Checking for rear spring sag

Question:

Remedies for a sagging rear end Thad a shop replace the front coil springs in my 1954 Pontiac Star Chief a while back and that was an expensive proposition. Now the back end sags, and more on one side than the other. The car has done a lot of miles, so I know the rear springs need work or possibly even replacement. Is there anything I can do myself and save a little money?

Answer:

Usually, if your car’s back-end droops, it’s because its rear springs are tired, unless you have something very heavy in the trunk. If you have new springs in the front of your car that are too stiff causing it to sit too high, the back end will look too low. But Ican only surmise that the shop obtained the correct length and load-carrying capacity springs as the car’s originals when they replaced the front springs.

To determine whether your rear springs are sagging, measure from the bottom of the frame to the top of the axle along its centerline. If the dimension is smaller than the one specified in your shop manual, I recommend that you replace the springs. Leaf springs can be removed and re-arched, but in my experience the results will be temporary.

Also, if one rear spring is more fatigued than the other or is broken, it will cause the car to droop to one side as well. And even if only one spring is broken, you will need to replace the one on the other side of the car as well so the car will ride level and evenly, and handle properly. You must always replace springs in pairs, unless they are new. If you replace just one spring, your suspension will be fighting itself, and you will be the one who loses.

Removing and replacing leaf springs is not a particularly tricky or difficult thing to do, and you can do it with standard hand tools for the most part. Block the front wheels, then jack up the rear of the chassis and put jack stands under its frame. The back axle should now be hanging down from the frame with the tires off the ground, held in only by the rear leaf springs.

Now place your jack under the differential housing and jack up the axle just high enough to entirely support its weight. Support the axle on jack stands as well. Never let the axle hang by its brake hoses and other connections. And don’t try to shirk when it comes to jack stands. I lost a friend in high school because he got impatient and got under a car supported only by a bumper jack; and another acquaintance recently had his chest crushed when a hydraulic jack blew a seal while he was under a car working on the transmission.

Remove the nuts on the U-bolts holding the axle to the springs. Lift off the U-bolts and attaching plates. If you get the load of the car off of the springs and get the differential height just right, removing the shackle assemblies at the back will be easy. If you have problems though, use a spring spreader and spread each spring until it assumes its normal shape. Remove the pins or bolts at the shackles, then remove the spring from the car.

Make certain you order the correct springs for your particular make, model, and year of car. Whether the car has a big engine or the small one, whether it is a four-door or two-door, and whether or not it is a station wagon or convertible are all important considerations. And while you're at it, order new shackle kits too. And then when your new springs arrive, compare them carefully with your old ones to make sure they match.

Your new springs will not come with metal covers, regardless of whether your old ones had them or not. If you are a stickler for authenticity, you can put canvas boots saturated with grease on the springs, and then put your old covers back on the new springs. But if you aren't particular you can leave the covers off. Modern cars don’t have spring covers on their springs, but if my memory serves me correctly, your 1954 Pontiac does have them.

To install your new springs you may have to use a spring spreader tool, available at rental yards, to spread the spring and attach it at the front pins and back shackles.

Most of the time you can apply enough pressure to the rear axle to get the springs within reach of the rear shackle

just by using your floor jack and jack stands, but if not, use a spring spreader to stretch the spring until you can mount the shackles and pins. Replace the rubber bushings, pins and other hardware at this point too.

Once that is completed, reattach the back axle, making sure that it is correctly aligned in relation to the frame. Of it isn’t, your car will pull to one side or the other.) Typically you should tighten the U-bolts to about 50 ft.-lbs. of torque, but check a shop manual for the correct spec for your Star Chief. Also hook up the rear shocks and sway bar if your car has one.

With new springs your classic will perform much better than it did with its tired, sagging originals. Instead of hitae bottom you'll be sitting pretty.