Ultegra Front Deraileur Chainring Size Compatability
 

I recently swapped my 52/42/30 Campy chainrings for to TA 46/40/30 chainrings to test out a crossover gearing setup. My front Deraileur is an Ultegra triple. I can't seem to shift into the largest chainring because the front Deraileur is sitting too high for the 46t ring. But if I lower it, then it won't clear the middle ring. Are front deraileurs designed for a specific size of chainrings? Are my chainring sizes not compatible? Help!

Yes, they are. I use an Ultegra triple for a 50/34/24 combination but use friction for shifting the chainrings. STI's were too fiddly, wouldn;t do the job consistently.
Shimano makes several cross FD's that would work.

Front derailleurs are sized both for the large chainring but also by the teeth difference between the large and middle rings. If the difference is not enough you will run into your situation, where the derailleur will need to be set too high for the large ring to avoid hitting the middle one. The curve of the derailleur cage may also not be ideal for the large ring. You may find that shifting to/from the large ring is not ideal but usable. Or failing that a different derailleur may be required.

While front derailers are sized for a range of outer chainring diameters, the range is very broad. I've used Ultegra for 52/40/30 and 50/39/24 triples and have used Tiagra for 50/39/28 and 46/34/20 without issues. They all shift just fine.

Shimano's tech bulletin on the Ultegra 6703 derailer says that the minimum difference between the outer and middle ring is 13 teeth which puts your combination well within the range. Something else must be off for the derailer to hit the middle ring. The alignment may be off.

On a side note, I've never found STI to be that fiddly to set up and use. The Tiagra front derailer is actually a better derailer for triples than the Ultegra, however. It's easier to set up, is more forgiving, and can be used over a wider gear range than its more expensive brothers.

I tried an Ultegra triple FD with a 22/32/48 and it was an utter failure.
The arc of the cage was too large for the 48, putting the tail of the cage too far from the chainring.
I replaced it with an IRD Alpina FD which worked like a charm.
Since then I've seen a lot of recommendations for the Tiagra FD for such applications.

Modern front derailleurs aren't designed for half-step gearing as the inner cage plate is too deep to clear a middle chainring nearly the same size as the outer unless the derailleur is mounted way too high.

Half-step was more common when 5 and 6-speed freewheels were the norm as it filled in the big gearing gaps between the cogs. With newer 8, 9, 10 and even 11-speed cassettes, the need for half-step pretty much went away and current front derailleurs won't work with it.

My difference between outer and middle are only 6 teeth with the new 46/40/30 setup.

"Ultegra 6703 derailer says that the minimum difference between the outer and middle ring is 13 teeth"

And the difference between 46 and 40 is 6 teeth, which is less than the specified minimum; this is why the cage is hitting the middle ring.

I just read an article by Jan Heine that describes front derailleurs and chainring sizing. Trouble with STI Triples | Off The Beaten Path I think I'm going to get the Shimano CX70 or another cross FD.

Keep in mind Heine is selling cranks so his analysis can be a bit self-promoting. Also, he never mentions the difficulty of finding a front derailleur capable of shifting the half-step gearing he advocates.

Yes Shimano sells 50/39/30 triples but the current 105 triple (FC-5700) has a 74 mm bolt circle for the granny so you can fit as small as a 24T granny ring in place of the 30. That will give a plenty low gear for almost any rider if combined with a wide enough range cassette.

I'm not going for lower gearing but to optimize my gearing for a better shift pattern and better chain line. I'm trying to achieve a 46/40/30 or 46/40/26 gearing so that I can have true crossover gearing for my outer and middle rings. This would allow me to avoid double shifting since I like to use down tube shifters. Do you think an Ultegra 6800 double front derailleur would be compatible with this gearing?

No, it wouldn't work any better. You have the same problem that the inner cage plate won't clear the middle chainring unless it's installed too high. These derailleurs are designed for at least 10 tooth differences (52/42) in the chainrings and often 11T (50/39) or 14T (53/39) or more.

So the Shimano CX70 it is? Is a cyclocross front derailleur that is designed for a small tooth difference work with a larger tooth difference for the inner chainring?

D'oh! I read 46 and 40 and thought 46/30. I will say, however that the difference my 50/39/28 falls outside the range as well without any issues.

Many things that Jan Heine writes should be taken with a grain of salt. All of my road derailers are set up with STI and they shift very well indeed...including the 46/34/20. There is no need to go to a friction system or barend shifter. The CX70 is a double front derailer. The Tiagra is a triple.

If you can find an Ultegra 6503 FD it might work because it's built for a 10 tooth difference between the big and middle rings and a 12 tooth difference between the small and middle rings.
My experience with TA rings is that it may be difficult to get the chain to drop over the teeth due to the thickness of the teeth at the tips. If you have this problem filing the backside of the teeth can help a lot.

I currently have a Shimano 6503 FD. Actually, as of today it is working but not 100%. I have the derailleur set so close to the middle ring that the teeth on the middle ring barely touch (by a hair) the back plate of the front Deraileur when it is in some positions. I have TA chainrings. I was thinking that a double front Der would work because the only difference between a double and triple is that the back plate of a triple extends lower. I think the Shimano 6800 would work as well as a CX Deraileur.

Read this: http://velo-orange.blogspot.com/2011...leurs.html?m=1

I suspect that you will just be trading one problem for another one. The problem with the Ultegra triple (FD6503) is that the distance between the inner plate and the outer plate is fairly narrow. It is further narrowed by indentations put in both plates to stiffen the derailer. The narrow gap causes other problems but in your case, the deeper stiffener pressed into the inner plate leaves you with less space to nestle up to the middle ring.

This is a problem with all of the more expensive Shimano front derailers...mountain and road. The more expensive the front derailer the harder they are to set up properly. The inexpensive front derailers...Tiagra and Sora, for example...have a wider gap between the plates and the stiffing indents are shallower. They are more forgiving in gear combinations and, in your case, would allow you to run the derailer closer to the middle ring.

Going to a double front derailer may cause issues with lifting the chain off the inner ring. You'll have to overshift the derailer to force the chain to the inner ring. Overshifting isn't something that STI likes to do nor is it that easy to make it do it. The likely result will be that you'll shift towards the middle ring but the chain will move to the outer and you'll have to shift it back. Doable but futzy.

The good news is that the Tiagra is cheap...$15 to $30...so if it doesn't work, you aren't out much money. Ultegra isn't nearly as cheap to experiment with.

Just an update on my experiments with a new front derailleur and my current 46/40/30 triple setup. I replaced my Ultegra 6503 triple front derailleur with a Shimano CX70 cyclocross derailleur. I want to remind everyone that I use a down tube friction setup for the front shifting.

So far, the CX70 works better than the Ultegra derailleur that i replaced it with. The next thing that I will experiment with is expanding the range of chainrings with this new derailleur. It is only rated to handle a max range of 16t, but I want to see if I can push it to 20t. It looks like there is room in the derailleur cage to handle a bigger range. I will probably go 46/40/26 or 48/40/28.