I think I’ve messed up….

Bought an XT Rd 786 rear mech a while back. Got it in Med cage (GS). I think I asked the guys at CRC (When u could still talk to someone on phone), about the right size for running a 1 x 10 set up. I was looking at using a 28 tooth front ring, as I rarely find a need for massive high gears.

I have a used Sunnrace 12-42 (may be 11-42) cassette.

I’ve now seen some info that suggests that it’s not just the equation of 42 – 11 = 31 and therefor a medium cage mech should suffice….but it’s actually about the max sprocket size!!

And this is based on which ‘Model’ of rear mech, not just the cage length/size.

Apparently it’s only the m6000 range (if dealing with Shiamno) that allow x large top sprockets like 42 and above on the rear cassette!

So my RD780, although an appropriate cage length (?) is not the right ‘model’ and won’t get onto the 42T 🙁

Any wisdom amongst you hive of Owls?

RD-M786-GS spec is 36t max

https://bike.shimano.com/en-EU/product/component/deorext-m780/RD-M786-GS.html

You can use after market options like a wolftooth goat link or a one up rad cage etc to make the mech clear a 42t.

Try it and see? Might well be ok.
I used a zee mech with a 40t cog for a good few years. It worked ok.
Then I got a goat link and it worked better.

GOAT LINK???? Whaaaa?? Just looked it up. It’s brilliant. I love that kind of thing. I guess it’s slightly cheaper than a new rear mech. Good tip

Shimano are conservative (with a small ‘c’) about the maximum sprocket size. Basically they are covering themselves, I’ve found you can go about 4 teeth greater than the stated maximum without needing to do B-screw hacks.

Try it. If it doesn’t work get a Goat Link and it should be fine after that.

I think I just didn’t realise about the whole MAX thing… It was never really an issue when running 2 x set ups (which I did for years). I don’t think I ever tried anything bigger than a 36. My first 1 x experiments were always based on existing drivetrains and only for riding local (relatively flat or short climb) trails. All my 1 x set ups came pre installed on new bikes… so I just didn’t think. Anyway, those extenders look great.
Just have to weight up getting back £15 – 20 for my unused 785 and buying a new M6000 for £50
So an outlay of £30- 35 vs £20 for an extension link.

Any views on the difference? Will I even notice one?

Rd-M6000-GS is only £41 on CRC.

If you are on a budget you could just try it, if you have a long mech hanger it might just clear. Most of the long b screws, extenders etc I have seen dont shift as well as OEM setups as the jockey wheel ends up to far away from the rest of the cassette once it has been adjusted to clear the 40/42t.

If its on a full sus make sure you take into account chain length as the suspension compresses.

@Stablebarns: if you go for a bling Wolftooth goat link it’ll be around £20, but as a proof of concept you can get similar things cheaply on eBay (search for “Bicycle Rear Derailleur Hanger Extension” on eBay and you’ll get plenty to go at).

So it may be worth trying your existing mech with an eBay cheapie, and if it works out OK for you upgrade to something nicer?

All very good advice.

It’s shame that there’s always this super fine line (of cost effectiveness) between buying again and the goatlink type of solution.

It should be 80% more to buy a new mech and then the choice would be simple. But £22 goatlink vs £41 new mech???

Plus I have to accept that loss of value in trying out the existing mech, which is currently boxed and unused.

Anyway, I think I got my question answered.

BTW it’s for a slackline build. Really wanted it to be super durable and all seasons hence the 10 speed.