# School me on master cylinders, please!

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My next major purchase for my bike is going to be brake master cylinder and caliper. I'll probably get the master cyclinder first and use it with the stock calipers until I can afford an aftermarket.

Can one of you fart smuckers explain to me how the master cylinder size (12mm, 13mm, 16mm, etc.) affects braking? For instance, does the larger size move more fluid resulting in less effort required to work the brake? And what exactly does that number indicate? Piston diameter? Stroke length? Something else?

Thanks for the lesson!

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You more or less have it but the larger master also requires a significantly stronger squeeze on the lever.

The goal of a balanced setup is to have the right volume to not ever require too much travel, have 'feel' and minimum effort.

A bigger master uses less travel but at the expense of effort and feel/control. A too small a master can result in not being able to fully squeeze the caliper.

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Pauly,

I am no expert. Others will staighten me out as required. Here my understanding.

The brake caliper requires pressure to squeeze the pads on the brake. The higher the pressure, the greater the braking action. Really what the caliper does is to transform the brake fluid pressure into the force that squeezes the pads.

Now some physics: Force = Pressure x Area. Or if we revrese it: Pressure = Force / Area. The area is the cross-section of the caliper bore. It does not change. So again, stronger braking = higher pressure.

The master cylinder generates the pressure in the brake line. Again, pressure is the result of a force applied over an area. The force is your hand pulling on the lever. The area is the size of the master cylinder.

You get more pressure in the line by squeezing the brake lever harder (you knew that :-)). Say now that you max out on the force with your hand you can still get more pressure. Use a different geometry of the lever (to multiply the effort from your hand) or use a smaller size master cylinder. For the same force and lever, a smaller size generates a higher pressure.

Now comes the volume of fluid that needs to be moved. This volume depends of the play at the brake pads (normally very close to zero) and the expansion of the brake line under pressure (this why we use SS braided hoses, they swell less).

Volume = Area x Length. The area is the cross-section of your master cylinder. Length is the stroke of the master cylinder. You can get the same volume with: small area and big stroke or big area and small stroke. There is of course a limit to the stroke (displacement) available from the master cylinder.

To sum up. You can increase the braking action by going with a smaller master cylinder but this will result in a longer stroke (everything else staying the same).

This is theory. I hope I got it right. Others will chime in with practical experience. Unfortunately mine is limited to installing SS braided lines. They make braking crisper (less spongy) and also reduce somehow the travel of the brake lever.

FL

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they need to matched.

what caliper are you going to get?

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William and Francois, thank you for you explanations. Nice technical explanation, Francois.

they need to matched.

what caliper are you going to get?

Eddie,

I plan on getting the Brembo billet caliper from QTM. I have an S model so I think the axial mount billet caliper is my only option.

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either one of them will need a 16mm master cylinder.

bigger makes the lever rock hard and no feel.

smaller makes the lever spongy and no power.

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either one of them will need a 16mm master cylinder.

bigger makes the lever rock hard and no feel.

smaller makes the lever spongy and no power.

Thank you, sir.

If I pick up the 16mm master now would it work with the stock caliper in the interim?

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the lever will be rock hard and it will stop worse than the stock master.

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the lever will be rock hard and it will stop worse than the stock master.

Well, that's not what I want. That means I need to purchase/install the 16mm master and billet caliper at the same time.

Out of curiosity, about how large is the factory master cylinder? Any aftermarket masters made to work with the factory caliper? Do they make a difference in braking?

Thanks again!

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stock on a sm is 13 or 14mm.i forget off the top of my head.

its all about matching the pair correctly.

with the stock caliper there is really no sense in changing the master.

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stock on a sm is 13 or 14mm.i forget off the top of my head.

its all about matching the pair correctly.

with the stock caliper there is really no sense in changing the master.

Very good. Thanks for the info, Eddie.

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FWIW, the PO of my DRZ swapped on 2007 GSXR levers, including the master cylinder. It works gangbusters, and it's using the original caliper. Granted, he also put the huge rotor on the front, the Braking rotor that is nearly as big as the wheel. Also has Vortex branded levers on there, they seem like a nice improvement over the stock levers I have tried on other DRZ's.

Not sure if all the money he spent was worth it, but it works pretty darn well and I like it overall. Only problem is that the reservoir is above the bars quite a bit, and it's plastic.

I haven't put the dirt wheels back on yet, or used the original sized rotor, so I can't comment on that yet. I could report back in about two weeks though, hopefully I will have some dirt wheels and a trip behind me.

Currently it has supermoto wheels on, 17 inch.