MECHANICAL OVERHAUL
of AC's post-war 2 Litre SaloonFront Springs
These are a little different from the average leaf-spring you might meet on other vehicles. Its forward fixing is a conventional gun-metal bush, while its rear end slides in graphite slippers. That means that the spring's span remains unchanged as it bends. The slippers are free to rotate in the slipper housing. There is a steel disc either side of the slippers to take any side thrust from the spring. The axle is attached closer to the rear end than the front end of the spring, which (I presume) helps to keep the castor angle constant under heavy braking (with axle wind-up). That also explains why the castor angle is specified across a range: 2.5 to 3 degrees.
There were at least 2 versions of this spring. The original version features short rebound leaves above the main leaf, and has 5 bent-over type clips. The later version has the main leaf as the top leaf and only has 4 bolted clips.
Leafspring removal
Thankfully, this is much easier to do than the rear springs and even simpler if the axle has been removed for attention. With the chassis supported safely on stands (and the axle supported separately, if still on the car), the U-bolts should be removed from the axle. The pivot bolt at the front of the spring is a tight fit in the chassis, but should come out with a soft drift and hammer. Then the side-plate on the slipper can be unbolted (bringing with it a side-thrust disc and possibly some brass shims), and the rear of the spring should come out sideways. Alternatively, you can slide the spring forwards to release it from the slipper.
The spring might refuse to pull out of the front mounting if the second leaf has shifted sideways, so a bit of persuasion with clamps should line them up.
Bushes
The bushes on my springs were still a perfect fit on the pivot-bolts. I understand that it takes quite a bit of force to press new bushes in, and they require reaming to size after fitting (due to slight compression). The bushes are 2 inches (50.8mm) long, but after installation, they - along with the end of the spring - are ground to between 1.980 and 1.990 inches (50.29 to 50.55mm). The outside diameter of the bush is 11/16 inch (17.46mm). Inside diameter (after installing and reaming) is 0.5 inch with a tolerance of -0.0005 to -0.0015 (12.70mm, -0.01 to -0.04).
I measured the width of the front mounting (on the chassis) as 0.032 inch (0.8mm) wider than the spring-eye. I don't have any data on what this clearance should be, other than some general (not AC) figures that are slightly smaller. There were no side washers fitted to mine, but adding these is an option if the clearance seems too large.
Slippers
The rear end of the spring (main leaf and the leaf below it) is ground to the same width as the front end, as detailed above. The graphite slippers resemble a solid cylinder that has been sliced in two lengthways, but reduced in thickness to accommodate the spring. The diameter of the cylinder, I measured to be 37.6mm (1.48 inches), which equals the diameter of the steel side-thrust discs. The thickness of each slipper is approx. 14.0 to 14.5mm - the sum of those thicknesses plus the spring, adding up to 37.6mm.
Above is the slipper housing with parts removed. Below are the slippers and side-thrust discs.
A rubber gaiter keeps the dirt and water out, and the grease in. At least, it is supposed to! In practice, after applying grease every 500 miles, the gaiter sags under the weight of excess grease. I would recommend fabricating an alternative design of cover, that will permit excess grease to oose out and be wiped off during servicing. Nitrile rubber is ideal for contact with grease, but use a thin sheet since nitrile is less flexible than other rubbers.
One of my slippers had several brass shims under the side cover, while the other slipper had none. If fitting new springs, you might need to add or remove shims for an accurate sliding fit for the spring between the side-thrust discs.
Camber
The early type of spring has a free camber of 3 7/8 inches (98.4mm). The later type of spring has a figure of 3 1/2 inches (88.9mm). Mine were the later type (apparently installed by AC circa 1960), but I measured the camber as 3 5/8 inch. It is possible that there was another camber figure for the later spring design? If your springs have less camber than specified, then it is time to get some professional help to reset them.
Camber is measured from the top of the main leaf (level with the centre bolt), to a referance line that passes through the front pivot-eye, and a point 1.5 inches (38.1mm) from the rear end of the main leaf.
Dismantling
If your springs have bolted clips, then dismantling is easy. If the clips are the bent-over type, then it might be simpler to get the springs professionally serviced? Ideally, the bent-over clips should be renewed. I dismantled my later type springs by G-clamping them before releasing the centre bolt. In fact, this spring does not expand much once released. Make sure that the leaves are kept the same way round by making a mark at one end of each (not too sharp or deep a mark, as you don't want to start a crack!).
