sacrificing gears for the sake of front mech rubbing?

daffyavfc

Hi

I've a compact 18 and 16 speed bikes and if I want to stop the rubbing on the outer front mech when in the largest chainring then I lose gears going back up the cassette as again it rubs on the inner side of the front mech.

This is blindingly obvious I know, so when I drop into the lower chainring when it rubs on the large one, the gearing is too easy. I've just increased the 34T to a 36T in the hope I'll get more gears when dropping down.

Should I put a 38T on? managed with a 39T a while ago but the 52 was too big for me on a standard double.
Just seem to be changing up and down more than I'd like because of the rubbing. I've set the derailleur as close to either side of the 2 chainrings in order to give me the biggest range and am happy with what I have done.

Any thoughts? or is it as obvious as I'd thought that I'm sacrificing gears unless I change the chainring or cassette? (have a 12-25 on best bike and 13-26 on other)

Perhaps I should have just changed the 52T to a 50T!

thanks

sandyballs

For starters what make and model would be handy. Both SRAM and Shimano have models that have a Trim function, that is whilst in the outer chainring, activating the front shifter once moves the FD a small amount the next action moves it into the inner chainring position. This means when moving up and down the rear cassette you use the front shifter trim function when necessary to stop chain rub.

Give us what you are using and someone will help.

daffyavfc

Ok

tiagra shifters, 9 speed, 50/34 shimano ultegra compact (fc r700), 105 rear mech, tiagra front mech. Is this the problem, only shifters and front mech match?

cycleclinic

The trim function will not get around the issue of cross chaining which is no no in my book. So to me it does not matter what shifter you have. If you are experiencing chain rb you are not using your gears correctly.

Big big or somewhere close is a ratio that is found in the inner ring and the middle of the cassette.

So surely the solution is the change gear differently. The best way is not to shift up the cassette and then drop to the inner chainring. Chain the chainring first. Your cadence will jump but this is fine then as the gradient climb shift up the cassette. This way you never need to cross chain.

Other than that a 50T outer may help abit. The one thing I do not like about compacts is the big jump or drop in cadance when shifting chainrings. I use a racing double now becase of this. Of course if you had a 14-25 9 speed cassette instead of the 12-25T with 36/50T the change in cadance when shifting chainrings would be less. Close ratio gearing is must I think for road cycling. 50/14 as a top gear works so long as you are not riding at 30 mph alot.

sandyballs

I disagree with your initial comment, I use SRAM Red, Full Tiagra, and Full 105 on my bikes. Whilst I agree cross chaining is not recommended it should be and is possible on all my set ups. I built all my bikes so am adept at setting up the driveline, and owning, riding, fixing for 25 years means most probably like yourself you get a feel for driveline awareness and gear selections.

As for the OPs problem, in my opinion it will be a case of set up vs experience and awareness of driveline and controls.

cycleclinic

I agree you can (and I do all the time) set up a drive trains so it does not rub when cross chaining but hey you should 't be doing it anyway so the point is moot. To me it means use another gear there is always one that gives a straighter chainline.

StillGoing

Whilst I agree that cross chaining is going to put additional stress on the chain and shorten its life, SRAM is certainly designed to allow it and mentions it in the set up instructions. SRAM's new yaw control goes further allowing the FD to move on the diagonal, but doesn't get around the fact the chain will be running out of square. There are times when on a climb, the last thing you may want to be contemplating is changing from the outer to the inner ring and then having to change up on the cassette to a similar speed losing you momentum, when it is easier and less taxing when you're fighting for that last bit of breath, to just change to a bigger sprocket on the cassette. I wouldn't suggest that one should ride along on the flat cross chaining, but the few odd occasions when you don't have the time to change with the front and rear derailleurs would for me be acceptable.

In short don't cross chain unless you have to and set the FD up to allow it. Just in case.

daffyavfc

I try not to cross chain due to the problems you've all highlighted. I just find myself not having enough gears in the largest chainring when moving halfway up the cassette due to the rubbing. When I drop to the inner ring it is too easy.
My thoughts are this- please let me know your opinions please
Increase the inner ring to at least a 36 if not 38T or look at a larger cassette other than my present 12-25.

Which will make the bigger difference, or both?
Thanks

MichaelW

Rubbing whe cross chaining can be annoying and can often be eliminated but it can also be a warning of crosschaining.
To eliminate, check the position of your front mech, vertically and in angle. Parktools will show you how to do it.

Your two chainrings have considerable overlap in gear ratios. You can see exactly how they overlap if you work out the gear inches. A little excel spreadsheet can show you in a jiffy.
When you shift the front ring, you need to shift the rear sprockets by 2-3 to pick the gear you need. Practice simultaneous shifting. If that doesnt work, try one after the other.
I tend to use a different shifting pattern going down the gears from going up. You will soon develop an autopilot shifting routine.

StillGoing

I run a compact crank set with a SRAM WiFli RD. I alternate between an 11-26 and an 11-32 rear cassette depending on how hilly the route is going to be as I'm crap at climbing. I get no interference on the FD on the odd occasion I have to cross chain from the big crank to the large cassette sprocket simply because the FD is set up correctly. It is possible to set it up with no interference so I would recommend trying an alternative LBS to have a look at it for you.