Witterings

Can anybody tell me apart the newer R7000 from being rated to 34T cassette as opposed to 32T for the RD 5800 what other differences you'd notice between and if you particularly wanted to go that route so you could fit the larger cassette is it just a matter of changing the Rear Derailleur for about an extra £40??

The other option I guess is to go Ultegra which again I believe is rated to 34T but would cost about £75/80 and would there be a noticeable difference between that and the 105's.

Retro Grouch 

Rear derailleur specs tend to be on the conservative side. I wouldn't do it on a customer bike but you can almost always "stretch" them a little on a personal bike.

My core philosophy is that, if I already own parts, I'll bolt up almost anything and see if I can make it work. If I'm buying parts, I'll only buy stuff that's supposed to work together even if I have to pay a little more. I think that it being November is significant too. This time of year I'm more inclined to try something that I'm "iffy" about because waiting another week or so for some needed part to be delivered doesn't bother me so much.

If it was my bike I'd start with just the larger cassette and see if I could make the derailleur that I already have work.

Lazyass

Disregard, I read the post wrong.

wipekitty

Ditto. If all you want is lower gearing, it's worth a try.

If you already have the 5800GS ('long' cage, well, technically medium cage) RD, there's a good chance that the larger cassette will work. There's guys out there running this RD with 36T cassettes. I've successfully run the 5800SS (short cage) with 32T, although it's only spec'd for 28.

As always, YMMV.

sdmc530

I have a charity ride that is hilly as heck for my liking and I put a 34 on my 105 5800 for that ride. No issues, its a bit tight back there but it does work. You would always get a hanger bracket too, then you can even go bigger. GCN did a hack with it to go to a 36 I believe. I would try the cassette before I would buy a new rear.

Witterings

My other thinking in terms of pushing the limits ... if the 5800 you couold possibly push to 34 but the 7000 rated at 34 you'll probably push to a 36

This is what they did with a aftermarket cassette on the Specialized Seq Elite
https://www.specialized.com/gb/en/se...elite/p/129126

Maelochs

Also, you can possibly get a "Wolf's Tooth" derailleur hanger extender for much less than the cost of a new derailleur.

Personally i'd try the 5700 GS and see it if fit .... crank out the B-screw or even reverse it.

Witterings

I was asking as the bike I was looking to buy had the old 105's in one size and the new ones in the other size and when I went for the pre order fit wanted it clear in my mind what my options were if my size had the older version.

As it happens I was right on the cusp of the two sizes and could of gone with either but they felt I'd be too "squashed" with the smaller of the 2 even with a longer stem so have gone for the larger which has the new 7000's anyway so happy days.

Cheers for everyone's help!!!!