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I've got a gravel bike, the Planet X London Road, with SRAM Rival 1x11 groupset.
I'm thinking of converting it to a flat bar. From what I have read, any SRAM 11 speed shifter should work, as the cable pull is the same, but I don't know how best to handle the hydraulic disc brakes.
Is there a lever that I can just plug the existing hydraulic hose into, or will I need to replace the whole hose from caliper to lever? Is there a specific model of lever I need to get?
The hose diameters (inner and outer diameter) differ between different brands of hydraulic brakes. When choosing a lever you need to make sure the olive/insert (the parts you install on the end of the hydraulic hose in order to create a seal at the lever) are suitable for the (diameters of the) hydraulic hose that is currently installed on your brake caliper. I've found the following sized for different brands: (ID=inner diameter, OD=outer diameter)
Here is an overview of the different hose sizes per brand :
Shimano (BH90) + Avid + Formula + Sram:
ID: 2.1mm , OD: 5mm
Shimano (BH59) + Magura MT:
ID: 2.3mm , OD: 5mm
Tektro/Hayes/Promax:
ID: 2.5mm, OD: 5.4-5.5mm
From what I've found online Sram, Avid and Shimano BH90 (shimano hoses come in two versions/sizes IIRC, BH90 and BH59) use the same size (ID/OD) hydraulic brake hose. This means you should be able to use any lever suited for flat bars from Avid/Sram (as long as they do indeed use ID = 2.1mm and OD = 5mm hydraulic hose (there might be some exceptions)) and all shimano brakes which use BH90 hydraulic hose.
If you use a hydraulic hose which is not the correct sizing (ID/OD) you have a high risk of the brake leaking.. so I would advise against this.
To get a similar 'lever feel' to your original brifters you'll need a new brake lever which has approximately the same mechanical advantage.. (advantage due to brake lever arm itself and advantage due to different piston size in lever and caliper). You'd be best off sending Sram an email regarding this I think since this info (especially hydraulic piston size) isn't readily available for most brake levers (in my experience).
There are examples online of people mixing and matching different brands of shifters/levers together, I don't have any personal experience with this yet..
Example:
any SRAM 11 speed shifter should work, as the cable pull is the same
Sorry, that's not the case. SRAM MTB 11 speed uses 'X-Actuation' pull ratio (3.48mm / shift) whereas road 11 speed uses 'Exact Actuation' (3.1mm / shift). See here for extensive documentation of cable pull rations, sprocket spacing etc.
However, SRAM makes trigger shifters for at least the Force groupset.
As Argenti mentioned, SRAM has the S700 series of trigger shifters that work with the road derailleurs. Here's a SRAM Q&A explaining derailleur/shifter compatibility:
SRAM Trigger Shifters On A Road Bike
Regarding brakes SRAM states the road Hydraulic levers are not compatible with their mountain calipers so I would not expect the mountain levers are not compatible with the road calipers.
SRAM Road Lever Mountain Caliper In-compatible
So if you want to go down this path you'll need the S700 trigger shifters and a complete set of MTB brake levers & calipers.
