Hi guys!

I am thinking about fit a thumb rear brake lever on my bike...

The motivation is that makes possible to use rear brake in right-handed turns in order to scrape and adjust velocity without upset the front suspension.

Question is... could I use two masters cylinders on one slave? or this is impossible?

My concern is the fluid goes into the other master cylinder reservoir when one is under use...


Could anyone put some light on these thoughts?
My idea is to fit a second hand rear master cylinder on the handle bar.. like the video below.. or something else

Easiest route I could think of would be to run a separate caliper on the disc creating a stand alone circuit and leave the original as is.  Not sure how you'd be able to pressurise the other master cylinder without having them both on a feed to a slave cylinder to create the braking pressure.

Yep.. like stunts riders do...

However I think is kind of dumb... to sometimes carry three calipers... maybe is the only way... but I like to think that is always another way...
who knows...
I saw some Brembo solutions... not wanna or have that money to spent in my lab rat (gs 500).. but they are very elegant.



Now I am looking for some type of one way brake valve....  who knows what God Google will return.

The reason stunt riders have multiple rear brake callipers is because they use them constantly and one would get stupidly hot and fail. Generally they have the original caliper operated by the foot pedal and another pair of larger (front) calipers operated by a lever near the clutch. That keeps the foot and hand levers on seperate fluid circuits.
Why not just have a thumb brake and do away with the foot pedal? if you were using it all the time you would get used to its operation/feel and lessen the chance of over reacting in a corner and locking it up while still leaned over.

Actually, I use the rear brake so rarely that I am feeling I am losing something... so this is one of the reasons that I started to think about it a thumb lever...
About get rid off the foot lever...I though in this way already.. but for some situations, the pedal is very useful... in a ramp.. for instance.

Nudie... are you sure that stunt break systems get that hot? Neither Superbikes or MotoGps use more than 1 caliper per disk... I still think that is only the practicality of the system installation.... just a thought

Yes the stunt riders use their rear brake a lot for wheelies in particular and they overheat.
A friend of mine builds custom bikes and supplies a lot of billet parts for some of Australias top stunters, I asked him about the need for multiple callipers that's how I know.

Another Idea that I have thinking is same as this one that I found just now

"I've also seen stunt brake kits with a piece that replaces the brake fluid reservoir for the rear brake with a new brakeline that hooks up to a handbrake

UPDATED images (suzuki fitting)
http://www.hoheydesigns.com/store/index.php?main_page=product_info&cPath=12_38&products_id=199

pic:





source: https://www.reddit.com/r/motorcycles/comments/6mwd1r/help_with_fitting_two_brake_levers_to_one_caliper/

some images to help evelop the idea

Quote from: Kito on January 06, 2018, 12:06:24 PM
The motivation is that makes possible to use rear brake in right-handed turns in order to scrape and adjust velocity without upset the front suspension.

Rear brake application would still shift weight onto the front suspension during your turn.  You're not going to be able to reduce speed without transferring weight forward, its just physics. And as you do that, you'll get less and less weight on the rear wheel, which makes it that much easier to lock the rear wheel up and crash (or at least have a bit of a close call, if you can recover it).

Do what you want to your levers, but if your motivation for doing this is that you want to be able to use your rear brake in turns, I think you're a bit misguided.

Hey QcBaker... Thanks for posting and contribute to this topic...

I agree with you 100%...but note that I didn't mention "no weight transfer", what I said was "upset the front suspension"... the way that the bike move when you use front or rear brake is completely different... but still is weight being transferred to the front.

What you can achieve with a thumb lever is to use the same approach to right and left-handed turns..AND the finesse that just the hands can provide.
But hey.. I had never used thumb lever...
so This is a project to have fun.. to learn and to test...

Below some lines from the Nick Ienatsch's book

cheers

I wasn't trying to imply that you should NEVER use rear brake in turns, although re-reading my post I see how it would have come off that way. When you said "without upsetting the front suspension" I assumed you meant that you thought applying the rear brake wouldn't affect the front suspension at all, which is simply not true. So, my post was more trying to dissuade you from doing something for the wrong reasons. If you're doing it just to experiment and have fun, then more power to you. I realize you probably already understand this, but if you do proceed with the the thumb lever, please be VERY careful not to overbrake. You will not be used to how sensitive (or not) the new lever is and thus will not have a proper sense for how much brake pressure is too much (or too little).

