DESIGN ANALYSIS
of AC's post-war 2 Litre Saloon (page 2)
SUSPENSION
The front suspension is where the AC comes in for most criticism, but quite unjustly. AC had perfected beam axle/leaf-spring suspension to exclude all the old troubles people associate with that system. At the same time, there were none of the unwanted side-effects that early IFS was suffering on many other cars. In other words, the AC avoided the worst of both worlds!
Writers tend to measure engineering design competence based upon the trends of the day. Most post-war cars were appearing with independent front suspension (IFS), so the AC is dismissed as inferior. In reality, the AC's front suspension deserves merit. The same cannot be said of its rear axle! But, since most other makers also fitted live rear axles, writers do not even mention the AC's biggest fault!
Had AC gone for IFS, then a much more substantial redesign of the chassis would have been needed. Much greater torsional stiffness is required to gain the advantages of IFS, which either means a weight penalty in the chassis, or a much more efficient design. Even with sufficient torsional stiffness, the localised layout of the chassis around the suspension pivot mountings can ruin the suspension's performance. There were also new problems that would have to be resolved - or more realistically, compromises made. The lower roll-centre allowed more body roll when cornering. The roll axis would be steeply inclined, assuming a beam-axle was still used at the rear end. Suspension movements affect the steering and wheel-alignment, at a time when steering-racks were not very common. So many problems to iron out, which could be worked on experimentally for possible future models.
Incidentally, some of the most competent looking IFS chassis designs of the day came from MG. I must hitch a ride in an example one day and find out how good it is :)
While the AC's suspension may get criticised for appearing outdated, it is in fact the assumptions made about it that are outdated. The AC's suspension is devoid of those once common problems of axle-tramp/wheel-wobble/shimmy. This is partly due to the transverse stiffness of the chassis referred to on the previous page. It is also helped by the transverse mounting of the inclined dampers on the front axle. The leaf-springs themselves are virtually straight under static load, and are fairly low slung relative to the axles. This latter point reduces the effect of axle torque reaction on the springs. Straighter springs provide less under or oversteer induced by body roll. The front springs did away with conventional swinging shackles. Instead, sliding shackles (or "slippers") were employed, giving a more rigid location for the springs under varying loads. The AC is also blessed with a wide track (4ft. 7in.) and this minimises the tilting of the wheels as they pass over uneven ground.
Articles and textbooks both ancient and modern (and countless websites) highlight alleged problems caused by gyroscopic effects from the rotating wheels. To a certain extent, this is a fallacy, even though a major reason for changing to IFS by most manufacturers was to reduce this 'problem'. Low speed wheel wobble was often cured by decreasing castor angle and had nothing to do with gyroscopic precession. At low or high speeds, other factors (listed above) affect the stability of the front axle. The fact that wheel wobble troubles largely disappeared in the new era of IFS cars, helped to perpetuate the misunderstanding about the causes. In fact, IFS usually helps because it controls castor more accurately, but the other reason for the improvement was the change to wider tyres and lower tyre pressures. This latter feature provided extra resistance/damping to any potential wheel wobble characteristics even if the wheel diameter remained unchanged.
An excellent assessment of this can be read in an article by L. M. Ballamy in the journal of the 750 Motor Club, January 1960, entitled "Some Thoughts on Vehicle Suspension". He had extensive practical experience of building suspension systems, and this article was mainly about swing-axles for IFS. Swing-axle IFS is of interest here because AC converted a 2 Litre Saloon to this arrangement in a simple but effective design (retaining the leaf springs). Ballamy maintained that he had never encountered any gyroscopic kicks with swing-axle IFS, as wheels strike bumps or potholes, despite experts theorising that this system should be awful! Apparently the experimental setup on the AC worked extremely well. In the 1950s, swing-axles were reputed to give the greatest road-holding. Sadly, swing-axles have been cursed with a bad reputation due to problems with many front engined cars using it for the rear suspension.
The gyroscopic effect I referred to is known as "gyroscopic precession". If you've ever played with a toy gyroscope, you will know that you can support it with a piece of string at one end, even though the gyroscope's axis is horizontal and it appears to defy gravity. Note that at the same time it will be rotating (precessing) about the string. If you stop it precessing, the gyroscope will fall. This rotation about a vertical axis, is caused by any external force that tries to rotate it about a horizontal axis. In this case, gravity is the force. For a car front wheel, a bump in the road provides the external force, and the wheel then tries to turn about the kingpin. At least that's the theory, but as mentioned above, it is rarely the true cause of steering troubles. Even if the toe-in is not set quite right on the AC, the faster it is travelling, the more stable its steering is, only being affected by bumps at lower speeds. With everything adjusted correctly, there are no problems. You will still notice slight kickback through the steering which is due to the steering offset designed to give plenty of 'feel'.