The next task is to scrape and sand off the accumulated debris and rust until you have shiny metal surfaces. The tips of each leaf may dig into the leaf above, so file off any sharp edges at the tips. Any cracks might show up while degreasing with solvent, but if in doubt, run a crack test with a suitable dye kit.
Painting
There seems to be a common practice of painting spring leaves in their entirety. This seems odd, to paint working surfaces which would grind off the paint and contaminate the grease (unless you use slip-paint designed for this purpose). The grease should provide the required rust protection. I painted the exposed parts of the spring (prior to assembly) with epoxy-mastic.
Lubrication and Reassembly
The lubrication requirements for leaf-springs varies between different makes/models of vehicles. AC specify regular lubrication, and this would be normal practice for a passenger car with effective hydraulic damping. Although the friction doesn't affect the spring-rate, it does affect ride comfort, by controllng the level of friction damping. And not forgetting the level of static friction which might create a jiggly ride if the springs are dry and rusty.
My old design and maintenance books provide no clues for the best form of lubrication, and so I opted to use graphite grease (or grease with graphite powder added), spread over all the working surfaces.
Some of the AC handbooks say that the springs were wrapped after greasing, while the other handbook says springs need regular spraying with oil. Wrapping the springs might work well if they are not neglected for years afterwards. I prefer to leave them unwrapped and lubricate once a year, by working oil in while all weight has been lifted from the spring (allowing the leaves to spread open slightly). At one time, you could get special tools for prising the leaves apart to assist with getting the oil in. If you don't have one of these antique tools to hand, then find something else to prise open the leaves, with the clip bolts removed from the spring.
When reassembling the spring, make sure that the 2 main leaves are aligned (laterally) by clamping them before finally tightening the centre bolt.
Refitting
Slide a new gaiter over the rear end before spring installation, unless you plan to omit the gaiter and make some form of dust cover. Ideally, new U-bolts should be fitted, but that assumes that you can find any of this size?
The side-thrust discs for the slippers should have their bevelled sides facing away from the spring. Fit the first disc and then the lower slipper (coating all slipper parts with graphite grease). Insert the spring into the front of the chassis and swing its rear onto the slipper. Then slide in the upper slipper. Refit the side plate with side-thrust disc (and shims if needed). The front pivot-bolt can then be tapped in (fitted with slotted nut and split-pin) and the U-bolts refitted (with self-locking nuts). Check that the spring slides freely in the slippers. The photo below shows the end of the spring between the slippers when the axle had been remove, so there was no weight pulling the spring down. Once the complete axle and wheels have been added, this weight pulls the spring further out of the slippers, and they might become skewed and block the spring's travel. The shock absorbers normally prevent this from occurring, so if they are to hand, fit them before the axle is bolted back on. Alternatively, remove the side plate of the slipper and check that all is well as the car's weight is put back on the springs. A large screwdriver can be used to hold the slippers in place as the car is slowly lowered on its jack.
Normally, you would fit a new rubber gaiter prior to installing the spring, but I decided to experiment with alternative rubber covers. The above photo shows the spring reinstalled with the upper slipper just visible. Below is my attempt at a nitrile-rubber boot. It's a slack fit around the slipper housing, so that old grease can escape while pumping in fresh grease. It is a flat sheet that has been folded and held in shape with insulation-tape. The only testing I've done on this is jacking up the chassis, and also bouncing the suspension! I'll update what happens once back on the road. If you come up with something better, please let me know.
Theory - in case you're interested...
The leaf (or laminated) spring is based upon the theory of a freely supported beam with constant bending moments (and thus constant stress and radius of curvature) along its length. To achieve that in a single sheet of metal, would require it to be rombus shaped. If you slice up the rombus and stack up the strips, you end up with a leaf spring, with spear-shaped tips to each leaf. A variety of designs for the leaf tips were tried, before plain ends were standardised.
Prior to assembly, the leaves (or "plates") have different radii, the shortest leaf being the most tightly curved. This is to make sure the leaves remain tight together without having to perfectly match the curvature of each leaf.
Clips are fitted to keep the leaves together during severe rebound, and also preventing them from swivelling about the centre bolt.
Shorter plates might be fitted above the main plate, known as damper plates or rebound plates. These reinforce the main plate during severe rebound, since if the plates spread apart like a fan, the reverse bending load would otherwise be placed solely on the main plate. The AC's rear springs have an unusual flat damper plate. Since its natural shape is flat, it reduces the spring rate during positive camber, but increases it for negative camber. Or in other words, a progressive spring rate, in addition to a leaf-spring's normal variation in rate.
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