Simple ideas are often the best...

What Are Rubber Springs?

  Logical progression: A spring stores energy as we deflect it, then releases this energy as it is allowed to return to it's previous shape. Given that a lump of rubber does this too, then we could try using a lump of rubber as a spring. So they did, and it worked.

  Most famous use of rubber to provide the suspension spring on a car is probably the original Mini, with it's system designed by Alex Moulton. Moulton is sometimes credited with "inventing" the rubber spring, but that isn't quite true - his family's business had been involved in the fitment of rubber suspension systems to train carriages, and he had long hoped to use the technology in a car. He got this chance when the Mini's designer, Sir Alec Issigonis, asked him to develop the suspension for the groundbreaking new car in the 1950s. Incidentally, during the '30s, Issigonis and a friend built a single-seat racer based on an Austin Seven which also used rubber suspension. It also had a monocoque chassis, which from a technological point of view eclipsed contemporary F1 cars. Not bad for what was basically a good old two-blokes-in-a-shed effort!

  Back to the Mini. The original intention had been to use Moulton-fangled Hydrolastic suspension (don't worry if this means nothing to you, we'll be covering the system in the next section), but the system wasn't compact enough at the time, so they used a rubber-cone system similar to that which Issigonis and Moulton had collaberated on previously.

  On the right we see the rear suspension set up from a Mini (the front uses rubber springs as well, but everything's easier to see at the back). The actual spring is a lump of rubber about the size and shape of a big Christmas pudding, and it is compressed by a metal cone (usually called the "trumpet", due to it's shape) which is itself loaded by the suspension arm as it moves. As you can see, it makes for a very compact system, but it also has the advantage of being rising-rate. Yep, that suspension-gets-firmer-the-more-it's-compressed malarkey which is so useful for car applications.

  About the only issue with this design is that the rubber degrades over time, with attendent lowering and stiffening of the suspension, though it must be said that steel springs in their various incarnations hardly last forever, either. Replacement rubber cones are fairly easy to get hold of though, and the expected life is something in the region of a decade, which is fair enough. Going back a while, you could even get stiffer "competition" springs, made from firmer rubber, but as far as I'm aware they became unavailable a long time ago.

  With it's compactness, light weight and simplicity, it's not really a surprise that rubber springing is useful for building trailer suspension, such as the units made by AL-KO. From the outside, they look like the torsion-bar system found on the rear end of Peugeots and the like, but inside they're quite different. Rather than having a metal bar that twists under load along it's length, these suspension units have two shaped tubes running inside each other, with rubber in the gap between them. As the suspension is loaded, the inner tube rotates, crushing the rubber between itself and the outer to provide the springing action.

  For a simple demonstration, click on the picture of the AL-KO unit on the left to see an animated version. This design of suspension unit is a common sight on trailers of all sizes, with AL-KO making them in various ratings of between 1/8 & 2 tons per wheel. Similar designs are made by various manufacturers.

Back to Suspension Basics 5 - Coil Springs

Forward to Suspension Basics 7 - Hydrolastic & Hydragas Springs