Theme : Evolution – Or Metamorphosis?

We trace a direct descendent.

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In 1922, against great opposition from his board, Herbert Austin introduced his Seven into a market dominated by the rudimentary cyclecars that had sprung up in the wake of the First World War. The Seven was a proper small car and, unlike other ‘people’s cars’, it had no radical and untried solutions.

It used a small 4 cylinder, front mounted engine, taking drive through a clutch and gearbox to a rear axle. The solid axle was suspended on elliptic springs but, because everything was so light, the springs did not need to be supported behind the axle and were just quarter springs flowing rather elegantly out of the line of the chassis to which they were fixed. In the event, the car was hugely successful, easy to maintain and has a loyal and committed following to this day, but it didn’t stop there.

Simple But Effective : The Austin Seven Rear Suspension
Simple But Effective : The Austin Seven Rear Suspension

In late 1927, a German company, Automobilwerk Eisenach, started building their version of the Austin Seven, The Dixi, under license. At the same time, the BMW Company, formerly plane builders, had been building motorcycles, but wanted to move into car production. In 1928, they bought Automobilwerk Eisenach and thus appeared the BMW Dixi, soon improved and renamed as the BMW 3/15.

BMW were ambitious and, with the licence to build the Seven lapsing in 1932, started developing their own designs. However, after trying more radical solutions, some aspects of the Seven layout remained. In particular the rear suspension. On the 326 saloon of 1936, its frame stopped in front of the rear axle, just like the Dixi. However, hanging the axle off a couple of cartsprings on a far heavier six cylinder car would have been entirely inappropriate.

Still, taking inspiration from the recent Citroen Traction Avant, BMW replaced the quarter elliptics with a torsion bar arrangement leading back to the axle on both sides, ensuring decent location of the axle by attaching an A bracket at the centre point. At the same time, by the late 30s, the original 747cc Austin Seven unit from the Dixi had also changed into a 1971cc six cylinder but, as each development on this route referred to its predecessor, aspects of the pre-War BMW six can be traced back to the little Austin unit.

Clever Derivation. The BMW 326 Chassis
Clever Derivation. The BMW 326 Chassis

After the Second World War, looking around for an inroad to becoming car manufacturers, the Bristol Aircraft Company ended up developing the BMW 316 chassis and the six cylinder engine into the first Bristol, the 400. The subsequent slow evolution of Bristol is well documented but, although other aspects of the cars changed, notably the replacement of the BMW based Bristol Six with a Chrysler V8, the final Bristol Blenheim stayed true to this rear suspension layout. It was an unfashionable live axle, but it worked very well indeed. And without Herbert Austin’s baby it would never have existed.

Bristol 400
Further Refining – The Bristol 400

2 thoughts on “Theme : Evolution – Or Metamorphosis?”

  1. That´s an informative photo. I had no idea that that could be done like that. I notice there´s a CV joint connecting the final drive to the fixed part. You´d wonder about the stresses imposed on that. I am not familiar with what the Bristol live-axle looks like: they didn´t have leaf-springs?

    1. Except for the Fighter, the Bristol rear axle stayed true to the bottom illustration. You’ll see the fixed torsion bars running forward with arms transferring the springing to the axle. Although I say the inspiration for using torsion bars might have come from Citroen, BMW might just as likely have been inspired by Tatra. In any case, they mounted them in a totally different way. Despite being technically a live axle, the BMW / Bristol system is by no means a crude, floaty cart-sprung solution. It’s also space efficient – there are no coils going up and the whole area behind is clear for boot space or whatever.

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