Unsprung weight; what is it, and why is it important?
A lot of people replace the factory suspension in their 4WD with something aftermarket. This is done for a huge number of reasons, but I feel that a lot of changes that get made aren’t always fully understood.
Every single modification you do has a long list of downsides, and its important to consider things from a balanced perspective.
Today, we are looking at what unsprung weight is in relation to 4WD’s, and why its so important that you think about it before making changes.
What is unsprung weight in relation to a 4WD?
Unsprung weight refers to the weight of your suspension and anything supported by it. This includes the suspension itself, your differentials (and lockers if you fit them), rims, tyres, diff guards, brakes and so on.
Is lighter better?
In general, the lighter the unsprung weight the better your 4WD will handle, and the less hard your suspension has to work.
Your suspension has a very hard life trying to remove the shock of bumps, corrugations and knocks from entering the cab of your vehicle. Every time the wheel goes up or down, the suspension is working to give you a nice ride.
Now, imagine that the weight that is going up and down (IE your wheels, differentials etc) is doubled, and what do you think happens to your suspension? It too, has to work twice as hard to control the extra weight being thrown around.
What increases unsprung weight?
Probably the easiest and most common way to increase unsprung weight is to fit heavier tyres and rims. Changing from the OEM setup to a quality light truck all terrain or mud terrain tyre will increase your weight per wheel by a fair chunk.
Then, if you fit heavy duty steel rims it makes it even worse. Our previous 80 series Land Cruiser factory steel rims were about 6mm thick, and weighed a huge amount.
Coupled with heavy mud terrain tyres and you have a huge amount of extra weight going up and down at every single bump. This is one of the reasons that heavy, aftermarket rims and tyres can be nasty for your suspension components.
Fitting other accessories like diff guards, lockers and heavy duty panhard rods also makes the unsprung weight increase.
Unsprung weight vs rotational mass
This increase in weight makes your unsprung weight heavier, makes your suspension work harder and increases the rotational mass, which decreases your brakes effectiveness, and makes you use more fuel.
Every modification you make affects quite a few other things, and its so common for them to be overlooked.
It’s impossible to avoid
When you set a 4WD up for touring, you will increase the unsprung weight one way or another. You can’t tour on the OEM road tyres that the dealer supplies. However, you can be smart about it, and fit rims that are light weight where possible.
We went from the OEM tyres on steel rims to quality all terrain tyres (Toyo Open Country AT2), but swapped the rims to OEM aluminium ones which meant the overall weight increase was only a couple of kg per wheel, instead of 10 – 15kg. One of the things people don’t consider is the actual 4WD tyre weight, and you’d be surprised at how different this can be between various 4WD tyres.
You’d be mad not to install a locker to keep the unsprung weight down, but its just a necessary evil that you need to be aware of.
Like many modifications, once you do one you’ll have things change that may need further attention.
If you increase the unsprung weight considerably you may find that the factory suspension is no longer able to dampen the heat generated from badly corrugated roads, and then you need a new, aftermarket suspension setup that has better heat dissipation.
Either way, it’s in your best interest to keep the unsprung weight as low as possible for most 4WDing.
