GRX 2×11 30/46T – 11/42T compatibility.

I need more gearing for my Trek Checkpoint ALR5. Ideally going from my current lowest gear 34Front-36Rear, to 30Front-42Rrear

GEARING
LOWEST GEARING with 35mm tyres;

ARANGEMENT INCHES
Checkpoint STOCK 27.24 (34F/34R)
105Chainset 34/50 36 rear 25.61 (34F/36R) (Current bike)
Absolute Front 30/46 +36T 22.61 (30F/36R)
Stock Front + GRXRear42T 22.00 (34F/42R)
GRX Recommend 30F 34R 24.00 (30F/34R)
IDEAL GRX setup 30F 42R 19.40 (30F/42R)

So I want to use;
GRX Chainrings 30/46 FC-RX600-2 £85
GRX Front Mech FD-RX810 £35
GRX Rear Derrailleur RD-RX812 £75
GRX 42T Cassette CS-M8000-11 £60

The rear mech however is only recommended for use in a 1×11 drive train although it is specced for a 42T cassette. I’m not sure that even with the long cage it will have enough throw to take the slack from 30/46T up front.

Has anyone ever used a long cage GRX 1×11 specific rear derailleur, in a 2×11 set up and if so, what happened?

My local shop forward the information that Shimano give on their website – ie “You can try it, but we wouldn’t recommend it!”. Currently they do not know of anyone else that’s tried this arrangement

Shimano are notoriously conservative when it comes to rear mech capacity. I’d be tempted to try it out. If not, a long cage mech should cope.

I’m about to put together a 50/34, 11-40T combination using an Ultegra rear mech. I’ve seen videos of it working so fingers crossed.

I use a GRX rear mech medium cage

It has been faultless used with a 34-46 chainset twinned with 11-28 11 speed cassette
I now use it with a 1x set up
38t absolute black oval chain ring and a 11-34 cassette also been faultless (gust a little noisy on the cassette extremes)
No chain device
No droped chains yet!
I know this does not exactly answer your question but i hope it helps.

There is only one cage length in RDRX812 with a capacity of 31T.

Mister P – I’m getting my information directly from eh Shimano website;

1×11-speed wide RD-RX812 / RD-RX817 (Di2) – RECOMMENDED CASSETTE – CS-M8000-11 (11-40T, 11-42T) CS-M7000-11 (11-40T, 11-42T)

From https://bike.shimano.com/content/dam/productsite/shimano-northamerica/pdf/GRX%20DOC%20CONSUMER.pdf

The capacity is the difference between smallest & largest front rings + the difference between the smallest and largest cassette cogs. So for 30/46 and 11/42 you would need a mech with a capacity of 47t.

Mister P – I’m getting my information directly from eh Shimano website

So am I.

Yes that rear mech is designed to run with an 11-42 cassette but not with a double chainring. The capacity of the mech is 31T which is the difference between the largest and smallest sprockets on an 11-42 cassette.

By adding a double chainset you are adding another 16T to the total difference which takes it to 47T.

https://productinfo.shimano.com/#/spec/Gravel/Rear%20Derailleur

Yep. The RX800 mech has a published capacity of 39T but seems to handle a couple more.

There is only one cage length in RDRX812 with a capacity of 31T.

Big to big (28-46) when 2x my chain was quite tight 😁

Thanks everyone.

I’m using this bike a lot to do the local short journeys that I would normally have done in the car (shopping etc!) and so it’s already got racks and panniers. Also I’m cycling to the Alps (solo and camping) on this bike, hence the need for the extra gearing. Apparently buying a separate touring bike is not an option…

So back to the original question…
Has anyone tried a 30/46T Chainring + 11-42T combo?
If not GRX, then what else has anyone toured on that works?
I’m slightly reticent to buy the GRX components on a ‘maybe’.

30/46 with 11-42 is asking the mech to handle the equivalent of an 11-58 cassette. I wouldn’t.

So back to the original question…
Has anyone tried a 30/46T Chainring + 11-42T combo?
If not GRX, then what else has anyone toured on that works?
I’m slightly reticent to buy the GRX components on a ‘maybe’.

Back to my original answer. Despite published max capacities, folk are getting Ultegra mechs to work on a 50/34, and 11-40 combo. That’s a 45T range, you’re looking for 47T. To get that you will need a MTB SGS rear mech and some sort of ratio converter. Some folk have had success using a 10 Speed MTB mech on an 11 speed Road groupset. It might be worth investigating that option.

