Combine Shimano flat mount road calipers with MTB levers?

[NickJP]

My wife likes flat handlebars on all her bikes, and I'm building her up a disc brake bike around a Carbonda CFR505 frame, which takes flat mount calipers. Is there any problem with using, for example, 105 hydraulic calipers with Deore XT levers? Shimano do actually make a flat mount flat handlebar set (Metrea, intended for "urban" bikes), but purchasing the combination of separate components I suggested above is actually cheaper than buying the Metrea set.

[Brucey]

there are small differences between piston sizes in various shimano calipers (of which I don't know all the details but IIRC they might make up to ~20% difference in mechanical advantage) and some differences in hose terminations too (ditto about specific models) but apart from those things (which can be overcome anyway) there is no reason why you can't mix and match as you intend.

BTW I have not checked but it might be that the possible variations simply don't exist in flat mount calipers since they were all designed at about the same time.

cheers

[Redvee]

I've gone the other way with R7020 levers and M7000 callipers. Not ridden the bike yet as I've only just got the R7000 front mech sorted after reading the instructions this time you need to set the mech up in a different order to previous generation Shimano front mechs. Wraping the bars with the small added bulk of the hydro pipes coming off the levers took four attempts, with cable brakes in the past it's only needed one attempt.

[thelawnet]

there are small differences between piston sizes in various shimano calipers (of which I don't know all the details but IIRC they might make up to ~20% difference in mechanical advantage)

my understanding is there are 3 different versions for MTB:

4 pistons of 16mm and 18mm
2 21mm pistons
2 22mm pistons (either resin or ceramic)

These measurements are likely not precise

21mm vs 22mm is around 10% more force, and the 4 piston is another 20% on top of that.

however they sell the same levers with both 2x22mm and 4 piston, so it doesn't seem to matter very much.

For road RS405 calipers are resin, the rest ceramic. Presumably they are all 22mm.

[CJ]

Is there any problem with using, for example, 105 hydraulic calipers with Deore XT levers?

I've built a bike for my wife with those components and they work perfectly well together. She's ridden it far enough with no problems to now be on the second set of front pads.

[Redvee]

I took my bike out for it's first ride on Sunday evening, only a short 3 miles just to test everything out and can report back that initially I had my doubts about the calliper/lever combination but then remembered that the pads were brand new and needed beeding in. towards the end of the ride the braking was starting to come good and biting when expected but later than what I'm used to having cable operated discs on this bike previously and on my other bike.

[Brucey]

the biting point of all hydraulic brakes (apart from 'closed system' ones) is later than you might expect on the basis of experience with other brakes. The reason for this is that the master cylinder seal has to move far enough to cover the compensation port before the hydraulic circuit will build any pressure.

If in doubt, compare with other hydraulic brakes; if they are not bled properly you can have sponginess and/or a late bite too.

cheers

[Redvee]

Well aware of that fact and I went through all the hydro braked road bikes in Cycle Republic to compare to my brakes before I rode it and compared mine in the LBS who did the bleed on my brakes with his bike. Will give it a few more miles before I take it back and let him do the final bleed.