Ride height on a semi truck is measured from the ground to a specific reference point on the frame rail or axle, depending on the suspension type. Getting an accurate measurement requires the truck to be on level ground, at its normal operating weight, and with properly inflated tires. The process itself is straightforward, but knowing exactly where to measure and what numbers to compare against matters more than most drivers realize.
Why Ride Height Matters
Ride height isn’t just about ground clearance or how level the truck sits. It directly affects your driveline angles. Even small changes in suspension ride height can create incorrect driveline operating angles, which produce torsional vibrations that you may not even feel while driving. Over time, those vibrations damage transmissions, U-joints, power dividers, and potentially the engine itself. If any of those components fail unexpectedly, improper ride height is one of the first things a shop should investigate.
Beyond the drivetrain, incorrect ride height changes your fifth wheel coupling angle, affects trailer kingpin loading, alters braking geometry, and can accelerate tire wear. Keeping ride height within spec is one of the cheaper forms of preventive maintenance on a class 8 truck.
Tools You Need
You don’t need specialized equipment for a basic ride height check. A tape measure or a straight ruler long enough to reach from the ground to the frame rail is sufficient. A level surface is more important than any tool. If the ground slopes even slightly, your numbers will be off. A concrete shop floor or a flat parking lot works well. Some technicians use a laser level or plumb bob for more precise frame-to-ground measurements, but these are optional for routine checks.
Preparing the Truck
Park the truck on a flat, level surface. The vehicle should be at its normal loaded operating weight, or at minimum the weight condition specified by the suspension manufacturer. If you’re measuring unloaded, note that, because ride height specs often differ between loaded and empty conditions.
Make sure the air suspension system is fully aired up and at operating pressure. On air ride suspensions, the leveling valves need time to stabilize after the truck has been moved. Let the truck sit for a few minutes after parking so the bags settle to their normal position. Tires should be inflated to the correct pressure as well, since low tires will throw off your ground-to-frame measurements.
Where to Measure on Air Ride Suspensions
Most air ride suspensions use a measurement from the top of the axle to the bottom of the frame rail, taken directly above the axle. This is sometimes called the “gap measurement” or “air spring height.” The suspension manufacturer’s spec sheet tells you the target dimension, which typically falls between 5 and 7 inches on most common rear air suspensions, though the exact number varies by make and model.
Some manufacturers specify measuring from the ground to the bottom of the frame rail at a specific location, such as directly above the rear axle centerline. Others reference the distance between the axle seat pad and the frame bracket. Always check the spec sheet for your particular suspension. Hendrickson, Reyco, and other manufacturers publish ride height specifications and the exact measurement points in their maintenance manuals, usually available free online as PDFs.
On steer axles with air springs, the measurement is typically taken from the axle pad to the frame rail as well, though the target dimension is different from the drive axles.
Where to Measure on Leaf Spring and Walking Beam Suspensions
For leaf spring suspensions, ride height is generally measured from the ground to the bottom of the frame rail at a point specified by the manufacturer, usually above the axle centerline. The measurement reflects the combined condition of the springs and shackles. Sagging or broken leaves will show up as a lower-than-spec number.
Walking beam suspensions (common on Mack and some older Kenworth models) can be trickier. Ride height on these systems is sometimes set by torque rod length rather than air pressure. Even small changes in torque rod length create driveline angle problems, so the measurement procedure for these suspensions is more involved and often requires checking multiple reference points.
Step-by-Step Measurement Process
- Level the truck. Park on flat concrete. Set the parking brake. If the truck has air ride, let it sit until the suspension fully stabilizes.
- Locate the reference points. Check your suspension manufacturer’s documentation for the exact spots to measure. Mark them with chalk if needed.
- Measure both sides. Take the measurement on the left (driver) side and the right (passenger) side at the same axle location. Record both numbers.
- Compare side to side. The left and right measurements should be within 1/4 inch of each other. A larger difference means the truck is leaning, which could indicate a bad air bag, broken spring, collapsed bushing, or a leveling valve issue.
- Compare to spec. Check your measurements against the manufacturer’s published ride height for your suspension model and load condition.
- Repeat at each axle. Measure at the steer axle, each drive axle, and the tag or pusher axle if equipped. Each may have a different target height.
Checking Frame Rake
While you’re measuring ride height, it’s worth checking frame rake at the same time. Frame rake is the angle of the frame rails relative to level ground. To check it, measure from the ground to the bottom of the frame rail at the front of the truck (behind the bumper) and at the rear. The difference between those two measurements tells you the rake angle.
Most trucks are designed to sit slightly nose-down (about 1/2 to 1 inch lower at the front than the rear) when loaded. This positions the driveline angles correctly. If ride height is correct at the axles but frame rake is off, you may have a bent frame, shifted body mounts, or other structural issues that affect driveline geometry.
Adjusting Ride Height
On air ride suspensions, ride height is adjusted by repositioning the leveling valve linkage arm. Lengthening or shortening the link between the valve and the axle changes the point at which the valve maintains air bag pressure. This is a relatively simple adjustment, but getting it wrong by even a fraction of an inch has real consequences for driveline angles and component life.
Some newer trucks have height control valves that can be adjusted with a built-in turn mechanism rather than changing linkage length. Electronic air management systems on premium suspensions may allow height adjustments through a dash-mounted control or a diagnostic tool.
On leaf spring suspensions, there’s no easy adjustment. If ride height is out of spec, the springs are likely sagged or broken and need replacement. Adding helper springs or shims is sometimes done but doesn’t truly fix the underlying problem.
How Often to Check
Ride height should be part of your regular preventive maintenance routine. Checking it every time the truck goes in for a PM service (roughly every 25,000 miles or quarterly) catches problems before they cause expensive drivetrain damage. Any time you notice uneven tire wear, a vibration at highway speed, or the truck sitting visibly lower on one side, measure ride height as a first diagnostic step. If the truck has had suspension work, frame work, or a fifth wheel repositioning, ride height should be verified before the truck goes back into service.