It’s important for every 4×4 driver to understand the concept of unsprung weight and rolling mass as they look to bolt go fast or protection bits to their 4wd.

So what is unsprung weight?

Unsprung weight is the total weight of everything that is NOT supported by your suspension. So that means your diff, diff housing and all the other driveline bits inside, your rims, your tyres, your brakes and so on.

The net effect of a larger than normal unsprung weight is that as the springs and shocks have to raise and lower the extra weight and things happen a little slower. This means you are more likely to loose contact with the road or track when the suspension needs to respond quickly.

You’re not going to feel this as slow speeds but you will discover a more noticeable a difference on corrugations or even on the blacktop as you head to your destination.

The main culprits for extra unsprung weights are:

  • Steel Rims
  • Mud or bigger tyres
  • Diff guards
  • Lockers

Now all these things have upside when you are off road and I’ve at times installed all of then on my 4x4s. But it’s important to realise what you are sacrificing when you bolt these accessories on.

Heaver unsprung weight will be detrimental to on road performance and in some faster moving off-road circumstances.

If you want a 4×4 that maintains some comfortable on road handling, perhaps think twice about your unsprung additions, but if you want an off-road weapon that will go anywhere, then who cares how hard it is to drive on the road anyway!

So now Rolling Mass, what is that?

Rolling mass shares some similar aspects to unsprung mass and is seemingly more often that not confused as being the same thing.

It’s actually very different.

Rotating mass are parts that rotate during the acceleration and deceleration of your 4×4. We’re talking here about wheels, tyres, driveshafts and diffs but also engine and transmission components. The critical thing about rotating mass is that it is around 3x times harder to accelerate or decelerate per kg than a non rotating mass.

So for every 1KG of extra rotating mass you add, it’s the equivalent of 3KG of extra normal weight to your 4×4.

Of course the same is in reverse. So whilst your heavy steel rims and big tyres will effect handling (unsprung weight), it will have a bigger effect on performance and fuel economy.

Remember though, this is only for rotating mass not unsprung, so things like diff guards wont impact acceleration any more than having them sitting in the back of your 4wd.

So that’s unsprung weight and rolling mass and the impact they have on the performance characteristics of your 4wd. Don’t you feel smarter now 🙂

Thanks to the racing industry for figuring out that one for us.