Tangentially related: I do know of track coaches and racers who literally do not ever use the rear brake when on the track because when braking really hard, the rear wheel doesn't have much traction, so it's very difficult to keep the rear from locking up. I don't know how fast you ride, but it's something to think about.

Hey Qc! Is always nice to change ideas... but I agree that I missed something too in your previous post.. because now we are talking the same

I am one of those who you have mentioned that has stopped using the rear break...
And furthermore,  I have a problem with symmetry.. (lol) if I can not use the rear brake on both lean sides.. I rather prefer not use at all... in this way I wanna start this project to really be able to test rear trail breaking and see if will be useful for me or not... 

The leverage that I think I will achive on the thumb and the master cylinder efficiency ( I will buy a shitty master cylinder on ebay to test do project) will hopefully not have the power to easily lock the rear brake.... but if it happens... one more fall of the bike, besides some unavoidable pain and leather damage.. its ok 

Soon I will start to machine the master cylinder splitter (red metal piece above) because 60 dollars here in brazil is already kind of expensive... lets to keep it on the budget!

Quote from: Kito on January 08, 2018, 11:23:04 AM
And furthermore,  I have a problem with symmetry.. (lol) if I can not use the rear brake on both lean sides.. I rather prefer not use at all... in this way I wanna start this project to really be able to test rear trail breaking and see if will be useful for me or not...

I think maybe you should think about what it is about right turns that prevents you from using the foot brake while turning. Is it the risk of scraping the peg/lever? If so, maybe hang off the bike more and try to reduce bike lean angle.

You might want to go onto the 2017 MotoGP site where there is an explanation of use of the rear barke and thumb brakes.
Using the rear barake transfers a very small amount of weight to the front during a turn but can be the difference between not enough and the right amount.
Using a modulated rear can help control both wheel lock and the vertical balance of the rear. In most cases not using the rear in corners at all means that ALL of the braking load is taken on the front and risks overloading the front tyre and front suspension. It also increases the tyre deformation on the front. It also increases the risk of front breakfree if you hit a bump im mid corner because of the increases deformation will allow greater vertical bounce and unload. This is even more important on the road where surfaces are less than ideal.
The amount the rear can load the front is limited but the fact that the weight transfer lifts the rear and reduces the contact patch until it breakes free ( extreme). The use of a HAND rear allows greater control ( do an experiment with you ability to control someting with your foot with a boot on and your glove hand. Hand wins every time.
Foot brakes are a hangover from when we had drum brakes that mostly used a rod to the rear brake. Many early motorcycles actually had hand levers for front an back and used cables.
Bicycles ( notably MTB's) have both brakes on the bars and allow for extremely fine control fo modulation. Personally I would much prefer both brakes on the bars and a thumb CLUTCH.

Attached solidworks model of the splitting piece
https://www.dropbox.com/s/9u05izaw4ad8qoe/thumbrake.SLDPRT?dl=0

FWIW all the braking should be done before the corner. It is sometimes feasible to do a little trail braking while in the corner, but you have to be very careful. Too much front brake will stand the bike up; too much rear brake will cause the rear to slide.

Most of your stopping power is with the front brake. Learn to use it to your advantage. Learning to use the rear brake takes a lot of practice. Watch racers "backing it in" to corners with the rear brake - it's not nearly as easy as it looks. 

Quote from: alpo on March 18, 2018, 09:33:20 AM
FWIW all the braking should be done before the corner. It is sometimes feasible to do a little trail braking while in the corner, but you have to be very careful.

The older style twin shock bikes where great once you learnt how to trail the rear brake and stiffen the rear end in the corner, the bigger GS's and XS's where easy to get round corners fast like that, even more so with the original Vmax.

More details

The first image was what I imagined to be the first step in the turnlathe machine...
And then .. milling

 

ebay item
Universal Motorcycle Pit Dirt Bike ATV Rear Brake Master Cylinder Oil Reservoir