Thanks Scotroutes. I’ll go for the Ultegra Mech and an 11-40 cassette with 30-46 up front.
Good luck and let me know how you get on with yours once you install it!

Just found a decent article for those that are following this thread!

https://road.cc/content/feature/246424-how-get-ultra-low-gearing-gravel-bike-adventures?qt-more_features_sidebar_tabs=0&page=1

I think you may need rhe full chainset as the PCD on the GRX will be smaller to accommodate the 30 tooth ring .

Yep, thanks. I’m going for the full set. The kit on my gravel bike (Chainset, cassette, chain, Front Mech, Rear Mech) will be donated to my road bike which currently is grinding away a very old drivetrain.
Having done more research (thanks to the advice above), I’ll be going for…

Ultegra Rear Mech RD-RX-800-GS £67
GRX Chainrings 30/46 FC-RX600-2 £85
GRX Front Mech FD-RX810 £35
40T Cassette CS-M8000-11 £60
Ultegra/XT Chain HG-701 £19
£266

The Ultegra rear mech also has the clutch for when I go off road next summer.

The stock ratios for the bike 34F/34R gave 27.24 inches and this combo will give me 20.43 inches. I’d like to have got a 42T on the back, but this will be a massive improvement over stock. I hope this helps others looking to do the same!

PS I might actually try it without the GRX front mech (penny pinching!) to see if the current 105 incarnation will cope.

IIRC the GRX chainset has the rings mounted further out than 105/Ultegra so your old front mech may not work.

And you need a SGS rear mech, not a GS.

scotroutes

Shimano are notoriously conservative when it comes to rear mech capacity. I’d be tempted to try it out. If not, a long cage mech should cope.

I’m about to put together a 50/34, 11-40T combination using an Ultegra rear mech. I’ve seen videos of it working so fingers crossed.

seen similar and just did that with mrs antigee’s new specialized ruby 105 – used an 118link chain and just kept taking out a link (think 3 in the end) until worked ok – no b screw issues and seems 100% problem free across total range

I can confirm that my 50/34, 11-40 also works fine. I needed 118 Links though.

I’m running the 1x GRX mech with a 40t ring and a 11-45 cassette (added expander ring I found to a 11-42 std) works fine. Not sure if this helps the OP. 30-42 would be way to low for where I ride.

11-42T + 46-30

After more research I went for Absolute Black 46-30 oval sub compact chainrings, which were a doddle to set up with the standard 105 front mech.

I swapped the 11-34T cassette for a 11-42T. This was a bit of a leap of faith. I knew (having read the roadcc article), that a 11-40T would work, but I wanted to get as much as I could so I gambled with a 11-42T. I had to swap out the 105 rear derailleur for an Ultegra GS (medium cage) derailleur and as I was putting it on my gravel bike, I went for the RX model with the switchable clutch mechanism.

It works!

I had to use pretty much all of the ‘B-Screw’ adjustment, but everything else was plain sailing with the set up. I used a straight out of the packet 116 link Ultegra chain and here is very little tension when in 30F 11R. Also the Rear derailleur is almost maxed out when in 42R 46F, but you shouldn’t be riding in those gears! At least I know that should I inadvertently choose those gears, I won’t die!
For most of my riding, I am now using the larger chainring a lot more, which means they’ll wear more evenly than before, so that’s a plus!
The larger steps in the rear cassette will take some getting used to, but it’s worth it for the end goal of 42Rear and 30Front.

GEARING

LOWEST GEARING with 35mm tyres

ARANGEMENT INCHES

Checkpoint STOCK 27.24 (34F/34R)

105Chainset 34/50 36T rear 25.61 (34F/36R) (Current bike)

Absolute Front 30/46 +36T 22.61 (30F/36R)

Stock Front + Rear42T 22.00 (34F/42R)

GRX Recommend 30F 34R 24.00 (30F/34R)

IDEAL GRX setup 30F 42R 19.40 (30F/42R)

Cost –

Absolute Black rings 30/46 £145

XT 40T Cassette CS-M8000-11 £60

Ultegra Rear Mech RD-RX-800-GS £67

Ultegra/XT Chain HG-701 £19

£291

This seems expensive, but you have to replace a worn out drivetrain at some point, and the clutch mech works really well off road. I can now ride up 14% climbs fully loaded and remain seated and spinning. For me – it’s worth the effort and cost.

Good to know.

Rear derailleur position and chainstay length might rule it out for some but it’s great when folk take the leap and try stuff. Do let us know how it all fares once it’s a bit worn – especially chain/cassette.