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Chassis Ride Height

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Have you ever been confronted by a vehicle with a bad attitude? The kind of attitude we're talking about here is not a chip on the shoulder, but a physical attitude that's not what it should be. The chassis may be sagging or slightly askew thanks to the unbalanced forces of gravity that are working upon it. The underlying cause of this kind of bad attitude is usually one or more sagging springs, and the way you detect it is by measuring ride height prior to doing a wheel alignment.

Ride height problems are not always obvious, and may be overlooked unless a vehicle is leaning like a battleship that's taken a torpedo, or is tilting like the Titanic on its final plunge. It's hard to see variations in ride height if the difference is only an inch or two. You can compare the height of the fender wells on both sides to see if the suspension is leaning or not, but a simple comparison won't tell you if the overall chassis height is correct or not. And with the nose-down aero attitude of so many cars today, it's impossible to tell if the vehicle has the right attitude unless you measure ride height front and rear and compare the results to the factory specifications.

WHY THE PROPER RIDE HEIGHT IS SO IMPORTANT

Ride height determines where the control arms operate within their normal range of travel. This is especially important with SLA and wishbone strut suspensions. When a suspension is designed by the vehicle manufacturer, the control arms are positioned to operate at a specified ride height. That position determines the amount and direction of camber change that occurs as the suspension moves from jounce to rebound. If the springs are weak and the suspension has sagged two or more inches below the specified ride height, the arms are forced to operate above their normal plane and beyond their normal range of travel - which can cause undesirable changes in camber and toe. Unequal camber can cause a vehicle to lead to one side.

Differences in ride height front-to-rear can also upset the steering geometry of the front suspension. Raising or lowering the rear of the vehicle will change the angle of the steering axis (caster). Depending on the amount of change, it may have an adverse effect on steering stability, effort and returnability.

There's a safety angle, too. Ride height can also affect the aim of the headlights, which in turn affects nighttime driving safety. A nose high attitude can blind oncoming drivers while a nose low attitude can reduce visibility and the time a driver has to react to a curve or obstacle.

MEASURING RIDE HEIGHT
A vehicle's attitude is difficult to gauge by sight alone unless it is sagging badly or listing to one side. So ride height should always be measured and compared to specifications to determine if it is within an acceptable range. Some manufacturers allow up to an inch or more of sag (but generally no more than two inches) before ride height is considered out of specification.

The point where ride height is measured is critical. Vehicle manufacturers specify various reference points on the frame, body, fender, bumper or suspension. On some applications, ride height may even be specified as a distance between two points on the vehicle's suspension, such as the distance from the frame rail to the axle or a suspension control arm. So make sure you know where the measurement is supposed to be taken.

For conventional ride height checks, a tape measure or ruler is usually all that's needed. Just remember that changes in the wheel and tire size will affect your readings. If tires or wheels on a vehicle have been replaced with ones that are taller or shorter than the originals, it will increase or decrease ride height if you're measuring between some point on the vehicle and the ground. So always check the wheel and tire size on the vehicles prior to taking your measurements.

Aftermarket ride height specification charts are also available that use the distance between the edge of the fender opening and the center of the wheel as a common reference point - which is a much simpler approach. Special aftermarket ride height measuring tools are also available that make this job even easier. The tool snaps on the wheel and helps you find the exact distance between the center of the hub and the lip on the fender.

On some vehicles, the ride height specification is for a loaded vehicle. Adding weight may therefore be necessary to get the correct dimensions. On vehicles with electronic or automatic load leveling suspensions, you may also have to add weight to see if the system is maintaining ride height within the specified range.

Ride height should be measured both front and rear, and on both sides. Comparing the side-to-side ride height dimensions can help you identify sagging springs as well as frame and body misalignment. A difference of more than an inch side-to-side may be enough to cause the vehicle to lead to one side.

Other clues to look for that may tell you a vehicle has a bad attitude problem:


ATTITUDE ADJUSTMENT: SHIMMING SPRINGS
To correct a bad attitude you have to identify the underlying cause and fix it. So anytime ride height is found to be below specs, repairs should be performed before any alignment work is done.

When coil springs are an inch or less below specifications, shims may be installed in or under the springs to restore ride height. But shims do not restore spring rates, so ride harshness may increase with this approach. Installing a shim under a spring to raise ride height also reduces the working range of the spring, which may increase the risk of the spring bottoming. A better way to restore a good attitude is to recommend new springs.

If a vehicle is used for towing or to haul heavier than normal loads, you might also recommend a suspension upgrade such as installing variable rate springs or air springs to boost the vehicle's load carrying capacity.

With leaf springs, longer spring shackles may be installed to compensate somewhat for sag. But the best approach here is also new springs. With torsion bar suspensions, the torsion bars can usually be adjusted to compensate for sag.

For a utility vehicle application, you might also recommend installing some type of helper spring, air spring or elastomer coils that fit between the rear axle and frame to handle the extra weight and to minimize changes in ride height when the vehicle is loaded.

SPRING REPLACEMENT

General spring replacement recommendations:

If ride height is unusually low, a spring may be broken. Breakage may be the result of metal fatigue, overloading and/or corrosion.

Chassis sag may also responsible for a loss of ride height if rust has weakened a strut tower or frame rail. A vehicle in this condition may be unsafe to drive!

If an attitude problem is due to structural damage or misalignment, a trip to a frame shop or body shop may be necessary to make the required corrections.

Related items of interest:

Alignment Guide
Wheel Alignment & Suspension Diagnosis Guide

Alignment Guide is a quick reference program that covers all the basics of wheel alignment and steering/suspension inspection.
It covers basic toe, camber and caster alignment, causes of various kinds of steering and tire wear problems, and tells how to correct these conditions.




More Suspension Articles:

How To Inspect Your Car's Suspension
Diagnosing Shocks & Struts
Servicing Air Ride Suspensions
Ball Joints: Inspection & Replacement Tips

To More Technical Info Click Here to See More Carley Automotive Technical Articles

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