If you understand modern disc braking systems, you would know the importance of a master cylinder. But what size master cylinder for 4 wheel disc brakes would work perfectly? Let’s find out.
The bore diameter of the master cylinder plays a vital role in your vehicle’s braking performance. The problem is that there are numerous ranges of master cylinders available on the market, and all have different bore diameters.
For instance, the most popular sizes of master cylinders are 3/4 inch, 7/8 inch,1 inch,1-1/8 inch, and various others. So you may get confused about what size master cylinder for your four-wheel disc brakes.
I cannot give you a simple answer to this question. You just have to try the hit and trial method to know which bore-size master cylinder is perfect for your vehicle.
In this article, I will discuss more about the master cylinder, how to calculate bore size, and brake: pedal ratio. So, keep on reading the article for more information.
What Is The Purpose Of Master Cylinder In Disc Brakes?
Most of the vehicles available today have a disc braking system. The master cylinder plays an essential part in this braking system. The brake fluid from the reservoir starts entering the master cylinder and gets compressed whenever you press the brake pedal.
The pressurized hydraulic fluid starts entering the caliper and exerts a force on the piston from all directions. The piston squeezes the brake pads and comes in contact with the disc or rotor. The friction between the brake pad and rotor stops your vehicle.
Why Is It Important To Choose The Right Master Cylinder Size?
Many people think that if they choose a large bore-size master cylinder, it will produce more hydraulic pressure. But in actuality, it is not true.
Selecting a small bore-size master cylinder will produce more pressure at the caliper or wheel assembly. You will even require less effort to press the brake pedal. But you have to press it down to create enough braking pressure.
If you choose a large bore-size master cylinder, then the brake pedal will not travel enough distance to produce enough pressure, and you will have to apply more effort to press the brake pedal.
You might like to read: Which Is The Front Brakes On A Master Cylinder?
How Can I Calculate The Bore Size Of A Master Cylinder?
A proper braking system will create enough braking pressure to stop your car comfortably at the desired location. It will increase your stopping distance. Even it delivers a sufficient fluid volume to produce a firm brake pedal.
I am describing below a table that shows the caliper piston size and its area in inches.
|Area of piston (in inches)
The second table shows the master cylinder bore size and its area in inches.
|Area of the piston(inches)
Most of the master cylinders used in vehicles have bore sizes ranging from 0.625 to 1.12 inches. I am sharing below an example that will help you determine the correct master cylinder bore size for your vehicle so that it can stop comfortably.
Let’s say you want the pedal ratio in your vehicle to be 5.75:1. Now, if you select the bore diameter of the master cylinder by inch and the caliper piston diameter of 2.75 inches, let’s see how much is the pedal ratio obtained.
The master cylinder with a one-inch bore diameter has an area of 0.79 square inches, and the diameter of the caliper piston with 2.75 inches will produce an area of 5.94 square inches.
Now divide the area of the caliper piston with the area of the master cylinder to determine the pedal ratio. So, in this case, it will be 5.94 divided by 0.79, which will be 7.52.
So, the pedal ratio for a one-inch bore diameter master cylinder will be 7.52 :1, which varies greatly from our desired pedal ratio, i.e., 5.75:1
Now, let’s change the bore diameter of the master cylinder from 1 inch to 1.12 inches. The area of the piston will be 0.99 square inches.
We will keep the caliper piston diameter the same, i.e., 2.75 inches, which produces 5.94 square inches.
Now diving into the caliper piston diameter area with the master cylinder bore dia area; we will get 6. So we are getting the effective pedal ratio as 6:1, which is far nearer to our desired pedal ratio. So you can choose a 1.12 inches bore diameter master cylinder for your vehicle. It will help in stopping your vehicle safely.
How Can I Calculate Brake Pedal Ratio?
Whenever you find it too hard to push your brake pedal, your driving becomes difficult and may put your life in danger. You need to install a master cylinder with the correct bore size to decrease your effort in pressing the brake pedal.
The brake pedal ratio has quite an essential role in the braking system. It affects how far the brake pedal will travel to generate enough pressure so that your vehicle will stop at your desired location.
An average person can press the brake pedal at 70lbs force, which is insufficient to create enough pressure. The disc brake requires 800-1200 psi to stop the vehicle.
So the force you apply on the pedal needs to be multiplied by several braking system components to stop the vehicle. The pedal ratio is one such factor that helps in amplifying the force applied on the pedal. The brake pedal ratio should ideally be between 5.1 :1 to 7.1:1
If the mathematical calculations done above are pretty complicated for you, you can determine your vehicle’s pedal ratio with the help of your measurement tape.
However, you need to follow the below steps.
- You have to take a tape and measure how far the pedal’s pivot is to the center of its pad. Let’s say you got the distance as x
- Next, find out how far the pedal pivot point is to the pushrod’s connection. Let’s say you got the distance as y
- Now divide x by y to get the pedal ratio.
Suppose you got x as 12 inches and y as 3 inches. Then the pedal ratio will be 4:1
Now, if you apply 70 lbs force on the brake pedal with a 4:1 pedal ratio, then 280 lbs force will be applied on the pushrod of the master cylinder.
If you increase the pedal ratio, then it will increase the pedal travel and leverage. Similarly, decreasing the ratio will decrease the pedal travel but simultaneously increase pedal effort. So, make sure that the angle of the pushrod to the master cylinder should not exceed 5 degrees in your pedal ratio.
Frequently Asked Questions
#1. What size master cylinder do I need?
The size of the master cylinder depends on the pedal ratio of your vehicle. The pedal ratio usually lies between 5.1:1 to 7.1:1. If you increase above the recommended ratio, it will increase the pedal travel. Likewise, if you decrease the pedal ratio, then it will increase your effort in applying the pedal.
#2. Should you get a proportioning valve with four-wheel disc brakes?
Yes, you need a proportioning valve for disc brakes. A proportioning valve help in minimizing the pressure on the rear brakes.
Sometimes you will find your rear brakes lock up after applying brakes. It is an indication that the proportioning valve has gone bad.
#3. Does the size of the bore of the master cylinder matter?
When you use a master cylinder that has a smaller bore diameter, then it will increase the pedal travel and decrease your effort in applying the pedal. Similarly, choosing a smaller bore diameter cylinder will reduce the pedal travel and increase your effort in applying the pedal.
#4. Can you use any master cylinder with disc brakes?
Yes, you can use any master cylinder with disc brakes.
You might like to read: Brake Fluid Pressure Is Measured In What Unit?
A Few Final Words
The Master cylinder plays a vital role in the braking system. Its proper size matters a lot for your vehicle because it will affect your pedal travel distance and the effort you apply to the brake pedal.
There are different sizes of master cylinders available in the market. You need to choose the one that stops your vehicle comfortably.
Try to calculate the pedal ratio either by tape or by mathematical calculation, and then select the right bore size master cylinder for your car. We thank you for reading the article and hope you like it.