A Quick Explanation of the BMC Hydrolastic Suspension

We recently restored a 1275cc-powered 1967 Mini Cooper S, one that was originally equipped with Hydrolastic suspension. What a fantastic design.

Designed by Alex Moulton, this unique suspension system uses hydraulic displacer units filled with water and what is effectively antifreeze. Lines run between the front and rear displacer units; when one wheel hits a bump, the liquid raises the other end to reduce pitch. This system totally replaces the conventional springs and shock absorbers found on most cars. 

In fact, at the rear, the displacer unit was mounted horizontally with a rocker arm-type device to save room and make the chassis even more compact. This increased the interior space even further.

This hydrolastic suspension setup wasn’t limited to the Mini Cooper. The standard Mini as well as other BMC small and midsized, front-wheel-drive sedans from the era—cars like the Austin America, MG 1100, Morris 1500 and so on—featured this setup. The Mini used this suspension from 1964 through 1971; some other BMC cars used it deep into the ’70s.

Despite the large production figures, parts have been hard to find. As a result, many cars were converted to the more conventional “dry” setups used before and after the hydrolastic system.

Since this system was so praised by the day’s testers, we have decided to keep our car original.  Kip Motor Company claims they can fix the now-unobtainable displacers, so we’re going to give it a shot.

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efahl New Reader
11/10/20 2:39 p.m.

My first car was a Hydrolastic Mini Cooper S 1275 (1293 really, full race SCCA C Sedan car).  I kept a gallon of suspension fluid around and raised or lowered the car on a whim by pumping in or out enough fluid to suit my fancy for the day.

Brian_13 New Reader
11/10/20 5:38 p.m.

The Skill-Lync video is truly terrible, not showing the displacer unit function at all. It includes a cross-section (starting at 1:20), and adds labels, then never shows it working; the description in the audio is even mismatched to the illustration, misidentifying the spring. At one point the later silly graphics even show fluid flowing into front and rear units at the same time, which is not possible because they are simply connected and have no other source of fluid.

To be fair, although there are some good descriptions avaialble online, I wasn't able to find a decent illustration at all.

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