UNSPRUNGMASS
Removing weight from your classic Ford helps improve handling, but removing unsprung mass is even better. Let’s find out why.
Todd Crooks, Jamie King
When it comes to talking about a car’s suspension setup, the term ‘sprung mass’ means everything inboard of the springs; that includes the bodyshell, engine, gearbox, anything inside the car (including the driver), and so on.
The ‘unsprung mass’ is everything on the other side of the spring. For the most part, that weight is made up of things like wheels, tyres, hubs, wheel bearings and brakes.
Other suspension parts where one end is inboard and the other outboard (such as suspension links, control arms, driveshafts and so on) can generally be considered as 50/50 unsprung/sprung.
LOW INERTIA
Along with the general benefit of reducing overall vehicle weight, reducing any unsprung mass in turn reduces the inertia of any component parts attached to the wheel.
Inertia, put simply, is the resistance of something to acceleration – so when your wheel sees an input, such as from running over a kerb on track, a heavier unsprung end will apply more force to the body (via the suspension springs) as it moves over that kerb. Then, to make it even worse, the spring and damper must work harder to control it all again.
All in all, this means that a heavy unsprung mass is much more disruptive to your suspension (and in turn, the body of the car) than a lighter one.
And that’s important when we consider the effect it has on the tyre’s contact patch with the road, or more specifically the load applied to the tyre’s contact patch (more load pushing the tyre into the road means better grip).
FEEL THE DIFFERENCE
As a side effect, unsprung mass can have a huge impact on the car’s ride quality too. This is often as a result of the body disruption effect – the heavier unsprung load applies greater forces to the body.
The consensus is that every kilogram of unsprung mass saved is the equivalent to 3 kg of sprung weight. Therefore, saving weight from your unsprung mass will have a much greater effect on the car’s ability to accelerate than reducing the weight anywhere else on the car.
As a driver, though, the likely most notable effect of reduced unsprung mass will be felt through the steering;
most of the unsprung mass at the front of the car is also turned with the steering (hubs, brakes, wheels, tyres and such like), so the steering as a whole will feel generally sharper and more responsive to your inputs.
The downside of this is the car may be more susceptible to bump steer from the same road imperfections as before.
ROTATING MASS
A subset of unsprung mass is rotating mass: the wheel, tyre, brake disc and so on, which spin with the wheel. These parts clearly move along with the car, but additionally they also spin at a rate proportional to the car’s speed. Therefore these parts require accelerating in both direction of travel of the car and in their own rotational direction too.
This in turn means they require significantly more engine torque to accelerate compared to a non-rotating part of the same mass.
The only real downside is that you will find the line between hard braking and locking a wheel will be much finer; there is less weight to stop, so the same brake force will cause the wheel to lock. You might need to work on your brake modulation, but it will be worth it for the acceleration benefit.
WHAT CAN YOU DO?
So, what can you do to reduce unsprung and rotating masses? The best place to start is with your wheels, not least because any gains will be multiplied by four.
For example, a wheel that’s 5 kg lighter than the factory item will mean a 20 kg overall reduction in rotating mass and unsprung weight. Factor in the ‘1 kg unsprung is worth 3 kg sprung’ rule as mentioned earlier, and that’s the equivalent of removing 60 kg from the chassis.
Not only that, but motorsport wheels tend to be stronger than OE alloys as well as lighter. Many cast aftermarket wheels will offer a significant weight reduction over an OE alloy, but for maximum weight saving, motorsport teams often opt for forged alloys. The forging process results in stronger material properties, meaning the same strength can be achieved using less material, resulting in a lighter wheel.
Top-flight motorsports take things even further with use of exotic alloys and even carbon fibre composites, but these tend to be a little overkill for the majority of fast Fords – although if you’re in the market for the latest GT, they come with a carbon wheel option from the factory. For this feature, though, we’ll stick to lightweight cast and forged alloys.
OTHER EFFORTS
The tyres fitted to your wheel choice will all be of a similar weight for the same size, but other things like brake setups tend to make a big difference.
Obviously, the braking advantages of a bigger, more powerful setup is clear to see, but when you’re not slowing down, a bigger brake kit can be detrimental to performance. Think about it: you’re carrying all that excess weight about, and having to speed up that big, heavy disc every time to accelerate again.
The ideal brake setup is the lightest calliper (probably a monoblock design) with the smallest diameter and lightest disc you can get away with while still meeting your braking demands. There’s no point in fitting 380 mm discs and 12-pot callipers when a 300 mm disc and fourpot calliper setup will do the job; you’re just carrying the excess weight with no performance benefit.
Beyond wheels and brakes, reducing unsprung weight becomes much harder, and the gains are minimal for the costs involved. Hubs, knuckles, links and arms can be improved in places, but remember that only 50 per cent of that improvement goes to unsprung weight reductions, and it won’t change your rotating mass.
To improve both, even small things like lighter wheel nuts make noticeable differences. It may only be a few grams per nut, but when there’s 16 or 20 of them, the difference can soon add up.
Every last little part of the unsprung and rotational mass can make a big difference to the way a car not only handles, but how it accelerates and brakes too, so it’s always worth knowing about and making changes where you can.