Structural Corrosion - Safe Repairs
R H Johnston

Some articles on the A30 design taken from the Automobile Engineer (see Further Reading at the end of this article), have made me concerned about the structural integrity of some A30/A35s presently on the road. The A30/A35 is a tough car and maintains its structural integrity even when severely corroded, but this strength should not be taken for granted. If your car creaks, groans or distorts when jacked up at the front corner, then it is in poor structural condition and needs repair. Even if you do not repair the car yourself, attention to the points made in this article should help you to be ensure that any work you have done is done safely. When buying a car the same jacking test may help to reveal weakness or indicate that unsatisfactory repairs have been made, perhaps with body filler rather than steel. The original design philosophy (outlined in the article from Automobile Engineer) and the calculations (see Further Reading at the end of this article) suggest that some cars that have been repaired for, and passed, the MOT test are potentially dangerous.

Some Bad Cases.

A common fault is to fit new outer sills without remaking the small box section which goes forward under the front wing bottom. This box section distributes the load from the sill into the panels which form the side wall of the footwell and the inner wheel arch of the front wheel. Without it, there is a concentration of stress in the front bottom corner of the door opening which could lead to the floor cracking across from side to side. Also, without the sealing of the front of the sill, water can enter the sill section causing rapid corrosion.

One two door saloon had new outer and inner sills. The welding was good, genuine original manufacturer's sills with the forward sections that go under the front wing bottoms were used and correctly fitted with one exception. The sill repair section ends at the rear of the door, and the outer sill was not attached to the panel to the rear of the door, but had a quarter inch gap. (The original is spot welded and leaded over on the saloon.) The sill is designed to act as a box section, but without this joint to ensure continuity it is three quarters sawn through and most of its strength is lost. The gap, at the bottom rear corner of the door was at a particularly bad place because it was at a point of stress concentration. Only the single sheet of the inner sill and the floor pan were holding the car together. In time a fatigue crack might develop and run across the floor of the car. This car had passed MOT tests at two different testing stations.

Structural repairs must be in steel and properly welded on. Sometimes pop-rivets have been used, or even glassfibre outer sills. Such a car is a death trap.

Design Principles Affecting the Repair of A30/A35s

As explained in the Autmoobile Engineer article, the A30/A35 body was designed by T K Garrett, an aeronautical engineer, and was the first chassisless mass-production car. It was designed using proper estimates of the loads that the structure would have to bear. This means that the body (except for the detachable front wings and roof, which was ignored for design purposes) was designed to act as the structural strength of the car. This fully stressed body made the car very light, and very stiff, so that there is very little twisting of the body on rough roads. Although the contribution of the strength of the roof was ignored in the design, it would be unwise to remove the roof without extra strengthening of the floor pan, because the design calculations were necessarily approximate and the cars were proved by testing in arduous conditions.

The implications of this design philosophy is that the detail design, particularly of the lower part of the car is not accidental. When repairing, care should therefore be taken to ensure that repair panels are the correct thickness, those which are supposed to be flat are without buckles, as buckling can lead to "oilcanning", which can be noisy and cause metal fatigue. Where there are multiple thicknesses of metal, or shaped sections or box sections, repairs must reproduce these adequately. If this is not done, the part may fail in service, either dramatically, or more probably by the development of potentially catastrophic fatigue cracks.

Some Places Where Special Care in Repair is Needed.  

The following gives some places where care is needed in repair. It is not exhaustive. Most of the body was made of 20 swg steel, though some 18, 16 and 14 swg steel was used for suspension mountings and other critical areas, as explained in the article.  

Front valance and front panel This is an area which rusts, particularly in the outer corners of the panel each side of the radiator grill and on the valance on the underside where it joins the two sections which come forward underneath on each side of the engine. (The suspension lower wishbones are mounted on these sections each side of the engine). This part of the body takes the lateral forces on the front of the car together, and ensures that the steering is precise. The corrosion in the valance is particularly common and is often ignored or filled in with body filler. The connection between the front panels and the inner wheel arch is also important to provide stiffness.  

