Time was when pretty much the only modification hot rodders made to their rear suspension was to add longer shackles to lower the rear on a transverse leaf-sprung Ford, or throw a set of lowering blocks between the axle and leaf springs on a Chevy or other car with parallel springs.
Things are a lot more specialized these days, and though lowering blocks and long shackles still have their place, the aftermarket now offers bolt-in and weld-in kits for everything from relatively simple upgraded leaf springs to complete IRS systems. Space restrictions mean we'll bump IRS systems for now and concentrate on live axles; or, more specifically, how that axle is located in the car. Spring rates and the benefits and disadvantages of coilovers versus leaf springs versus air springs is also another subject for another time. However, before we delve into the various types of rear suspension, there are a couple of important subjects we should first mention-scrub line and pinion angle.
Imagine the consequences if this tire were to suffer a blowout at speed. And we don't mean the spider webs would blow away!
Scrub Line
Imagine your car-any car-sitting on the street without any tires. The wheels are still there, just without the rubber. Is anything touching the ground? Suspension parts, any brackets, exhaust system, fuel tank, transmission, or motor even? If so, they are below what is called the scrub line, and while you may not worry too much if those massive 6-inch lowering blocks meant the nuts on the U-bolts are now halfway down the sidewall of your rear tire, you'll be in for a nasty surprise if you get a flat on that tire at speed. Those U-bolts will make a nice gouge in the blacktop, and you'll be fighting to stay in control of the car ... if you're lucky. You'll more likely be in the ditch or facing oncoming traffic. Tall tires don't help either, but ensure no part of your undercarriage hangs down below an imaginary line drawn between the bottom of all four rims. In fact, an inch of clearance would be a good idea.
Speedway Motors, among other suppliers, sells these wedge shims to fit between the axle and lowering block to adjust the pinion angle if need be, though obviously setting the angle before welding the spring pads to the housing would be preferable.
Pinion Angle
We'll now open one of our favorite cans of worms, as whichever expert you consult will give you a different answer, depending on their particular automotive area of expertise. The pinion angle is the angle of the flange on the front of the rearend (the pinion) in relation to the angle of the driveshaft. That everyone will agree on. Well, most everyone anyway. It's how you measure that angle that causes disagreement.
But first you should understand why pinion angle is important. As power is applied, and the drivetrain is under load, the rearend tries to rotate around itself, meaning the pinion will rise. The goal is to create a straight line from the back of the crankshaft through the transmission, driveshaft, and the pinion of the differential, under load, so to achieve this under power, the pinion has to be angled slightly downward from the driveshaft when the vehicle is at rest. The awkward part is determining that angle. Different suspension setups will require different angles, though parallel leaf springs will require the most, owing to their tendency to "wind up." While all this may seem overly scientific, and the stuff of drag race chassis tuners, it is important on street cars, to eliminate driveline vibrations or ultimately component failure, and to ensure the longevity of your universal joints and bearings.
So what's the ultimate angle then? How long is a piece of string? Classic Performance Parts suggests mounting the engine and trans at a 4-degree-rearward angle, so the carburetor flange is parallel with the ground, then setting the pinion at 2-3 degrees upward from the ground, so that under load it will move upward, aligning it with the driveshaft and making the pinion angle 1-2 degrees. Currie Enterprises, who has manufactured more rearends than anyone else for the hot rod market, suggests a 1-3 degree angle between the transmission tailshaft and the driveshaft, and an equal angle between the driveshaft and the pinion. The centerline of the tailshaft and the centerline of the pinion shaft will be parallel, the angles mentioned above allowing the driveshaft to run between the two components using universal joints, which have a safe operating range of between 1 and 3 degrees. Obviously, zero degrees is the optimum, but U-joints work at their best when running through a slight angle, hence the 1-3 degree measurement.
If this is confusing, take a look at Inland Empire Driveline's Web site (www.iedls.com) under powertrain setup guides, where an excellent explanation with diagrams will clear up everything.
This '46-48 Ford rearend uses a transverse leaf spring and a torque tube for location. You can clearly see the solid driveshaft inside the half "ball" at the forward end of the torque tube, which is enclosed in a matching "cup" at the rear of the transmission.
Location, Location, Location
As with real estate, location is important, and by location we mean how your live axle is attached to the chassis. And there are plenty of options from which to choose: ladder bars, trailing arms, three-links, four-bars, four-links, triangulated four-links, and parallel leaf springs. Then there are torque tube-equipped axles. We'll deal with each application in turn.
Torque Tube / Transverse Leaf Springs
This style of suspension, the staple of early Fords up through 1948, is located using a cup-and-ball arrangement at the forward end of the torque tube, and on early Fords, apart from the spring and shock absorbers, that's the sole method of locating the axle. The spring limits any sideways movement, with wishbones (radius arms) triangulating the system, which bolt to each end of the axle casing, and to the torque tube toward the front.
If you're using an axle so equipped, you'll most likely be sticking with the stock system of a transverse spring. Even with such ancient technology, POSIES can supply new springs in varying rates.
POSIES Super Slide Springs feature a domed pocket at the end of each spring leaf that houses a moly-nylon button.
Torque Tube / Parallel
Leaf SpringsOn an early Chevy, the torque tube also locates the axle at its forward end, but it uses parallel leaf springs instead of a transverse spring. The axle obviously travels in an arc about the front of the torque tube, as the axle and tube are a single assembly with no universal joint between them, which necessitates pivoting mounts between the axle and the springs. The clamp pins on the leaf springs are 1 inch ahead of the axle centerline, meaning it will sit too far forward in the wheel openings if you want to upgrade the axle and simply drop the new one on the original leaf. You have two options in this instance. POSIES offers new leaf springs with the clamp pin on the axle centerline that is designed to work with the stock perches, or Fatman Fabrications has a kit that lowers the car at the same time as mounting the axle in the correct place.
Parallel Leaf Springs
Parallel leaf springs are probably the most common type of rear suspension for '50s- and '60s-era cars. As the name implies, two leaf springs are mounted longitudinally in the car, one supporting each side of the axle. The front of the spring is mounted directly to bracketry on the chassis or unibody of the vehicle, while the rear of the spring is mounted on shackles that usually hang down from the chassis, but which can also be mounted so the spring eye is above the shackle bracket, as on early Chevys. Why the shackles? Because the rear spring mount has to be able to move, as the spring will increase slightly in length as the suspension compresses. Because the springs are mounted fore and aft of the axle, and in four places, there's very little sideways movement of the axle, though antisway bars may be fitted to reduce body roll.
The Fatman Fabrications kit for early Chevys includes an antisway bar and mounting hardware, lowering blocks with the repositioned clamp pin (not those shown), U-bolts, lower plate, and new shocks, which mount in the stock upper location. | 
Chassis Engineering offers complete leaf spring kits to fit a huge variety of cars. |  |
If you're swapping leaf-sprung axles or just want to lower a car already so equipped, Speedway Motors and Summit Racing stock a number of relevant parts, such as weld-on spring pads, lowering blocks in various sizes, and longer U-bolts. | | |