Shimano GRX Drivetrain to 42t cassette?

[boroboy151]

I have the following drivetrain:
Rear derailleur: Shimano GRX RD-RX820, 12-Speed
Front derailleur: Shimano GRX FD-RX820-F
Integrated shifters: Shimano GRX ST-RX820
Crankset: Shimano GRX FC-RX820, 48×31T
Cassette: Shimano 105 CS-HG710, 11–36T
Chain: Shimano CN-M7100
Groupset: Shimano GRX

I want to modify to a 42T cassette and know ‘officially’ Shimano says it’s not possible; but seen conflicting advice to say it can be done albeit I know there will be limitations linked to cross-chaining. Would appreciate some definitive advice please.

 

[Jameshow]

So long as your chain is long enough to cover 48/42 then you should be ok, you might find it's a bit loose on 31/11 but you shouldn't be riding in that gear anyway? (Crossing chain)

Shimano have always been cautious with road and MTB specs not sure about grx.

 

[fossyant]

Your issue isn't just chain length. The upper jockey's may foul the bigger sprockets.

You could always try it. Shimano normally get away with 2 Teeth more.

 

[T4tomo]

Can you pedal fast enough to keep the bike moving forward in 31-42?? That's a hell of a low gear.

 

[ktmbiker58]

It should work with the proviso that you must set the chain length for 48-42 and accept that the chain will become loose as you approach 31-11, that way you eliminate the risk of breaking the mech if you end up in 48-42, while at 31-11 you just have an untensioned chain which shouldn't do any damage

 

[boroboy151]

It should work with the proviso that you must set the chain length for 48-42 and accept that the chain will become loose as you approach 31-11, that way you eliminate the risk of breaking the mech if you end up in 48-42, while at 31-11 you just have an untensioned chain which shouldn't do any damage

Chain length is 126 links; but again seen conflicting advice that this will work, or may need 3-4 extra links. Also that a different rear mech will be needed. Any extra thoughts please? Thanks

 

[Dogtrousers]

Chain length is 126 links; but again seen conflicting advice that this will work, or may need 3-4 extra links. Also that a different rear mech will be needed. Any extra thoughts please? Thanks

According to Shimano https://bike.shimano.com/en-NA/products/components/pdp.P-RD-RX820.html
The capacity of a RD-RX820 is 42
Your required capacity is (48+42) - (31+11) = 48.
So you are already at the capacity limit with the 11-36 cassette.

It also has a low sprocket max of 36, so you'd be exceeding that as well.

So technically you'd be exceeding the limits of your rear mech both on capacity and max sprocket. So you might end up with a saggy chain if you ride in little-litte (which you shouldn't do, but might do by mistake) and you may get horrible noises if the cage fouls the big sprocket, which you may be able to resolve by adjusting it.

Lots of people say that Shimano are cautious with their limits and it might be OK. I don't know, I don't have any relevant experience. I've just done the sums, that's all.

As to chain length, that's dependent on your bike so you have to figure that out using one of the various methods (like wrapping it round big-big then adding a bit, I forget what)

 

[T4tomo]

What sort of riding are you doing that you feel you need as gear as low 31- 42? heavily laden touring / bike packing or trailer towing?

You will need the 822 series long cage RD for sure. shimano's generous allowance usually mean two teeth above recommendation but YMMV.

 

[teeonethousand]

My lowest on the same mechs is 30 front 36 rear. When I am in that gear up steep hills it is very little more than walking pace. I have decided that if an even lower gear is needed then I will walk I don't care what people think if I do and it's probably much safer for me...and others. It's only the 25% and more than get me...not many of those even round here...if it ever spoils my day I will go electric.