MECHANICAL OVERHAUL

of AC's post-war 2 Litre Saloon

Front Axle


Accessibility to the front axle is considerably easier than for the rear axle, and it is also a much simpler task to remove/change seals and bearings. At the time of writing (2017), the wheel bearings have been discontunued, and old stocks held around the world are extremely expensive.

Dismantling

With the chassis safely supported by stands, remove wheels and brake-drums. The drums are held on by a pair of countersunk screws. If removing the entire axle, dismantle the brakes, (being careful not to injure yourself while pulling the spring-loaded shoes off). Use a suitable solvent cleaner to remove brake-dust safely. The hubs have screw-on end caps. Remove these to reveal the slotted nuts that hold the hubs on and also adjust the bearings. Remove the split-pins and nuts, and the outer bearings (inner cones plus races) will come away, then the hubs will pull off by hand (bringing the inner bearings with them). Remove brake backplates and the dampers.



The ball-joints on the drag-link and track-rod can be removed with a suitable puller. The lefthand steering arm has both rods connected to it, close together, and my puller would not fit onto the lefthand track-rod end. Therefore, I unscrewed the track-rod from the ball-joint and waited until the stub axle was off the car, and tackled it with a hub-puller. The original style of ball-joints have leather washers rather than the more modern rubber boot. I had to split the leather washers and remove them, before the puller would fit.

To remove the stub axles, remove the nuts from the kingpin cotters, tap them out, and then tap the kingpins out.

To remove the axle beam, undo the self-locking nuts on the U-bolts.

Checking and Testing

I like to strip off the paint before doing any further work. Knowing AC, the original coating is likely to be bitumen, which can be scraped off, and/or dissolved with solvent cleaners. The axle beam can then be checked to see if it is true. Kingpin inclination should be 7.5 degrees, and needless to say, the kingpins should be parallel to each each other when viewed in side elevation. Check that the pads for the springs are in the same plane.

I would recommend crack testing the stub axles. I've heard reports of pre-war ACs having cracked stub axles, which are a similar design (slightly different). I also crack tested the hubs on my AC. You can get dye penetrant crack testing kits. The parts to be tested have to be very clean, and the kit includes a solvent cleaner for the final clean up. Then spray on the dye and leave for about 20 minutes or so. Then wipe off the excess with tissue paper moistened with the solvent cleaner, and spray on the developer. Leave for a few minutes until the dye turns almost white, and any cracks should become visible. If they fail the test, reach for some tissues - for your eyes! Then seek out some secondhand replacements that pass the test, unless new ones become available.



Kingpin Bushes

The kingpin bushes are of phosphor-bronze lined soft steel, with a split down one side. Before removing the old ones, you need to consider how the new ones will be finished. After installation, they have to be either reamed or honed to size, and my period technical books give conflicting information as to which method to use. In fact both will work, but for reaming, you need a pilot-reamer. For pilot reaming, you will need to remove and replace just one bush to start with, and use the other old bush as a guide. Then do the same for the second bush. My own approach was to make my own honing tool from a steel pipe wrapped with emery cloth, that was a tight fit in the new bushes. I wrapped smooth tape around part of the pipe to allow the other bush to act as a guide. It is more laborious, but I don't have access to special reamers. I'm not sure if adjustable reamers give a satisfactory finish? The kingpin should be a snug sliding fit, with no slack at all. Aim for perfection!





As to removing the old bushes, if you are lucky, you will be able to press them out. Failing that, you can hacksaw through opposite the slit, being very careful not to damage the axle stub. I had obtained new bushes from AC at Thames Ditton back in the 1980s (along with new kingpins, cotter-pins and thrust washers), but they resemble many pre-war car bushes, so they might be available from other sources? My new bushes needed cleaning up with a needle file along the slit, and in fact I filed out the slit a little before the bush would fit. Otherwise, I found that the outer steel part got gouged when trying to press it in. Be careful to fit them the right way up and around. The lubricating holes have to line up. The inside of the bushes have channels for the grease, and these feed to one end of each bush, which should direct grease towards the axle and thrust washer. The lower bush needs to project upwards far enough to accommodate the phoshor-bronze washer.

