How do You Balance a Flywheel?

Why Precision Matters in Flywheel Balancing:

When balancing a flywheel or even a car wheel precision mounting is absolutely critical. A seemingly minor imperfection, such as a burr on the flange, can cause the part to sit off-centre. Even when there’s no visible defect, there’s always a margin for error when using generic cones to mount components. These cones grip inconsistently depending on how they’re tightened, which affects the repeatability and accuracy of the balance.

To eliminate that variability, I use a custom-made mandrel tailored to each flywheel type. It’s a press-fit into the central hole, ensuring perfect concentricity. Yes, this approach involves more time and effort—what some might call a “buggeration factor” but it guarantees precise, repeatable results.

Demonstrating the Difference

A colleague of mine machines and lightens a significant number of flywheels for performance customers. However, he lacks his own balancing setup and outsources this service, spending £2,000 to £3,000 annually. I recently visited his shop to show him how to build an in-house system and stop relying on subpar third-party balancing.

We started by testing a couple of OEMs (original equipment manufacturer) flywheels to benchmark their factory balance. As expected, they were spot-on. OEMs invest heavily in highly accurate, dedicated balancing systems for their engines. My system measures in gram at the flywheel rim, and both OEM samples were within 1 gram at a 5-inch radius virtually perfect.

In-House Machining vs. Outsourced Balancing

Next, we examined two flywheels that my colleague had lightened but not yet sent for balancing, along with four others that had already been balanced by the external specialist.

The unbalanced but freshly machined flywheels showed roughly 2 grams of imbalance at the rim. That’s quite good, and it confirmed that his lathe setup was concentric and removed material evenly. These flywheels would likely have performed fine without any further adjustment.

The outsourced flywheels, however, told a different story. The best one was 3 grams out, while the worst was a shocking 8 grams off-centre enough to cause significant vibration in a high-RPM engine. That’s 8 to 10 times worse than the OEM balance.

In short, the balancing service he had been paying for was making the flywheels worse, not better. Watching his reaction as we tested each one was sobering he realized he’d been spending money year after year only to degrade the quality of his product.

What Went Wrong?

How could a “specialist” balancer consistently return worse-than-factory results? The likely culprit isn’t the machine itself, but the method of mounting. Most balancing setups rely on tapered cones, and if those cones are worn or imprecise, the flywheel won’t sit true. Every slight misalignment introduces error and worse, it’s a repeatable error across all the work they do.

There’s an old saying: If you want a job done right, do it yourself. Of course, not everyone has the tools or expertise to do that which is where people like me come in. But it does raise a broader question: What standard of work are you really getting when you send your parts to “reputable” engine builders? Based on our findings, not a particularly good one.

Rethinking Balancing Best Practices

Let’s also clear up a common misconception: flywheels do not need to be balanced on the crankshaft they’ll be installed on. Unless you’re dealing with a very high-revving or extremely delicate racing engine, that level of precision isn’t just unnecessary it can be counterproductive.

Why? Because even when fitted, a flywheel isn’t perfectly fixed in place. There’s always a tiny amount of movement between the crank spigot and the flywheel’s central hole, as well as between the bolts and their corresponding holes. This play might only be a thousandth of an inch, but that’s enough to shift the centre of mass and affect balance.

The best approach is to balance the flywheel on a mandrel that fits perfectly in the central hole. That way, you’re ensuring the flywheel itself is perfectly balanced, independent of the crankshaft. This is how all production flywheels are balanced, and it provides consistent results across multiple installations.

Measuring Real-World Sensitivity

My own system is sensitive enough to detect tiny imbalances caused by those thou-level misalignments. Even so, the effect is typically less than 1 gram negligible in most engines. Interestingly, none of the poorly balanced flywheels that my colleague had been unknowingly sending out ever led to customer complaints. That suggests most engines tolerate a few grams of imbalance without obvious symptoms.

But just because a small error doesn’t because immediate issues doesn’t mean it’s acceptable especially in performance applications where engine life, vibration, and power delivery all matter. A balanced flywheel isn’t just a nicety it’s a standard of quality.

Final Thoughts

Balancing is as much about how you do it as it is about the tools you use. Relying on universal cones and shortcuts might be faster, but precision and repeatability suffer. Custom mandrels and careful measurement might take more effort, but they deliver results you can trust.

If you’re machining flywheels or building engines, and you’re still outsourcing your balancing work, it’s worth asking: Are you really improving the product, or just paying to make it worse?