11-40 or 11-42 On The Back
 

I'm going to change my current 105 - 32/48 on the front with 11-34 rear to either 11-40 or 11-42 on the rear.

I'm caught between a rock and a hard place of preferring the lower gearing the 42 offers over the 40 but it does start to become a bit "gappy" on the bigger rings so interested in people's opinions generally and also if anyone else has done similar and if you'd choose the same again or a different cassette for either of the reasons I've mentioned..

When in doubt, more chainrings!

I went with a CS-M8000 (XT) 11-42 cassette.

Gaps between gears isn't an issue.

The first seven sprockets are the same on both the 11-40 and 11-42.

If climbing something which required 30 gear inches (30 front/28 rear) or lower then I'm looking for low gears and don't care about gaps. Bigger gaps just mean lower gears. I don't even think about it.


-Tim-

So there is a 39t max on the 105, correct? That can be fudged and whatnot since Shimano is conservative, but a 32/48 mated to an 11-42 would mean a total wrap of 47. That isnt too much? I guess never going into the 32-11 combo will help too.

I was hoping that may have been the case and part of the reason I asked so thank you for that !!!

Did you use a roadlink or anything I was looking at the Sunrace extenders???

I'm not sure exactly what you're asking but is this answered with my comment above about using a roadlink or is there something else I should be looking out for??

A roadlink will help drop the RD down to be able to not run into the massive cogs when moving up and down the cassette.

Every RD has a chainwrap capacity, meaning how much chain it can 'take up' and 'hold'. Basically, if your bike is in the small-small combo, the chain would be super slack when compared to the bike being in the big-big combo. The reason the chain isnt super slack in the small-small is because the RD has a pivoting arm that is on a spring and pulls the chain tighter as you get to a small-small combo. If the amount of chain needed to be 'held' by the RD exceeds what it was made to handle, then the chain will be sloppier and looser than desired in the small-small range of gearing.
Shimano says 39 teeth is as much as its GS RD can handle. They are conservative though with all their printed limitations.

Well, gaps not being a problem is just one man's opinion. YMMV and others may disagree. I've never been bothered by wide spacing but then again, I ride 48x16 fixed gear on the road and that rarely has the "right" gear. .

I did not use an extender. A CS-M8000 11/42 cassette with 46/30 chainrings works with an Ultegra GS Di2 shadow derailleur on my bike.

Again, keep in mind that my comments are specific to my bike - AKA disclaimer that it may not work with anyone else' bike. YMMV. Use only as directed. Objects in mirror are closer than they appear. No user serviceable parts inside. Past performance does not indicate future results. Etc.

One of my fears is saying, "Yeah, it is great" and then someone spends lots of money and it doesn't work on their bike. I would feel bad. At a certain point one needs to just try it. Getting a worn out MTB cassette from a friend might be good to test with. I don't have one to send you or I would.

Anyway, here it is. Roadies are like, "Look at that Cassette!!!" and MTB guys are like, "Is that a mountain cassette???" Nobody really knows what to make of it.

https://cimg5.ibsrv.net/gimg/bikefor...e18ac8ef69.jpg


-Tim-

If you swap out the RD to an XT you won't have a problem. That's a huge gap for the 105 RD to cover. XT RD is easy with a tanpan.

I haven't been bothered by the gaps in the 11-42. I also ride a lot of road and road group rides with this setup.

Even the 11-40 gaps are annoying to me, YMMV but 11-34/36 is the sweetspot for me for a 2x gravel setup that balances gaps and range. 1:1 ratio is the lowest i generally need with a measly 3.2w/kg FTP

Brilliant and cheers for everyone's help .... some very useful info and much appreciated! :thumb:

I'm going to try the 42 and see how it pans out, I'd be miffed next time I'm struggling to get up a hill if I go for anything less and the only way I'll find out if I do like it is to try it so will dive in and see how it works out .... worst case I sell the cassette 2nd hand and change it later on but at this bike will be a "keeper" and a go to bike I think it's worth getting it right ... even if it means a bit of trial and error / expense :)

.

Nice! I think you'll be surprised how great the 11-42 is.

By gap I meant the tooth difference in big to small for the RD to handle. The 105 RD is not designed to handle that big of a tooth gap. I foresee it shifting like absolute garbage if you can get it to shift cleanly at all.

40 or 42, won't make much difference. It's only about 5%.