Keep in mind that it is the force (or more correctly, the turning couple set up) rather than the change in wheel camber, that causes precession. The fact that the wheel camber does change shows that the gyroscopic effects are too small to to keep the wheel steady (which would have meant at least one wheel airborn), which in turn means little or no precession? It also needs to be noted that when a wheel hits a bump (or drops into a pothole), the initial force on the wheel reverses as soon as the vertical acceleration of the wheel changes from positive to negative (or vice versa). The car needs to be going fast for gyroscopic effects to become significant, but at higher speeds, the reversal of vertical forces when riding over bumps, will occur within a timescale minute enough for the tyres to damp out. Added to this, the effect of steering offset also acts against any tendency towards gyroscopic precession. This is from the wheel striking a bump and being knocked rearwards (giving a steering kickback), or dropping into a pothole and accelerating forwards. This paragraph gives my own interpretation of the topic as applied to a front beam axle.
The AC has a relatively modest castor angle of 2.5 to 3 degrees, although this has to be related to the rolling radius of the wheel to find the castor trail at ground level (about 0.6 to 0.75 inches). Actual castor trail is reduced by the distortion of the tyre as it rolls forwards. Note that if you fit wheels/tyres of a smaller rolling radius, castor trail will reduce and steering offset will increase.
It is often claimed that IFS reduces unsprung mass, but I am not convinced! Most of the unsprung mass is made up of the wheels, brake assemblies, and stub axles. In case you're wondering, a low ratio of unsprung mass to sprung mass (i.e. the chassis/body) helps to keep the tyres firmly on an uneven road. Thus road-holding on bumpy roads is helped. It can be a disadvantage to reduce unsprung mass too much, because (depending upon tyre choice) it can give a harsher ride as the tyre fails to soak up road irregularities.
One noticeable drawback of retaining front beam axle/leaf-springs is that the distance between the front springs is limited due to steering lock clearance for the wheels. With springs close together, they need to be stiffer than the rear ones to provide roll resistance and the required effective spring rate at each wheel, for single wheel bumps. This makes the ride harsh if both front wheels strike a bump or ramp. It also promotes pitching of the body on some road surfaces.
So how does the AC achieve a comfortable ride, despite not being 'soft'? Quite a large outside diameter to the tyres (although a common sort of size in its day) reduces road shocks compared to cars with smaller wheels. This is an issue overlooked by all the textbooks I've read. The rearward slope of the front leaf-springs also helps marginally, although the reason for the slope was to achieve the required castor angle. The AC's long wheelbase and wide track also makes the ride less choppy.
Using a live rear axle may have been common for years after the AC 2 Litre was phased out, but it is still one of its worst features! Firstly the high unsprung mass. Secondly, the mass of the differential in the centre. This acts like an imaginary pivot, because of its inertia. When, say, the right-hand rear wheel strikes a bump in the road and rises, the mass of the differential is reluctant to move. The actual 'pivot' point will be somewhere between the differential and the left-hand wheel. Therefore, that left-hand wheel will be pressed downwards and loaded more heavily. Not a bad thing, one might think? Until, that is, the right-hand wheel descends the other side of the bump, becomes more lightly loaded, and the reaction on the left wheel is reversed. The left wheel is thus unloaded at the same time as the right wheel and roadholding is reduced for a moment. If one is cornering, then the rear end may hop to one side.
Another drawback of the live rear axle is the indirect effect it has on the front axle design. The rear wheels have to be upright (i.e no wheel camber), and this limits the choice of front wheel camber. Positive camber of the front wheels was partly a compromise to reduce steering offset (distance between centre of tyre/road contact patch and the point at which the king-pin axis meets the road) without an excessive king-pin inclination. This front camber was also needed to help provide understeer, since there was no option to apply negative camber to the rear wheels.
Ideally, a De Dion arrangement would have been much better. A dead beam axle would solve the above problems, and possibly permit greater axle to chassis clearance. However, given a fairly smooth road, the road-holding of the AC is far better than most people expect.
