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, 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 independent front suspension (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While the AC's suspension may get criticised for appearing outdated, it is in fact the assumptions made about it that are outdated.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 and 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. 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.
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.
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.
Rear Axle/Suspension
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.
