I have been looking into running 50-34 compact cranks with a 12-27 and had a few questions.
I plan on going with long cage rear.
Dura Ace 7800 FD spec sheet says the standard derailer only has a 15T capacity.
Dura Ace 7803 FD spec sheet says the triple derailer has a 22T capacity.
I would like to go with triple up front to play it safe, so (here’s the question part):
Can I set the limits on the triple front to accomodate the compact cranks?
Will it have a shorter throw, and if so is this an issue?
I run 50/34 with a 12-27. Ultegra FD (double) and short cage Ultegra RD. No problems, just make sure the chain length is correct, FD height is correct (maybe a problem w/ braze-on… mine’s clamp-on) and the RD is adjusted properly. I can’t do 34/12 without rubbing but I also wouldn’t want to because it doesn’t make any sense. I would be wary of trying to make a triple FD shift a compact double properly, though Zinn would probably tell you it can be done. Also, you might look at FSA’s website. If memory serves me correctly they make a FD specifically for a compact double (16T capacity).
Can be done and sensible are tow different things. Please understand that running Ultegra will not make you slower. Get parts that shift well by being designed for the job. One day, you will be on a stupidly big hill and you will have to downshift and you will thank yourself for getting the right stuff. Also, don’t forget that you should never run 50x27 or 34x12 combinations. Those gear ratios are on the other chainrings anyway. Here is a gear chart showing the comparative sizes of your gears: 345027 34 50 24 38.25 56.25 21 43.71 64.29 19 48.32 71.05 17 54 79.41 16 57.38 84.38 15 61.2 90 14 65.57 96.43 13 70.62 103.85 12 76.5 112.5You can see that the 50x27 combination is about the same size as the 50x19. Also, once oyu shift higher than the 34x16, you are better off on the bog chainring. The amount of drag with bigger chainring and bigger cog on the rear is lower, and your chain will last longer too. The chain will not have to bend as much around the larger cogs.
I haven’t measured the widths but I’m pretty sure that the cage on a triple front der. is wider than a double. When you use a triple derailleur on a compact crankset or normal double crankset the chain won’t shift as quickly up or down. If you were using bar end shifters you could get away with it as the front der. uses friction and you would just need to set it up so that the high limit has a little more room than normal. If you use STI shifters it will be sloppy and not shift well.
For the rear derailleur it won’t really matter. The max size cog for a short cage shimano derailleur is 27t but we have set them up on a 28t SRAM cogset.
I’ve currently running an Ultregra triple on a compact 50/34 front with a DA long cage reare on a 11-26 and it all works fine. I have to say my LBS set it up so I’m not sure what adjustments had to be made but it does work.
Shimano compacts cranks are designed to work with their double FDs and they work well when correctly adjusted.
Your frame does need to allow the FD to go low enough to get the correct 2mm clearance.