Test fit the hub assembly on the axle to be sure that it doesn't bind at any point as it swivels. A tiny amount of vertical movement is permitted, but felt and/or metal washers can be added if required.

Oil Seals

A simple felt washer seals in the oil for the front hub. The old one on my AC was 6mm thick, but since they compress in use, I made a new seal 8mm thick. The part of the hub that the washer fits over, is just under 2 inches in diameter, at 1 31/32 inch, so I made the ID of the washer a tiny bit smaller. Outside diameter is 2 7/8 inches. You need to obtain the correct type of felt, made largely of wool (rather than synthetic). I bought my felt from Stationary Engine Parts. Before installation, the felt seal needs to be soaked in grease.



Reassembly

You can tell which way around the axle beam should go, by test fitting a cotter-pin next to the kingpin boss. It fits in from the front side of the axle. Fit new self-locking nuts to the U-bolts, and ideally new U-bolts too. Don't forget the aluminium plates that are sandwiched between springs and axle. To stop the axle weight pulling the springs (almost) out of the slippers, refit the shock-absorbers, or else support the axle weight. Grease the kingpins and thrust washers (don't forget the phosphor-bronze washers too), and mount the stub axes. Make sure that the flat on each kingpin is aligned for the cotter-pin to go in, then tap the cotter-pin in with a hammer and tighten up the nut. Fit the dust caps on top of the kingpins. They should press onto the stud, but the stud size seems to vary with different kingpins, so you might need to adjust the cover hole size.



Bolt the backplates on (high tensile bolts, spring washers and full nuts).

Adjusting Wheel Bearings

If the bearings are in good condition, then scoop out the old grease and repack them with fresh grease, but don't get carried away with the quantity. Test fit the hub without the seal. Check that the inner bearing is snug on the stub axle, otherwise a suitable adhesive might be required to prevent it from slipping. With the hub in place (without seal), fit the outer bearing, washers and adjusting (slotted) nut, and adjust the nut to give the specified bearing clearance. AC stated this to be about 0.003 inch (0.08mm). With a splitpin to insert, each sixth of a turn equates to almost 0.012 inch, so if you are unlucky, you might not get to within 0.006 inch of the desired clearance. You could add a washer, and sand it down as required. I measured with a vernier caliper between the back of the hub and the backplate. I noted the position of the nut, and then removed the hub, fitted the grease-soaked felt washer, then refitted the hub. The new felt is a tight fit, and makes it hard to feel the bearing tightness, hence that pre-fit dry run. I measured again by pressing the hub against the seal. With the wheel fitted, it magnifies the clearance if you rock the wheel vertically.

Don't forget to fit a new split-pin on the hub. That is all that stands between a happy drive and your wheel coming loose! Replace the end cover tightly too, as this acts as a back-up if the nut did loosen.


Ball Joints

Interesting little chaps, steering ball-joints! These seem to last a very long time if greased regularly, and if the seal is intact. The seal on the originals is made from leather, in the form of a washer. It is 5/16 inch thick, with an ID=7/8 inch and OD=1.5 inch. Unfortunately, I had to split the leather seals when extracting the joints with a purpose-made extractor. As a replacement seal, I decided to use the off-cuts from the 8mm felt used for the hub seals. I'll have to report back in the future on how effective these prove to be. A later replacement ball-joint, I fitted to my drop-arm, and purchased it from AC at Thames Ditton. It features the more modern rubber bellows to contain the grease. The old one had lost its leather seal, hence its premature failure. I presume that the leather seal provided some degree of friction damping, so hopefully my felt washers will achieve the same effect. And presumably the modern rubber boot version loses that damping characteristic?



Toe-in and other checks

With the trackrod refitted, you will need to adjust the toe-in. This is specified at 3/16 inch measured at the wheel-rims. With the axle hanging low, you can get a tape-measure under the engine sump. Make sure the wheels are running true and that the steering is exactly in the dead-ahead position. Fine tuning of the toe-in can be done when road testing the car. If the steering wanders and tramlines badly, then it is worth adjusting, turning the trackrod a quarter of a turn at a time (then tightening the clamps), and then road testing on bumpy roads (low speeds). The steering should remain straight even with hands off the steering wheel.

