Ride height (also called ground clearance or simply clearance) is the amount of space between the base of an automobile tire and the lowest point (typically the axle); or, more properly, to the shortest distance between a flat, level surface, and the lowest part of a vehicle other than those parts designed to contact the ground (such as tires, tracks, skis, etc.). Ground clearance is measured with standard vehicle equipment, and for cars, is usually given with no cargo or passengers.
Ground clearance is a critical factor in several important characteristics of a vehicle. For all vehicles, especially cars, variations in clearance represent a trade-off between handling and practicality. A higher ground clearance means that the center of mass of the car is higher, which makes for less precise and more dangerous handling characteristics (most notably, the chance of rollover is higher). However, it also means that the car is more capable of being driven on roads that are not level, without the road scraping against and likely damaging the chassis and underbody. Higher ride heights will typically adversely affect aerodynamic properties. This is why sports cars typically have very low clearances, while off-road vehicles and SUVs have higher ones. Two well-known extremes of each are the Ferrari F40 and the Hummer.
For armored fighting vehicles (AFV), ground clearance presents an additional factor in a vehicle's overall performance: a lower ground clearance means that the vehicle minus the chassis is lower to the ground and thus harder to spot and harder to hit. The final design of any AFV reflects a compromise between being a smaller target on one hand, and having greater battlefield mobility on the other. Very few AFVs have top speeds at which car-like handling becomes an issue, though rollovers can and do occur. By contrast, an AFV is far more likely to need high ground clearance than a road vehicle.
Lowering a car's suspension is a common and relatively inexpensive aftermarket modification. Many people prefer the more aggressive look of a lowered body, and there is an easily realized car handling improvement from the lower center of gravity. Most passenger cars are produced such that one or two inches of lowering won't increase the probability of damage significantly. On most automobiles, ride height is modified by changing the length of the suspension springs, and is the essence of many aftermarket suspension kits supplied by manufacturers such as Eibach and H&R.
Some cars have used underslung frames to achieve a lower ride height and the consequent improvement in center of gravity. The brass-era cars of the American Motor Car Company are one example.
Self-leveling suspension systems are designed to maintain a constant ride height regardless of load. Vehicles not equipped with self-leveling will pitch down at one end when laden; this adversely affects ride, handling, and aerodynamic properties.
Some modern automobiles (such as Audi's Allroad Quattro) have adjustable suspension systems, which can vary the ride height by locating the suspension mounting points, depending on road conditions and/or the settings selected by the driver.
Other, simpler suspension systems, such as coilover springs, offer a way of manually adjusting ride height (and often, spring stiffness) by compressing the spring in situ, using a threaded shaft and adjustable knob or nut.
18-wheel tractor-trailers also have to take the ground clearance of both their tractor and especially trailer into consideration on certain areas of uneven terrain, such as raised railroad crossings. Their extremely long wheelbase means that such terrain could potentially catch the undercarriage of the trailer in the wide space between the axles, potentially leaving the truck stuck with no means to extricate itself.
In some areas buses are required to have a ground clearance of at least 100 mm (3 15⁄16 in).