Project Mini Cooper S | Magazine Series Part 6: Addressing Our Mini Cooper’s Suspension and Brakes

Update by Tim Suddard to the Austin Mini Cooper S project car
Oct 5, 2020

[Editor's Note: This article originally ran in the March 2014 issue of Classic Motorsports; for more updates on our 1967 Mini Cooper S, visit here.]

Photography by the author unless otherwise credited

Which car has no springs and no shock absorbers yet still handles like it’s on rails?

The answer is a mid-’60s Mini Cooper.

Photograph Courtesy Mini

When Sir Alec Issigonis first envisioned the Mini, he was working with legendary suspension designer Alex Moulton on an entirely new suspension system. Eschewing standard shock absorbers and traditional live axles suspended by leaf or coil springs, this so-called Hydrolastic suspension instead used fluid-filled rubber bags–one at each wheel. The fluid mixture in these displacer units was essentially water and alcohol.

Pipes linked the front displacer units to their rear counterparts. That means the right-front displacer was connected to the right-rear unit, while the left-front displacer was connected to the left-rear unit.

Here’s the theory behind the system: When the front wheels encountered a bump, the front displacers would collapse, sending their fluid rearward. As a result, the back of the car would rise and be ready to face that same bump. It was unconventional, but the system combined a comfortable ride with good handling.

The Hydrolastic suspension first appeared for 1962 on BMC’s smaller sedans–the ones that fell under the ADO16 chassis code, like the MG1100, Morris 1100 and Austin America.

Since that setup wasn’t ready for the Mini’s 1959 release, it featured an equally ingenious rubber cone suspension. This time, the traditional coil springs were replaced with rubber cones–picture the business end of a toilet plunger, but in solid rubber.

The Hydrolastic suspension first appeared on the 1964 Mini, but it didn’t stick around for the duration of the model’s run. The standard Minis reverted to the simpler rubber cone suspension starting in the spring of 1969, while the Cooper S finally did the same in June of 1971.

In today’s Mini world, models with Hydrolastic suspension are “wet” cars; the rubber cone cars are “dry.” Generally speaking, enthusiasts perceive the wet suspension as more comfortable yet more complex; the dry setup is said to handle better, but the ride can be harsh. Since the Hydrolastic system is more involved and expensive to maintain, many cars have been converted to the dry setup.

Our 1967 Mini Cooper S was produced with the Hydrolastic suspension and still had it in place. Before we tackled that part of the restoration, we talked with two of the country’s leading experts, Don Racine of Mini Mania and Kip Lankenau of Kip Motor Company. They both agreed that if we wanted to restore this classic, first-generation Mini Cooper S to perfection and achieve the best ride quality, we would need to be patient and revitalize the original wet suspension.

Hydrolastic: Stock or Not?

We intend to keep our Mini rather original, but being gearheads at heart, we couldn’t leave the Hydrolastic suspension totally stock–especially if the modifications could be easily undone.

The vast treasure trove of extra parts that came with our car included a set of Leyland factory competition progressive bump stops. As Mini Mania’s Don Racine explains, these bump stops are longer than the stock pieces; as a result, the chassis tends to dive less. Don says they’re a must on a Hydrolastic car. In fact, Mini Mania sells replicas. Figure around $100 to convert an entire car.

Although we restored our Mini’s suspension to stock specs, we did exchange the stock bump stops for these factory progressive ones. It’s an easy way to reduce the pitch that sometimes accompanies the Hydrolastic suspension.

While visiting Heritage Garage, we learned about another common modification made to these Hydrolastic cars. Heritage Garage owner Graham Reid explains that it’s pretty easy to mount shock absorbers to the front of a Hydrolastic Mini. He says it goes a long way toward offsetting the pitching associated with the wet suspension. Simply fabricate a mount similar to the ones used on the dry cars, and add shock absorbers to the front end.

This modification involves welding external mounts onto our body shell, so in the interest of originality, we decided to wait. Once the car is back together and on the road, we’ll decide whether we want to add the front shock absorbers.

Getting Our Feet Wet

Today, there’s one big problem surrounding the Hydrolastic suspension: The displacer units are no longer available. New old-stock pieces occasionally come up for sale, but demand has significantly driven up their prices.

