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Proactive National Car Care Month Plan Could Lead To More Business

As gas prices continue to drop, ­consumers will have a few more dollars in their pockets to spend on essentials, like vehicle repair and service. Getting customers to spend some of those dollars in your shop should be one of your goals this spring. With...

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Do Training, Technology And Parts Sourcing Issues Keep You Up At Night?

We often hear that the things that keep shop owners awake at night pertain to profitability, productivity, training, keeping up with technology, shop operations/expenses and parts quality/availability. This month, we hear directly from one of your...

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Top 10 Automotive Repair Shop Pet Peeves

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Pinpointing the Source of Mercedes-Benz Fuel Leaks

If you’re paying attention, there are more and more European cars on the roads these days. And, if you’re doing repair and service on ­European vehicles, there’s a good chance you’ll see several Mercedes-Benz E-Class cars in your shop. Between...

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Engine Coolant Temperature Diagnostics

While old-school cooling system service often ­focused on coolant leaks and overheating engines, let’s begin thinking “new-school” by looking at modern cooling systems through the eyes of the engine coolant temperature (ECT) sensor. Coolant temperature...

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Volkswagen: Parking Indicator Sounds Off with No Obstacle

Customer complains that the parking assistance issues warning sounds for both the front and/or rear of vehicle with no obstacle in range. Water intrusion into sensor holders is causing corroded plug contacts and sensor electrical pins, which results in...

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The Right Diagnostic Tools Save You From Extensive Disassembly Time

By Andrew Shulgin, Vasyl Postolovskyi and Olle Gladso Contributing Writers and Instructors at Riverland Technical and Community College in Albert Lea, MN   A frequent reason for ­customers showing up at a vehicle repair facility is...

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Know the Specs for Your Social Media Accounts

By now, we should be well past the things I heard as recently as two years ago: “Social media is just a fad,” “No one will follow a (repair) shop on Facebook” and “I don’t care what you had for dinner, and I’m sure you don’t care where...

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MAHLE Service Solutions' New R1234yf Recovery Machine Named 'Best Use Of Technology' At MACS 2015

The ArcticPRO ACX1280 machine from MAHLE Service Solutions received the “Best Use of Technology” new product award at the 2015 Mobile Air Conditioning Society (MACS) Worldwide Trade Show. The award was selected by an impartial panel of automotive...

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Ford: Smoke From Vehicle When Starting

Vehicle: 2005 Ford Taurus SE, 3.0L Complaint: The customer says smoke comes from vehicle when starting. Cause: Confirmed the customer’s complaint and found smoke coming from the vehicle when the engine was started. Inspected the vehicle and found...

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R-1234yf Safety Procedure Checklist

Remember that R-1234yf is only mildly flammable. To become flammable, the mixture of air and refrigerant in a closed area like a vehicle cabin would need to be between 6.5% and 12.3% of the chemical vapor. This mixture must then experience a significant...

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Refrigerant Revolution: What R-1234yf Means for Service, Equipment, Safety

The new R-1234yf refrigerant is more than just a new jumble of numbers and letters on a label. For your shop, R-1234yf means several new procedures, a certification and new equipment in order to properly handle these new systems. Why the difference...

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Home Brakes BRAKE MATH: CALCULATING THE FORCE NEEDED TO STOP A CAR

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I made a mistake probably because of my distaste of math, now it is time to learn from it. In a three-part series, we will look at the math of brakes. We will start with the driver pushing the brake pedal and end with the pad contacting the rotor and bringing the car to a stop. Along the way, we will explore the math of boosters, quick take-up master cylinders and caliper sizing. 
 
On a mechanical level, it is easy to understand how brakes work. We all understand that brake fluid transfers force from one hydraulic component to another. But, how does this apply to how a brake pedal feels? This is where math is required.
 
You need only two simple math equations to commit to memory. First, the equation for calculating the surface area of a circle (caliper or master cylinder piston) is p(3.14) x radius2. Second, pressure is equal to the force divide by the area or pounds per square inch.  The rest of the math is just multiplication, division and addition/subtraction.

Pedal Ratio
Lets start with the driver. In a sitting position, the average driver can comfortably generate 70 lbs. of force on the rubber pad at the end of the brake pedal. The brake pedal is nothing more than a 
mechanical lever that amplifies the force of the driver. This is where the pedal ratio comes into play.
 
Pedal ratio is the overall pedal length or distance from the pedal pivot to the center of the pedal pad divided by the distance from to the pivot point to where the push rod connects.
 
The optimal pedal ratio is 6.2:1 on a disc/drum vehicle without vacuum or other assist method. This means that the 70 lbs. the driver has applied now is amplified to 434 lbs. (6.2 x 70 lbs.) of output force. The problem is that the travel of the pedal is rather long due to the placement pivot point and master cylinder connection.
Brake Boosters
A booster increases the force of the pedal so lower mechanical pedal ratio can be used. A lower ratio can give shortened pedal travel and better modulation. Most vacuum boosted vehicles will have a 3.2:1 to 4:1 mechanical pedal ratio.
 
The size of the booster’s diaphragm and amount of vacuum generated by the engine, will determine how much force can be generated. Most engines will generate around -8 psi of vacuum (do not confuse with inches of HG or Mercury). If a hypothetical booster with 7-inch diaphragm is subjected to -8 psi of engine vacuum, it will produce more than 300 lbs. of addition force. Here is the math:
 π(3.14) X radius(3.5)2 = 38.46 sq/inches of diaphragm surface area X 8 psi (negative pressure becomes positive force)= 307.72 lbs of output force
To keep things simple, let’s return to our manual brake example. The rod coming from the firewall has 434 lbs. of output force. When the force is applied to the back of the master cylinder, the force is transferred into the brake fluid.
The formula for pressure is force divided by the surface area.  
If the master cylinder has a 1-inch bore, the piston’s surface area is .78 square inches. If you divide the output force of 434 lbs. by the surface area of the piston, you would get 556 psi(434 lbs. divided by .78 inches) at the ports of the master cylinder. Not bad for a 70 lbs. of human effort.
If you reduce the surface area of the piston you, will get more pressure.
 
This is because the surface area is smaller, but the output force from the pedal stays the same. If you used a master cylinder with a bore of .75 inches that has a piston that has .44 inches of piston surface area, you would get 986 psi at the ports for the master cylinder (434 lbs. divided by .44 inches).
 
But, how is this force transferred to the calipers? How does the size of the caliper piston change the force needed to push the brake pad to the rotor? We will explore this next month. 
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Andrew Markel

Andrew Markel is an ASE Certified Technician and former service writer, and he brings this practical knowledge to the Brake & Front End team as editor.

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