Bugatti installed De Ram shocks on a variety of models in the mid-1930’s, the sophisticated hydraulic dampers offering a dramatic improvement in handling and ride quality over the simple friction devices that were previously available. When they are working properly, that is. Given the complexity of the design and the relatively small number of them that exist in the world, getting a set of De Rams to work as designed is no small matter. I’ll talk here a little about the history and what lives inside the rounded steel housings.

A Quick Refresher on Taming the Bounce

Automotive shock absorbers, or dampers as they are known in many parts of the world, have come a long way in the century or so since they were first invented. Horse drawn carriages used leaf springs for at least a hundred years before the automobile, and they relied on the friction between the spring leafs to dampen the bounciness of the spring. Binding the spring leafs tightly together could increase the friction, and a variety of methods were used.

When the internal combustion engine upped the speed of the carriages, more damping was required and friction shock absorbers came into use. Basically a stack of discs, squeezed together and operated by a pair of arms, the added friction slowed the bounce and rebound of the steel spring. The main limitation of a friction damper is that the force required to break free the mechanism is much greater than the force required to keep it moving, once unstuck. The more tightly the discs are squeezed together, to increase the damping effect, the more force is required for the initial motion.

That results in damping action that is extremely stiff at low speeds with small displacements of the suspension, if they are to be stiff enough to work at high speed or take big hits.

Oil Comes into the Picture

There were a number of attempts at overcoming the limitations of a friction damper by using hydraulic fluid as the damping medium, with the Houdaille shock absorber being one of the most successful. Still a rotary device, it had vanes moving inside a chamber of oil, and the vanes forced the oil through an orifice engineered to slow the mechanism to dampen the spring motion. But there was only so much range of damping available with such a simple device. (Suzuki used a similar design on its TLR sport bikes in 1998, with more sophisticated valving, but it was almost universally disliked by critics.)

In the period between the world wars, Georges Henri Ernest De Ram thought he could combine a friction damper with hydraulic control to make a vastly better damping mechanism, and he did. His patent application filed on September 11, 1928 was the basis for the unit that was manufactured and used on Bugattis in the early 1930’s.

Motor Sport Magazine reported on the new dampers in April of 1935, which opened with the following:

WITH the first appearance of the new 750 Kg formula racing cars, 1934 will always remain a red letter year for the followers of motor racing, and particularly a year of triumph for the German racing cars with their independent suspension, which undoubtedly has given them a great part of their superiority over the orthodox Monoposto Alfas. Bugatti however stuck to the rigid axle lay-out, and much to the surprise of the critics these cars proved themselves in road-holding little if at all inferior to those built according to the new ideas. The familiar Bugatti suspension system, half elliptics in front and reversed quarter elliptics at the rear was still retained. How then had it been adapted to deal with speeds in the neighbourhood of 170 m.p.h. ? Simply by changing over to De Ram shock absorbers, the merits of which have come to be appreciated fairly generally on the Continent and which have several times been referred to in these pages.

https://www.motorsportmagazine.com

I’ve had some experience with these extraordinary devices over the years, in a period of my life where I was also doing suspension work on my KTM dirt bikes, which required making valving changes to hydraulic dampers in a much simpler and modern design.

The De Ram’s Mechanism

One way to think of the De Ram shock absorber is as a mechanical brake, the braking force slowing the motion of the suspension, with the discs squeezed together by hydraulic pressure to vary the amount of braking force. In many ways, the mechanism resembles a wet clutch as used in motorcycles, or in fact, like the multi-plate clutch Bugatti used in many of its cars. The De Ram uses oil pressure and several small pistons to squeeze the plates instead of coil springs, or the cam mechanism in the Type 35 clutch.

The hydraulic pressure is generated by a large piston that is moved by a cam attached to the operating arm of the unit, and different cam shapes were used to vary the rate that the pressure increased. A steeper cam face would increase the pressure more quickly than a gentler slope, for example.

On top of the fixed hydraulic pressure that is generated by the size of the pistons used and the shape of the operating cam, the units are adjustable with a needle valve that controls the flow of oil from the main piston. Or more precisely, how the pressure is vented from the cavity, as it limits the peak pressure. Different springs can be used in the control valve, as well as behind the small operating pistons, which influence the amount of friction when there is no hydraulic force applied.

The disc stack can also be assembled with preload, by changing the spacing arrangement of the discs, and in fact at least one Bugatti specialist eliminates the hydraulic feature altogether, removing the control valve and relying on disc preload for them to work! A terrible option, in my opinion, because as soon as the plates wear, the damping effect is lost. Not to mention, you are basically left with a very expensive friction damper that works just as poorly.

The mechanism is incredibly complex and must have required some very good machinists to manufacture. Given that, it was also very expensive. In 1935 they cost 200 English Pounds for a set, including installation, or £12,415 in today’s money! ($16,263)

Getting Inside and Making it Work

A number of special tools are required to work on De Ram shocks, many of them requiring a machinist to make due to the fine splines that are required. Some tools are more simple, such as the four pin wrench that unscrews the main housing. Originally, the four pin holes in the cover plate were filled with lead at the factory… for aesthetics or to prevent hack mechanics from working on them… who knows?

None of the parts inside are available to purchase, as far as I know. At Phil Reilly & Co, we had new bronze friction discs manufactured, and had some special Teflon seal rings made. Anything broken needs to be reverse engineered and made from scratch, or robbed from unused spare units (unlikely!)

More than once, I’ve found destroyed springs inside the valve body, and a wide variety of springs have been tried by folks over the years. I spent quite a lot of time playing with different springs in the valve body, which influences how far the valve opens and the volume of oil that passes. If you were to plug the valve entirely, the main piston generated enough pressure to lock the disc pack together solid.

More fundamental is the initial setup of the disc stack, which influences the low-speed action… how stiff it is without hydraulic pressure. Theoretically, you could set it up with zero preload, but it would take a larger movement of the suspension to begin to make pressure and squeeze the discs together.

In my work, I used a simple torque wrench to measure the effect of changes, which is a crude method that only gives limited feedback. It would be useful to setup an actual shock dyno to see the effect valving changes make at speed. But it is interesting to feel how the shock responds to how quickly you try to move the arm. A slow push gives solely friction damping, while faster motion begins to get the hydraulic action into play.

You can also get substantial changes in action by changing the viscosity of the oil used. 5 weight motorcycle shock fluid will flow more easily through the valve body and reduce the operating pressure, while heavier fluid, like 15 weight will make more pressure and result in a stiffer damper.

The front and rear De Rams have different internals, primarily changing the number of small pistons and the shape of the cam. The fronts typically have twelve pistons while the rears have eight, though the main piston is the same diameter and the stack contains the same number of discs.

Now that we’ve taken a look at what makes a De Ram unit work, the next installment will take a look at some of the common failures and what can be done to keep them working properly. Sign up below to get notified when part two comes online.