I have an angle meter, so (after the wheels were back on) I did a quick check of wheel camber (2.5 degrees), kingpin inclination (7.5 degrees) and castor angle (2.5 to 3 degrees), all of which matched the specifications.





Further Reading: Theory

If you've ever wondered about kingpin inclination, wheel camber, castor angle, toe-in, and steering offset, then these have to be considered how they work together as well as individually. Firstly, a point overlooked by most articles is that as a tyre rolls forwards, its contact patch with the road shifts forwards, relative to the axle. This affects the castor action of the wheels. Overall, I don't think it causes negative castor, since the width of the tyre's contact patch, creates a self-centring torque.

Steering offset: The axis of the kingpin, if extended down to the road, may or may not intersect the centre of tyre contact at road level. If it meets the road to the inside of this point, then the distance between is the steering offset. A large offset would mean that under heavy braking, the steering arms, trackrod and joints would be put under a lot of stress. Bumps in the road would also cause kickback, giving the steeringbox a hard time, as well as the driver's hands. Steering would also be heavy. The AC has a small amount of positive steering offset, (about 0.95 inch) which provides some feedback through the steering wheel. Combined with kingpin inclination, it also provides some castor action to self centre the steering. Front wheel drive cars (with transverse engines) normally have negative steering offset. This is so that the offset at axle level is zero, to reduce torque-steer when accelerating. It also helps braking stability if one front wheel slides - at least for drivers who have no idea what "feedback" means!

Wheel camber: Positive camber of a wheel means that it leans outwards. One of the reasons for this, is that with a live rear axle, the rear wheels have to be parallel with each other, and so the front wheels have to help provide some degree of understeer. It also helps to reduce steering offset without excessive kingpin inclination. Too much camber can cause an anti-castor effect. This is because a cambered wheel is trying to roll along a circular path, and if this radius is smaller than the path you are trying to steer along, it may try to steer more tightly. Castor angle causes camber to increase on the inner wheel, when cornering, and reduce on the outer wheel. Camber on the AC's front wheels is 2.5 degrees positive.

Kingpin inclination: The kingpins lean inwards to reduce the steering offset. They also produce a self-centring effect for the steering, provided that there is some positive steering offset. In such cases, when the steering is turned from the straight ahead position, the axle beam, and the front of the car, is lifted slightly, its weight then tending to straighten the steering. On the AC, kingpin inclination is 7.5 degrees.

Toe-in: The positive camber of the wheels makes them try to steer outwards. Apart from wear of the tyres, this can make the steering wander, as a slight anti-castor action can apply if the steering is deflected slightly from the straight ahead. To correct this, the wheels are aligned with a slight toe-in (3/16 inch at the rim, for the AC), which steers them back on course. It also affects the side loading on the tyre walls, since pre-loading the walls influences how quickly the tyre reacts to a change of direction. Toe-in also compensates for any elastic deflection in the rods and joints, etc. as the car moves forwards, but that was more of an issue for older cars with adjustable spring-loaded trackrod ends.

Castor angle and trail: To create a sufficient self centring effect for the steering, the kingpins may be inclined (viewed from the side). This brings the kingpin axis forwards where it meets the ground, so that it is ahead of the tyre contact patch centre. This distance is called the castor trail, while the angle of the kingpins (in side elevation) is called the castor angle. Too large a castor angle can lead to severe wheel wobble at critical speeds, because turning the steering from the ahead position, lowers one end of the axle and raises the other. Wider and softer tyres can help to hide such problems.

Ackerman steering: Ackerman geometry for the steering of the two front wheels, aims to keep the axis of every wheel aligned with the radius of the car's curved path, when cornering. It doesn't quite work in practice, because of the slip angles of the tyres means that the wheels don't travel in the direction they are pointing, while negotiating curves. Also, the steering linkages would be too complex if they attempted to follow this geometry perfectly.

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Website started 29th December 2006