Making the restoration process even more difficult is the fact that there are several different types of these displacers created for different models and model years. It’s nearly impossible to find the right ones. (Each of these displacers is marked with color bands that indicate the effective spring rate; our Cooper S displacers, for example, have yellow bands.)

Here’s the common failure point: the line running into each displacer. Fortunately, the folks at Kip Motor Company have devised a way to save this suspension.

The first step in the displacer restoration process is to cut off the old line. Next, a new fitting is silver-soldered on. This is done under water to prevent the heat from melting the rubber displacer.

The restored displacers look new and feature fresh hydraulic hoses.

The final step of restoring a displacer system is to pressure-test the unit to make sure there are no leaks.

There is some good news, though: Unless a car has been repeatedly jumped like a Hollywood stunt machine, these displacers seldom go bad. However, the lines can crack–right where they join the displacers. Once a crack forms, the suspension leaks, and the car quickly becomes a lowrider.

Fortunately, our friends at Kip Motor Company can solder new fittings onto the original displacers. That may sound simple, but it’s a delicate process since the solder joint is surrounded by the rubber displacer unit.

Kip’s answer is to do the soldering underwater to keep things cool. As he points out, the process is not an easy one to master; he and his team ruined a lot of displacers while learning how to do it.

Kip Motor Company charges about $1500 to restore a set of displacers. It’s a very valuable service that keeps these unique cars original and on the road. Once the displacers are repaired, a system of bump stops needs to be replaced. Fortunately, Mini Mania has most, if not all, of these parts.

More Familiar Waters

The rest of the suspension is rather straightforward. While ahead of its time, the Mini was built using bolt-on subframes in the front and rear that support the suspension pieces.

While our displacers were being restored, we dismantled and straightened our subframes–they tend to get bent up a bit through the years. East Coast Ornamental Welding, located near our offices in Holly Hill, Florida, then sandblasted and powder-coated the subframes.

From there, it was simply a matter of ordering new bump stops and brake hardware and beginning the reassembly. We found a factory shop manual that made this process simple. Mini Mania has everything you need to rebuild your Mini suspension, so sourcing parts turned out to be pretty easy as well.

The rest of the Mini’s suspension restoration is pretty straightforward. A pneumatic pickle fork helps remove the upper ball joints, and new ones were sourced from Mini Mania.

Rear control arms carry conventional drum brakes and backing plates. As usual for our project cars at this stage, everything looked this nasty.

With everything disassembled, we had a stack of parts to take to the powder coater. The rest of the pieces were painted with Eastwood Chassis Black paint. Small tip: Always photograph and itemize any parts you take to a powder coater, so you can quickly make sure everything comes back.

Once we refinished our subframe, we could begin reassembling it. Working with nice, clean parts is so much easier. Another tip: We use some old prison cafeteria trays–Import Carlisle finds–to hold hardware.

Next, we fit the restored rear displacer units into the subframe. Eventually, the Hydrolastic system will be refilled with a mixture of alcohol and water.

We carefully disassembled and cleaned the steering rack, purchased new tie rod ends from Mini Mania, reassembled the rack, and adjusted it properly. Then we painted any black parts not sent to the powder-coater with Eastwood Chassis Black. Then we restored the cadmium-plated parts with Eastwood’s cadmium paint system.

We redid the brake system at this point, too, and again Mini Mania had nearly everything required. The front brakes were restored to stock specs aside from EBC pads. This is an easy way to make a major improvement over no-name stock products. This modification didn’t affect originality either.

Our completed rear subframe sits next to our restored drivetrain. Those coil springs don’t support the car; they’re just limiting springs that come into play if the car ever gets airborne.

We’re trying hard to keep this one stock, but we did retain these period-correct Mini-fin alloy brake drums that came with our car.

We completely restored the front subframe, too. Everything from the wheel bearings to the axle shafts was professionally redone.

Finally, we carefully disassembled the steering rack, cleaned all the bits, and properly put it all back together, making sure to shim the preload correctly.

We used Mini Mania’s rear shoes, but we kept the cool Mini-fin alloy drums that came with our car. We’re trying to keep this Mini close to stock, but these period-correct accessories were too cool to discard–and we’ll take any extra braking performance we can get. Besides, you can’t really see them without getting on the ground to look behind the wheels.

Now that our engine, transmission, differential and subframes are all done, we can turn our attention to the metalwork. Blasting the body shell revealed some horrors, and now it’s time to face them.