My Roadbike with Shimano di2 and mid cage has 52/36 chainrings and I used to have 11-28. For three years ago I changed to 11-32. Now I am about 70 years old and I make at least 40 kilometer in fast pace almost daily, that is why despite of a complicated operation om my heart I can maintain minimum puls at 50 bits per minute. Recenty I feel I need more cogs to do the climbs where I really have problems. The question is should change my cassette combination to 11-36 or 11-40 without making any change in the frront chainrings?
Changing the cassette will increase or decrease the range of available gear ratios, changing the chainrings will move the current range up or down.
What is the capacity of the medium cage Di2 derailleur? Currently you have 37T range (21T difference at the back plus 16T difference at the front). Shimano typically state two values: one for 1x and one for 2x.
If you want lower gearing overall I’d swap out the front chainrings, maybe something like the GRX set. If you do this you might also need to change the front derailleur as the cage will be matched to the 52/36.
On my Ultegra, 50/34 in the front and 11-34 in the back are the smallest configurations. If 1:1 in the lightest ratio is not light enough, I‘d also consider switching to GRX.
@thebluesky You’ll need to find out what your rear mech can cope with. If it can cope with an 11-36 that’s probably the easiest/cheapest change. (I doubt your current set up would cope with 11-40). If the mech can’t cope the 11-36 changing your chain rings for a 50/34 will be easiest. You may prefer to do the latter (50/34 with your existing 11-28 cassette) anyway as the spacing between the cassette cogs wont be as large a jump; that is what I’d prefer to do if I lived in a hillier area but Rutland is as about as hilly as it gets here (short, sharp climbs) so your existing set up is what I use.
Just checked, you don’t say if it’s Ultegra or Dura Ace but the Ultegra capacity is:
Cassette Size Capacity: Short Cage: 11-25T to 11-30T; Medium Cage: 11-28T to 11-34T
Total Capacity: Short Cage: 33T; Medium Cage: 37T
So with the 11-34T you can only have a 14T difference between the two chainrings. Shimano are conservative with their capacities, I’ll push 2T and occasionally 4T over the recommended maximum but that’s with mechanical systems not Di2.
If you don’t mind losing the high end then fitting a GRX 2x chainset will drop you from 52/36 to 46/30 so your lowest gear will go from 36:32 to 30:32 but your top gear will also drop - 52:11 to 46:11. The Di2 front mech will handle a 30T chainring.
Like @Dave already posted, you just need a rear derailleur extender to make this work.
However, given the OP is 70, I’d really kiss the semi-compact chain rings goodbye and get e. g. 46/30. 46:11 can still easily get you up to 50+ km/h pedaling at 100 rpm.
Agree with others here that you would be well served with smaller chainrings. GRX as mentioned goes as low as 46/30, then there is the 48/31, or an ultegra/105 50/34. You would get lower weight and could have a tighter cassette on the back. If you went this route I would still pair with an 11-36 cassette - the SRAM PG-1170’s cog array is much more sensible than Shimano’s 11-34 (I have it) - you get smaller jumps in the small end, and a lower low gear.
I have Ultegra 6800 with compact 50-34 medium cage. I can run 11-40 on it with the 11-36 length chain. Just cant go to the 40 with the big chain ring. I run the 11-40 on bikepacking or hilly terrain. I managed 18% loaded on gravel in Oregon…so sketchy. I always say Ive never had too many gears on a hill. Dont have a great power to weight ratio and use 165 cranks which I have to compensate with more gearing as well.
He’s going to spend ($200 in parts??) money on a Chain, cassette, and RD.
This is a slam dunk for GRX 46-30. The ($150) GRX crank should work fine with the rest of the setup - if not, the FD is $50. He’s lose the equivalent of the 50x11 gear, but I’m guessing he wouldn’t miss it.