The sections each side of the engine These are the sections which hold the front suspension lower wishbones. Corrosion in this area is unusual, as it is saved by the oil mist from the engine. The only frequent problem is buckling in the region where the holes for the brake and clutch pedals are located, caused by a front end accident. Severe distortion can lead to a tendency to steer off course and difficulty in centralising the tight spot of the steering box. (Lack of centrality (but not pulling steering) can be remedied using the adjustable A40 farina drag link in place of the right hand drag link). Professional body-jacking equipment is required to remedy this distortion.  

Sills I have already discussed some typical problems. The inner sill should be joined to the side panel of the footwell to give a two layer thickness with the latter and joined both to the floor and the side wall. The floor in the corner and the inner wheel arch often rusts here and these should be repaired if needed. The small box below the floor was used to support the body during assembly and is not designed as a jacking point, and should never be used as such. It is of no structural significance. On the outer side of the sided panel of the footwell, the outer sill has a tapered box section behind the wing bottom joining the wheel arch and the side wall which must be welded to both of these. There was a rubber strip attached to this tapered box, which was intended to keep water out, but actually holds water and mud and causes corrosion of the front wing bottom and the tapered box. This rubber strip on the tapered box should be removed to allow water to escape. Mud in the bottom of the wing should be cleaned out periodically to avoid corrosion. The jacking point in the middle of the sill is not a good feature, and its use is not advisable, as the inner strenthening is prone to corrosion.  

The outer sill should be properly welded to the inner sill along its length and attached to the panel of the rear window (2 door cars) and to the lower reveal of the rear door (4 door cars). At the rear end in both cases, the inner sills are multiple thickness and involve some complex shaping to give extra strength in this vital area near the front rear-spring hanger. These complexities need to be reproduced so far as is practicable. Where reconstruction of a very rusty car is contemplated, an original non-rusty car should be examined to see the form of construction - it is too complicated to explain in detail here.  

Inner rear wheel arch, rear spring hangers and damper mounting points This is an area which is prone to rust. The construction of the rear-spring hangers is complicated and should be reproduced as exactly as possible, as these sections are designed to take the load up into the body work. Holes can develop in the front and rear corners of the rear boot floor and these should be repaired with steel as these areas contribute to the strength of the spring hangers. The inner wheel arch often fails where it joins the sills: this should be repaired with steel as this area transfers the load from the sills to the inner wheel arch and hence to the spring hangers. Leaks into the car can result in water settling under the padding on the inner wheel arch which can then become dangerously thinned by corrosion. The damper mounting points can also fail, and the double thickness of steel must be reproduced, together with the flanges.  

Vans - sides The upper and lower panels of the van sides suffer corrosion at the seam. This is not only unsightly but if extensive, it must be repaired with steel to ensure rigidity of the body.  

Further Reading  

Compared with the owners of most cars, we have the advantage that some of the actual design calculations for the A30 were also published in "The Automotive Engineer". The full papers are too technical for republication in Spotlight, but for those who are interested they are:  

"Austin '7' Structural Design: A Body-chassis Structure Unique Among Quantity Produced Cars", (Anon), Automobile Engineer, December 1952, pages 3-8. (A non-mathematical review of the A30 design)  

"Automobile Dynamic Loads: Some Factors Applicable to Design", by T K Garrett, Automobile Engineer, Feb 1953, pages 60-64. (This paper gives the loads to be considered in the A30 design calculations. There is some mathematics, but the ideas can be grasped without it - it is not necessary for those only wanting information on A30 structure.)  

"Structural Design. Part 1: An Analytical Method for Chassisless Vehicle Design", by T K Garrett, Automotive Engineer, March 1953, pages 103-111.  

"Structural Design. Part 2: The Front End Structure", by T K Garrett, Automotive Engineer, April 1953, pages 152-157. (This pair of papers gives some of the detailed design calculations used for the A30 design. These papers are more difficult, involving stress calculations, but the conclusions and drawings are useful for repairers)  

All material on this site is copyright, with all rights reserved
(c) R H Johnston, 1998.  Material may not be published for profit without permission.

Whilst great care is taken to ensure that the technical information and advice offered is of the highest standard, neither the author, nor The Austin A30/A35 Owners Club, or the Officers of the Austin A30/A35 Club accept any liability at law for any death, injury or loss whatsoever arising from the use of advice contained in this article.