I currently run 11-28tT with 38 upfront which is great for the gravel in the flatlands but I need something lower for a hilly event. The rear derailleur is a 5800 SS which from Wolftooth’s website I can see will work with 11-40 using a road link but it says 11-42 won’t work.
11-42 is easier to come by and cheaper has anyone got it to work?

can’t say for sure but I have an XT mech on an 11-40 with a road-link type thing and no B-screw extension needed so I would have thought it would be possible to stretch that by 2 teeth.

If cost is an issue then save money by getting a cheap hanger extender from ebay and spend the extra on a Sunrace 11-40

Just don’t wont to spend much as it will only be used a few time a year.
Cheapest 11-40 I’ve found is SLX for £50
Sunrace extender £10
New Chain £15

Using an MTB mech requires buying the mech and getting an adapter on top of the cassette cost.

The cheapest option is to fit my 30t SLX chainset and fit the 11-32 cassette I have in the spares box but I think that losing the top end will be frustrating.

https://www.bike-discount.de/en/buy/sunrace-mx8-11-speed-cassette-11-40-530158?currency=3&delivery_country=190&gclid=Cj0KCQiA-JXiBRCpARIsAGqF8wVhx7zfbQhzgHVVT5Y0iFIYpqWj_0nxJipXFjc7_HtRPb8DiPo2HZkaAkNrEALw_wcB

£41 for sunrace – Or fit the 11-32 and get a 34 or 36T chainring for a compromise

I’ve used a standard short cage 9070 di2 rear mech, with a road link and a 11-40 SLX cassette

I thought road & mtb were cassettes had different spacings?

If cost is a determining factor and it’s not going to be used much then try ebay for a a dog link and an 11 speed 11-40 cassette from China/Taiwan.Both can be had for £35.00 delivered.Very happy with a 9 speed 11-40 cassette I got last week and I’ve been using one of their dog links for ages.

This might help….

https://road.cc/content/feature/246424-how-get-ultra-low-gearing-gravel-bike-adventures

If you want to use an XT rear mech with road shifters then Wolftooth also do a sort of pulley device that attaches to the back of the mech that converts the pull ratio. This seems a better solution to me compared to a road link as report with road links suggest it is not uncommon to get poor quality shifting or noisy transmission with some gear configurations.

Alternatively if you can find a broken XT mech that has an undamaged or re-usable jockey wheel cage it looks like a simple job to swap over the jockey wheel cage so you can put a long XT jockey wheel cage on a road mech. That would be the best solution I think.

New Ultegra RX series 11 speed mech will handle 11-40 out the box (no extra B screw or link required). Has a clutch too for 1x setups.

I wanted to get 11-42t on my new gravel bike build however I am going to settle with 11-40t as close enough.

Might help you out to know!

Ultegra RX and 11-40 here – works really well without roadlink. Don’t forget that 11-speed shimano MTB cassettes are 1.85mm shorter than road cassettes so a spacer is needed.

Edit – if you search then a couple of people are saying that it works with 11-42 as well..

I thought road & mtb were cassettes had different spacings?

No I think the spacing is the same. It’s the pull ratio of the shifters that is different

My setup is Ultegra short cage, roadlink, 11-42 11v Sunrace, 44t NW ring.

Shifting works OK, it’s not amazing. Chain is tight on the 42, I measured it without going through the mech and added one complete link as usual, but +2 would have been better.
The 42t rattles on the Sunrace. Over rough ground it’s annoying, unless you shift to the 42.
I only went 1x to avoid replacing a bust left shifter, will probably go back to 2x when this all wears out.

I run a 50×11-42 sunrace on my ‘gravel ‘ bike and it’s great but it’s worth noting that as the sunrace is an mtb cassette it’s marginally narrower than a road cassette (although the spacing is the same) it took me a while to work this out but was easily fixed with a very narrow spacer.

On my vaya: 105 shifters and compact 34/50 up front, xt sgs rear mach and 40t on the back using a jtek shiftmate 8a to sort out the pull ratio. Works great