The short version
- C is the dynamic rating: the load a bearing survives for one million revolutions, with ten percent expected to have failed.
- C0 is the static rating: the load that permanently dents a raceway while the bearing is not turning.
- Life scales with the cube of the load ratio on a ball bearing. Ten percent more load is roughly a quarter less life.
- These are laboratory numbers. Real wheel bearings die of contamination long before fatigue arrives.
Every rolling bearing carries two numbers that describe what it can take. A wheel bearing load rating is quoted the same way as an industrial bearing. Almost nobody explains what the letters mean.
They are worth knowing, because one of them explains every argument on this site about load.
The two numbers in a wheel bearing load rating
| Symbol | Name | What it means |
|---|---|---|
| C | Basic dynamic load rating | Load for one million revolutions, ten percent failed |
| C0 | Basic static load rating | Load that permanently deforms a contact point |
| P | Equivalent load | What the bearing is actually seeing |
| L10 | Basic rating life | Life ninety percent are expected to reach |
| p | Life exponent | 3 for balls, 10 over 3 for rollers |
The dynamic rating is a reference figure, defined by the standard, not a limit you should approach.
A bearing run at exactly C would reach one million revolutions with a one in ten chance of having already failed. Nobody designs anything that way.
What C0 actually protects against
The static rating is set by a permanent deformation of one ten thousandth of a rolling element diameter. That is at the most loaded contact.
That is a dent, not wear. It happens under shock or under load while the bearing is stationary or barely turning.
This is exactly the mechanism behind pothole damage and behind bearings damaged while a car sat on a transporter.
Those are on the pothole page and the sitting page.
The exponent that explains everything
The rating life relationship raises the load ratio to the power of three on a ball bearing. On a roller it is ten thirds.
So a ten percent load increase does not cost ten percent of the life. It costs roughly a quarter of it.
This single exponent is why spacers, towing, heavy wheels and overloading matter so much more than people expect.
The applied version is on the lifespan page, and the spacer case is on the spacers page.
Why a wheel bearing load rating does not predict mileage
The standard describes rolling contact fatigue in clean, well lubricated laboratory conditions.
A wheel bearing on a road car almost always dies of seal failure and contamination first. The rating does not model that at all.
That is why this site quotes ratios rather than predicted mileages. The relationship between loads is sound. The absolute number is not.
What actually ends them is on the failure causes page.
Where the rating is genuinely useful
- Comparing two bearings of the same size, where a higher C means more capacity.
- Understanding load changes, because the exponent tells you what a modification costs.
- Judging shock risk, since C0 is the number impacts act against.
- Reading manufacturer claims, which usually quote C without saying what it means.
- Ignoring life predictions built from it, which are laboratory artifacts.
How sizes and designations are read is on the sizes page.
Common questions
What is a wheel bearing load rating?
Two figures. C is the dynamic rating for one million revolutions at ten percent failure. C0 is the static rating against permanent denting.
Can a bearing carry its C rating?
Not in service. C is a reference figure from the standard. Running at it implies a one in ten failure within a million revolutions.
What does C0 protect against?
Denting under shock or while stationary. It is set by a deformation of one ten thousandth of a rolling element diameter.
Why does load matter so much?
Because life scales with the cube of the load ratio on a ball bearing. Ten percent more load costs roughly a quarter of the life.
Can I calculate how long mine will last?
You can calculate a fatigue life, and it will be wrong. Real bearings fail from contamination long before fatigue arrives.
Is a higher C always better?
Between two bearings of the same size, it indicates more capacity. Across different sizes it tells you very little on its own.
Where do I find these numbers?
Manufacturer catalogs list them for industrial bearings. Car hub assemblies rarely publish them at all.