Your truck steers fine most of the time, but there is a bit of a wobble when you hit a bump at speed, or the front end shimmies after a pothole. These are not random events. They often point to wear in the king pin and steer axle assembly: the pivot system that lets your front wheels turn.
At Ressorts Laval, king pin inspection is part of every complete steering check we perform. Here is what you need to know about how this system works, what wears out and how to catch problems early.
What is the king pin?
The king pin is the steel pin that runs vertically through the steer axle knuckle, allowing the wheel to pivot left and right when you steer. Every time you turn the wheel, the king pin rotates inside its bushings. Over hundreds of thousands of kilometres, those bushings wear down and introduce play into the steering system.
On a heavy truck, the steer axle king pin assembly includes:
- The king pin itself: a hardened steel pin pressed into the axle beam
- King pin bushings: bronze or iron bushings pressed into the steering knuckle that the pin rotates in
- Thrust bearings: the bearing that supports vertical load and allows the knuckle to rotate smoothly
- Draw keys: locking keys that hold the king pin in place in the axle beam
Signs of worn king pins
Front-end shimmy at highway speed
A slight shimmy or oscillation in the front end, especially after hitting a bump at 90 to 110 km/h, is a classic sign of king pin play. The worn bushings allow the knuckle to wobble slightly, and at speed that wobble amplifies into a shimmy that you feel through the cab and the seat.
Steering wheel wander on rough roads
Worn king pins allow the front wheels to wander independently of each other. On a rough surface, you end up fighting the wheel constantly rather than holding a steady line.
Clunking when turning at low speed
A dull clunk or knock when you turn the wheel at parking lot speed, especially when going from straight ahead into a turn, is often a worn king pin bushing or a seized thrust bearing.
Uneven front tire wear
King pin play changes the effective camber and toe of the front wheels dynamically. Even with a recent alignment, worn king pins can cause rapid or uneven tire wear on the inside or outside edge.
⚠️ Warning
King pin play is a SAAQ inspection fail point. A front end shimmy that appears above 80 km/h should be inspected immediately. It will not correct itself and gets worse over time.
How we measure king pin wear
In the shop, king pin wear is measured with a dial indicator. With the wheel raised and the brake applied, we place the dial indicator at the top of the tire and apply lateral force. The measurement tells us exactly how much play is present in the king pin and bushing assembly.
The SAAQ limit for acceptable play varies by axle rating, but as a rule of thumb: more than 3.2 mm (1/8 inch) of lateral movement measured at the tire is cause for replacement. On a loaded truck at highway speed, that small amount of play translates into real instability.
💡 King pins should be inspected at every major service. On trucks with grease fittings, greasing the king pins at every oil change significantly extends bushing life.
King pin replacement: what is involved?
King pin replacement requires removing the wheel, brake drum, hub and brake components to access the steer axle knuckle. The old king pin is pressed out, new bushings are pressed in and reamed to the correct fit, and the new pin is installed with fresh thrust bearings and draw keys.
It is precise work that requires the right press and reamers for the specific axle. An incorrectly fitted king pin bushing that is not reamed to the right diameter will wear out in a fraction of the normal lifespan.
What does king pin repair cost?
| Repair | Price range |
|---|---|
| King pin kit replacement (one side) | $400 to $800 |
| King pin replacement (both sides) | $700 to $1,400 |
| With wheel alignment after | + $200 to $350 |
| King pin play measurement (diagnosis) | included with steering inspection |
After every king pin replacement, a wheel alignment is mandatory. The new bushings and pin change the knuckle geometry slightly, and you want that corrected before putting the truck back on the road.
Frequently asked questions
How often do king pins need to be replaced?
It depends on use and whether the truck has grease fittings. On a greased king pin with regular maintenance, replacement intervals of 400,000 to 600,000 km are achievable. On a neglected or non-greased assembly, wear can occur much sooner.
Can I drive with worn king pins?
Short-term and at lower speeds, yes. But worn king pins affect steering precision and braking stability. At highway speeds the shimmy can become severe. Do not delay the inspection.
Does king pin wear affect wheel alignment?
Yes, directly. Worn king pins allow the front wheels to move outside the alignment spec under load. An alignment done over worn king pins will not hold. The play in the system overrides the adjustment.
Do you replace king pins on all steer axle brands?
Yes. Dana, Meritor, Hendrickson, Spicer, Eaton: we have the tooling for all common steer axles on North Shore trucks.
Front-end shimmy or steering wander?
We measure king pin play in the shop and tell you exactly what needs to be done. Ressorts Laval, steering and suspension specialists since 1971.
Call us at 450-661-5157.