. Had a little ride around and found the chain rubbing against the front mech when I was using the big chainring on the front and 7th gear (10th being the biggest sprocket) on the back. I took it back to the shop and they said that was normal and that I should only expect to use the highest/smallest 6 gears on the back. (To be honest, this is giving them the bod as well as I'm sure it was rubbing in anything but the 5 high gears on the cassette..)

My question is - is that correct? I do understand you're not supposed to use a big to big or small to small combination but telling me I can't use pretty much half the gears on the cassette seems a little off to me. I've had other road bikes before and tended to ride around in the 2nd or 3rd biggest gear on the cassette when on the flat and didn't notice this problem.

I posted a question elsewhere and some suggested 'trimming' was the answer? Thought it didn't hurt to get some suggestions from this forum.

Grateful for any help or clarification. Thanks.


  • keef66
    keef66 Posts: 13,123
    Well I have a 9 speed Tiagra double on one bike and 10 speed 105 triple on the other, and by using the trim function can use all the sprockets from all the chainrings without any rubbing, so it can be done. You certainly shouldn't be losing half the cassette.

    Have they shown you the trim function and how to use it?
  • No, they didn't but I have some idea after experimenting a little - seems like when youre on the big chainring, there's like an intermediate position for the front derailleur before it shifts to the small chainring. Is that correct?

    They did actually adjust it a bit when I took it back but only because the chain was rubbing when I was in the lowest gear (i.e. small to big combination), which even I know definitely shouldn't happen. When I collected, they mentioned 'trim' but I thought this was something they'd adjusted themselves, not something I could do whilst riding..
  • keef66
    keef66 Posts: 13,123
    Yes, when you shift into the big ring it should be aligned so you can get the smallest of the sprockets, and a gentle click back the other way shifts the cage inboard a tiny bit to allow you to select the bigger sprockets.

    Di2 does the trimming automatically...
  • OK, I'll try that out later. Doubt I'll ever be serious enough to warrant Di2... :)
  • Nick Payne
    Nick Payne Posts: 288
    Well, without bothering to trim the front derailleur, on all of my bikes I can use the big chainring with all cogs on the cassette except the two largest without getting chain rub, and on most of them I can use all except the largest cog without chain rub. It's a different matter when on the small chainring, as when I get somewhere beyond the midpoint of the cassette as I shift to smaller cogs, the chain starts hitting the shift pins/ramps on the large chainring because of the angle it is running at.

    Chainstay length will also affect this - the shorter the chainstay the larger the angle the chain has to make for a given sideways displacement.