The purpose of suspension is to isolate a vehicle's contents (you) from undesirable effects of the surface a vehicle is being driven on, but without losing feedback and control for steering etc. And that, basically, is it. You need to be disconnected enough from the road surface not to get shaken about every time you go over speed humps, potholes or the neighbour's cat, but not so disconnected that you have absolutely no idea what the car's doing beneath you.

Why Suspension Exists

  Flat surfaces are hard to come by, and most roads don't get anywhere close. Although this is obvious when looking at the kind of rutted cart tracks used for centuries, it's still true for a fresh tarmac surface, even before the famous British roadworks take their toll.

  The first problem encountered with a vehicle with four or more wheels is making sure that they all touch the surface. A wheel off the ground is no good to you; it can't support any of the vehicle's weight or help you accelerate, brake or steer. If you have no suspension, and your wheels are rigidly attached to your chassis, then the only way to have all the wheels touching the ground is by the chassis flexing. Bad idea. Of course, a vehicle with three or less wheels doesn't have this problem, though you still have to contend with the issues found once you start moving.

  So you've got what is quite possibly the simplest vehicle available: A horse-drawn cart that is little more than a flat wooden surface between two wheels. As you roll along, every movement up or down of a wheel is transmitted straight to the load you're carrying, which is not in the least bit comfortable for your passengers, and a liability if you're carrying anything delicate or fragile. On top of this, the wheel itself (most likely made of solid wood) is trapped between the weight of the cart pushing down, and the force of hitting a bump pushing up. Given a big enough bump combined with a heavy enough cart, at some point you'll break a wheel - so what's needed is a way of absorbing these shocks as much as possible.

  It's not just impacts themselves, though: Once you start travelling at speed, not only do you have to deal with the effects of these shocks (which get worse the faster you hit the obstruction), but you also have to deal with vibrations caused by smaller surface imperfections, that you may not have noticed at lower speed. All in all, the problem is very simple: The surface is transmitted directly to your arse, and it's not at all comfortable.

  Looking at your cargo being people (on which the effects of rough surfaces are more instantly noticeable than on inanimate objects, because the latter don't complain as much), your first step in guarding against bumps is to put a nice soft cushion 'twixt buttock and seat. This stops a lot of the complaining, but doesn't really help with the shocks affecting your vehicle's structure. If you build the part of the vehicle that you want to ride in as a seperate unit, and then hang it with leather straps or chains from the frame which has the wheels attached, you go quite a way towards smoothing out shocks, but the downside is that you tend to sway around a bit too freely. No, what is needed is something that will absorb shocks and impacts, but still be stiff enough to hold everything together.

Spring Has Sprung

  If you take a metal strip and lay it across a gap, then put a load in the middle, it bends. Remove the load and, provided it wasn't too heavy, the strip bends back - you've got a very simple spring. Using the same principle, if you put a similar metal strip in place to link your wheels to your cart or carriage, the strip bends to absorb shocks and loads. Because it's thin in one direction and wide in another, you get lots of bending (and therefore shock absorbtion) in one direction, but rigidity (and therefore no swaying about) in the other. You now have a very simple but effective suspension system, the basic principles of which are still in use.

  In the following sections, we will be taking a look at how car suspension design evolved from these simple ideas, and the reasons behind the advancements made.

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