Gravel Bike Gearing

[GrahamGB]

I am building up a new gravel bike and want to have a wide range of gears. I have an Ultegra 11 speed 36-46 chainset and a 11-42 11 speed SRAM road cassette but need a suitable derailleur to go with a set of Shimano RS505 hydraulic shifters and handle this set up. My issue is total link capacity. Shimano have the new GRX derailleur in 810 or 812 format. The 810 is rated as max sprocket 32 but 40 or 42 link capacity and recommended for 2x11 set ups whilst the 812 is 42 max sprocket but only 31 link capacity and recommended for 1x11. I am uncertain which to go for. I am leaning towards the 810 and maybe using a Wolftooth type derailleur dropper to accommodate the 42 tooth sprocket but to have the overall link capacity to handle the length of the chain. Has anyone experience of a set up like this and can offer any advice. Many thanks

[NetworkMan]

It seems a little strange this transmission you are proposing. It is not one of Shimano's standard offerings but if you are designing a special I'm sure several people on here could suggest a design that would offer some or all of:
Lower cost
Lower weight
Smaller gap between gears
More durability
A lower bottom gear

Did you already have the proposed cassette perhaps?
Just curious really.

[Brucey]

the 810 model has a guide pulley which is concentric with the pivot, and a slant parallelogram which is angled for an ~11-32 cassette. The 812 model has a guide pulley which is not concentric with the pivot. This gives extra capacity for the rear mech, because in addition to the slant, the guide pulley moves up and down with variations in chain slack and can thus track a wider range cassette than the slant angle alone might suggest.

However the 812 mech is designed as a 1x mech; if you use it with a double chainset (with any reasonable chainring spacing) it cannot maintain an optimum performance on both chainrings, since the guide pulley is at different distances from the sprockets with each chainring. Having said that a 10T interval is not huge, modern chains and sprockets are very slick shifting and you might get away with it better than you might expect; a difference of ~8mm or so in guide pulley height might be anticipated between chainrings.

Using the 810 mech with a +10T sprocket will require the guide pulley to be ~20mm lower than normal vs the smaller sprockets. This may work OK with everything is new but when it is worn the shifting on the small sprockets will probably go to pot earlier than it might otherwise.

cheers

[RickH]

I am building up a new gravel bike and want to have a wide range of gears. I have an Ultegra 11 speed 36-46 chainset and a 11-42 11 speed SRAM road cassette but need a suitable derailleur to go with a set of Shimano RS505 hydraulic shifters and handle this set up. My issue is total link capacity. Shimano have the new GRX derailleur in 810 or 812 format. The 810 is rated as max sprocket 32 but 40 or 42 link capacity and recommended for 2x11 set ups whilst the 812 is 42 max sprocket but only 31 link capacity and recommended for 1x11. I am uncertain which to go for. I am leaning towards the 810 and maybe using a Wolftooth type derailleur dropper to accommodate the 42 tooth sprocket but to have the overall link capacity to handle the length of the chain. Has anyone experience of a set up like this and can offer any advice. Many thanks

How fast are you wanting to pedal?

I have a 1x with a single 36T chainring & 10-42 cassette. 10th gear (12T) is, for me, a 25mph gear (up to around 28mph for short stretches) & 11th (10T) I only use if I'm going faster than that AND want to pedal (which isn't very often).

[mcshroom]

I'd be tempted in your situation to get a MTB rear mech and a shiftmate. The M7000 SLX RD has a 41t capacity and a 42t maximum sprocket, without the extender.

Otherwise the GRX RD-RX400 has a 41t capacity. I know it's nominally 10sp, but I imagine, like with 4700 kit, it should work with 11sp.

Just a couple of ideas