AIR SPRINGS FOR MY BN1

Ever since restoring my very early BN1 the rear suspension has been “difficult”. The root cause of this is actually the suspension configuration of these cars and is something that B.M.C. struggled with on Big Healeys until a complete rear suspension redesign was introduced in 1965.

The problem is a consequence of the under-slung chassis which is very unusual in any passenger automobile design. This configuration has the rear axle positioned above the chassis which seriously limits the amount of suspension travel available.

In order to prevent the rear wheels from leaving the ground as the car passes over a drop at speed sufficient clearance between the axle and the chassis is required and, to prevent the rear of the body and exhaust system from contacting the ground when a bump is encountered, the camber and rate of the rear springs are very critical.  In their attempts to get this correct the design of rear spring changed three times during the early days of production of the Austin Healey 100.

The springs finally adopted were reasonably satisfactory when only a driver was in the car but, add a passenger and a load of luggage and the fact that suspension leaf springs tend to sag over the years, it doesn’t take too long before things turn pear shaped.

In my efforts to come up with a solution I spent many hours building various spring assemblies from the dozens of spring leaves that I had collected over the years but a suitable compromise between satisfactory ride height and spring rate always alluded me.

My Carcamel uses air springs for the rear suspension and with the experience gained in designing and operating that vehicle for some 25 years I decided that perhaps supplementary air springs could provide a solution to this Healey rear suspension problem.

I found a suitable size of air spring and installed a pair between the axle and the rear bump stop boxes on B.#174.

The result has been fantastic.

On the early BN1 rear suspension the rubber “bump stops” are mounted on the underside of the bump brackets (commonly called bump boxes) inside the rear wheel arches and directly above the rear springs. (Later cars have the bump stops mounted on the axle). When fully compressed, the air springs selected are designed to work as bump stops and their compressed length is very close to that of the rubber bump stops on the BN1.

These air springs have a mounting thread on the upper end which was used to mount the air spring directly onto the bump brackets after having removed the original bump stops.

For the axle end a pair of mounting plates were fabricated which were bolted to the rear spring mount gussets on the axle and the air springs were in turn mounted to these plates using the pair of studs on the air spring’s lower end.

Each air spring has an air inlet on the upper end which uses 1/4:” diameter air tubing.

These air tubes were run to a “T”  directly above the differential and the third leg of the “T” was connected to an elbow and then to a bulkhead mount Schrader valve carefully hidden under the Armourcord of the floor of the tool tray inside the boot.

Once installed this modification cannot be seen without looking into the battery compartment.

The leaf springs fitted to B.#174 produced the correct ride height when the car was unloaded however, adding just two passengers dropped the rear end by almost ¾”.  By inflating the air springs to around 15 p.s.i. the ride height, with passengers, returned to spec and 24 p.s.i. achieved the same result with two passengers and 100 lbs. of luggage on board.

I carry a very small rechargeable air compressor with me these days which is great for adjusting tyre pressures and now rear ride height.

The car has now travelled several thousand miles with the air springs installed and all the rear suspension woes are behind me, so to speak, and I plan to add the same system to my fuel injected BN2 as this modification with some very small changes will adapt to any Big Healey, even the late BJ8 with the improved rear suspension.