Anything that is attached to the suspension of the car and moves under compression and extension is defined as unsprung weight. This includes wheels, tyres, brakes, hubs, suspension components, even a portion of the driveshafts and wishbones are under this umbrella. When you hit a bump in the road, everything that has to move in order to absorb the bump is classed as unsprung weight.
Reducing unsprung weight means that when you hit bumps, kerbs and bigger undulations, the wheel and tyre can move more easily out of the way and extend faster to remain in contact with the ground, therefore increasing grip.
Imagine holding a heavy dumbbell mid-air, and having to lift it vertically from one point to another within a set time frame and back again, let’s say a foot within a second. To initially move the dumbbell requires a lot of energy, then when it returns back down, it naturally wants to go straight past the initial point so controlling it is very difficult, and normally ends up in your arm movements affecting the rest of your body, as the energy and inertia forces you to move the rest of your body to keep it controlled.
Now do the same thing with a tin of beans, suddenly it’s much easier, faster, and requires less energy so you can probably do it without translating those arm movement into the rest your body. The same thing is happening with your car. Lowering the unsprung weight is like going from the dumbbell to the tin of beans. Your arm and the thing you’re holding are the unsprung weight and the rest of your body is the chassis of the car.
In its most basic form, the spring rate refers to how much force it takes to compress a spring a certain distance. For example, a 50N/mm spring requires 50 newtons of force, to compress it 1mm. There is no straight forward equation for calculating the correct spring rate for your car, as there’s a lot of variables that must be considered when deciding on which spring rates to go for.
Generally speaking, a stiffer spring rate will induce more mid-corner grip as the car will transfer less of its weight to the outside wheels, meaning a better distribution of load between all 4 tyres. The contact patch of 4 tyres combined will be bigger therefore lateral grip is increased. The same applies during braking, when you hit the brakes less weight is transferred to the front wheels, so you have a larger contact patch on all 4 tyres to utilise for braking. Whereas with a softer spring rate you unload the rear tyres reducing their contact patch with the road and therefore making them less effective during braking.
A disadvantage to running stiffer springs, is that once you are at the limit of grip there’s less progression and the car will tend to “fall off the cliff” once it does break traction, rather than progressively losing more grip the harder you push. There’s less “middle ground” between having grip or not which can make the car more difficult to handle on the edge which doesn’t suit all driving styles. This can be particularly problematic in the wet as being able to feel when the car is about to let go is crucial, however this can also be managed with adjustable anti-roll bars not just spring rates.
In comparison, a softer spring rate will provide more road comfort on uneven surfaces, bumps and pot holes, as when the wheel hits these imperfections the wheel moves with little resistance and without translating those forces into the chassis of the car. When the wheel needs to make longer travel movements to follow the road, a softer spring rate will allow the car to remain stable, and the tyre to move out the way with ease, whereas a stiffer spring would not only move the wheel, but move the chassis of the car too which could affect comfort and the balance of the car equally.
When it comes to choosing the correct suspension set-up for your car, it can become a minefield. In a world of double digressive valves, inverted monotube dampers, 4-way adjustability, TVCLH-A, DFV, lots of other jargon and terminology, it can be hard to work out which kit is best suited for your needs.
If you're looking to upgrade your car and not sure where to start, the best bet it always to contact us directly so we can advise and inform you of the information you need to make an informed decision.
However we understand this may not always be practical, and we are extremely busy dealing with customer projects so we've put together a simple guide to help you learn the technology used in modern day suspension systems, why they're so good and which suits your application best.
We can supply, fit, set-up and advise on a huge range of suspension products, beyond the brand and what's listed in this article. So if you have a more specific enquiry or need more info, we're here to help with any queries no matter how technical or simple they may be!
We're official distributors for Ohlins, BC Racing, KW Suspension. We have a small selection of what is avaiable in our webstore, if you vehicle isn't listen please get in touch. We ship worldwide and can source suspension to suit almost any make and model!