How to Measure Brake Drum Out of Round

Brake drums are an essential component of a vehicle's braking system, especially in older models and heavy-duty vehicles. Their primary purpose is to provide a surface for the brake shoes to press against to slow down or stop the vehicle. Over time, however, brake drums can become out of round, leading to decreased braking efficiency and increased wear on brake components. Measuring and addressing this issue is crucial for maintaining optimal performance and safety. In this article, we will discuss how to measure brake drum out of round and what steps to take if you find that your brake drums are indeed out of spec.

Understanding Brake Drum Geometry

Before we dive into the measurement process, it’s important to understand what it means for a brake drum to be out of round. A drum is considered out of round when its diameter varies at different points around the circumference. This can happen due to wear, heating, excessive use, or improper installation. When a drum is out of round, it can cause vibrations during braking, leading to a pulsating effect in the brake pedal and uneven wear of the brake shoes.

Tools Required

To accurately measure whether your brake drum is out of round, you'll need the following tools

1. Micrometer or Caliper To measure the diameter of the drum at various points. 2. Dial Indicator This precision tool helps measure small variations in the drum's surface. 3. Turntable or Brake Drum Lathe If you find the drum is out of round, a lathe can restore its shape. 4. Safety Equipment Always use safety glasses and gloves when working on brake components.

Measuring the Drum

1. Safety First Before beginning any measurement or inspection, ensure the vehicle is securely lifted, and the wheels are removed. Always engage the parking brake and use jack stands for safety.

2. Clean the Drum Remove any debris, rust, or dust from the drum surface. This ensures that your measurements are not affected by dirt or corrosion.

3. Initial Visual Inspection Look for any noticeable cracks, grooves, or excessive wear that might indicate damage. If you see significant issues, it may be prudent to replace the drum rather than simply measure it.

4. Use the Caliper or Micrometer Start by placing the caliper at one point on the inside surface of the drum and take a measurement. Rotate the drum about 90 degrees and take another measurement. Continue this process at multiple points (approximately every quarter of the drum) to gather data.

5. Record Your Measurements Write down each measurement for later analysis.

6. Set Up the Dial Indicator If you have a dial indicator, mount it securely and make contact with the surface of the drum. Rotate the drum slowly and observe the dial. Ideally, the reading should remain consistent; variances will indicate how out of round the drum may be.

7. Calculate the Variances The difference between the largest and smallest measurements indicates how far out of round the drum is. Most manufacturers specify acceptable tolerances, usually within 0.005 inches for passenger vehicles. Consult your service manual for the specific tolerances for your vehicle.

What to Do If the Drum is Out of Round

If your measurements indicate that the drum is out of round, you have several options

1. Resurfacing If the out-of-round condition is within the tolerances but still noticeable, consider a professional resurfacing at a brake shop. Resurfacing can restore the drum to a uniform shape.

2. Replacement If the drum is significantly out of round or beyond resurfacing limits, replacing the brake drum is your best option. Always replace drums in pairs (front or rear) to maintain balanced braking performance.

3. Further Inspection Check other components of the braking system, including brake shoes and wheel cylinders, to ensure they’re not contributing to or exacerbating the problem.

Conclusion

Measuring a brake drum's roundness is a crucial maintenance task for ensuring vehicle safety and performance. Regular inspections and maintenance can prevent many issues associated with out-of-round brake drums. By following the steps outlined in this guide, you'll be better equipped to keep your braking system in optimal condition. Remember, safety should always be your top priority, and if you're unsure about any aspect of brake maintenance, consult with a professional mechanic.