MOTORING: Car ground clearance: What you should know

If your car is a speed-bump scraper, or you want it to be less vulnerable on rough roads, there are several ways to increase its ground clearance.

The simplest and cheapest are to adjust the suspension height, or fit larger wheels and/or tyres.

There are a few keynotes to bear in mind: First, a little increase makes a lot of difference. So don’t try to change a lounge lizard into a rock rabbit. A suspension lift of two or three centimetres (by adjusting the setting, fitting a spacer) will make a car significantly more competent; more than that will be technically complex and expensive, and could have some unwanted side-effects on the suspension and steering geometry. If you need to go more up-in-the-world than that, don’t change the clearance. Change the car.

Second, if you go for more height using bigger wheels, or even just a higher profile tyre, you will need to check the space in the wheel arches for both suspension travel and steering lock. And even if there’s no problem there, the change will affect the car’s gearing … perhaps awkwardly.

BEST RESULTS

A bigger tyre/wheel diametre that gives a higher ride also increases the tyre’s circumference. And that changes the gearing ratios, which affect acceleration (less good), top speed (higher), and distort the speedometre and odometre readings. The difference may be small – between five per cent and 10pc – but that can be noticeable, and ultimately annoying.

Third, recognise that a car’s clearance is not a single measure. It varies depending on whether the vehicle is static or dynamic (standing still or when the suspension is flexing during movement), and on the load it is carrying. Not just the height, but also the hardness of the suspension matters. There are also distinctions between the so-called entry, break-over and departure angles, determined by the wheelbase and the length and height of front and rear overhangs.

With these keynote factors in mind, and with some consideration for cost, the inevitable question will be whether bigger tyres or a lifted suspension are the better solution.

The answer is “neither”, because the answer is “both”.

The best results will be achieved by making a small change to the suspension height, and a small change to the tyre size. That way you will minimise the unwanted technical side effects (because the change is minor) and optimise the desired benefits (because the effect is cumulative).

Those considerations and principles apply not only to increasing a car’s clearance; and not only to tinkering with suspensions and tyres for any other reason.

They are practical wisdom when making almost any alteration to a car’s original design – engine, bodywork or whatever.

Changing one thing, even positively, almost always affects another, usually negatively. If there was an assured win-win alteration, the designer would have made it before the vehicle was built.

For your very personal preferences or usage needs, some modifications can give you a plus that you want with a minus you are ready to tolerate. Change settings within their design limits; don’t try to reinvent the system. When you do what you do, ponder why the manufacturer didn’t.