Installing wheels spacers is common within the tuning scene. They are a cheap and easy way to improve wheel fitment and increase the track width of your car. However, they are also a fast and easy way to ruin the geometry of your car and can badly affect your handling.
The main issue with a wheel spacer is that it moves the wheel to a wider position whilst the rest of the suspension arms remain in the same place. In particular, the king pin inclination axis remains in the same place. Therefore, fitting a wheel spacer moves the wheel outboard and alters the scrub radius of the car. It is important to know whether your car currently has negative or positive scrub radius and ideally how much it has. Fitting a wheel spacer will increase the positive scrub radius at the front wheels which has some unfavourable consequences.
If your car already has a bit of positive scrub radius then the wheel spacer will increase it. An increase in positive scrub radius increases the forces on the steering rack under braking conditions. Any bumps in the road will also be amplified through the steering wheel, making the steering jerky and more unpredictable. Twitching wheels when braking can cause the tyre to lose grip and ultimately can cause the wheels to lock up and slide. An increase in positive scrub radius also causes the wheel to gain positive camber when the wheels are being turned, reducing grip in corners on the front wheels and producing understeer.
If your car has the more favourable set up of negative scrub radius then installing a wheel spacer can cause bigger issues. The outward movement of the wheel can make a negative scrub radius become zero. This is the worst position for it to be in. A zero scrub radius can cause squirm. This is where a scrubbing action occurs on both front tyres in opposite directions which can create unpredictable handling in corners and can lead to understeer too.
Secondly, installing wheel spacers on a car increases the leverage on the wheel bearing. The larger the spacer, the larger the leverage becomes. This reduces the life of the wheel bearing as they are not made to take leverage forces and can therefore wear faster, increasing rolling resistance and power loss of the car.
Finally, the wheel spacer moves the wheel further away from the damper mounting point. This means that the wheel will have a larger effect upon the damper and will act upon it with more leverage. This effectively makes the spring and damper softer and less effective on the wheel. This means that the car can roll more on corner entry and exit and feel generally softer out on track or on the road.
However, there are some positive aspects to fitting wheel spacers to your car that can prove beneficial. Moving the wheels outboard increases the track width of the car. The increase in track width also lowers the roll centre of the car and therefore increases the amount of cornering force that the car is able to carry through its tyres. Due to being able to manage an increase in cornering G force, it means that it is possible to corner at higher speeds.
Installing the spacer on the rear wheels means that the negative aspects take less effect because the scrub radius effects are not present at the rear wheels. Therefore if your car is suffering from oversteer then installing wheel spacers might be a quick and cheap way of reducing that.