You'll need the GS cage rear derailleur as opposed to the SS, although I suspect that you may already have this if you're currently running a 28t cassette ring.
You'll also need to add a couple of links to your chain.

Other than those, there's no reason that you can't run a 32t cassette with your Di2 set up.

Hi Everyone. I'm new here. Please be gentle!

I love my new BMC GF01 with Di2 Ultegra (electronic shifting). It has a compact and 11-28 gears.
I am tackling the Lake Tahoe Death Ride this year for the first time, and am worried about not having an easy enough first couple of gear ratios.

On my tri bike I am not ashamed to admit I have a 32 gear (SRAM) on the back and 50/34 compact on the front, and have often been very glad of that climbing gear on long, climby rides at altitude.

Does anyone know (or point me to an existing thread?) if I can use an 11-32 or a 12-32, or even a 30 on the back, with Di2 and a 50/34 on this bike?

Many thanks,

R

When I was looking into this, yep, you need the GS, which I assume did not come with the bike.

Not correct- you do not need a long cage derailleur to make this work with a 32 cog.
You will need to do a couple things.
1.) Increase b-tension so that derailleur clears the 32.
2.) Ride and shift carefully. Don't go big/big.
3.) You may or may not need a longer chain. If your chain was installed using the "Shimano method"- chain on big/big (not through the rear derailleur) plus 1 link- you might need a longer chain.
If chain was installed small/small (through the rear derailleur) with slight tension on rear derailleur- you should be fine.
I have used and 11-32 many times and it works great with the above caveats.

I see this posted all the time and it's the worst piece of advice that I see posted regularly.

I didn't say it was foolproof.
II said that it works fine if you know what you are getting into.

Except the event that the OP mentioned features long mountain passes- so he(she) will be in the small ring. Not cross-chained on the big.
And it is not a race.

My opinion is still like others, no way would I ride a bike with safety limitations because I was too cheap to buy the correct parts.

Is this not what folks gave the guy a bad time about when his P5 broke?